The AM Forum

Picked up a sweet pair of tested 811's from a guy in last ER issue (Sylvania) for a modulator I am building. 811,not 811A. Can run them at 1700 or 1500 (have multiple taps on the S-49 transformer). I know that you can push 811A's to 1700, lots of folks do. Not so sure about the 811's. These are going to drive a single 8000 tube a.la. W8ACR/K9ACT circuits (triodes are nice). For the 811A the bias is -4.5V at 1500V. For the 811 the bias is listed at -9V at 1500V.

1. Should I try to run it at 1700 or be happy at 1500?
2. I have a free 6.3V tap on the grid trans that can give me 8V easy. Is that enough bias or will I really need the extra volt?
3. Should I skip the 6.3V hardware and just use a diode string or Zener for the bias?

John W9JSW

John,

The bias point depends on the plate voltage applied to the 811's. 0v@1250v, -9v@1500v, -??v@1700v, etc. The unused 6.3v winding should run a bit high, being unloaded by filament current. It will probably give you all the voltage you'll need with the light load of the bias. I think you'll just have try it, and see what you get. A small variable power supply would be handy for testing, or even just a 9v battery.


Don

If you use a voltage doubler, you’ll be able to get 12v from that 6.3 winding and be able to use a pot to set whatever bias you need.

Ed, KB8TWH

Good call, Ed. I did that in my 813 amp. Worked fine.

Don

Hi John,

Will the modulator grids be pulling any current? (AB2, B2, etc., )  If so, you will need to regulate the bias supply.  Even a few tenths drop will add to the distortion with a hard-driven high Mu tube.  IE, why add distortion, even a tiny amount, if you can avoid it?

That said, in every big rig I have ever built to date, (linear RF, audio AB2)  I have always used a string of diodes in the fil center tap and a rotary switch to select the exact bias I needed, no matter what plate voltage (or screen voltage with tetrodes) I chose.  Figure on ~0.5V drop per diode, diode arrow direction towards ground, forward biased. 20 diodes in a string should give enough range.

The diodes technique is rock solid self-regulated and adjustable. I love using fil CT diode bias for directly heated cathode tubes.

BTW, run as much plate voltage as you dare on the rig. Higher voltage helps efficiency and the tubes will be driven easier with less grid current for a given output. (cleaner)    Make them sing for their supper, but cleanly!  Add some audio negative feedback around the modulator and you're golden. 6-10 dB  NFB  - experiment.

T

I'm with Tom Vu on this one. I've built many many amps with 811, 811As and 572Bs. All used Zener diode biasing. Simple, cheap and effective.

Tom please explain how to do this - Add some audio negative feedback around the modulator and you're golden. 6-10 dB  NFB  - experiment.

1700v it is, with a diode string instead of the battery.

Attached is my schematic. Let me know if you see anything messed up. Been reading a LOT of old threads on how to get this simple design correct.

John - W9JSW

I have always preferred the center-tapped filament transformer with a zener because one can sample plate current in a simple fashion as shown in the file below:


Phil - AC0OB

OK John -

I looked over your schematic and it looks very FB.  

The only change I would make is to use a bigger "glitch" resistor in the plate HV, like 5-20 ohms.  If I read it right, there is only 0.82 ohms in there now?

Just ground the grid CT for the modulators and put the diode bias string in series with the modulator fil CT and you're all set.   Just to be sure you understand, use a rotary switch to short out a few diodes at a time to change the resting current. You can use a clip lead to find the best choices before wiring the rotary switch  in.


Audio negative feedback for the modulator:

This can be an advanced subject - I am covering only highlights here -  you might do some reading on the web. I'll get ya headed in the right direction.  

Basically what I did for my pair of 4-1000A grid driven modulators with regulated screen current:  Start with a HV resistor ladder from one modulator plate cap to ground. It might be 1-2 megs with a smaller 1K resistor at the bottom of the ladder to ground. Tap a small audio voltage (like 3 volts) from the 1K to ground and feed this audio voltage back to a low level 1 volt audio point in your audio chain.  Use a capacitor in series with this lead that will have to be experimentally determined... 500 pF - 0.1 or whatever works best doing audio sweeps.  You may also need a small HV cap, like 30-50 pF ACROSS the ladder to fine tune things. It's all about running audio sweeps and trying things to get the best scope picture.

For a while I tapped off the modulator transformer secondary but later found it better over all not to push my luck and stick with the tube plate cap.  Doing NFB through a mod xfmr with all its phase shift is not easy.

At the low level output at the ladder use a small value experimentally selected cap to ground to limit ultra-sonics.  The phase needs to be right - pick the proper plate cap so it will be negative feedback, not positive. If positive it will take off and may do damage.  Use a variable pot to adjust the level of audio into the 1 volt preamp input, just like a mixer. This pot determines the amount of NFB in DB.  Too much NFB and the system may become unstable and take off. I was able to get 12 DB after doing a lot of experimentation and settled for about 8-9 DB NFB in the end.

For 6DB of negative feedback you will have to have 6dB of excess system gain available because this gain will disappear once the NFB is working with 6 DB of NFB. There are surely some good 811A NFB designs out there on the web, so look around. When running right, the NFB will greatly smooth out the extreme lows distortion and increase your high end limit to reach extremes in a cleaner manner.  On my 4X1 modulators, I was able to get the response down to 20Hz and up to 12 KHz before it fell apart.  Without NFB it was not as clean. The 4X1 rig now approaches the class E rig I used to run, though the E rig had a wider and more transparent range for sure.

You should get the modulator working perfectly before adding the NFB. That's the good thing - you can add NFB at any time.


Phil:

Your schematic looks OK, except I'm not sure if I am correctly reading the fil CT 10 ohm metering resistor? Is that a safety resistor with a missing direct reading mA meter?   Excuse me if I am, but a 10 ohm resistance in the modulator CT will cause a voltage drop across the 10 ohms that will mean a couple of volts fluctuation as the class B cathode current flows up and down. This will result in distortion. There should be a low resistance current meter in place of the 10 ohms, just like John's circuit.   I just want to be sure -

T  

I usually meter everything with a cheap 10k/V voltmeter across a resistance since it is difficult to find decent meters of the value you need or inconvenient to drill 2 inch plus holes in thick aluminum.

I don't know about distortion or how much there would be but there would definately be negative feedback voltage at the cathode (filament).

As you suggested, one could simply monitor plate current with a current meter at the zener or drop the resistor to a 1 ohm value and use a DMM to determine plate current in which one would see 100 mV/100 mA of current.

In the new schematic below, I don't think 200 mV of cathode feedback voltage would cause any problems.


Phil

   

Phil,

Sure, 200 mV is much better compared to 2 volts of cathode fluctuation. Though 200 mV is still about 4% regulation which is "acceptable."  (using 4.5 volts of bias)    A direct reading meter might still be  a better solution.

A cathode resistor would work fine in a class A rig since there is no fluctuation, and use to be a common practice in low level tube audio amp circuits. But class B -  NG.

The reason I am so anal about this is because for a year I ran a pie-wound 15 ohm small wire, series cathode center-tap choke in a big indirectly heated cathode RF linear amplifier. (This was a "trifilar" choke, resulting in 30 volts fluctuation) The tube was known for its cleanliness and specially designed for linear service.  I could never understand why I could not get better than -25 DB 3rd IMD using it. Splatter problem.  I tried everything. Then one day it dawned on me and I wound my own choke using #18 wire. The choke was now < 0.25 ohms, 1/2 volt fluctuation. The IMD immediately shot up to -45dB++ where is should be. Clean as a whistle. HUGE difference on the airwaves.

I'm on a crusade not to let anyone else suffer thru what I went thru... :-)

T

As the circuit is drawn, plate current meter A2 will show the combination of plate current plus grid current.  In order to register only plate current through A2, connect the positive side of the bias supply and the bottom of the fixed bias adjustment potentiometer R14 to the center tap of the 810 filament transformer secondary.  With this change, grid current will no longer flow through meter A2.  An alternative would be to just ground the center tap of the filament transformer, and float the negative side of the high-voltage supply.  But when using a single high-voltage supply for both modulator and final amplifier, the first option is simpler, thus requiring no change to the modulator minus supply circuit.

i assume meter A8 is a voltmeter, intended to show total negative grid bias.  If that is your intention, you might consider moving the meter connection to the bottom (cold end) of RF choke L3.  You really do not want RF on the meter.  

Also, if you have any difficulty neutralizing the 810 final, you may consider reducing the value of C10.  This capacitor forms a voltage divider with the neutralizing adjustment capacitor C9.  5000 pf for C10 seems a bit large.  Another option to make it easier to neutralize is to use a split-stator capacitor for C5, grounding the rotor, and eliminating C10.  The handbooks discuss various methods of neutralization, with pros and cons of the individual circuits.

To reduce the possibility of parasitic oscillations, it is a best practice to ground all the bypass capacitors in the RF amplifier to a common grounding point on the chassis.

Good luck on your project!

Did you mean to say 9 volts for bias? I was going by the Amperex spec sheet for Class B and a Vp = 1500V.

https://frank.pocnet.net/sheets/084/8/811.pdf


Phil

Hi Phil,

I was going by what the guys mentioned earlier at 4.5 volts.  9V looks fine with 1500V as you said.
I usually use the switchable bias diodes, so don't worry much about bias until the rig is operating and gets the optimization dance in real time.

Good eye, W8KHK...  Yes the neutralization divider would be better at .005 - .001.  Good find on the grid meter/ RF connection change.   

The other minor thing is the grid bias is labeled screen bias... no screen in the 811A... :-)


Now if I can just find the motivation to get on the air. The 75M band has been so long it might be time to call for west coast.

T

As I learned long ago from Dennis, W7TFO, in a triode, you cannot hear it screen. 

Yes, get on the air, Tom.  I will be away from home for about ten days, so I will miss the Heavy Metal Rally.  I will take my SDRplay RSP2 so I can eavesdrop, but I will be back in time for the AM Rally.  Hopefully John, W9JSW will be ready for that event in February!  At the rate he is progressing, he will not have to push an AM button on a plastic radio!

Thanks guys,

On C10, that was to be used across A8. If I move the meter down, the cap goes with it and solves the divider problem.

