The AM Forum

Decided to post this, as a sacrificial cow
This is REV 0
This is my first junk box AM transmitter.
Poured over the designs on this Board
The handbooks of old, and pieced this togeather
if something is flawed, let me know

HTV

It looks like the blocking cap (1000, 5Kv) is in the wrong place. At the moment it will block B+ from reaching the plate of the 813. It should block DC from the Pi network.
There is something unusual about the grid connections, but I don't remember the pinout of the tube well enough to put a finger on it. Go here and take a look: http://www.amwindow.org/tech/htm/813/813.htm

Bill

BTW, I like K-1 and R-2 to bias off the tube. I use the same setup on my 813 and 4-400 transmitter. I think you forgot to draww the ground at the end of R-2/K-1.

Bill

Yes, Bill is correct, move the top connection of RFC 1 over to the left side of
the .001/5kv cap.

Also the ground is missing from the relay shorting area of the cut-off (stand
 by bias) resistor there in the center tap return...

I think you are slightly overdoing ( making it too complicated, why?) the bias
with the cathode resistor as well as the VR controlled grid bias circuity.
Remember KISS... There are better, simpler ways...

Pin 4 is the control grid, pin 3 is the screen and pin 5 to ground...

it looks like your using the beam forming plates as grid one? They are usually grounded as I remember. (Pin 5) I never seen this grid used in this way.

Pin four is the control grid.

The control grid connection pin # needs to be corrected as well as the plate blocking cap placement  as mentioned. also I believe the bypass on the grid rf choke needs to go to the bottom of the coil and maybe reduce the 1000uuf cap some if you don't get enough feedback for nuetalization. The neutralizing cap and the bypass cap form a voltage divider with the rfc isolating it from the bias supply. After you get her up and running you might experiment with some grid leak as well as your regulated bias. Has worked better for me on modulated amps. Good luck with your project seems, like you've got it down pretty well,
                                    Jay-

center tap of filament transformer shown on wrong winding.

The neutralizing capacitor can be connected on the cold side of the blocking capacitor, at least that's the way I have done it.

VR tube in bias line seems needless.

If you want adjustable bias, R4 should be downstream of the VR-90.  An adjustable resistor at the point where you have it is worthless because all it's doing is adjusting the input voltage to the VR-90.  When the VR-90 fires, that's what your bias voltage will be.

+1 on the other comments.

Always good to see guys design their own stuff and give it a go.

I noticed the grid L1 / 250pf parallel circuit appears to be floating (thru the RF choke) with no RF reference to ground to drive the grid.

Add a 500pf or 1000pf cap from the cold end to ground to create a voltage divider for the neutralizing cap to work into - and also give the RF grid circuit a ground return. This may then require more than 10pf of neutralizing capacitance - try and see.

As Bill suggested, here's a sample neutralizng circuit to work from:
http://www.amwindow.org/tech/htm/813/813.htm


Update: (whoops - Jay mentioned this already in a few posts above)


T

how about getting screen voltage from the modulated b+ from a dropping resistor?

very good point brandon, or maybe using a small choke in series with the screen supply.

You have many mistakes in your layout and design.  First you need to improve your drawing of schematics.  The grid is always just above the filament and to the left, the screen grid is next and to the right, then the surpressor grid also to the right (which is grounded), then the plate at the top.

The dc blocking cap goes between the plate choke and the pie network.  You have it between the choke and the plate.  You're missing a 250-300pfd cap from the junction of you're grid RFC and input coil, the cap goes to ground.

You need to supply screen grid voltage directly from the modulated 1500V B+ through a 40K 150watt resistor.  You can use a combination of power resistors to make up the 40K 150 watter.  You should add a clamp tube circuit to the screen grid to protect the 813 if there is no drive.

You're missing the center tap on the filament xfmr.  I see you're placing the plate current meter in the B+ lead.  I do not like that design although commonly used.  I would place the current meter in the filament center tap lead to ground.  You will be reading the total of the plate current, screen current and grid current.  This design keeps the HV voltage off the meters.

You do not need the bias supply.  Just put your grid resistor from the grid RFC to ground, through your grid current meter.  The grid resistor should be 13K about 10-15watt.  Also can be made up from a combination of resistors.

I'll have to go back and take another look at your drawing to add more comments.

I just finished building a single 813 rig, completely my own design including winding the coils, the 813 worked perfect on the first try.  With proper design and construction there is no reason why your 813 rig shouldn't work on its first try.  Just to add a note; if you see smoke or hear sparks, you made a mistake ;D

I'm not sure how you're placing the rig into standby.  I shut off the HV power supply (with relays) on standby.  It is fired up, on transmit, through a step-start circuit.

My comments are based on the fact that you're building a plate modulated AM rig.

Fred

Took another look at the drawing.

I see how you're putting the rig into standby.  You're biasing off the 813 with high value cathode resistor and the relay.  You're missing the ground connection at this relay.  Also place a arrow head on the relay's moving contact arm.

Using this method, how are you shutting off the modulators??  The commonly overlook problem with this design is what happens to HV voltage power supply when the rig is on standby.  The voltage on the lightly loaded HV supply will increase, maybe to an unsafe level.  Something you have to think about.  Are the HV filter caps of a high enough voltage rating to handle the higher standby voltage??  This is the reason why I like to shut off the HV supply on standby.

Fred

Fred, we wanna see a real schematic or it didn't happen  :D

Fred actually showed up on the WFDN about a week or 2 ago running the "imaginary 813 rig". It's no longer imaginary!! (I didnt think I would live long enough to get to hear it! ! )   :o  :o  ;D

Guys, thanks for the comments, I will try to mark the drawing up with your comments I will reply to this with a new drawing showing integration of your design ideas into the sketch, I will be wiring soon I hope, with proper caution if I zorch something nobody will get hurt.

HTV

  Very good Fred...

  Very comprehensive and I agree with using a clamp tube on the
  screen to pull its voltage down on loss of drive.
  
  A 6Y6 is what I use, plate connected to the screen side of the
  series resistor and grid connected at about 10K above the ground
  side of the  grid load of the 813.  Cut the clamp tube off with drive;
  no drive on clamp grid, tube conducts pulling final screen down to
  safe value on final tube...

  You want to select the series feed resistor of the 813 for around
  400 volts at about 15/16 ma when the tube is fully loaded. And yes,
  a high wattage dropping resistor is required there.  

   In one 813 final I built 60+ years ago I used a fixed 40K/150W in  
   series with a 15K/150W with an adjustable tab to set the screen voltage.

   Be very carefully in there, remember that's lethal voltage on that tap!

   Fred, scratch out your schematic for us and lets see what your final
   looks like at this stage.

   I missed that filament transformer CT on the primary side on my scan
   of the original drawing, sorry...

I have read over everyone's suggestions and agree with all of them except for the biass / cutoff scheem.

Keeping with the KISS theme, why bother with a clamp tube?? Simply biass the 813 well down below the cutoff point and if it loses drive it will simply cut itself off and go to sleep. That is what I did with my 4X1 transmitter and it has been happy as a clam for more than 10 years now. It just requires a little more RF drive to the grid.

Also the heavier grid drive and increased biass will really help make the munky swing!! (and improve plate efficiency a bit as well)

Also, if you're building the thing why not use a pair of 813s instead of a single. Even
if you dont run any more power than a aingle, it helps with the "loaf along" factor, or will allow you to make more power out with a lower plate voltage.

