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THE AM BULLETIN BOARD => Technical Forum => Topic started by: KB2WIG on October 28, 2021, 04:00:06 PM



Title: This should be interesting
Post by: KB2WIG on October 28, 2021, 04:00:06 PM





After a decade or so, I'm finally got around to working on my "BigRig". (The name "Fabio" has been taken so I'll come up  with something else.) Following a consultation with an unnamed op from near Mirror Lake, I'm giving up on the Freed transformer for now; I'll be using a Peter Dahl transformer for the Modulator and RF deck power supply.

In the recent past, I've cooked the Mod transformer overnight, and the Mod Deck w/o HV for a few hours. See the pic, how nice the 810s and the 6CA7s glow in the dark.
The Peter Dahl is rated for 2135 V , .5A CCS. It's a split primary, and I got ~ 1 KV, no load. Keeping the transformer at 1 KV, I have an unknown choke, and a 23uF oil can that I propose to fire everything up and see what 'appens. The audio load consists of (3) 2K ohm, 200 Watt resistors. I'll feed the HV to the Pri and 2nd of the mod transformer so the won't feel shunned and decide to crap out. I'll scope it an see what thinks look like at 1 KV.

So, do anyone have any input/comments/ridicule r/e this thing of mine?

klc


Title: Re: This should be interesting
Post by: KB2WIG on October 28, 2021, 05:20:20 PM
eL Mod deck at night.


Title: Re: This should be interesting
Post by: KB2WIG on October 28, 2021, 05:23:15 PM

Mo power.


Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 09:58:22 AM
What is the overall approach? Got a schematic?

What is the history? Did it get that patina from your decade of inactivity or is it a chassis from another brother...

John


Title: Re: This should be interesting
Post by: W7TFO on October 29, 2021, 11:15:33 AM
Kev, your problem is not enuf old paint on the workbench. :(

73DG


Title: Re: This should be interesting
Post by: KB2WIG on October 29, 2021, 11:54:32 AM
Yes, a brotha from a different muther.

The plan is to throw together what I have and see what sticks.. I started this around 2006? For Now, I'd like to put this on the air on 160m and 75m, hopefully before The Rally in February.
https://amfone.net/Amforum/index.php?action=gallery;su=user;cat=14;u=714

I picked up a 'kit of parts'. i.e. partially disassembled home brew transmitter, along with some misc. parts and paperwork that sometimes matches what I have. So, originally, an 810 modulator, BC 610 mod transformer, some power iron. For the RF side, two 813s in  Push Pull,  link coupled. The circuits are apparently from The Radio Handbook, 14th ed.

The mod deck  as found;  mic in, 12AT7 then a 12AU7 that tickles the grids of two 6CA7, which then feed the 810s.

The Plan.   Power supply design for the mod deck and RF Deck. Then then the RF deck.

"JS"  a full wave rectifier, with the transformer pri. series wired to provide ~ 1 KV, add the unknown choke and use a 10 uF 2KV oil can. This should give me a bit of juice to fire up the 810s. I have 8 K ohms of load which I'll use for a mod transformer load, and we'll see what happens. Various AM Gurus suggest feeding the HV to both sides of the Mod transformer to keep it happy.

Probably  I should convert this to  a parallel 813 Rf Deck with a pie output, but, I have most of the parts from the previous design, so I will stick with it. For now that is..... I have some 8000s which might make a nice 20/10 rig for the upcoming solar cycle.

D,
Please be kind, as I suffer for my art.

klc


Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 12:50:01 PM
You can get some more juice from going with a CLC power supply. Get Duncan's Amp Tools and try out your PS design there.

I have some oil can caps that you could have for a song. I have a 8mfd at 3K one I did not use on my 813 rig from the Gates iron I obtained. May be just right for the first C in the sequence. Duncan can tell you true...

I made my PS variable by using a 15A 125V variac on one leg of the primary. If yours is a 220 primary, that works really well.