Just about ready to start bending metal. Is there a problem with having the modulator and the rf deck side by side? I have a short 9U (15in) encased metal rack and there is not enough room for a stacked config with the rf deck close to 12 in tall. I don't want to place the 8000 horizontal as I am leaving room for a second one (just in case  ;D ). My mod iron is the gating factor keeping me at 1 final. Hoping to score a better one eventually. I know I could put in a heising reactor but those are hard to find and will still leave me with a modulation transformer with a limited audio range.

So, my plan is to build the mod on a 7x12x3 chassis, and the deck on a 10x12x3 chassis. Work with them on the bench then join them together into a 17x12 unit and mount them on a 7U front panel. I plan to build my HV power supply as a separate unit and control it from a 2U power control that will have 115v switching, circuit breakers and soft start units. The plan for PTT is a dow-key sequencer. I just picked up a 12V one with an extra relay on the outside. I ordered the ER issue with Don's article on a simple sequencer.

My thoughts on PTT are to cut off the exciter and the audio and let the resting bias keep the 8000 in cut-off with the HV left on. Is this a good idea? Of course, I could cut off the HV if I want to if a really long span of RX is happening.

My exciter will be sandy. I have a digital VFO that I plan to run into one of those cheap QRP-Labs 20W amplifiers. I will have filters on the exciter to insure good clean signal into the transmitter. Later I may decide to replace this with a tube exciter. RX is an SDR.

For audio, I have 2 options. One is an old SS PA amp with mic inputs. Have to check it out more. It may be 70V output. The other option is that I bought one of those cheap Class D audio amps on Amazon. It is a 100W mono subwoofer and has instructions on how to remove the low pass to open up the audio range. I have a mic preamp that I can use if I go that direction.

Hope to start actual assembly real soon now. Been waiting long enough.

John W9JSW

Do I need the clipper that Phil shows in his schematics? I read about them but do not understand how this one works. The Zeners add up to 580V. Does it just flat top the audio if over-driven? I see some transmitters that have splatter +/- 10kc or more and that is pretty bad. Will this prevent that? On my SDR transmitter (see QRZ for pics) the passband is brick wall sharp. Don't want this transmitter to be a band hog...

See attached revised schematic with all of the above suggestions (except NFB) implemented.

John W9JSW

Yes, move the meter down.  With the meter where it is you probably will not be able to neutralize, and you will have RF on the meter.  After you move the meter, you may want to add an additional bypass capacitor across the meter which is now connected at the bottom of the RF choke.  But you must keep a cap where C10 is presently, however a smaller value, such as 500 to 1000 pf (not 5000 pf).  This cap and the neutralization variable are an RF voltage divider that enables neutralization.  You may remove C10 if you use a spilt stator capacitor for grid tuning, (grounded rotor) but without a spilt stator you must have C10 in its present location to neutralize.

A few simple thoughts on negative feedback - neglecting phase angle.
The important thing to consider about feedback is that if it starts to become positive, you have
an oscillator.

That feedback resistor, if you put a cap in parallel then at some frequency that is a function of
the reactance of the cap, the cap "looks like" a wire, aka very low resistance. So that cap sets
an HF rolloff based on the value and so the reactance vs. the impedances. Usually this cap is
set to control HF overshoot, for a hifi type amp. It could be set to provide a 6db/octave rolloff
at some lower frequency, like -3dB@ 6kHz. for example. You'd be down 9db @12kHz.

There are other in circuit means by which bandwidth can be limited.

The point that you come back to with the feedback must be a point that is out of phase
with the point that you source it from - they need to cancel. Also the circuit needs to have
"excess gain". That means that if one needs to have everything running full out to reach clipping
of the 811 tubes, then adding feedback will not give you enough swing, not without increasing
gain, or increasing the input level, if that does not overload the input...

The small caps to ground are generally for the purpose of limiting or eliminating ultrasonics
and/or RF from the feedback and/or gain stages...

It's a good idea to look at something like a triangle wave once the modulator is built, with first
a resistive "dummy load" on the secondary, then with the actual final to be sure that it looks clean
and linear up into clipping...

                           _-_-

OK, Rick. I have it correct now.

On the grounded variable cap. I have a 10-100 breadslicer cap in the junk box. Will that work as a direct replacement in a single ended LC or will I have to recalculate the LC and also center tap the inductor? I have this 21uF Air-dux that just works out single ended. I did not want to go with a center tapped inductor due to ease of build for 4 bands. I will if it makes better sense. The input LC calculations were pretty easy using a LC calculator and 15k for RL. Not sure how to model the other circuit.

Pls review my rewiring of the bias. I forgot to bring the +V off of the supply. I think I have it correct now.

John

Thanks Bear. I understand.

I have a good scope and plan to take this stepwise starting with the modulator and testing it separate. That is why I wanted to have separate but joinable chassis. Easier to work with as I build it up.

Going to hold off on the feedback idea for now. Already a full plate.

Just realized I am going to need a bigger dummy load. My MFJ 300W unit is only good at that level for a few minutes. Wonder if I can strip it and insert it into a can of mineral oil and make it a cantenna?

John

Hi John -

There's a lot to comment on, but I will chime in on two areas dear to my heart....

1) PTT Keying the modulators and final:  Yes, they need keying. Keeping the modulators running will burn up power for nothing - and might even generate hash into the RX if not stable. With modulators, even though they are biased into "cutoff" class B, they still need to idle at a recommended current so that the crossover point of the sinewave is clean - preventing crossover distortion.  To key, just stick a 30K - 50K resistor in series with the cathode circuit and use a set of contacts to short it out during key up. IE, 50K biases the modulators into deep-cutoff - and then this shorted resistor lets you idle the modulators anywhere you desire using the biasing diodes as described before. Do the same thing with the RF final.

See my 813 circuit for an example:   (There should be a .001 cap from the meter/RFC junction to gnd in the screen circuit)

https://www.amwindow.org/tech/htm/813/813.htm



Negative peak limiting and clipping:

There's a lot of opinions on this and I'll give you mine:  I feel all clipping and limiting (and processing) needs to be done in the low level audio stages. There was a time when I used high level clipping/limiting in all seven of my homebrew rigs. But one by one I removed these circuts due to side-grunge, splatter reports whenever I leaned into them. High level to me is more as a safety net.  Since then, I use just one master low level audio system and it works FB. (528E, PMC-300A, 31 band EQ...)   For hard negative peak limiting I use Steve/ QIX's clipper circuit taken from his PDM generator design. Just a simple diode and resistor circuit to limit the negative peaks to 95% and let the positive peaks soar to where the rig can handle... 140%, etc.

Add in a couple of 19" rack audio boxes to help audio processing and you are covered. If everything is running well in the one volt low level stages, (SDR is a good linear amp solution too) there is no reason to need splatter clipping in the high level mod transformer stages - except for arc-over protection, which is a valid reason.   Again, some use high level clipping and it works for them.  Be sure to have ball arc-gaps for mod transformer protection...  I use gaps on my Heising reactor too and even on the screen choke to quench collapsing fields when unkeying.

BTW, if you experience excessive arcing across the gaps during unkey, there are a couple of great solutions I will tell you about if needed later. The collapsing mag field has to be dissipated  somewhere and hopefully not in the form of damaging mod transformer turns arcing.

T

At 80m, you can use a variety of things for a dummy load - including appropriately sized heating elements, water or
mineral/transformer oil can be used. (if needed!)

A minor concern to me is that the secondary of the mod transformer is unloaded in "normal" operation, until
the grids start to draw appreciable current... I think I'd opt for a light resistive load on the secondary of the mod iron.
Dunno if this is standard ham practice or not...

                           _-_-

Hi John,

In the beginning of the thread I wasn't sure how much B+ was being run on the Mod transformer and the original circuit was a Soft Knee Limiter for supplies running about 700V.

The soft knee limiter was suggested by Bonomo in ER #111 to save the Modulation secondary from failure by loading it on negative peaks, reducing any high voltage
 V = - L di/dt inductive “kickbacks.”

I use it for any upgrades to EF Johnson and Heathkit type transmitters and a variation of it for homebrews'.

For your application at 1700 volts, the circuit has more 200 volt Zener diodes.


Phil

One additional comment John on your latest circuit:

It appears your grid current ammeter is shunting the grid and protective bias.

I think it should be in series with L3 and R1 since you are measuring the current in a series circuit there.


Phil

J,

Throw a few high Wattage lamps in // and test away.

Wall Whirl has bulbs (don't task for lamps as no one knows what one really means).......

GE LIGHTING Incandescent Light Bulb,A21,200W 200A/CL-1     $2.97.

 Get 'em while they're hot.


klc

Phil,

On the grid meter. A8 is a voltmeter. My bad. I fixed it to properly be identified as such.

On the knee limiter, what is the target knee voltage to shoot for at 1700v?

Tom,

I added a ball gap on the mod trans.

I also added PTT of the filaments. I guess this also gives me a soft start on the filaments as well?

KB2WIG - I have a few 300W Farm bulbs that will work nice for a dummy...I live on a farm...

New schematic...

John

The rule of thumb is 1.2XHV B+ so in the new schematic I posted the limiting starts to occur at ~ 2000V.


Phil

Good on the ball gaps. They gets adjusted by trial and error. You want no arcing during normal speech but yell and depending on your low level limiting, the gaps may arc. Or they may just arc occasionally during PTT unkeying. All adjustable to suit your fears.  :-)



Filaments keying, soft start???

You meant you are keying the center tap of the fil transformer which is cathode current, right?  The filament voltage  from the transformer goes directly to the tube fil pins and stays on, no keying. The fils do not get keyed, just the fil xfmr CT.  Maybe that's what you meant. Look at my 813 schematic for an example if not clear.    

https://www.amwindow.org/tech/htm/813/813.htm


UPDATE: I just looked at your schematic and you have it correct. It was just your description that was unclear to me.

T

John,

Some more suggestions...  I took a look at your power supply schematic....

You COULD put the HV filter choke in the plate transformer's  center tap to ground. It will do the same job but the high voltage to ground/frame will not be there.  It will still experience current induced voltage spikes as usual across the windings.  If you keep it in the HV lead, then let it sit on some Plexiglass or a block of wood to isolate it better from ground. You never know how well old choke internal isolation to frame is holding up.