Just my $.02 worth.

Why not a pair of 813's, Hey I only scrounged up an art-13 mod transformer, I sold a 350Watt unit 2 years ago, thought I would never get around to it, but here I am back at it, maybe it will burn up and need to be replaced.

Have a hole in the chassis for the second 813, put a piece of aluminum over it as a patch. I will try to draw this animal up after work and post a second version of the thing I need to carefully read all these comments and absorb them. some are obvious flaws or misdrawn stuff, other stuff I need to study further as I attempt to correct the schematic. Thanks again all, for taking the time to consider and correct my humble efforts

HTV

Is it me or are the screen voltage connection and control grid bias connection reversed?

I agree with K1JJ regarding the grid circuit. I think there needs to be a ground point in there somewhere. I would use a split stator capacitor with the rotor grounded. Of course to get 250pf one would have to have 500pf per side. Alternatively, one could use a smaller variable cap with a padder. This is what I do with my single band 160 meter rig. I believe that grounding the rotor also helps suppress VHF parasitics. 100pf should be enough grid tank capacitance. I believe my B&W 160JEL grid coil is resonated by about 80pf at 1.9 mHz.

One can also ground the center point of the grid tank coil. There was a very interesting thread a while back started by K9ACT regarding the drive requirements being less if the coil is grounded vs the capacitor. Do a search on the topic "Mystery Grid Drive" to see this thread.

Ron W8ACR

Top Secret ;D

I'm having trouble putting up an antenna (for the time being)...so
I think I'll hang my dummy load from the balcony and try to raise you...
after all we're only 12 miles apart. I've got two homebrewed xmtrs
and a dx-40 I repaired and no frigging antenna >:(

Don’t get me wrong, I am not trying to be a jerk or insult you but the amount of errors like the wrong placement of the plate blocking capacitor, driving the suppressor grid instead of the control grid and no DC return in the cathode circuit all remind me of the very old test you had to take for the second class commercial license and have to identify the problems with the circuit, once again I am not poking fun at you but I found it fun to identify the errors. Would almost like to see someone draw up some more circuits with errors and see who catches them. Thanks for posting this and wish you success with your project!
Ray F.

If this amplifier is to be run in class C plate modulated service, you do not need or want the VR tube to regulate the negative grid bias to the 813 grid.

In a plate modulated amplifier, you really want the grid bias to be completely unregulated; this serves to improve the linearity of the modulation process in the final amplifier circuit. The only purpose of the fixed bias supply is to provide some degree of protective/safety bias, and to support the bulk of the grid bias which should come from the grid leak resistor.

A good rule of thumb is that approximately one-third of the grid bias voltage should come from the fixed bias supply, with the balance derived from the grid leak resistor.

And I agree; why run one 813, when you can run two of them instead? 813s are still inexpensive and readily available, as are their sockets.

73,

Bruce

I think he was testing us? Did we pass ;D ;D ;D

Mr. HTV (I am sorry, I do not know your real name ;))

First congrats for taking the incentive to build a home brew transmitter.  When you put a schematic on here you have some of the best minds looking it over for error's and suggestions. 

A couple of things I noticed in the postings:

Some folks that posted to you said the same thing that others before them have said.  Some of the comments were simply repeats. 

Don't be discourage by the comments that you made so many mistakes and the inference that you should already know better.  I take it that your new to home brewing these type of circuits and making mistakes is part of the learning curve.  The technical comments were right on, but rather blunt. 

Oh and finally, I believe you answered very well why you were not running a pair of 813's was due to a modulation transformer limitation.  That's smart planning to allow for another jug to be put in later when you find suitable iron. 

A very good reference design is from Tom, K1JJ although with the good technical comments here, your on your way to making a working amplifier.

Have fun!

Joe, W3GMS     

Some corrections, rev1 schematic


HTV

  You forgot the cap to resonate the grid coil...

Ralph, true enough, Will add correction  to fix,
How do I set the idle current in the modulator, resistor not shown on modulator

HTV

  If you are going with the typical pair of 811A's, they run 0 bias up
  to about 1250 VDC on the plates.  I've run them around 2 KV with
  a 4.5 volt dry battery in the CT of the driver xformer...

here's a handbook design (from '57).

Greatly improved schematic.   I still see some problems with your design layout.

The bias supply is not correct the way you have it.  First, it is going to put hum on your grid.  I still think you should not use the bias supply and protect the 813 with a clamp tube circuit.  But if you want to use it you need to change the design. 

From your diode connect a resistor about 1000ohms, then connect the negative terminal of your filter cap 20-50mfd 200V to the 1K.  DO NOT ground the winding on the xfmr, return that side of the winding to the positive terminal of your filter cap.  You now have a half wave resistor input bias supply that is still not grounded, it is floating.  Now connect an adjustable resistor, about 5K, across the filter cap.  Connect the junction of the negative side of the filter cap and 5K to your grid circuit and then ground the adjustable arm of the 5K resistor.  This circuit will have proper filtering on the bias supply regardless of where you adjust the bias.  The bias is adjustable from zero bias (when the ground point is at the negative side of the cap) to full bias voltage (when the ground point is at the positive side of the filter cap) or anything in between.

The 2.5mh RFC in the screen circuit is not needed and can cause problems because of the (same) 2.5mh RFC in the grid circuit, may cause the 813 to oscillate.  Replace the RFC in the screen circuit with a 1-2K wire wound 10W resistor.  Put the screen bypass cap at the tube pin not where you have it.

W3GL already mentioned the missing grid tuning cap (usually about 100pfd).

The grid bypass cap, at the junction of the grid coil and grid RFC, is too large,  about 250-300pfd is more correct if you expect to get any feedback voltage from the neutralizing cap.

Add a filament bypass cap on the center tap of the xfmr.  For the filament bypass caps you can use .01mfd disc ceramics.

You'll need some way to put the 813 in low power to tune.  We can get to that in another post.  Not hard to set up.

Good luck with the project,  myself and many others are more than happy to help you through to a successful project.

Fred

That one looks MUCH better! ! !

I agree with Ralph, you need a cap to resonate the grid input coil.
You also need to relocate the cap at the low side of the biass adjusting pot. The low side of the pot should be grounded. Also add an electrolytic cap on the outpoot of the bias supply for filtering. (where the diode meets the pot.)
Hook the bias supply to the high side of the pot (so the pot acts as a bleeder.)
then hook the bias feed to the grid to the wiper of the pot.

Screw the clamp tube!! It's not worth the bother if you got it biased properly.

And like Fred said, remove the RFC fron the screen feed and run it straight to the screen with a .001 bypass cap.

   As shown, when keyed, and with no excitation, the 813 screen voltage goes to 1500v. Is this of any concern with an 813? I doubt if the protective bias will be enough to keep the static plate current within a reasonable value.

  Also, if plate modulated, don't we want grid bias more than fixed bias?

Jim
WD5JKO

Jim,
     You're right I didnt catch that, there is no grid leak. that is an often debated subject,
whether or not to use a grid leak on a plate modded final. With a grid leak, however he will have to drive it into class c operation.