Good idea to use the same HV on both Mod and RF. Safer on the mod trans. If you lose HV on the RF deck, it will also go away on the Mod, saving the Trans.

Spark gaps! See my pic on QRZ. Look for red standoffs...

Use a few strands of coax shield for a fuse for the HV. Mine is 3 strands of RG-213 shield suspended between 2 ceramic standoffs. Has not blown yet in 1yr of operation.

Don't forget a step start. A resistor and a relay works FB.

How about this - https://www.ebay.com/itm/324690117876 for your output link.

8000's in the rf deck work nice. No screen to mess with. The 8000 is the brother to the 810... HamTips has some good discussion on the differences.

If you decide to go with the 813's anyway, I strongly suggest that you wire the grid with a -75V PS plus a grid leak. Provides safety bias in the loss of drive. On the screen, use a screen dropping R. I did and is works FB. Much easier than the choke approach. I shut off the HV on RX so no wasted heat on the big R then. Math is easy. (HV-400)/80ma = R. e.g. for 1500V the R will be 13,750 ohms. Wattage is 88W. Use a 100W ceramic Ohmite/Clarostat tube R. Although, using the screen R removes some of the nice from a variable HV.



Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 01:50:55 PM
That trans looks like it is for the Amp Supply LK-500. I don't understand how you are only getting 1kv. You should be getting 1.4 * 2135 minus whatever the choke wants to regulate it with a bridge. I would guess somewhere around 2200-2300V. That feeds 3-500's nicely which is what that amp had.  Too bad it does not have a CT. You may really want to do the variac. You then can use anywhere from 1500V on up.

Here is a pic of the LK500 PS - http://user1623492.sites.myregisteredsite.com/sitebuildercontent/sitebuilderfiles/power-supply-schem.pdf


Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 03:14:05 PM
You could try these for your bridge. I just bought some...

https://www.amazon.com/gp/product/B087PRYGRT/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1


Title: Re: This should be interesting
Post by: KB2WIG on October 29, 2021, 04:07:49 PM
J,

Yes, it's the same transformer used in the LK 500.  I want to run the modulator system at a reduced voltage for a while. Later, I'll bring her up to full V.  This thing hasn't been fired up for ~14 years; I haven't put the 810s under HV at all- today I'll hook up the mod tranny with a  6k+ ohm load and see what happens at 1 KV. ( maybe i'll try the cut wire spark gap method for insurance).

The power supply  'bridge' is on some cheep perf board, and the four 1n4007s  are only good for 4KV piv,so it will do for now.  I'm going to see if i can source locally  fiberglass board to make something more substantial with the drill press. If not, I have some thin plexi, or grab some thicker stuff from the Homely  Despot. The stand off insulator fuse is a good idea. I'll probably grab some microwave fuses later.

I have a 21.3 uF/ 5KV oil can, which I should put to use. I do appreciate the offer of the 8uF. Today, I'm using a 10uF 2.5KVcap; 930V DC across 24 K Ohms /76W load.

The RF deck will get started in a week or two. That baby will be the time killer. I have done some preliminary work  on the metering/filament/screen supply, along with layout in the cabinet. I do have a few  2 A powerstats handy, and a timing ckt for the step start. So I've got stuff to do. For now,I'll be using a B&W TVL base for the swinin' inductors. The base looks OK ish, but the other coils are suffering with polystyrene disease.

Those diode bricks look nice.

Thanks fer the input,
klc


Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 04:20:23 PM
I used Lexan on my diode support. Should be able to get a scrap at HD. can also get small cuts on Amazon.

So, are you running 1kv by using the 230V windings on 120v? That will get you there at 1kv. Good idea.

1n4007s are a bit light. I believe that in a bridge you still have 1000V PIV, not 4K.  You may pop one on startup. 