Another point - about your ART-13 mod transformer:   It has a tiny core and is  an amazing transformer for the job it does. The WWII guys really knew their stuff. It's an anomaly.  I once had a pair of 813s modulated by a pair of 813s using a 50 pound broadcash mod transformer with a Heising choke. It ran 3KV and sounded great.  One day I decided to clip lead in an ART-13 mod xfmr for a test. It did well and sounded fine except the lows were greatly lacking for reasons I will explain. It did not blow up...

Anyway, you are driving the 811As with a backwards hi-fi 8 ohm audio transformer and system so it will have great audio except for the ART 13 bottleneck. If you cannot find a better mod xfmr, then it's highly recommended you add a Heising choke to the circuit. Say 20H or higher - with a 1-2 uF 4KV coupling cap will keep the DC off the ART-13 xfmr and improve its saturation figures greatly. Better low end and safety.  If interested, look at my 813 schematic for the hook up details.  

In the meantime, saturate the media with some "wanted" ads on the various ham websites for a broadcast mod transformer.  BC 250w or 500w xfmrs with matching Heising reactors are available. There are lots of older hams who have them in the cellar and will never get around to using them. They'd love to have your strong back take one away. :-)   You will never regret taking the time and effort to locate a quality xfmr. It will last a lifetime, let you run hi-fi audio with a big low and high end - and do well with NFB.  There is probably someone on this BB willing to help you out.

T
  

Tom's suggestion of a Modulation Reactor will insure the Modulation Transformer doesn't saturate easily.  8)

Annnnndd, if just by chance you decide you need one I happen to have one from a
BC-1T 1kW transmitter.  ;D


Phil

Wow, that looks like a beauty, Phil!   Made for 833As X 833As from 1957 era.

I use a similar looking black "décor" mod xfmr/reactor set from a 1KW BC transmitter for the 4X1 rig. They are built like tanks.

Well, there ya go, John. That's the big league  - 1/2 way there!   Don't settle for anything but a matching 1 KW mod xfmr now.    

T

FWIW, I’m running my 811A modulators at 1400v and -2.4V on the grids via battery bias. I’m using two 1.2 V rechargeable size AA batteries. I have about 40 mA of idling current. Very simple system and seems to work FB. The mod transformer is a UTC S-22 with a Dahl mod reactor (reactor highly recommended). The transmitter is currently running a single 810 in the final position.
This transmitter is my “learning curve” transmitter, and overall has worked much better than I could have hoped for. I am now in the process of planning my next transmitter which will be PP 254W’s (I think) modulated by 838’s. I’m drawing the schematic now, will post it when ready. Good luck John. Yes, triodes are nice.😉
Ron W8ACR

I really appreciate all of the help I am getting here!

Ron, your thread history is the genesis of my circuit design, along with the circuits posted on Jack's K9ACT web site. And with all of the help I am now getting, the circuit is getting better and better. This is a great group of hams!

Phil, want to make a deal? You go to any hamfests in the area? We live 275 miles away from each other.

If not, I am finding a few chokes at 12H and ~300ma. None in the 20H range that are not horribly expensive. If I don't go to a BC class choke, can I pair 2 relatively inexpensive filter chokes together to get around 20H? I don't want to spend a couple hundred bucks on an interim solution.

John

John,

I have a spare mod reactor that is in tough physical shape, but i believe it is in working condition. I believe it is either 40 or 50 HY. I can measure it if you like. I have been keeping it as an emergency spare, but I’ll probably never need it. I also have a single spare 8000 tube that I know has a good filament for sure, and I’m also pretty sure it is a good tube. Again, I was keeping it as a spare, but I now doubt that I’ll ever use it. You can have these if you want them. I’m in Montana now at my cabin, but when I get home on Tuesday, I can dig them out and send them your way. They will be very affordable.😉 just let me know. Do you need any other parts? Do you have parts or tubes for trade?
73, Ron W8ACR

I have some stuff to trade. Send me an email to Ke5ssh@gmail.com. My old call sign.

Hi John,

I have the matching mod transformer as well. Sent you a PM.


Phil

Hi John,

Yes, chokes in series will work to increase inductance as you would expect. However, as I understand it, the chokes designed and wound specifically for audio work, like Phil's BC beauty, will do a better job with audio (30 Hz-12 KHz) vs: common power supply chokes designed for 60 Hz.  At least this is what I have been told over the years. Is it winding placement or gapping, I dunno.   So go with Phil's BC iron and you will be covered.

Don't worry about impedance ratios on these big BC mod transformers as long as they are reasonable like 1.1:1,  1.5:1, 2:1, etc.   The optimum match for a single tube  final modulated by a pair of the same type  (one 813 X 813s) is a 1:1 ratio.       For a pair X a pair (813s X 813s) the ratio becomes 2:1.    The lower ratios usually mean bigger audio peaks, though sometimes less efficient from my own experience.

Anything between 1.1:1  >  2:1  step down ratio is common and will work FB in the many different rigs you may build over the years.  IE, don't get hung up on perfect matching of ratios. It will affect the peak audio capability, but with the right tubes, you will have plenty of clean audio available.


Probably you will get a 1KW set of iron, use it for this rig and later build a bigger 1KW rig.  Then use this iron for the bigger rig - and later find a smaller set, like 250W/ 500W, etc, for this present rig..


Update:
 I see Phil has offered you the 1 KW matching mod iron!  You don't know how lucky you are, OM... :-)   What is the impedance ratio on the mod xfmr, Phil?  The BC-1T web notes show 1.12:1 and 1.33:1   (I'm assuming they mean impedance ratio, not turns ratio, but not sure)


Here's a 2012 AMFone thread that has an interesting mod xfmr and reactor discussion:

http://amfone.net/Amforum/index.php?action=printpage;topic=31556.0


BTW, Your idea about using software (in another thread) for most low-level audio processing is a great idea. This is the present trend with SDRs and will eventually be the only way to go in the future - so knock yourself out... :-)


T

That sounds about right. Here is the simplified schematic which also shows the tertiary secondary winding for the driver stage:


Phil

The grid meter circuit looks ok now.  The only confusing thing is the "A" on the meter.  Makes it appear at first glance to be an ammeter, where a V would indicate a voltmeter.  It is common to have a grid ammeter, but rare to also have a voltmeter. 

As for the cap, if you use a split stator, that is actually two capacitors in series.  To obtain the same capacitance in the parallel resonant circuit, each half of the split stator should be double the pF value of a single capacitor.  Remember how to calculate capacitors in series......

Look at the diode string in the filament circuit of the modulator.  If these are standard diodes, then you have the symbol reversed.  If they are zener diodes, then the symbol is in the correct direction.   For standard diodes, they should have the cathode end grounded.

Rick,

Glad you caught that. That was my fault. John followed my description from a post I made.  Been corrected.



John,

If you later decide to get rid of that driver transformer, (phase distortion) consider this WA1GFZ-designed MOSFET audio driver that will drive your 811As or even 833As directly - later on. It will handle NFB right at its input. Cleanest driver system I ever used on my 4-1000A plate modulated rig.  You then have just one transformer, the mod xfmr, to worry about.

http://amfone.net/Amforum/index.php?topic=23632.0

T

Made all of the changes noted in the last few notes and removed the voltmeter. Did not have one anyway.

My latest quandary is metal layout. I am not sure how the modulator will lay out if I add a large reactor. I may just have to lay them both out on separate 17x12 chassis and forgo the cabinet that I have that I thought was large enough. Finding rack style cabinets in southern Illinois is almost as hard as finding broadcast iron!

If I add another 8000, how much more power can I expect? I have the iron for a separate 1050V power supply at 300ma. I could run the mod on that. I have a Thorardson 1180-0-1180 transformer and a 12H 400ma Thorardson choke. Would just have to find some caps. If I use cap input before the choke I could get 1400v with less regulation.

What else in the overall design would have to scale up?

Scope creep is occurring!

John

Update - if I don't find a rack, I have a backup plan. 2 37in high storage rack end frames from Menards ($17 each), then mount 20U rack rails ($19 pair) to them. Put the power supplies on a wood shelf on the bottom then 2 7U high spaces for the mod and rf deck. Going to keep the exciter and AF amp outboard of the rack, probably next to the computer that will run my audio processing chain and the SDR RX. 

Mention of the ART-13 transformer being subbed in made me think  - humorously, that if transmitters looked like this, one of those big blue levers would hot-switch from the BC transformer for fidelity to the ART-13 transformer for punching through the QRM.


Dennis W5FRS uses one of those ART-13 units to mod a 250TH running just about 2KV and it sounds great. Think of it as high-level analog signal processing. Another transmitter uses a BC-610 transformer. All that stuff works great.

There was a transformer article by Menno Van Der Veen a while back that stated generally that if the audio transformer's power rating is doubled, the low frequency limit is divided by SQRT(2). Another reason to use a large size unit.

John -

You were looking for an actual negative feedback circuit for the modulators. I didn't realize it, but this MOSFET audio driver includes the working NFB circuit I currently use in my 4X1 rig. It will give you a good idea what is needed for your rig.  
http://amfone.net/Amforum/index.php?topic=23632.0



BTW, the way you're scaling that rig up - and you're just in the schematic design phase, why not bite the bullet and  build a pair of 813s X 813s for a 500 watt carrier and 140% modulation?  (3KW+ pep)  Phil has the perfect mod iron for you.  A single bigger plate transformer would be easy to find. (2500-3KV DC is robust for 813s) You know how hobbies go -   a year from now you'll be thinking bigger and wished you had... :-)

Can you hear it whispering in your ear?
https://www.amwindow.org/tech/htm/813/813.htm


The JJ promise and money back guarantee:  ;)

Build this rig using 813s X 813s, 2800VDC, with BC mod iron, the GFZ MOSFET audio board with NFB, low level audio processing/ limiting, proper voice EQing -   and you will be in the top 1% of the cleanest AM plate modulated rigs on the air.   I did just that and have the actual IMD spectrum and triangle audio wave tests to prove it...  This combination works like magic when set up right.  The 813 modulators in triode config with the GFZ audio driver board  and NFB are a "perfection anomaly."