As far as the screen goes, in standby that shouldn't matter in his circuit. With the filament center tap opened up, there is no DC path to ground for it to draw any screen current, so it shouldnt matter as long as the voltage isn't high enough for it to arc over to another element in the tube.

Back to the clamp tube for a second, it should only be necessary if you are only using grid leak biass. The fixed biass should be set at a level that would protect the tube in the event of loss of drive.

  Yes I agree, but as G2 rises towards 1500v in the case of lost excitation when rig is keyed, the -100v or so fixed bias will not be enough to keep the tube from overloading. On a tetrode, the G2 voltage has a very pronounced effect on the G1 to Ip characteristic.

  Isn't it more common with big tetrodes, and beam power tubes to get the G2 voltage from a different source than the plate voltage? I'm thinking the Globe King 500 where the screen gets a high impedance 500v G2 supply, and with a choke it self modulates G2 along with plate modulation, or the ART13 where we have separate supplies, and where the modulation transformer has a separate screen winding.

Just my 2 cents..
Jim
WD5JKO

Jim,
      If the fixed biass is set to a point that would cut the tube off with the screen voltage and plate voltage appiled, it shouldn't matter much. However, I do agree that a tertiary winding on the mod tranny or a self modulation choke would be a better way to fly.

Since he allready has an ART-13 mod tranny this would be an easy way to go.

He could also tap down the HV on a voltage divider instead of just using a dropping resistor.

If he was to change the biass rectifier to a simple half wave doubler this would give him all the biass he could ever need.

This is kinda fun, all the fun of doing the design work without having to do the drilling and blasting.............................

Although it is done that way, Jim, I don't know ifn it's more common. Having the scrren and plate come on together limits the chance of the screen being on without plateV which, of course, usually will only happen once!  :o

I would get rid of the bias supply and use the clamp tube.  I have a clamp tube in my 813 and it worked perfect.  I shut off the drive (with full plate voltage) and the screen voltage dropped a very low value, plate current also dropped to a low level.  It's better to get the grid voltage from a grid leak resistor.  I used 13K about 10w.  At 13ma of grid drive you get about 160-170 volts on the grid which is the recommended amount.

I also have a tune position which is controled by the screen voltage.  I switch in a 7.5K 25w resistor from the screen side of the 40K screen resistor to ground.  This drags down the screen voltage but still leaves enough plate current to easily tune the grid and plate.  The OP is using a screen current meter shunt resistor, so, the 7.5K 25w resistor should be switch in at the 40K but before the screen shunt.  Same thing is needed if he uses the clamp tube circuit.

I also think the plate tuning cap is too large.  The drawing shows a 500pfd.  About 200pfd is more than enough to tune on 80M with his tank coil of 19uh.  You need a high voltage tuning cap.  I use a 80pfd tuning cap (9KV 1/4" spacing) and a 118pfd 15KV RF doorknob on 80M, but a 7.5KV cap would also be enough.  I mention this because of the large size of these caps.  Also with a 500pfd cap it makes tuning very touchy.  Using a vacuum cap is always better.

IIRC from I what saw on the drawing, the plate blocking cap of 5KV should be increased to at least 10KV, I'm using two 510pfd 18KV doorknobs in parallel.  The plate bypass at the plate RFC should also be at least 10KV .001pfd.

The loading cap of 1000pfd probably is just enough for 80M.  Any low voltage type tuning cap can be used.  Usually most often used are the BC type tuning caps.  I use a 4 gang 2100pfd on 80M, 2 gangs on 40M.  Not sure how much of it is in use.  I think about half meshed on both bands.

Fred

Thanks Budly,

A very often overlooked problem.  That's why it's better to get the 813 screen voltage through the dropping resistor from the HV supply.

Even with my 6550's that I use for the 6146 the 650V is always on the plates of the 6550's even on standby.  The 300v screen voltage is on the PTT line.  But, there is a safety relay that will open the screen circuit (within 1 sec) if the plate voltage is removed.

Fred

Hello all,

Made the suggested change on the grid bias supply, should have 80 ma. max. in the floating circuit , the tube table suggests -90 volts here, some of you suggest this be higher. the limit would be the voltage of the secondary of the bias transformer, should I go ahead and add a series resistor to increase this.
the tube table lists a grid current of 12ma. can this listed value be used to size the grid resistor? if so what total bias should I be shooting for then I can back out the resistor size. ( what about the 1/3 rule?)

The air caps I have for the tank circuit are of an unknown value, and I have additional air caps in the junkbox, I was shooting for the values on the schematic. the plan was to use old russian doorknob caps to increase the value of the air variables if needed, thinking the tune range would not have to be all that wide for this thing to live at 3870kc and never move I have several air caps with almost 1/4" spacing, I can sort of guess at thier value comparing them to ones of known value.

here's rev 3
will a broadcast cap live in the load position, I don't know

HV supply

Sizing the HV Bleed resistor:

Unknown : HV transformer about 40 pounds worth no output capacity in ma. data printed on it

using 1500-0-1500 taps on it has also 2000-0-2000 taps.

Known: Full wave rectifier with Big crazy Silicon valley rectifiers Full wave not Bridge.
they are sized at 5KV each for 2 amps. (I have 4 of them using only 2)

Mombo unknown value HV choke about 30 pounds, after the rectifiers in the HV line, followed by a stack of 47Uf electrolytics 8 I think 450V each.

with a resistor stack across the top (25k 15watt ea.) (from cap input arrl supply)

before adding the choke (Cap input supply) the thing climbed to 3200kv (no load) and I shut it off with a stick on the breaker
Need to size bleed resistor, and add the choke to stop this. do I do this by trial and error not knowing what output the transformer is good for or say use a 15k 150w resistor an pull 100ma. right off the top ans see what it does.
if I just add the choke make it a Choke input supply what will it do, the caps won't see the rectifier ripple and it may not pump up to the ac peak.
or will it?
HTV

Look at your grid circuit / biass supply closely in rev3.pdf. You still dont have a grid leak or a DC path to ground for the grid / biass supply.


Fred,
       You ALWAYS take the long way around the block. Do you also walk around with a shirt that says "Kick Me" on the back! !  ! ! !  ;D  ;D  Lets face it, a 3' tall Budd cabinet stuffed to the gills for a 50w transmitter is a bit of overkill. 

Why don't you really simplify things by using an 810 instead of an 813. Get rid of all that screen circuitry mumbo jumbo. Use the 813's as triode connected modulators. To quote K9ACT...... "triodes are nice" ;D ;D

Ron

I haven't look at the rev drawings but just to answer a few questions.

BC radio tuning caps are fine for the loading caps, you made need a three gang one if you have one,  if not, use a two gang.

Finally you told us what freqs you are building the rig for.  With that 19uh coil you'll need less than 200pfd to tune the tank circuit on 75M.  Try to use a 100pfd wide spaced tuning cap and another 100pfd, or something close, HV RF doorknob, if you have one.

I'll have to re-read your antics with the power supply testing.  Try not to electro-plate yourself.  FORGET the capacitor input supply.  That type of supply is far too unstable for what you're building.  Especially with the high-bias-cathode-resistor-relay scheme you're using for standby.  Use the choke input filter.