Title: Re: This should be interesting
Post by: KB2WIG on October 29, 2021, 04:48:10 PM
I've 16 of the 07's, 4 per leg. Buy 'em by the 100's get good price. I'm down to 40 of the 1n5408s; these are 3A at 1KV PIV. Don't want to waste them. I have a scrap piece of lexan or whatever it is. I would prefer pc board or G10 fiberglass, 'caus it looks so nice.

Yes, I series up the windings for now.  I'll probably put a switch on the front panel to control the 120/240 aspect; comes in handy for testing, or so I'm told.

The 8000s have a bit lower capacitance where it counts, and I got 'em. Should pick up some sockets fer the tubes.

klc


Title: Re: This should be interesting
Post by: KD6VXI on October 29, 2021, 05:52:53 PM
I stopped buying anything but 6A10 diodes for power supplies years ago.  Last order was 10A10s.  They are a beast of a diode, 10A at 1kv.  Sitting in storage now, I'd of sent you some for free before my move to KP2.

With the primary series up and running on 120v a step start may be a moot point....  However a resistor and relay are cheap.

Looks like a fun project.  I already miss building but hopefully I'll be bringing the shop to me.....  Just have to find a house with a hurricane proof shop!

--Shane
KP2/KD6VXI



Title: Re: This should be interesting
Post by: w9jsw on October 29, 2021, 07:16:41 PM
Data Point - I am running my 813s at 2kV in parallel. Get 550-600W out with 750W input (2000*.375).  I can't push it harder because my plate load cap is only rated for 5Kv. Still, plenty of power.

First time on the air I caught my antenna on fire. Was a DX Engineering doublet kit fed with twinlead. Had to stop and go build a better one. Now it is just fine running a fan dipole with rg213. QRO stresses stuff a lot more that 100W.

Sounds like you are well on way to the Rally. I agree a step start is moot. I thought you were using 1 diode per side. 4 is FB, for sure. Fire it up!

How are you going to drive it?

John


Title: Re: This should be interesting
Post by: KB2WIG on October 29, 2021, 08:31:04 PM

I'm plan to put a 240 V line to the basement shack sometime this year, and run things off of this  (I said this last year). Somewhere in the back-stock there are a few of the Amperite delay relays in-a-tube which  are planed for the 813 power supply. I'm finding things all over the place...

S,  Yea,  the beefier diode... The 10A10s are $.31 per 100 at mouser. Sounds like a purchase.


J,

 Drive it? it needs a ride?... I could use a DX60, or I have an Icom 706. The 706 has 160m, where the Heathkit doesn't. Or a Johnson Viking 2.

thanks guys,

klc


Title: Re: This should be interesting
Post by: KD6VXI on October 29, 2021, 09:28:59 PM
You can't beat the 6 and 10A10 diodes.  They are pretty indestructible, short of a short :) ....

As to the step start I gave up on the Amperite relates quite awhile ago.  In private conversations with W8JI I found the Ameritron method much easier in practice.

Put 10-15 ohms in series with the primary.  Contacts wrapped around that resistor to short it out when step start 'clicks'.

The coil is put on the transformer side of the resistor to neutral.

As the capacitor bank charges the voltage drop across the resistor lowers.  This increases the voltage the coil of the step start relay sees.  At minimum Vdrop, the coil will latch and the step start closes.  Easy Peasy.

If you want protection for the resistor in case something goes wrong, wire a small 5ish A fuse in series with the step start resistor.  If you are in to lighting effects, replace the resistor with a larger incandescent light (just don't go too large or the relay won't reach V to latch).  You can watch the cap bank charge now :)

Hard to argue with a 2 or 3 component step start.


--Shane
KP2/KD6VXI


Title: Re: This should be interesting
Post by: Opcom on October 31, 2021, 02:34:59 AM
Do you have a schematic of the modulator? I like the 6CA7s driving 810s and wonder about whether it uses any negative feedback to keep the signal linear over the half cycle when the 810s draw grid current.


Title: Re: This should be interesting
Post by: KB2WIG on October 31, 2021, 04:54:32 PM

A quick post as I'm at work....