Clean test audio 20 Hz > 12 KHz =  transparent audio =  Great sounding audio on the air

T

Then I will have to replace the S-49 HV trans, build a bigger bias supply, buy different tubes, etc.

Where does it end???

John

Commit to build it as described above and I will send you four NOS 813s and four NEW white ceramic sockets for FREE.  (free shipping)

T

J,



Its a Don Corleone offer.....  how can you refuse?

KLC

Is there a board or gerbers avail for the mosfet driver?

Hi John,

The MOSFET driver board thread had talk about making circuit boards but it never happened as far as I know.  I did the only prototype as pictured below.
I think Frank/GFZ has built one for his Valiant too.  There is a PCB layout by Rich/W7SOE shown below. It probably works. Needs to be checked and tested. He mentioned traces need more spacing.

But point-to-point wiring like I did is an easy build and is well worth the effort. Mine worked right away. Been in the rig for 8 years, no problems. All parts from DigiKey, etc. Cheap $3 MOSFETS.  This driver will drive most ANY modulator, big or small effortlessly, so is good for a lifetime.  1 volt in, mucho watts out, no transformers, clean as a whistle.

Now ask around for a HV plate transformer good for 500 mA and capable of generating 2800 VDC and you're dancing.


***  I'll even sweeten the pie and throw in FOUR brand new white ceramic 813 plate caps with silver plated inside cups for FREE.


If you're game, I'll give you up to 60 days (or sooner, your choice) to show some solid progress on the project as described -  and then all parts are yours, OM. (4) 813s, (4) sockets, (4) plate caps

T

Under serious consideration

Tom,

Deal. Phil and I have an agreement. I will be getting the 2750VDC PS as well. Guess I need to start on assembling the rest of the BOM and get started on the driver.

Gonna need a bigger rack!

I won't get the hardware until late January as Phil has a trip planned then which will allow me to get the iron without having to pay shipping. A real nice deal as well.

John

Will be selling some now surplus to my needs components. A NOS S-49 UTC Transformer, some tubes, sockets, ART-13 Mod Transformer, etc. PM me if anyone is interested.

THIS is ham radio.

Good on you Tom.

Take it from a guy whose been there and done that.  If Tom says you should go a certain way, you probably should. 

Or, you'll end up like Brett, and have a LOT of beautiful rigs and not enough time to talk on all of them.  Such a horrible situation!

--Shane
KD6VXI

John,

Good!  Then we can get started....

You should probably plan on a vacuum variable cap for C1 and turns counter - for plate tuning.  500 pF @ 10KV is common.  You can get away with a 4 section 365 pF variable for loading, C2 as long as it is of goood quality and has good plate spacing. Some are junk and will arc over. The more strapping ones will do 500 watts AM carrier, maybe arc with a weird load occassionally.  Ask around. You can also use a vac cap for C2 - 2000 pF @ 3000 - 5000V.   You may luck out and find a vacuum variable pair for C1/C2 with turns counters. You're having great parts luck lately!

For the GFZ driver printed circuit MOSFET driver board, I would contact Rich, W7SOE and see what he says. I think moving just a couple of PCB tracks for the +- voltage will suffice and give you a working PCB. He has the software already set up so could make it real easy for you to build it.


The 2750 VDC HV from Phil... is that a complete power supply or just the HV transformer?   Figure with a C.T., choke input power supply, expect to get about 0.9 X the CT AC voltage UNDER LOAD.   So 5,500 VAC  center tapped = 2,475 VDC under load.        If you can end up with 2500- 3KV, you will be FB.

Plate choke.... The common 7"? tall National chokes wound on white ceramic will work FB.   You can also wind your own using heavier wire. (recommended) Decide which way you wish to go and we can help you. I wind all my own single layer plate chokes using heavier wire.

Get some decent Russian 500 pF coupling and plate bypass caps. Do not use the cheap 500 pF/30KV TV doorknobs. You should use 500 pF for plate coupling and 500 pF for choke bypass so that the audio will not be shunted at the extreme highs.  

BTW, all of this will work down to 160M except maybe for the C2 365 pF loading cap needing a padder on 160M - have to see.


For a plate tank coil, unless you have something strapping made from copper band or whatever, wind your own using copper tubing. 1/8" is OK on 160M and 75M - 1/4" and larger on 40M and above.  I have also used a very strapping rotary coil, but it has to be heavy duty with heavy rollers.

You will need two 10A/ 10V CT filament transformers.  These are available on eBay, etc. or maybe here from the guys

There's more, but you have most of it covered already. Make a list of what you need and either the guys here will fill you in or try some ham boards with "wanted ads".


Shane, yes, helping to mentor others is a great thrill. If they have the spark to build a rig, then we must help them.  The help John is getting here reminds me of the Amish. When someone needs a barn built, the whole community comes in for a day and knocks it out - painlessly.

John, I may have some more critical parts you need.  Just to be clear, the 813 sockets and plate caps I have for you are brand new.  I have about (10) 813s, some are NOS and others are near new, all mixed together, all major USA manufacturer labels.  Amazing tubes.  Bottom line is I will pick out the four best ones for you. I don't expect to build another 813 rig here since I have the 4-1000A plate modulated rig now.

T

Oh, before I forget, John...

The plate bandswitch needs to be really heavy duty.  Something from a T-368 or BC-610 is the caliber needed.

The problem is that when you short the tank coil to change bands, the switch contacts carry very high RF voltages and will jump the gaps.  The test is to picture the RF path on the switch and see where it can jump and bridge switch contacts, etc.

A 4" diameter ceramic bandswitch for 813s with 3KV is not over doing it.

Take a look at the bandswitch I used in my 4-1000A RF deck. It's about  8" diameter and uses TWO sections in parallel for lower inductance and higher current capability.  It has never arced or failed.


Also, notice the technique I used to hang my vacuum variables on the front panel and the connections to the turns counters.  There are many ways to do it.

T

Ok on the caps

The ps matches the other parts 3500-0-3500 at 1.5a ccs, plus a matching swinging choke. It also has a 8uf 3kv cap. I have 2 10uf 3kv caps I can use. They also have a 2uf 3kv cap for the mod coupling. Is that a high enough rating?

On the board, i am fluent in board layout having built a 1/2 dozen various boards for other projects. I will contact him and see if I can pick it up. I usually have my boards made at Elecrow.

I have 1 fil trans already. I also have one new 813 ceramic socket.

Thanks much,
John

Hi John,

OK on the PCB and your experience there.

The 2 uF Heising blocking cap needs to be at least X2 the plate voltage
 
Anyway, you are using 240VAC primary, yes?  

The HV filter caps need to be at least 1.5 times the HV DC voltage.  I like X2 or more myself. Another issue is that the HV will soar upwards without a load, depending on how well your swinging choke works and how heavily you bleed the HV with power resistors. One way to keep the soaring voltage at bay is to use a step-start. Put a 5-10 ohm 200 watt power resistor in the 240 AC primary and key on the 240... then 1/2 second later have a set of contacts short the 5 ohm resistor. The HV will stay well below VDC MAX during key-up and un-key.  Plus, the soft start will be so much easier on ALL your power supply components.

The reason I mention this is because your filter caps are rated at only 3KV. Is there two separate HV terminals for plus and negative on these filter caps? If so, you can put your two 10 uF caps in series for a 6KV rating but only 5 uF.   That is pretty low filter capacitance even with a good choke like you have.  

My power supply uses a pair of 70 uF @ 10KV caps in parallel.  (140uF @ 10KV)  with a large 30H choke. Step start.  Rock solid and clean.  That is extreme, but WTF, I had the parts... :-)  So, you can use what you have for now, but look around for better caps.  There are "photo flash" caps out there about the size of a loaf of bread at 51 uF @ 5 KV. I see them all the time at flea mkts. I realize some say they cannot handle continuous service, but I have used them for 40 years and have never had a failure. I remember Peter Dahl used to sell them as filter caps.

The next logical step may be to pick up a heavy duty  Variac. It will be invaluable for initial testing at 1500 volts and later creeping the voltage up and up as you get more confident in the rig. You can actually use a 30A 120 VAC Variac across one side of the line. IE, you will always start at 1575V and turn it up to 3150 KV.   A  240VAC Variac @ 30A is good for 7,200 watts.  Once the rig is tested, you can turn some Variacs all the way up to the "stop" and it will be out of circuit.  You will always get SOME loss from a Variac, from voltage drop = less regulation, so the ability to take it out of the system is a good option.   The bigger the Variac, the less HV voltage drop under load at the HV end.

You will need a string of bias diodes in the modulator cathodes once you get above say, 1800 VDC or so. That's why a rotary switch is a good feature to add or short out diodes when needed.

That should do it for now, OM.

T

Here are the specs for the components as describe by AC0OB (edited out shipping details)

Power Supply Group from a BC-1T

Used with 833A’s in Push-Pull Class B service modulating a pair of paralleled 833A’s with 2600 Volts on the plates.

This Power Supply combo produces 2700V @ 1.5A CCS.

This item is known to be functional and was removed from a Gates BC-1T Broadcast Transmitter.

Dimensions Power Transformer: H = 12.75,” D = 6.5,” W = 8.5”
Pri: 230V, Secondary: 3800V CT@1.5A CCS;
Dimensions Swinging Choke: H = 10,” D = 7.0,” W = 7.5”
Dimensions 8 uF 3000V Capacitor: H = 7.5,” D = 3.75,” W = 4.5”

1kW Modulation Transformer

This item is known to be functional and was removed from a Gates BC-1T Broadcast Transmitter.
Dimensions: H = 9.75,” W = 7.75,” D = 5.75.”
Frequency Response: 40-15000 cps (40-15k Hz).
It is marked “AM-30469 C” and was made by Electro Engineering Works for Gates.

0.75 kW Modulation Reactor

Used with 833A’s in Push-Pull Class B service modulating a pair of paralleled 833A’s with 2600 Volts on the plates.

This item is known to be functional and was removed from a Gates BC-1T Broadcast Transmitter.