The 25K 15 watt resistors that you're placing across each of the eight filter caps will give about 10ma of bleeder current at 2KV standby voltage.  You can reduce the 25K's to something less 15-20K if you have them.  You will need to increase the bleeder current to at least 30-40ma total.  So you will need to add another bleeder group of resistors across the output of the supply to make up the difference from whatever the equalizing cap resistors are drawing.  I usually use two large power resistors in series for bleeders.  With the eight 450V caps in series the filter cap is good to 3.6KV.  No need to get close to the 3.6KV on standby, when the supply is lightly loaded.  30-40ma of bleeder current will keep you below 3.6KV even with the higher voltage taps on the xfmr (with the choke input filter).

With the grid bias supply you will need to add a grid leak resistor.  The grid leak resistor will need to drop the grid voltage needed ( with 13ma of grid current) minus the grid bias supply voltage.  The total grid voltage needed is about 160-170V for the 813 at about 1500 volts on the plate.

Fred

Are the filter caps 47uf or 470uf??  You said they are 47uf.  If so, you will not have enough filter capacitance with eight of them in series.

Let me know.

Took a close look at the two schematics.

I see you change and added all the improvements.  There is one thing you didn't get exactly correct.  You connected the grid lead to the wiper arm on the 5K resistor.  There is no ground point.  I said to connect the grid lead to the junction of the 1K, 5K, neg terminal of the filter cap and ground the wiper arm.  You can also do it the way you have it, but you then must ground the junction of the positive side of the filter cap, 5K and xfmr lead.  Either way will work.  You will also need to add in a grid leak resistor between the bias supply and the meter shunt resistor.

It's now 5AM, so will continue later today.

Your metering circuits will need some changes but will get to that in another post.

By the time you build this xmtr you'll either be an expert or as confused as all of us are ;D

Fred

Frank,

Here we go again, lets pick on Fred. (good title for a net)

The 3ft 18" deep open cube rack contains the 813, 811's, 1500V PS, 20amp variac, mod iron, 50hy mod reactor, and the whole 50watt xmtr.

I was just reading one of your other posts.  You bring up a good point.  The very high screen voltage that occurs  when you open the filament CT on standby. Without any screen current the screen voltage could rise to a the full unloaded power supply voltage, which may reach 2500V.  May arc over inside the tube.

Fred

cleaned up the HV transformer
labeled as follows:
1500
1250
1000
0
1000
1250
1500
so my max voltage with a bridge is 1500
a smudge over the label cleaned off to read 500ma
If I used a full wave bridge I could raise the HV voltage
somebody mentioned using a 810
plate voltage would be out of range of the 811 modulators at 2 KV I think
half an amp, at 1500 volts, wow, does seem like a waste to just use 1 tube
what is the output of only 1 813 at 1.5KV

I will fix the mumbo jumbo bias supply for Fred it was my fault, I drew it wrong.

oh, the power supply caps are 470uF

what else,  oh I have a bunch of large wattage resistors I will be able to cobble together the bleeder or maybe lower the valve of the capacitor bridging resistors
I am catching up, but still way behind you guys though.

this is like skipping high school, and going to college courses taught by a bunch of broadcast engineers, my tube theory in college was a long time ago, and I don't remember it as well as I would like. but it is coming back, Thanks to all for the time you have taken to comment.

HTV

Added some photos of this animal for reference

HTV

WoW!!  didn't know you had some of the rig already build.  I thought you were still in the thinking and planning stages.

FB,  I think your plate xfmr might be a UTC S series judging from the three plate voltage levels  Maybe a S-48, IIRC that one is rated at 500ma, most of the others are rated at 300ma.  I have a S-49 that has higher voltages but only 300ma.

Stancor plate xfmrs usually had only two voltage levels BUT the ratings as marked on the xfmrs are the DC voltage output after two stages of filtering.  The AC terminal voltages are higher.

If it is a UTC S series you probably can not use it with a full wave bridge rectifier.  The ceter tap may not be able to handle the high voltage above ground.  With a FW bridge your max voltage would be 3KV not 1.5KV like you say.

With a full wave rectifier you will only get about 1250-1300 volts DC after a choke input filter.  There is an easy way to bring that voltage up 300-400 volts, 1500-1600 volts is a good level to run the rig at considering the 811's

Fred

Thanks fred,
I went back out a looked at it it indeed is an S48 you were right.
I don't even want to ask how you knew
secondary hipot is 4500V but I suspect you are right, may not want to subject the thing to 3kv or even 2kv on the center tap
so I have gobs of current but am limited for HV, I don't have the modulation transformer for 2 tubes, there is a 125 watt on ebay but I suspect I need a bit more for 2 tubes,.
the ART-13 Mod Transformer seems small, It is smaller than the 60 watt transformer in my viking 1
How do I boost the output of the HV trans without doing a voltage doubler circuit?
maybe the smart thing is to add a second tube and run 1250V and find another modulation transformer would the multimatch on Ebay work?

73 HTV

you can still use the ART-13 mod xfmr. See

http://www.amwindow.org/tech/htm/shuntfedmod.htm

Simple way to bring up the final plate HV;

If you have another power xfmr in the range of 300-400 volt,  (700-800VCT) with a current rating in 250-500 ma current range,  do the following.

Using the xfmr with a set of solid state rectifiers just exactly as if your were building a small PS,  connect the output from the rectifiers directly to the center tap of your plate xfmr (S-48).  This will bring your final HV up by added amount of the smaller supply.

Remember, just the output from the FW diode rectifiers with nothing in between the diodes and the CT of the S-48. (no chokes, no caps) You could put a fuse in between if you wanted, that would be OK.

This will bring your CT voltage up 300-400 above ground but most any xfmr should be able to handle that without any problem (always check the CT terminal with an ohm meter to make sure it is NOT internally grounded).

This can be done with any xfmr you have on hand.  The small xfmr can be a FW bridge diode rectifier output,  same thing, just connect output of the FW bridge diodes to the CT of the S-48.  Even if a HV xfmr (not your S-48) was using FW bridge rectifiers, you could do the same thing,  the output from the small xfmr diode rectifiers would be connected to the diodes on the HV xfmr (the junction on the FW bridge rectifiers that is normally grounded) and you will get the same results.

If you want to lower your plate voltage, you just shut off the small xfmr and the voltage will just drop back down to just the S-48, NO need to disconnect anything, just shut off the AC to the primary.  Turn it back on to get the higher voltage.  You can do that while the supply is in use.  You could shut off the S-48 and you would be left with just the voltage from the small xfmr.

This method allows you to increase your HV by a smaller amount of added HV, instead of the double-the-voltage you get when going from FW to FWB.  You also mantain the full current capacity of the HV xfmr.  Normally when you go from FW to FW bridge you have to half your current load on the xfmr.

The current capacity of the small xfmr would normally have to be the same as the HV xfmr.  If you use a power xfmr that has filament windings, you can draw more than the rated HV current by not using the filament windings.

A note about diode strings,  You need to use enough diodes in each string to handle the PIV.  Xfmrs in the 350-400V range I use two 1KV PIV diodes in each string.  Xfmrs in the 1500 volt range I use seven diodes in each string.  When using this stacking-of-PS method like I described, there is NO need to increase the amount of diodes normally used for each xfmr.

Let me know if you have any questions about this.

A test on this subject matter will be given on Friday ;D

Fred

That's pretty neat... what kind of audio response can one expect with an ART-13 mod txfrmr?  Can one approach Hi-Fi or is that asking too much from them?