I wired everything up last night and got the Mod transformer to talk back to me.The scope shows a nasty 120Hz hummm across the modulator load, but at least I'm getting something to work with. Maybe more later tonight.

KLC
ps    i've a partial  " as found  "   schematic that I'll look at tonight.


Title: Re: This should be interesting
Post by: KB2WIG on November 07, 2021, 12:54:53 PM


Update.

I spent the last two days playing with DigiKeys Scheme-it  to generate a usefull schematic to post here..... It didn't upload to digikey, so its sitting on the home computor. I intended to print the schelmatic  here at work, scan and upload to da fone... no joy, so I'll try another way....

I don't like the 60 V p-p sitting on my power supply load, so thats gunna be the fun thing to do. BTW, the mod deck works with the low ~ 1KV on the 810s, but the ripple makes me sad.

33 uF total after the choke; dont know the Heneries, yet.

Mo to follow.

KLC


Title: Re: This should be interesting
Post by: W7TFO on November 07, 2021, 01:08:13 PM
Kev, you may have an iron deficiency.

Your project may have caught it from you.

If a heavy cure is needed, ring me up. Or some such, many cures for such lackings here.

73DocG


Title: Re: This should be interesting
Post by: WD5JKO on November 07, 2021, 03:23:22 PM
KLC,

  The ripple on the B+ is an RMS value, not peak to peak. As an approximation, 60v P-P is 30v peak, and .7X that for RMS, = 21v. Yes the ripple is not sinusoidal, so this is an estimate only.
Then (21v/1000v) = .021 Then take that X 100 to get percentage, or 2.1%
The B+ supply for a plate modulator, or RF final at 5% ripple or less is usually fine. You are way better than that. If you want better than that, add another choke, and another 33 uf for a L-C-L-C filter. :P

   A Push Pull Plate Modulator should null out the power line ripple from the modulator B+ so long as the tubes are balanced and driven equally. The 2% ripple on the RF PA B+ will modulate the RF 2% at 120hz, but most BA receivers do not reproduce frequencies that low unless they are "souped up". With a Flex, or other SDR, I routinely see the old BA transmitters with +/- 120hz sideband energy, but the level is usually quite low, and usually not audible. To me this is a signature of a vintage rig, and not necessarily indicating a problem.

Jim
Wd5JKO


Title: Re: This should be interesting
Post by: VE7RF on November 07, 2021, 08:46:22 PM
I stopped buying anything but 6A10 diodes for power supplies years ago.  Last order was 10A10s.  They are a beast of a diode, 10A at 1kv.  Sitting in storage now, I'd of sent you some for free before my move to KP2.

With the primary series up and running on 120v a step start may be a moot point....  However a resistor and relay are cheap.

Looks like a fun project.  I already miss building but hopefully I'll be bringing the shop to me.....  Just have to find a house with a hurricane proof shop!

--Shane
KP2/KD6VXI



Shane, are the 10A10  diodes the same diam and length as a 6A10 ??   Does the 10A10  use the same  16 ga wire leads on each end ( .051") ?
 The surge is the same on both... 400 amps.
There is another diode, called a 10AO8, which is rated for  1kv  @  10 amps... but has a 600 amp surge rating.
On all those diodes, they require 1.1" square inch of surface area on a  pcb  if run at rated average current.....and  2.2" square inches  at every series junction.

A while back, I was experimenting, using 49 x 6A10's on perf board.... for bias for a 3x3 tube.  It was in a test jig, with a 0-60 vdc lab supply..and a series
  25 ohm / 100 watt ww  resistor.   The  6A10's  ran pretty warm ( not quite hot)  with  2 amps  CCS flowing.    The  1N5408's ran pretty warm ( not quite hot, but getting there)...with  1 amp  CCS flowing.   I noticed the V drop increased by  12%..when increasing the current from  200 ma..to  1/2 amps.