Dimensions: H = 9.75,” W = 7.25,” D = 6.5.”
It is marked “AC-10458E” and manufacturer is currently unknown.
He also has included the C4 2 uF 3000V Coupling Capacitor from the transmitter

That is all I know. Phil stated that since these came from Gates, they should be good to go. (Phil, feel free to opine if you are monitoring the thread)

John

Gates....very good info to have....  I didn't realize you had a complete power supply design -  just assorted parts from mixed sources. Excellent!

2700 VDC will be perfect for the 813s. (Higher using solid state diodes.)  Looks like a lot of the work has been done for us.

What puzzles me is they're using only 3KV rated filter caps with a 2700 VDC HV.  Must have great swinging choke action. I'll bet the caps are "Gates" 3 KV rated and really good for 5KV... :-)   And the Heising coupling cap is only 3 KV - maybe they placed it in the negative lead..... hmmm.

It is good the constant carrier of AM helps regulation for the modulator variation vs: a linear amplifier.

It's hard to argue with success.  Go with it.  Use my power supply suggestions if you need to modify things later on.


T

http://www.wd8das.net/Gates/GatesBC-1Tmanual.pdf

Page 61 has HV supply.

Where can I find some good Diode blocks? These are all I seem to be able to find...not sure of quality. I have a pile of 1N5408 so I can make strings if I have to.

https://www.amazon.com/PRHVP2A-20-Voltage-Rectifier-Leakage-Current/dp/B07BPW8HJX

John

Easy as pie step start.....

Wire one side of a 120v contactor to neutral.

Wire the hot side of the contactor to the transformer side of the step start resistor.

As the cap charges, the current draw lowers, causing the loaded line side voltage to rise.  At some point, it will close the contactor.

You only have to size the resistor properly this way.  No RC circuits, no external timers, etc.


KISS



As to diodes?  Since k2aw passed, good luck........  And I never really liked those.  A string of 6A10 or 10A10 will never fail, 400A surge capability, etc.


--Shane
KD6VXI

Important safety note here:  Imagine a significant issue with the PSU. Choke shorted to case/ground for example.  Current will never decrease so contactor will not pull in. Be sure the combination of the resistor, overloaded power supply, and main breaker are sized so that the breaker can/will blow when the resistor is in line or you can have a runaway situation where the resistor limits current just enough to prevent the breaker (or fuse) from blowing while the power supply burns....

Ed/KB8TWH

John,

I have always used these 6A, 1KV diodes. Inexpensive.  Mount the diodes on a piece of drilled Plexiglas. (tiny holes for leads)  Mount the board on standoffs.  They are bulletproof and will take lots of abuse. They get much better air cooling than the blocks. I have been using the same stack of 100 (25 per leg in a bridge) for 25 years now.

https://www.parts-express.com/power-supply-rectifier-diode-6a-1000v--6a-1kv

For your situation, 15-20 diodes in each leg (15-20 KV, two legs for a full wave) should be way beyond safe headroom ratings - for both voltage and current. No equalizing resistors or bypass capacitors needed.

These SS diodes will bring your HV up somewhat (compared to the original Gates tube rectifier design) and the full current load will pull it back down. I will be interested to see what your key down fully-loaded HV is in the end, but should be FB for 813s.


T

Always size parts for the job at hand.  And, if you really worry about this failure mode (it can be a problem.....  I've repaired a couple Ameritron amps with broken step start), you can put a fuse inline w the step start resistor.

Or, spend the money and use a breaker along with a solid state zero crossing relay.  Just don't depend in the ssr to have the chassis at zero volts.

In my step starts, I use the ubiquitous Dale (chicom clones) and size it with a 10 to 25 Watt R.  Use heat sink compound and mount to a wall, etc. These I've found have NO problem sacrificing themselves 😁🤔.

Many ways to skin this cat......  Also, I DO use timers in the step start for a tube filament.

--Shane
KD6VXI

They did place the Mod. reactor cap in the negative lead to ground. All caps are oil filled and have at least a 20% surge capacity.

Yep, Phil Corleone made John an offer he couldn't refuse.  ;D

He had me at “modulation transformer”. Was love at first sight.

Took the generic circuit and transcribed it for my implementation. I like to show the exact circuit layout so that I can remember what I did a few years down the line.

I perused the Gates schematics and believe I am true to their design for the HV and modulator. Pls comment.

In reading an old thread from Slab Bacon SK? - what a great ham - glad his knowledge is still helping others. He warns about using the chokes on the center tap as perhaps being counter to the transformer mfg intent. I chose to follow that advice. Gates did not wire it that way, and the screen bias transformer that I have is circa 1960's so I thought it best to not do it there as well.

Added demarcation points on the interconnects to start thinking about how I will partition the design into separate units.

The PS on a wood platform near the floor, of course. A panel plate to cover the front of the PS with input voltage control breakers/fuses/switches. Next up will be the modulator, which I think will be a larger wood platform sliding on rails, with a smaller AL chassis to mount the tubes and a front panel to cover it up. That should give me short interconnect between the plates and the mod trans. Next will be a plate with a row of panel meters (I think). Lastly, on top will be the RF Deck on a standard large chassis with front plate also on sliders.

I am going to use a quality RG-8 with a poly dielectric for the HV direct connect. No Millen connectors. Please advise how to connect the shields to avoid excessive ground loops if that matters. Also suggest how to ground all of the chassis together. Was thinking of some copper strap.

I am attaching the schematic and a BOM. If anyone can help out on vacuum caps, doorknobs and bandswitches that would help. I will pay a fair price.

John W9JSW

Edit - looks like I need to add a string of diodes in the Mod cathode path. And add Bandswitches to the BOM. Just reread those suggestions...

Wow, looks NICE  John. I wish I could get the parts here in Costa Rica to do the same.
I have sufficient vacuum caps, but it will not be possible to get the HV parts here at all. Transport and import cost is completely prohibitive to import heavy stuff. I am planning to make a small switching power supply to generate 2 - 4 kV at 0,5 Amps for such projects. If that works out well, than I may follow your design.
Just a few points.
5kV for the output cap at the PI filter C19 seems quite overdone to me and 1000 pF too low a value. It will be completely meshed. Putting caps in parallel will give false resonances if the connection between these caps and the variable cap isn't VERY low inductance. I have been there. two caps of 1000 pF in parallel using short wires may result in parallel resonances well within the amatuer bands. So be carfull there. When there is 500 V RF at that cap, you already have 5 kWatts at 50 Ohms. In order to avoid arcing at modulation peaks, 1.5 - 2 kV seems quite sufficient to me.
I advice to put an RF decoupling cap between the RF choke for the G2 L3 to avoid RF flowing into the LF choke L4
The cap C20 for the bias is reverse.
I should use RG58 for the HV instead of RG8. The mechanical stress at the connectors of RG8 when moving chassis may result in bad contacts and losing up the shield connection. RG 58 is much more flexible and the HV connectors for RG58 are readily available and low cost and due to the fexibility results in a safer system. With RG58 and those HV BNC connectors you can safely transport 5 kV at 4 Amps
Also for the interconnection between the chassis, NO copper strip, something flexible so there is no stress at the wingnuts to connect the safety grounds when moving the chassis.
I am very interested to follow your progress. Thanks for posting

Thanks for your comments. I am a very lucky person to have been offered the BC Iron. And doubly so to be offered the tubes/etc. Otherwise I would be building a lesser rig. Still would have been ok, but this is much better.

I have a S-49 trans. How much to send 31 lbs to Costa Rica? It will do 1800VDC at 300ma or so.

Agree on C19. I have a 1150pf BC 4-gang variable now, but looking for one higher with slightly wider plate gaps. 1150 won't get me to 160M, I worry. Or a vacuum variable if found at reasonable cost. I want the deck to be able to cover 160,75,40 and hopefully be stable at 20M. I think 20M will be largely how well I lay it out and the care in assembly.

On L3/L4 is C7 and C2 not enough decoupling to preclude RF feeding back to L4 or are you suggesting another cap between L3 and L4.

I should also add caps across the meters, I think. Forgot those.

C20 is correct, I think. It is a -75V bias.

I recall a thread suggesting some good HV connectors. RP-BNC.

John

The problem isn't sending it, the problem is it not arriving.  Nico said in other threads customs can be very liberating......  So to speak.

Might need some cartel mules in reverse (tongue in cheek).....

As to the HV switcher, I believe Manfred (ludens.cl) has built one....  He runs a shack off a river in Chile, so his station is pretty efficient as well.  Last I heard he was doing a class e or similar amp, a red pitaya for sdr and was wrapping it around predistortion.....  Have to have super high efficiency on a water wheel stream powered shack!



--Shane
KD6VXI

Need a spec on the 3A Variac. Can I use a rheostat?

Is 2W ok for the 50K Cathode drop resistors in the PTT circuit?

John

Good job, John!  You are making progress.

I did a quick once-over and see a few things to comment on.  Once we address them we can look for more:


Yes, 2W is OK for the 50K cathode keying resistor.

All meters need .001 bypasses.

Use a Variac for the screen transformer AC primary. A rheostat will burn up power for no reason since there is constant current flow during keyup. (and screen power supply bleeders)

You have the C20 electrolytic  cap polarity backwards.  That is a negative voltage at the diode anodes, thus a negative cap polarity.

A .001 on left side of  L3 to ground should be added. Don't be afraid to sprinkle .001 disc caps around in the DC grid and DC screen circuits.. can't hurt.

OK on the transformer center tap precautions... that's fine - do it as you described.

Your 75V VR tube in the grid DC is a good idea -  to keep the RF from charging those filter caps and fixed bias drifting all over the place.  Check it during initial testing for stability.

The plate tank vacuum cap can be 500 pF at 5KV, though 8-10KV is a common rating.  Use what you can find with at least 5KV min.

The tank loading 4-section air variable should be 1KV if possible -   2000 pF will cover 160M barely.    A vac cap is better and can be 2000 pF at 3KV (common rating found) if you can find one. They are more rare.   Remember that these voltage ratings are min and larger is always FB.  The common 365 pF multi section air variables for loading will barely handle 500 watts carrier and might arc over during heavy modulation with funky antenna loads...   For 160M, I usually end up with a 500 pF or .001 padding loading cap that gets switched in with a relay or heavy duty switch. This cap needs to be physically large to handle the 160M RF current in amps. (like a golf ball size or bigger) The "G1, G2"?  versions of Santimago? caps from commercial BC rigs are good for this. I use them in my 4X1 rig for plate bypass and load padding.  They are best used on 160-20M. They may exhibit some inductance on the higher bands I have been told.