Chris (W2JBL) runs one like that (ART-13) somewhere around 200+w out and it sounds real good.

Or just simply change the filter to cap inpoot (C-L-C).
I know you loose some regulation.........blah, blah blah..................
But you gain a noticable increase in plate voltage, even under full load.

Yer takin the long way around the block again, Fred! ! ! ! ! ! ! !

Also, dont run an old piece of iron FWB unless it's rated for it!! I didnt listen and had to learn the hard way, I blew up a handful of them that broke down at the center tap to ground or center tap to the primary! ! !

Thanks Fred for staying with me on this, you are of great help.
You opened this up for questions so here goes.

I have seen resistors used across the diodes in solid state power supplys.
if the PIV of the diodes is high enough can these be omitted?
If indeed I need this precaution how are these sized?

Back to my mumbo jumbo bias setup, should I split the bias source some from fixed and some from a resistor, or delete the fixed bias entirely and risk the tube.
I could use the bias transformer for control relay voltage, or get that from the filament winding, allthough 10V may not operate a 12V relay.

I understand the concept of the small transformer and rectifier on the centertap of the Main HV transformer. I suspect that I do not have a small transformer with  the current capability, but I may I will have to look. I do have a large variac I guess I could use that on the primary of the HV Trans. Some folks seem to discourage the use of variacs in HV supplys though.

I want to confirm a basic assumption, In this circuit it is assumed the exciter is always on and the HV is always on, Keying the rig is done by removing the bias from the centertap of the filament transformer.

Today I will scrounge around my shop looking for  relays transformers and the small chokes I need I know I have them I just need to find where I put them.

I will put up a corrected schematic again soon after I correct the errors.
73
HTV

To start with, don't listen to Frank KB3AHE, here's a guy that doesn't know much of anything about radios, thats why he has blown up so many xfmrs ;D.  Frankie, I love ya.

Even though a lot of folks will tell you that with modern diodes you no longer need to use them, I still use them.  A 470K 1/2w and a .01 1KV disc ceramic across each diode.  I just feel better using them and I use 1N5408 3A 1KV piv diodes.

I would not use the bias supply as I mentioned before, but if you insist, your grid voltage should be around 170 volts total.  So, if you have say 80 volts of bias supply you then need another 90 volts of grid leak bias.  Figuring 13ma of grid current, 90v/.013a = abt 7K of grid resistor in series with the bias supply.  .013x.013x7K = 1.18w, so, use at least a 2W or larger resistor.

As far as your design for keying the xmtr into standby, I am completely against using the high-bias-resistor-relay setup.  It creates too many problems (in standby) that you're not seeing.

First, your assumtion that the exciter is always on, is wrong.  The exciter is OFF on standby.  If you left it on, to begin with, you would have so much of your own signal in your receiver you would not hear anything but your own signal.

Leaving the HV PS on during standby conditions is never a good idea and IMO poor engineering.  It may be done all the time with linear amplifiers but not too good an idea with plate modulated AM xmtrs.

Because you're using a series dropping resistor for the screen voltage, on standby the HV PS voltage will rise, without any current flowing through the screen resistor the voltage on the screen could rise to the full PS standby voltage.  This may cause the 813 to arc over inside the tube.  Frank, KB3AHE, correctly pointed this out in a previous post (only time he ever came up with a good point) ;D

Another thing is that this same HV will appear on your meter switch circuit.  Most likely exceeding the voltage rating on those disc ceramic bypass caps, in addition the meter switch may not be the right type to handle such high voltages.
Another point about the meter circuit for the screen current, you want to avoid too many bypass caps on the screen because you may begin to bypass too much of your higher freq audio.  It's better to meter the screen voltage than the current, you then don't need the extra two .001 bypass caps.  

There is probably other issues with leaving the HV supply on during standby.

Leaving the HV on during standby, in general, just waste energy, you're heating up bleeder resistors for no good reason.  It also can be dangerous, one can walk away from the station for a minute or so and forget the fact that there is a 2KV power supply left on.  Someone else (not you) may get hurt.

It's better to shut off the HV power supply on standby and key it up on transmit with a step-start circuit.  This is not hard or difficult to do and it eliminates a whole series of issues.

Fred

You asked about using a variac on the HV xfmr.  The UTC S-48 is rated for 115VAC on the primary.  It is also rated to produce 1300VDC after a single choke input filter.  It is rated for 500ma.  These plate xfmrs like most plate xfmrs are designed to be run with choke input filters.  Using a cap input filter (CLC) will bring up the DC voltage output of the supply.  But, it increases the VA load on the xfmr and you must reduce the total current drain by about 30%.  CLC filters are not as stable as a choke input filter.  Under light loads the voltage can increase close to the peak AC voltage.

It is not recommended that you try to run the primary at a higher voltage than normal AC line voltage, which in most areas is around 120VAC.

You can easily bring up the DC output voltage by using the extra xfmr and diode rectifiers like I described previously.  IMO, a few hundred volts on the CT of the S-48 should work fine.  This method is far less stressful on the xfmr CT than using the xfmr with a FW bridge rectifier.  With FW bridge the CT is operating at half the total AC voltage of the secondary. So, that would be 1500VAC if using the full 3000 secondary AC voltage.  There a big difference from a few hundred volts to 1500 volts.  Using this method also mantains good voltage regulation.

It would be good if you can tell me what choke you have to use with the supply.  Some choke are better than others, even with chokes that have the same ratings.

Fred

Fine Buisness Fred, Might as well do it right, I may have this thing for years.

The choke I have is unlabled about 8" tall and 7" wide with traditional End caps It is not a potted style. (has 2 wires coming from the windings #18 with cotton rubber insulation) in good shape weighs 30 LBs. don't know what value or if it is a swinging choke, hard to calculate critical value when you don't know the size of the choke

I need to study how a stepstart circuit works, If the bias is all passive either off the HV supply of derived from grid leak and the hv supply goes away along with the exciter, no harm done to the tube, But the exciter (a millen 90800 with a separate power supply) is going to turn off quicker than the caps on the HV supply will discharge, I need to study this. Analyzing what happens during this process is something I have never done.

Dale

Hi Dale,

Although my previous post was somewhat tongue-in-cheek, I really think you should consider using a triode or triodes in parallel for your final amplifier. 810's 100TH's or 250TH's are all plentiful. If you have two 813's, you could then connect them as triodes and use them as your modulator tubes. A single 1500-2000V ps could be used for both RF final and modulator.
I also recently built my first junk box transmitter, and I used two 254W triodes in parallel modulated with 811A's. For simplicity sake, I built it as a single band 160M transmitter, and it has been working great. I built separate power supplies for the modulator and the RF final. I run the 811A's with about 1400V on the plates, and the 254W's with about 2000V on the plates.
I am attaching a schematic of my next project, an RF deck for 80/40 using 8000's as finals. This RF deck will be used with the same power supply and modulator as my 254W RF deck. This circuit is nearly identical to my 254W RF deck circuit. Only the tank component values are different. I would like to point out that not having a screen supply simplifies the circuit significantly. Also note the simplicity of the bias supply.