I noticed an anomaly though.   The  1st and last diode ran stone cold.  ( diodes  #1  and  #49).   Diodes 2-48  were getting borderline hot.   With 6 amps CCS flowing through a 6A10, it would burn my fingers.   6 amps X .78 V  =  4.68 watts..so no wonder.

Then it clicked, with  test leads  connected to 1st and last diodes...the medium sized test lead clips are heatsinking the input of  diode #1..and the output of  diode  #49.

It's all semantic for the most part though.  The general idea is to keep the leads long..as they are doing the heatsinking.

In normal operation, like a FWD, or  FWB, the diodes are only running at a 50%  duty cycle..with a CCS load on the supply  ( cap input or choke input or  C-L-C  filter etc.   Only 2 of the 4 x legs of a FWB are conducting at any one time.  That explains why  Dahl  rated his FWB rectifier assys' at  12 amps CCS, when using 6A10's in all 4 x legs.   (he rated the  3 phase version, with  6 x legs, at  18 amps CCS..also using 6A10's in all 6 x legs).

BTW, the last batch of bulk diodes I bought were PM-600's.  These are identical to 6A10's.   I since found some cheap sources for  10A10's in bulk,  ( typ boxes of  100 or  250).

The  6A10 /  PM-600 is a good diode.   It's the same length as a 1N5408...and same  16 ga wire leads. The 6A10 is double the diameter of a 1N5408.  Point here is... a 6A10 /  PM-600  will drop right in...where a 1N5408  once stood. 

Later.... Jim   VE7RF


Title: Re: This should be interesting
Post by: KB2WIG on November 07, 2021, 09:29:03 PM

It's late, but, in the interest of Science.....Some pics and some actual numbers.

The load is 6 k, I used a .001 / 3000V cap and 10X probe fer the scope. The Fluke is a Fluke. 830 V dc.


Title: Re: This should be interesting
Post by: KD6VXI on November 08, 2021, 05:40:41 AM
Jim,

I use very long lead lengths on the diode banks I build. ARRL manual taught that decades ago.     When using silicon rectifiers the leads are heat sinks.

Truck is, I solder the leads together for the entire lead.  This gives more structural rigidity as well as a lot more surface area creating more heat sink.  The diodes are placed almost the entire lead length off the perf board. I do the dame thing when I used pc board material.

The 10A10 where probably the dame diode as the 10A08 you spoke of.

You have to be careful when bargain shopping for the big diodes.  I got some P600 diodes on a reel once that where 50 volt rectifiers!  Makes for a great turn in event when using to rectify 4.5kv ac!  Not sure why, but they looked identical to 6A10s, same on the diode meter, but after looking up the different numbers on the side of the reel, they where 50 volt.  Marked as P600, which cross referenced to 6A10.

So I then had a ton of rectifiers for bias boards.  Nothing really lost, other than that first bridge!

--Shane
KP2 / KD6VXI


Title: Re: This should be interesting
Post by: w9jsw on November 08, 2021, 06:08:53 AM
For 10 bucks you can get these on amazon...

https://www.amazon.com/gp/product/B087PRYGRT


Title: Re: This should be interesting
Post by: KB2WIG on November 09, 2021, 06:38:29 PM
 This was a pain


Title: Re: This should be interesting
Post by: KB2WIG on November 21, 2021, 04:47:51 PM

Quick update.

The BC 610 mod transformer talks to me. Not much of a problem for now. I wired up the P Dahl plate transformer, ~ 1650 V with choke input & 33 uF. Then the zorching began......Got rid of the clip leads,  and used ring terminals with 5 KV rated test lead wire. Now a lot quieter, and less chance for incontence.

A nice pattern on the scope, then the 31kHz parasitic decide to ride the waveform.  I'll assume that the original builder had this thing running correctly, so adding resistors or even ferrets will wait. Increasing the bias has a small effect on the parasitic ( or is it decrease the bias if I make it more negative???)

My scope has decided to play games with the volts/div, so I cant yeah a low to see what happens; the plate meter shows 50- 100+ mA when swinging. Somewhere i have a 100:1 scope probe, so more to come.