Good on wood insulation for certain HV iron mountings. You are thinking ahead.

Use decent relays for the antenna T/R switching keying. Open frame relays the size of a golf ball or larger, rated for the proper current and voltages involved are needed. No micro tiny crap.  An antenna relay that arcs across its poles can easily feed TX RF into your receiver. It happened to me and was a disaster. Trace it out and see what I mean.  On my biggest rigs, I use a vacuum antenna T/R relay in this position for safety.  The cathode CT keying relay can be smaller and handles less abuse.

This rig is being built using high quality, robust parts. Just like a drag racing car... if everything is rated for 2000 HP and you cheap out and install a 400 HP rated torque converter, the weak link will always blow first.  Try to use over-rated parts in every position -   and the longevity and trouble-shooting process will be GREATLY improved.

Nothing worse than spending your time blowing up and replacing parts, redrilling and messing up your work... all because of using parts below reasonable ratings.  That goes for the plate choke too. I always wind my own and cannot remember having blown one in the last 20 years. It is rare.  Winding a plate choke and having it be resonant in a ham band is HARD to do.  Like winning on a 36:1 roulette wheel.

The best outcome is to fire up the rig and most everything works - just requires basic testing and optimization.  It can happen - you blow just 1-2 parts and then it runs forever... :-)

I can sense you are doing things right. Now let's spend some more time fine-combing this schematic...

T

John,

Just to give you something to look forward to - we all need a carrot:   ;)

I looked back at some operating notes for my pair of 813s X pair of 813s. I usually keep a cheat sheet on settings from band to band for quick access later on.

My rig, similar to yours, uses BC iron and a  2.5KV, 3KV and 4KV switchable master HV supply with the GFZ MOSFET audio driver. Tank circuit: all vac caps and heavy tank components.  Audio NFB around the modulator and driver board.  I have glass lantern chimneys blowing a slight breeze thru all four 813s.  I believe they are now rated at 200W plate diss with the air, rather than 125W.     Regular uncooled 813s are REALLY 160W tubes in my opinion - bad ass tube.

Under full strap conditions, 3KV, 75M into a dummy load:

Output 1KW carrier with 120% positive peak modulation.  Monitoring on the SDR and scope. (Negative peak limiter to protect mod xfmr during tests)   100 Hz Triangle wave looks near perfect.  Finals dark gray, modulators JUST starting to show color. (carbon plates)  Grid biasing well into class C with larger grid leak resistor, approaching class D with heavy RF drive.  Final efficiency near 83%.  

I usually abuse the rig during the testing period and once it is sorted out, (blown parts)  I drop the power down to a reasonable level and it usually runs forever without failures.  Slam it and then baby it.

My next step is to keep the voltage up in the 3KV level and play around with loading. Lets say the carrier is 1KW with heavy C2 loading  (C2 unmeshed) ... what if we load it lighter (C2 more meshed) and drop the carrier down to 600 watts? The audio headroom increases and we see 150%+ clean audio.  We have the choice of keeping the voltage up at 3KV or dropping it back to 2500V.  Add more grid drive (a little past manufacturer grid current recs) and see how the audio peaks increase and clean up. A little more or less screen voltage (Variac) and see what that does.

After experimenting and all parts are stressed out and hold together, we will find a sweet spot where power, audio, efficiency and abuse all come together for optimization.

Using heavy duty parts lets us forget about "babying" the rig but rather shooting for best results.

T

All suggested updates incorporated.

I want to make some inductors. How much Cu to buy. A $1 a foot locally. How many feet do I need? How many turns on how big of form. Going to also make a K1JJ Tuna... Should I just buy a 50ft roll?
 
How to build the plate cap?

How to build the input inductor?

Antenna Relays - RFParts has quite a few clean pullouts in the 50-75 buck range. Are these robust enough? Too many to choose from. Which ones are right?

Thx for the help, Tom!

John

Hi John,

Below are some pics of tank circuits. I especially like the first one by Chuck / K1KW.

The easiest way to ruin a great rig is to install a shoddy tank coil circuit. The easiest way to make it look and perform super is to silver plate it, good coil placement and short strap leads with a robust bandswitch.  The tank and plate circuit is a piece of art and should be treated this way...  :D   There is nothing more stunning than a lot of silver plated shiny "chrome" that shines brilliantly from the filament light. It gives the rig a soul and depth.

You can use heavy duty Mini-ductor stock for 160M effectively like Chuck's rig below. Silver plating for the other coils using 1/8", 3/16" or 1/4" copper tubing is not expensive.  Notice the incredibly short silver plated strap leads from the switch to the coil as well as other tight parts placement.

Check around your area for silver platers.  They also have kits on the web where you can apply the plating yourself - not quite like the tanks but still very FB..  

Calculate each coil for Q=12 and install. Then in the testing stage, you will probably find you can do better by experimentally moving the bandswitch taps around a turn or two in either direction.

Ccalculate the coil lengths and buy the copper tubing by the foot in the various diameters you need.  Just keep the interconnecting paths short and make good use of copper strap that also gets silver plated.  Make a large rectangular panel window so you can see the tube plates and the silver plated coils.


The 50 ohm input RF link coil is simply 4-6 turns of high voltage wire wound on top of the center of the grid coil. You will have to see what works best on all the bands, 4, 5 or 6 turns.
That grid coil resonates with the grid variable cap and the whole assembly is floating at grid voltage above ground, so use an insulated tuning rod.

What is a plate cap? (I am sending you four plate caps.)  Did you mean plate choke?


There are a few good threads on AMFone for the "JJ" tuner.  Forty turns of 3/16" or 1/4" copper tubing, 5" diameter, will easily cover 160M (and higher bands). Make the tuner so it will config both parallel and series feed as shown in the thread... to cover ANY impedance match known to man...   Let me know if you have questions after reading the threads. Be sure to use #10 Home Depot black insulated stranded wire or equivalent for the dipole feeder and flat top. Good quality spacers are important, or just space the line 16" apart, pull tight and use NO spacers at all - for best OWL possible.

Do search on AMFone:  "K1JJ Tuner"

T

You will need one DPDT relay or can use two SPDT relays for antenna T/R.

The ultimate is the Jennings vacuum DPDT relay below.  Cost is $147.


Or you can get two SPDT  vac relays for cheaper to do the same job.

Or, you can get an open frame Potter and Brumfield  24V DPDT relay for $27.  It will "probably" never arc over at your power level.  I used them for years until I went to all vac ant relays for safety of my receiver in case of flashovers as I mentioned.

My opinion? I would use the Jennings sample below. Look around for a similar Jennings DPDT vacuum or equivalent relay. That way you are sure.

Consider using 24 VDC as your shack keying voltage. This will match many, many different BIGGER relay coils.  I have both, 12V and 24VDC here.

BTW, do you have the circuit for antenna switching? IE, the ant connects to the receiver during standby. During key up the antenna connects to the 813 power output while the 813 input connects to the RF driver.  Can you visualize how an RF arc can travel from the 813 output contacts to the receiver input contacts by bridging the relay?

Also, see the DPDT (or two SPDT relays) antenna relay and exciter switching below.  R3A / R3B   using  J1 RF in  and  J2 RF 0ut SO-239s.  


T

 

Back to the tank coil. My thoughts were along the lines of the second pic. A single tapped coil with a shorting bandswitch. How do you calculate these large coils with various types of cu tubing? On the first one, what kind of bandswitch can switch in different coils? Where can I find out how coil stock affects inductance?

Update - did a lot of studying this morning, getting a handle on it.

Tom,

Found a band switch - see PMs

John

Hi John,

To calculate the coils, go to an online coil calculator and pi-network calculators.   Between these two you will be able to enter a Q=12, and go from there.  

Enter the coil dimensions to come up with the number of turns per band.  You can short the coil for the various band taps from the loading capacitor 50 ohm point up the coil towards the plate tuning C1 cap. Put multiple coils in series with the smallest at the plate tuning cap end - with the bigger coils (160M) at the 50 ohm end. Know what I mean?

The type of coil stock will have a small effect on inductance. The coil diameter, turns per inch and coil length have the biggest effects.

The PM you sent with the various auction coils, bandswitches and other components are all in the ballpark to use for 813s  @ 3KV.  To save money, I would approach the guy with the picture showing the two white chimneys. Make a deal on the bandswitches, the plate choke, coils AND there is a metal turns counter sitting inside the chimney... See if you can buy the whole lot cheap.  Send him an email and once in a blue moon they will cancel the auction and do a special deal... :-)  Otherwise just win the auction and try to buy the other stuff too.

That strap/edgewound coil will be plenty for 160M and can be used on 75 and even 40M.  But for 40 M and 20M I would rather use one of the 1/4" tubing coils shown. They have better dimensions for the higher bands than the edge wound coil.

Coils like to be 1:1  to 2:1 length to diameter ratio for best Q.     IE, a coil that is 4" long and 3" diameter is in the ballpark. A coil 12" long and 3" diameter is NG.  Keep the coil turns spaced apart as "reasonably" wide as possible to keep inter-turn capacitance lowest.  However, wider turns spacing decreases the inductance. This may be good or bad when tailoring for the specific band.

I would clean all of that stuff up with solvents suited for the job and silver plate everything you can.  This is an example of how a dirty tank assembly can make a nice rig look like crap.

I was looking at the silver plating processes on the web and it's pretty easy. From what I briefly read, use a 12V battery with two chemicals in a big glass bowl and you're in business.  It's all on the web to read.  The chemicals can be purchased on the web.  

Can someone here chime in about their experiences /dangers with DIY silver plating ??