Transmit/receive switching is also relatively simple. When I apply filament power, the bias supply comes on and puts about -100V on the grids of the 254W's. This is enough to cut off the tube in case plate voltage is accidentally applied. When I key the driver (a viking I), the receiver is muted and drive is applied to the grids of the 254W's. Because I have the grid circuit metered, I can see that when I apply grid drive both the grid current and voltage go up to approximatelytheir working values. I then hit a foot switch to apply the RF plate and modulator plate voltages simultaneously. At the end of my transmission, I simply release the foot switch, and then turn off the driver.
This is a simple control system, and probably less than optimal, but it seems to be working fine so far. Regarding the use of tetrodes versus triodes, I think the main advantages are less drive requirements for the tetrodes and also some tetrodes may not require neutralization. If you have twenty to thirty watts of drive power available, that should be enough for most triodes. Good luck.

Ron

Try to put up a pic of the choke, I may be able to identify it.  30 Lbs seems like a big enough choke, the only thing you need to worry about is the high voltage rating of the choke.  What is the DC resistance of the choke??

The Millen 90800 exciter puts out about 50 watts, you only need about 5 watts to drive the 813.  I think that exciter was designed to drive the Millen twin 812 final.  You'll have to work out some way to reduce the output. and key it on and off with the with rest of the xmtr.  Probably lower the plate voltage to the output tube in the Millen and have a PTT relay to key this voltage on and off.  This same relay (through separate contacts) will control the step-start relays for the HV supply.

You're right about the timing of the driver and the HV supply turning off.  On start up, the step-start delays the HV a bit.  But on turn-off the HV caps will delay the HV from going to zero.

Best way to overcome this problem is to use the 6Y6 clamp tube circuit on the screen of the 813.  With the clamp tube you don't need the grid bias supply and you can just use a grid leak resistor (13K about 10watts).  The 6Y6 clamp circuit is really very simple to set up.  The other thing that needs to be keyed along with the whole xmtr is the antenna relay and the receiver.  What relay are you using for the antenna??  A Dow-Key antenna relay also has extra contacts that are usually use to mute the receiver.

If you design the control circuits correctly, you should be able to control the whole transmit-receive with a single PTT switch.

The step-start usually requires two 120v relays and a power resistor of about 20-30 ohms.  The contact amp ratings have to be large enough for the primary current of the HV xfmr.

We also need to look at your metering circuits, I'm not sure what meters are in the rig.  What meters (exactly) are in the xmtr??

Fred

I was trying to figure out why there were RFC's going to each control grid on the 8000 amp of Ron's. RFC chokes would be used to keep RF at one end and pass DC only. Yiou would need RF on input and output of the RFC's(maybe they are low value inductor used kind of like a snubber resistor to blow in case there was a short from plate to grid.Usually there is a snubber of 100 ohms, 2 watts to each grid.

Probably small RFC's (a few uh) to reduce any parasitics,  similar in purpose to the parasitic choke that is usually used in the plate lead to the final.

  I like Ron's method of fixed/grid leak bias. This method with diode isolation was debated on other threads with lots of opposition and misunderstanding. Glad Ron seized upon the advantage, and ran with it.

Jim
WD5JKO

Yes, those RF chokes on the grids of the 8000's are small VHF parasitic suppressors. I believe I used about 10 turns of #24 wire wrapped around a high value 1 watt resistor. I believe their value is something like 0.2 uH. One other small correction - all the meters are shunted by a 0.01 uF capacitor.

And Jim, I forgot to credit you for giving me the idea to try the diode isolation circuit in the bias supply. So far it has worked very well. As I said, the grid circuit is metered for continuous reading of both current and voltage. They are both very stable during transmission.

Ron

Ron,

your grid tank looks strange. With a split-cap I would have used grid circuit
C below.

Peter

Ron
the picture of you're FR deck really looks cool,  fine buisness nice Radio

I am going to press on with my 813 though, I do hope to continue to build stuff after this, as This is a work in progress. I can always add a second tube change tubes whatever, but at the moment this is a challenge for me.
but I am learning

Fred,
 I did not throw an ohm meter on the choke but I will, I studied the clamp tube circuit last night, it was either this or use a  grid cut-off bias scheme using a Bias supply and a relay, so I dug through my junk box and found a 6y6G and a ceramic tube socket and also found a 6.3V filament transformer 6.3V at 3 amps. I even have the parts.
I even have a Greenlee chassis punch the proper size, Lucked out.
The meters on the rig came from an old WW2 Army Signal Corps 6 meter RF deck
1 meter is 1-100ma the other is 1-10ma
I have a matching meter that is 1-300Ma to replace the 1-10ma
they say signal corps on the face inside the glass, also real nice jeweled movement, I will look further for any other stuff like resistance, voltage rating etc.
There is a picture of this I put up on a previous post, I am building the faraday cage you see mounted to it.

Also I have not started wiring yet but I will try to partition the input circuit from the rest of the stuff under the chassis.

I have a Dow key relay with 2 extra contacts and several other open reed relays capable of a fair amount of current, they are old school like what is in a KWM-2 not the modern mini type with the clear plastic box
I have 2 or three that have a 6 volt coil and about the same number with a 120Vac coil

Step Start

What about using the voltage drop across a resistor at inrush to hold in a relay at steady state current it opens up using the normally closed contacts to short across the resistor,taking it out of the circuit the problem is where to put the relay in a 1.5KV line, I wonder if the relay coil could be in the 120V line to the primary of the HV trans.
also what is the acceptable voltage drop across this resistor at inrush

also the discharging HV caps won't hold it closed,

it would be nice to see a second set of contacts on this relay key the exciter
after the inrush current event and unkey the thing and this is as far as I have progressed do I want to unkey the exciter before the HV dies or after I think before, then the clamp tube affair takes over cuts the tube off and then HV has leaked away we start over.
I will keep studying

Dale

Saw your post late in the game  :-[

Here is another 813 design:

http://www.raibeam.com/zl1axb/813_amp.html

Phil - AC0OB

I way overestimated my choke
I weighed the thing at 15 pounds not 30
measures 83 ohms across the terminals
my ohmmeter has a inductance setting said 23 henrys
finally drug it out and weighed it, where's the thordarson company when you need them

might work to isolate the secondary in the Mod transformer

any thought's, I need half an amp for my power supply

this is fun, I am having a blast guys, hey, ham radio this is where it's at
I need a swap meet, HTV

Dale,

I'll wait for the ohm meter reading of the choke, don't forget to put up a pic if you can.

The meters are OK and maybe not OK.  Keep in mind that some meters have a faceplate that is not exactly what the meter movement really is.  You have to look close at the bottom of the faceplate to see if there is any other info.  It may say something like full scale = 1ma, which means the meter is really a 1ma meter that is used with external shunts to make it read the full scale current that is on the faceplate.  If you don't see any such notations then the meter reads directly the current that's on the faceplate.  Many volt meters have these types of notation on them.  Example, a 1000v meter may read, down at the bottom, "1000 ohms/volt", which tells you that you have to add your own external multiplying resistance, in this case 1megaohm in series with the meter.  The meter movement itself is a 1ma meter.  In most cases meters that read small currents and lower voltages (under 1KV) have these shunts and resistances inside the case.  Most all the movements alone are 1ma.

In my HB xmtr I use meters that came out of the military ART-13 xmtr.  The meter has a number of different scales on the faceplate.  At the bottom of the faceplate it reads full scale = 1ma.  I use four of these meters, along with other meters.  The 1ma meters are used to read many different currents and voltages.  With switches and the proper resistors one (1ma) meter can be made to read any number of voltages and currents.