T

Take care, the process uses potassium cyanide or silver cyanide,VERY, VERY toxic. Also, when the PH isn't correct one can produce cyanide gasses. So when you smell almonds when opening the jar with chemicals or during the process, leave running
There is a cyanide free process, but the maximum thickness is about 20 uM. For the 10 meter band ok, for the lower bands not sufficient, the skin effect goes deeper at lower frequencies than 20 uM.
There is also a method to put a silver layer at isolation materials like glass etc.
But when you leave the solution for more than a few hours, a high explosive silver nitrite will form. There were quite some accidents when people just left the solution and moved or just touched it after a while. Silver nitrite is a high explosive, please take care.
I had children in the house and gave the chemicals to a waste center where they could dispose of it with great care. I rather use a plating center where they have the knowledge.

Thanks much for the info, Nico!  Scary stuff.

I guess if it were easy everyone would be doing it DIY- but they're not. I have a few friends who have had professionals do it for them in plating tanks. Chuck's rig is an example. Now I know why.

No DIY silver plating.

T

I've used those open frame contractors on some very large 10 meter amps.

Tetrodes with handles, twin 3cx3000, etc.  If you really want them to be bulletproof and drop to almost 0 ohms in a 4 wire tester, put some (multiple layers) KOOL AMP on them....  Poor man's silver whetted contactor.  Been there, done it, works a treat.

Biggest issue I had with them at the upper bands is you have two sides to detune.  I was never able to get 0 reactance on them with a single bypass cap.  I ended up with a bypass cap on each arm.  Probably not an issue at 40 meters and lower.

HOWEVER!  They are slow, and if you use them only for output keying, you will probably end up hot switching your output.  The 12 volt ones should be run on 18 to 24 volts with an appropriate dropping relay to speed them up.  To speed up the release, you can add a resistor to the ubiquitous anti spike diode.  That will still damp the inductive kick,  but provide a quicker release.  (not my idea on the release.  Don't remember where I read it, probably some amplifier forum).

I have a couple of those Jennings dpdt relays around here.  Total cats meow.  Also, look at the break rating.  The top contacts would make a darn good overcurrent break relay.

--Shane
KD6VXI

Would you trust buying a Jennings RB3 on ebay? Will give me 1 week to test it. Good price.

Sure. I buy stuff all the time off eBay.  It's a plus if he's a ham.  Google him and find out all about his activities and reputation and you can pretty well tell who you're dealing with.
Is it Steve, $85? He has 100% rating with 2K sales. Looks good.  

How would you test it in a week?  A hi-pot test could show bad vacuum as an open contacts fail.  But how about current test?  You could put some RF thru it at 50 ohms. Maybe Shane knows a good, easy way.

Just to be sure...  you could probably get away with the $25 open frame relay as Shane agreed... but I prefer the Jennings vacuum. It's real  overkill and like insurance.  Of course we must remember that a plate modulated rig  capable of 1KW carrier can generate 5 KW pep RF power, so it's really like a BIG linear.  I mean, WTF.

T

I have been buying a lot of stuff lately so will hold on the vac relay.

Just got done with the 2 threads of Fabio II and taming the beast and the audio games. Am I in for some fun or what! Hopefully twin 813's at 2700V is not as much of a fireworks show.

Put a bid in on the tank stuff. He had the other stuff for a project and was selling the parts of a lot he bought to get those other parts.

Made my first order from Russia on some doorknobs. Also got a 3A variac for $25 bucks plus shipping. Still need a bigger one for 1/2 of the 240 feed.

Winterfest 2019 is end of the month here in STL land. A big show. Hope to score a vac variable for the plate load there. Perhaps an antenna load cap as well. Most of the other parts are coming together.

Starting to think about sequencing. Is that 4 relay board you started with a good way to go, or should I take the discrete approach. Recall that I plan to feed the rig with a DDS VFO driving a 20W IRF510 push-pull SS amp. Do I need to put a relay on the drive input to the big rig or can I just plan to cut power to the amp to stop the drive?

On the big relay for the antenna, why DPDT? I get that half of it is switching the antenna between the TX and the RX. What is the other half doing?

Should I be planning on some failsafe sensors like you did on Fabio II to cut off PTT if bad mojo happens?

Later,
John

John,

Lest I forget...

The Jennings relay terminals are extremely fragile in regard to stress. I have broken one or two in my time as well as many other guys. Be careful when tightening the screws and support the terminal when doing so. A direct turn of the screw without support will usually crack the glass and bye-bye vacuum.  If you use strap, be sure it is pre-formed to align without stress.


The next frontier:

Since you are in the design phase, you will soon be designing your front panel layouts. If you want to take a quantum leap, study Chuck's front panel on his homebrew 4-400A AM rig. Notice the incredible balance and symmetry. Big lights, brass bezels, beautiful knobs, etc. Obviously he spent a lot of time figuring this out and getting the internal parts to mount correctly, etc.  This is like 3-D chess. Building a good layout for RF, making the panel's layout symmetrical and making it pleasing to the eye - is the mark of advanced work.  You might as well shoot for this kind of finished product. You will probably not get there on the first rig, but you will be far ahead in the end if you are aware of these things and make it a goal now.

As I've been told, "If it's worth doing, it's worth doing right."

** Over time I have found this is the difference between whether your rig eventually gets torn down for parts or someone begs to buy it for a few grand.  Over the years, I have noticed that Chuck always gets a good price for his rigs - the demand is there... all the way back to high school.  How much do you think his latest rig would bring... $15K? more?    :)

T  

 

Already been thinking about the panel layout.

Looks like I will not be getting a PCB from Rich. I see that he cancelled his license in 2018 around the same time of his last logon to this site. I was thinking he was SK, but found him alive and well on Linked In. Anyway, he has not responded to my email for over a week, so not holding much hope.

I may very well do the board myself. Looks pretty simple. I did a PWM driver on EasyEDA a few months ago. They are not very hard to do. I used to use Kicad but find EasyEDA easier - more straightforward. That is where I have been doing my schematics.

John

If you can give the diameter of the tubing you plan on using and the projected plate voltage and current, I have a MatLaB program that calculates this stuff in 0.1 seconds.


Phil

Hi John,

OK on all.  Yes, Fabio, the audio work and other things were a trip.  My guideline is this:  It usually takes as long (or longer) to troubleshoot and finally optimize a big rig than it takes to build it. Trying to get operating perfection takes a long time... ;)

See below a sample schematic showing the intended antenna / transceiver/exciter RELAY circuit. You will see why a DPDT or two SPDT relays are needed.  This schematic is a linear with cathode feed -  and yours will be feeding the grid instead. Look at relays  K3A / K3B.   J1 input, J2 output SO-239s.       You will have to figure out your transceiver, driver and/ or receiver switching or whatever outside of the rig.

Yes, sensors to prevent the grid or screen or plate from melting down are a good idea, but can be added later.

You might ask Steve/QIX or one of the e-rig guys what they are currently using for a sequencer board. I am still using the W2NBC design with variable, indepedent keying.  Yes, sequencing is important for sure.

T

Phil, I plan to use 1/4 in tube. For current my guess is 450 ma at 2700 volts. Tube sheets don't go that high.

Thanks,
John

I read something in the Fabio II thread that I am going to adopt. I am going to make my HV supply general purpose. I have a Drake L4B lineeer that needs a power supply as well. I think I will put a compatible cinch jones interconnect so that I can easily move the supply back and forth. Need to work out the details on this but I think it is doable. I will add a low voltage control line so I can control the hv from a sequencer when running from the 813 rig.

Hola John,

Good idea, since you will have such a truly magnificent HV supply, might as well use it for everything... ;D

To use a general purpose HV supply in the shack, in the HV supply I mounted a large ceramic "bandswitch" with a knob accessible from the outside.   This bandswitch connects to the HV supply and then distributes HV to each outside amplifier. I can switch the HV to the amplifier in use and this way there is HV only on the amplifier I pick. With a 240VAC primary step start, there is only HV when I key up and only on the amplifier I choose... safety features.

T
    

Won that big bandswitch auction so looks like the tank is coming together.

I have a 300pf and a 1150pf variable from my prior design. Think I will hook this up for grins with an RL of 3K and my MFJ antenna analyzer and see if I can get it to resonate on the 4 bands that I am shooting for. Probably will have to add 500pf of antenna load for 160M. I calculated RL assuming 2750v and 450ma. Is that the right current for this design, Tom? The tube datasheet does not go up that high. Also, should I be looking for a larger ammeter than 500ma full scale for the plate meter?

Got to clean it up first.

John

Status update - Parts gathering continues.

Still on the hunt for a 500pf vacuum variable and a turns counter. Perhaps a Millen 10030...that would be the cat's meow.

Picked up a Weston 1000ma meter for the plate. Placed a bid on a 500ma meter for the mod. Already have 2 250-300ma ones that will work for the bias and grid. Have a nice Weston 2.5kv meter that I want to use for the HV but my HV is a bit too high. Trying to decide if I mark it as Kv * 2 on the label and calibrate it to 5K. I really don't like to do this, I like meters to read proper correct values. Should I have a filament voltmeter?

Need to spend some time on the bench and check the meters to see if they need external shunts. Betting they do.

Tom, for the 25 ohm 20W glitch resistor, what type should that be? Also looking for a 100K 100W load resistor.

Thinking of building the RF deck on a 17x17x4 Hammond chassis. Honking big but will give me plenty of room above and below. Modulator will be a 17x8x3 or so to mount the tubes with the mod iron on a wood platform to the rear.

Big Midwest Hamfest on the 26th. Planning to attend.

John

Hi John,

The 300 pF and 1150 pF tests won't mean a lot since the in-circuit stray capacitance and your later-to-be-found  real-world loading sweet spots will be different. Spend the time looking for that 500 pF vacuum variable and other work instead.  Get the right caps and inductor and everything will fall into place once it is built.  (500 pF and 2000 pF)

Yes, 2750VDC and 450 ma is a good start, though near full strap at 1KW out carrier. Again, once the rig is built, then the meaningful testing and optimizing will start and you will determine where and how you want to run the rig..  My prediction: You will have less than 2750VDC when loaded up to a full 500 ma.  But that's OK - it is what it is. Maybe some added  bigger filter caps will stiffen it up later... we will have to see.

Usually you want the meters to read mid-scale in its normal operating mode, so  1A final and 500 mA mod meters will be OK.  Personally I like 60% of full scale scale operating for mine to give decent resolution while giving some room for "full strap" tuning, off resonance, etc.

T

W5UF has these listed as of October and not marked sold.

2 ea. 32 ufd 4.5kv, no PCB, GE, 4X4X10”, $45 ea plus shipping.

Thinking I should get one. That will give me 40uF of filter. I have a query out to him on a 10V/10A trans.

John

Keep looking - the caps and turns counters are around. Try to get a METAL turns counter. The plasdick ones work, but I have broken a few.  BTW, you can also use a right angle drive and mount the vac cap standing up on the chassis like Chuck's rig. That gives flexibility in placement and will free up some space if needed.

The grid meter should be 50ma full scale?... the screen meter  100 ma?     250-300ma is way too much and the needles will be reading too low for good resolution.  Look up the recommended operating currents and get meters to operate at 50%-60% of full scale or close to this when the rig is running normally.


The glitch resistor can be nearly any type of composition - but a wire wound is probably common and the best choice.  It has some inductance which is a good thing at that point in the supply to keep any remaining RF out of the HV supply.


Yes, the bigger chassis the better. You will always use more than planned, especially when you blow up parts and replace them with bigger, more robust parts. This is a common thing to homebrewing.  Be sure to use some kind of struts to stiffen the panel - chassis junction. Use horizontal rails for the module to sit in when in the rack. (and slide in and out to work on in the rack if possible)  Do not depend on the front panel to bear much of the load.  You probably already know this.   Heavy weight (iron) goes in the bottom of the rack if possible, of course.

4KV might be a good full-scale HV meter range for you when running 2750VDC. You will likely have to calibrate the HV meter and mark it so might as well pick a good voltage range.

Yes, a fil voltmeter is a good idea. I always use a Variac in my filament xfmr primary, so I need it to set the voltage each time.  The Variac is small, so worthwhile.

You are making good progress. Once you get "totally committed," road of no return....... have most of the parts and start the chassis layout phase, I will send you the tubes/ caps/sockets.  Should be shortly the way you are going.   ;)

T

Yes, those would be perfect! If you have the spare cash buy them both.  60% overrated 4500 VDC@ 70 uF in the HV filter is a righteous value!  
With that big plate transformer and big choke, and now big caps, your regulation would be as good as it gets. Build a big class B linear later on and you will be glad you did.

T

ok on the caps and meters. One of the meters I have is dual marked 50ma and 250ma so that works for the grid. I will look for a 100ma for the screen.

On the kv meter, it uses an external R so I can calibrate it to whatever I want. I wanted to avoid a printed replacement marking though. They look cheap and get yellow over time. I will keep looking.

There are some millen right angle drives on the bay now. I could use those but I really like the idea of a turns counter. i guess I could look for a vernier knob.

I am getting close. I think by the time I have the heavy iron from Phil I may be ready to start hammering.

I was committed long ago, by the way.

Thanks for the continued help.

John

PS - i need to find a high voltage probe for my knockoff china fluke meter.

Good - I think you have the right idea on most everything commented on...


Just to be clear: You will always use a turns counter with a vacuum variable, regardless if the cap is mounted on the chassis vertically or mounted horizontally at the front panel.  

A right angle drive is needed only when the cap is mounted vertically for the 90 degree turn.  


Spend a lot of time laying the rig out. Study again Chuck's rig layout.

T

The right angle drives I found are millen 10000 which have a 1:48 turns ratio. Are these the correct parts or will that make the turning painfully slow? i sent a email to Chuck to see if he perhaps has some pics of the underside of that deck pic as well as some for the modulator. His QRZ page only shows the same pic as yours.

John

Hopefully Chuck took some pics from underneath the rig too.  That would give you a lot of good info to study.

500 pF vacuum caps used for HF are slow enough already at 1:1 drive ratio. No need to slow them down further. You will be rowing around the bands enough already at 1:1...  ;) Same with the 1500-2000 pF loading vac caps.  

If you vertically mount one, get a 1:1   90 degree angle 1/4" shaft drive.   You will need a separate  1/4" shaft 1:1 panel mounted turns counter too.   (Most vac variables use 1/4" shafts, but not all.)   Be sure you use an insulated 1/4" shaft to the front panel (and/or an insulated 1/4" flex coupler) because there is often RF induced onto the metal shaft coming out of the vac variable casing. Potential RF burns even though the vacuum cap "rotor " may be grounded thru internal threads and finger stock. That goes for all metal shaft variable caps coming out to the front panel on a QRO KW rig.

T

Chuck is helping me with parts. Have right angle drives, now he is looking for caps. What a guy!

Yep, Chuck has helped many of us over the years.  (WA1EKV / K1KW)


Some background:

There was a time when I was building a lot of rigs and I needed to call him almost every night for advice. I'll bet we spent 100 hours on the phone and email over the years. He was always easy-going and more than willing to help. He could answer the questions that nobody else knew the answers to... the practical side of homebrewing that was not in the books.   He probably doesn't realize it, but he was my mentor for many years, though my work still doesn't compare to his.  The question is: Is the ham radio world a better place because of you?  In his case, yes, he made a huge difference.

It was back in 1965 when there was a knock on the door and there was 14 year old Chuck standing on the porch with his father. We had an eyeball QSO for the first time. He had his homebrew screen modulated 6146 rig under his arm built into a Bud Shadow cabinet. He pulled the cabinet off and I was amazed that anyone could build a rig, never mind make it look like factory work.  He went on to eventually build a 4-1000A plate modulated by 833As in high school. This young whipper-snapper was louder and sounded better than the old buzzard W1's of the 1960's AM era which probably made them crazy...   ;D

He went on over the next 50 years to build many homebrew rigs, some of which are still on the air in others' hands.  He was always on the "fun" leading edge of Ham technology. His 75M rotary beam in the mid 80s, stacked HF logs, homebrew amplifers, SDR, etc., etc., were a ham's dream. He's an engineer's engineer.  

Many don't know it, but he has another serious hobby -  a 6-bay garage full of 1920's/30's restored cars that he regularly drives to events, etc.  My guess is he has helped many of those hobbyists too.

What can I say? You are in good hands. Ask questions and get the right answers.  Glad you are finding some of the hard to find parts...  Some day you will pay it forward and help others build their rigs too.


T

Chuck came thru with the Vac Caps. Almost there. Just have to now wait for all of the stuff to arrive.

That's good news. With those right angle drives, you can mount the vac caps vertically on the chassis if you want.  Better tank placement and no support bracket needed for the vac cap. Now you need turns counters. They can sometimes be found on eBay. The metal ones are best and hardest to find.

There are plastic ones around made by a guy in OH? who has since passed on. I once bought 10 of them -  but one by one the counter gears stripped. All of my rigs use metal now. Also, try to get them with three digits. Two digits sometimes run out of range and you have to guess where you are.  IE, the vac cap requires a certain number of full turns from end to end.... be sure your turns counter is able to cover the full range without repeating itself.

What value caps do you have coming?  (uF / voltage)

T

I have 2 5KV caps coming, one at 500pf and one at 1000pf. Chuck said the 1000pf will be perfect at my b+ level (2800v). He was cautious on the 500pf. He had some 15kv units that were too large for a 813 class unit. These are new and hipot at 5.8kv. He said if I keep my B+ nearer to 2500V they should work fine. I recalled that you doubt that the supply will do 2800v at full power. I figured backing it down nearer to 2500V will have a negligible effect on overall power output. We shall see.

If it does not work well there I can always move it to the Tuna.

I also have the right angle drives and he found 2 turns counters. I am planning to lay out the deck along the lines of his.

I will have to pad the antenna load cap for 160M. He suggested some good caps to use. I will also use some of those spst russian vac relays like he uses. He says they are excellent.

I get my iron in a week.  ;D

John

Excellente!

Yes, the 1000pF @ 5KV will be fine for loading.  You will need some padding on 160M.  Nico had mentioned about padding caps and 10M / VHF parasitics due to feed-thru coupling of the relay or switch used.  If the padding capacitor relay has minimum feedthrough C, it will be OK.   Measure it when you receive the relay and see what it is.  I've never had any problems with padding cap generated parasitics on 20M and lower.

The 500 pF at 5 KV for the plate tuning will have to be watched carefully during testing and operation. When ramping it up to full power, use put a tall mirror behind the rig in a dark room to watch for arcing. (careful!)  Also, monitor carefully the vac cap shiny copper plates for signs of tiny black pits which is the sign of RF arcing.

You might as well get a Russian T/R ant switching vac relay (or pair of spdt) while you're at it.

Too bad you don't have a pic of the bottom of Chuck's RF chassis. Well, just use strap to connect the grid circuit components and keep it as close to the grid as possible, as he says.  Just think in terms  of as short and thick as possible for all the RF connections and layout.  Make sure no input components can see the output components anywhere in the amplifier.   (good shielding and bypassing)

The rest of the stuff isn't really that critical - the audio, AC and DC stuff.

T

Bandswitch showed up. It is huge! Going to throw it in the dishwasher and then test for inductance.

Hi John,

Well, it looks like you are really going through with this 813 project. You have reached the point of no return...  ;)  

As promised, I  packed up the four tubes, sockets and plate caps and put them in both bubble wrap and styro-pellets. Should make the trip from CT > IL  very FB, OM.

I checked my 813 tube supply and found I had only seven tubes. I forgot I gave four to another ham back in 1999 who built this same rig.   So I have three more 813 spares in case you run into any problems with these four.  They look pretty good and at one time I probably tested them in one of my rigs.

I assume your Salem, IL adr is OK on QRZ.com?

T

Yes, the addr on QRZed is correct. Thank you so much for the parts and the help so far! This is just the start of a journey I am sure.

I am going to start a new thread directed at the build. That way we start fresh and focus just on this design. I am amazed that this thread has had so many views! I feel like I am just as famous as AOC  ::)

New thread here - http://amfone.net/Amforum/index.php?topic=44558.0

We are about 1 week out on the delivery of the iron. Hoping that the weather does not mess this up. It is very kind of Phil to haul this along on his trip south. At 180lbs for the transformer and the weight of the other components, it is not a casual undertaking. Hope it does not weigh down the back of my Yugo too much...Ha! (I have a Ford F250, actually).

John
W9JSW