In your schematic you have drawn meter shunts in your grid and screen circuits.  Problem is that your meters are not going to work with a shunt type circuit. (now you'll understand why I mentioned earlier having to discuss your metering circuits)

Your 300ma meter (if direct reading) will work fine to read the cathode current (through the filament xfmr CT) of the 813, you'll need another same to read the 811 current.

To read the grid current you'll need about a 20-25ma meter.  Your 100ma meter could be used but the grid current will read at the very low end of the scale.  The meter would be wired directly inline (no shunts) with the grid circuit.  The 10ma meter would also work but you would have to shunt it with a shunt resistor equal to the resistance of the meter.  It would then read 0-20ma. Meaning that, a 10ma scale reading would really be 20ma.  Problem with this is, measuring the exact resistance of the meter and then finding that exact value resistor to place across the meter terminals.

Reading the screen current will be more problematic and probably could be eliminated as it is not really needed.  With the correct size screen resistor dropping voltage from the plate voltage, the screen current will be within the correct amount and there is no practical way to adjust it anyway.  Metering the screen voltage is better but you need the correct meter to do this.

I"ll describe the step-start in another post, it's not difficult but you"ll need the right relays to do it.  I think you said you have a few 120vac relays.  Don't worry about the discharging of the filter caps.  That will happen rather quickly with the 813 and 811 drawing current.  It's the charging of the caps on startup that you have to think about.

Fred

Glad you're having a great time as we slave over trying to get you to build this xmtr and have it work when you're done ;D

Kinda thought 30Lbs was a little high for a choke that has lead wires on it.  Usually, any of the big chokes have terminals.  Any model number anywhere on the choke??  Most any choke will read high inductance when there isn't any current flowing through it.

Yes there is a number on it, I  thought it would be buried in the annals of obsolescence. I will write it down and post it tonight. I did not notice last night if the end bells come off, if they do I could get some idea of wire size in the winding looks big enough to handle 350Ma or so.
Plate current on the single 813 is about 180ma. The modulator I have not looked up yet but it is not a constant load.
My relays are only good for 10A. need to get one good for 20A to switch the primary of the HV transformer.
Still digging through the junk boxes looking for parts, making a list, hope to make headway this coming weekend.

Dale

What is the brand name of that meter? I need one that will do hennry's.

Resistance of the Choke
the resistance of the winding is 83 ohms
and the winding to the case is open

Dale

Ron,

I like the grid bias supply circuit you're using with the diode isolation.  I guess Jim WD5JKO came up with the idea of the diode, credit to Jim.

It seems that the grid resistor offers a load on the supply, which is good.  I think the full grid bias is developed through the grid resistor without regard to the bias supply.  I guess the grid voltage would be greater than the bias supply voltage at full drive current.  If the grid drive drops, grid leak voltage drops,  the bias supply maintains a minimum voltage on the grid.  Seems like it should work.

I'll have to try this circuit.  With triodes you need some sort of fixed bias to protect the tube.  With screen grid tubes, using a clamp tube circuit on the screen voltage also works well.

Thanks for sharing the circuit.

Fred

Peter:  I am using a B&W JEL 160 grid coil which has no center tap, and the wire is so fine and wound so closely, that it would be very difficult to add one. So I simply applied the grid bias voltage to the end of the coil instead and it sems to work fine. I also have a JCL160 coil that has a center tap and fixed center link. But since the tubes are in paralel, not push pull, I thought the end link coil would be more appropriate. I always thought the link should be at the lowest RF potential spot on the coil. This would be in the center for a balanced push pull circuit, or at the end for a non push pull arrangement. In any event, it works.
Fred: My understanding is he same as yours. With no grid drive, the bias supply supplies cutoff voltage. With grid drive, all the bias is grid leak. When I apply filament power, I see about -100V on the grids. With grid drive but no plate voltage, I see about -450V. With grid drive and plate voltage, the grid voltage drops to about -350V, just where the book says it should be. Maybe I'm just lucky, but so far it has worked very well. :D

Ron

Ron, if it works it works! it's just that I've never seen unbalanced linking
tuned by a split-cap with rotor grounded. It almost looks like a pi-net
input.

I wonder, maybe one of the experts can chime-in here, could this kind of
grid tank help with tubes of high input capacitance (like, say, the 4D32) ?

73 de Peter

Peter,
Your post started me thinking and doing some research both here in the forum archives and in my reference books. It seems that you are right - a center tap grid coil is recommended whenever the grid circuit is balanced (which it is when grid neutralization is used with a pi network plate tank circuit). Since I do have a center tapped coil available, I'm gonna give it a try and see what, if any, changes occur. I'll let you know what happens.
Ron

Dale,

Any luck on digging out the choke and seeing if there are any numbers on it??

Waiting to hear,

Fred

Meters

weston 505
weston 506
simpson IS-129 model 125
no text as to internal resistance
 
Choke numbers
21210
138552

Hope to work on the radio today and try to redraw the schematic this weekend

Dale,

OK on the meter numbers, those are style type numbers.  You should be able to use the meters,  probably you'll have to use the 10ma meter for your grid current.  You'll have to shunt it with a small resistance equal to the resistance of the meter.  Not hard to do, just measure the terminals with a digital ohm meter and find a resistor equal to that amount and connect it across the meter.  The meter will then read 0-20ma, 10ma reading will be 20ma.

OK on the choke,  looks like it may be OK for transmitter service (just maybe).

The numbers don't tell much, except the 138 number is a manufacturer's code.  The 138 code is for Chicago Transformer.  The choke was made by Chicago for some other manufacturer of equipment. The last three digits are a date code, the last digit of the year (0-9) the last two are the week of the year (01-52).  These codes were used between 1940-1960. But, as you can see, no real way to know what decade the part was made, but, most of these codes were from the 1950's. The other number set has very little meaning, except Chicago did use those 5 digit numbers on some of their chokes to tell the ratings.  Example, Chicago's standard-catalog-model choke 12hy@105ma carried a number, stamped into the steel case, of 12105.

If we try to use that same method for your choke number 21210,  That would mean the choke is 21hys@210ma.  Problem is 21 henries doesn't make any sense because of the 80 ohm resistance you measured. A 21hy choke of that size would have much more resistance.  Double check the number because a number of 12210 could easily be what that choke is, 12hy@210ma.  If the 21210 is correct, then it's just a part number that usually can't be located in any Chicago catalog.  Only Chicago and the manual of the piece of equipment the choke was made for would have the ratings.

I would suggest that you try to find a better choke, one that was designed to run at high voltage with at least 300ma.  If you are going to use the step-start to turn on the HV supply on transmit, you need a high inductance input choke to limit the in-rush current, especially with those high capacitance electolytics.

In most all other cases, xfmrs and chokes that have 5 digit part numbers (no letters) are special units that were made by a transformer manufacturer for some other maker of equipment.  This is true for most all transformer manufacturers.  The manufacturer's code number did not always appear on the parts.

Fred

is it possible to separate the power supplies for the modulator and the transmitter after rectifier stage or does that cause problems
I have seen transmitters and modulators with separate power supplies  in the handbooks from the fiftys.
The henrys measurement I took was with my vom that has a lc meter
there are 2 little wire grabbers recessed into the front of the meter
so you have to solder some solid hookup wire leads to the coil being measured.
the meter is made for measuring small leaded components, and it was kinda bouncing around when I took the measurement. I have never really trusted it for big components, I wanted to be able to take this to swapmeets looking for capacitors and transformers.
Today is punch and mount the clamp tube day maybe do some filament wiring.

fine buisness on the meters may not even bother with the screen current, like you say, you cant really change it anyway.

there is a swapmeet next weekend, I will see what is there.

Dale

Dale,

As you were making your last post I added some additional comments about the choke in my previous post that you may want to go back and read.

Most will tell you that it's better to use one power supply for both the 813 PA and the modulators.  I agree, You would never want one supply to turn on and have the other supply to fail to turn on.  Especially, the modulators being on without any high voltage on the 813 PA.  No PA load on the mod. xfmr will quickly arc the mod xfmr.  Better to use one supply for both.

Fred

Hi Dale,

I have a nice NOS Triad choke, model number C-19A. Looks new. 10Hy, 300mA, 3000V test. It's yours for the price of shipping if you think it might help.

Peter: I put in the new center link/center tap coil and it also works fine. As far as I can tell, there is no difference in the tuning of the grid tank with the new coil. I'm gonna stick with the center link coil since that seems to be the consensus opinion of how to do it. I made a couple of other changes to the circuit as well. I took the second 254W out of the circuit so that I could get a larger spacing on the neutralizing cap. It was only about 1/8 inch spacing with both tubes, now it is about 1/4 inch. With both tubes, I was only running the plate current at 200mA anyway. With one tube, I can run about 180 mA, so I havent lost much power. 1800V @ 180mA - 325W input and about 225W output.  There is one added bonus - Now that I have one tube running at full plate current, there is a faint red glow that brightens with modulation. Nice visual effect. :) Lastly, I raised (lowered) the grid voltage to -400V after reading some archived comments and suggestions from K4KYV. All the changes appear to work FB.

Ron

Ron

That's a fine offer Ron, take it in to UPS have em weigh it and cost it out I will send you the money

Thanks

Dale

The Triad choke that Ron offered would be a good one to use.  I've found that Triad transformers are some of the better ones.  I have a 500ma 10hy Triad and it does a good job at filtering.  A lot of folks may be surprised to know that some of the worst chokes IMO are the UTC S line.  In contrast the UTC PA series chokes are some of the best.  UTC CG series are also good.  Early Thordarsons are good.  One of the last series Thordarson made (21 series) are fair.  Most any of the Stancors are good,  they made the same chokes throughout their history.

I have about a 1000 transformers and chokes (counting the small ones) hanging around and have been working with them for nearly 50yrs.

I'm not sure if the Triad is rated at 3KV or 2KV. I've seen Triads that were only rated 2KV.  BUT, you must understand what that rating really means.  Those ratings are the RMS TEST VOLTAGE.  As a rule, to find the DC voltage the choke will handle you must subtract 1KV then divide by two.  So a choke that is rated at 3KV RMS TEST is really only good for about 1KV DC.  Some chokes are rated at some "DC voltage working".  You can use the choke at the full DC working voltage.  Some chokes are rated at some "voltage insulation" not really sure what the manufacturer really meant by that rating.

You can probably run the Triad at a MAX of about 1200V-1500V tops.  Just think, if it fails and shorts, you'll be the first one to know ;D.

There is more stress on an input choke than there is on the second choke in a two stage filter circuit.

A lot of folks will tell you that you can insulate the choke from ground (chassis) to avoid it shorting.  Forget that, it's stupid and dangerous.  You can put the choke in the negative lead which greatly reduces the high voltage on it. Check any Handbook to see how that's done.

Insulating any choke or transformer from ground is a dangerous thing to do, even though it's talk about and done by many.

Fred

Fred,

What can I expect the efficiency of the HV transformer to be?
 Initially, I have to run 1250V at the moment so using the tube tables

The RF deck can be estimated to draw 160Ma (.160 X 1250V. = 200W input)

The Modulator deck is said to draw 260Ma peak.
so  for RMS .707 X 260 is 183ma
and we do not need 200 watts of audio so we can derate that and be conservative for a single 813 so say 100Ma

so total is 260Ma average
IXE=Watts
.260 x 1250 = 325W

W/E=I or 325/120=2.7A
* est 60%    2.7/.6 = 4.5A

fuse at 5 amps 120VAC (my relays may work)

what about the ground path back thru the chassis rack etc.
what about my number #4 copper going to a couple of ground rods
from the rack and the aluminum rack base of the power supply rack
I understand the 2 wire model  of having the choke in the return path.
but now Ive gone an mucked up my understanding of this given my setup.

Dale

Dale,

The S-48 (500ma) plate xfmr will handle all the current the xmtr will draw.  Remember the modulation current is not a steady load.  You could load the 813 to the full 170-180 ma (plate and screen current) at whatever plate voltage you end up with (probably about 1300VDC).  Your only limitation is the current running through the modulation xfmr secondary.  I think you are using the ART-13 mod xfmr.

I would fuse the primary at about 6-7amps.  Don't forget the higher in-rush AC current in the primary.

If the 120V AC relays are DPDT, you can bridge the two sets of contacts to control the xfmr primary.  You only need to relay the HOT side of the AC line.  The neutral can be left connected at all times.

In my previous post I mentioned putting the choke in the negative lead of the HV power supply.  There are some problems with doing this,  the choke is between the chassis ground and the center tap of the plate xfmr.  This raises the peak voltage on the CT to a high level, the xfmr may not be able to handle this.  I never use that method in any of my supplies.  I always put the choke (or chokes) in the positive lead.

You can put the Triad choke in the positive lead, at 1300 volts it should be OK, but I'm not a 100% on that.  You'll just have to try it and hope the choke doesn't short to ground.


You should always connect your equipment to the station ground.   You can use the ground rods BUT you should connect the station ground back to the building AC ground.  The AC panel ground and any other ground rods you add should be connected together with at least a #6 wire.  Also make sure that you have connected the HV PS chassis and the 813, 811 decks together.

Fred

Yes,

Will have a neutral and a ground back to the panel from the transmitter power supply, I will also check all the ground rods and the wire size I used old copper stranded wire from an above ground overhead ac supply to the mast on my shack that I demo'd to put all electrical to the shack underground
I had a dedicated panel put in my shack which also has a couple of ground rods for it alone. as well as the ones that I put in separately to ground radios
the transmitter will have a dedicated 120V circuit just for the HV supply
the Filaments and 120V control will come from a separate circuit

I am building a 500V power supply for the Millen exciter and an optional small modulator for it, if I use it standalone, so same thing here have to sum up all the loads, I found a good old military transformer with a 570V. primary and several Filament windings, Have you seen the schematic for the Millen?
I found the original artical for the design in the 1943 ARRL handbook.
This will help with winding the coils for the thing, it is an old balanced output
link coil rig pretty neat but old school compared even with the 50's era designs meant to feed twinlead, no coax connector. so that is what I am working on as well as hunting up big resistors, chokes and the like for the 813 rig. the clamp tube is mounted in the chassis, boy do those greenlee chassis punches work slick

73 guys having too much fun