I do wanna get started on the RF side of things, but thats a NoGo untill I get this thing to behave....

BTW, there feedback is from one pin on the S-9 driver transformer back to the
1st stage of the 12au7.

klc



Title: Re: This should be interesting
Post by: K8DI on November 22, 2021, 12:05:38 PM
Hello --

I'm looking at the schematic a couple posts up, and saw a few things that may (or may not) be contributing to your frustrations...

R2 has no point. 10M paralleled by the 470K R1?

first triode has no cathode resistor, and no grid leak. With no DC reference for the grid, it will just conduct. Maybe that's why the plate R is so unusually large? Look at the second stage, 100K plate/510R cathode/some part of 350K grid leak, vs 1200K plate/0 cathode/infinity grid leak. If you mod this section make sure that C2 ends up tied to ground and not the cathode, it's an RF bypass cap on the mic input to keep RF out of the audio.

Phase splitter. Usually the grid goes directly to the plate, OR a grid leak and an extra cathode resistance are included. Yours does neither. See https://en.wikipedia.org/wiki/File:Tube_push_pull_poweramplifier.PNG (https://en.wikipedia.org/wiki/File:Tube_push_pull_poweramplifier.PNG) vs. https://robrobinette.com/images/Guitar/Tube_Load_Lines/5E3_Cathodyne_Phase_Inverter_Schematic.png (https://robrobinette.com/images/Guitar/Tube_Load_Lines/5E3_Cathodyne_Phase_Inverter_Schematic.png)

6CA7's screens have no power source, just a cap to ground. The cathodes are tied together, but appear to go nowhere. They need fixed bias on the grids, or some amount of cathode resistance to set a bias point.

Maybe most of this is mistakes in the creating of the drawing, but, maybe worth checking...

Ed



Title: Re: This should be interesting
Post by: KB2WIG on November 22, 2021, 03:30:03 PM


I removed the embarassing schematic. What a waste of time playing around with the DigiKey freebe...... following Blutos advice to Flounder, time to start drink heavily.

I'll replace it with something hand drawn that should be more accurate as to what I actually have. Ed, I do apreciate the input.

klc


Title: Re: This should be interesting
Post by: w9jsw on November 23, 2021, 06:36:00 AM
I used to use EasyEDA on the web. It worked ok but did not like my schematics being hosted in China. I now use Kicad. Pretty straightforward if you have a cursory understanding of schematic editors. For some of the simple circuits, I take it all the way to having PCBs made. For the tube designs, I just use the schematic editing. All free.

It is really pretty easy. Install it. Create a project template. Find a suitable component. Drop it. Find another, and drop that. Change to line mode and connect the two. Do another. Rinse and repeat. Easy.

There are tutorials on youtube if you prefer. PM me if you want to know more. Be glad to help.

John


Title: Re: This should be interesting
Post by: KB2WIG on November 23, 2021, 02:10:40 PM

J,

Thanks fer the offer. For now, I'll stay with the hand drawn schematics. I've waisted far too much time with the digiKey freebee with very little return. I wanted to get this mod deck finished by November, and get the RF end going.Maybe sometime in the early spring when I've got some more time.

I found my 100:1 probe and hooked it up to my scope. I'm begining to think that some of the osc was from my rats nest of clip leads, scope probes and jumpers. Last night I was able to get usable output before the parasitics showed up, and they seemed to be somewhat better behaved. I was able to play around with the 810s bias a bit. I think that I'll remove my choke to get the HV up a bit and play with the bias more. With the "Q" point set  ~ 25 mA, the output is cleanish most of the time.

klc


Title: Re: This should be interesting
Post by: KB2WIG on November 30, 2021, 11:09:33 AM
Another Over Weekend Update.
 I added two 100 Ohm, 2 W resistors into the grid leads of the 810ís. Iíve been led to believe that this isnít the best way to do things on a Class B amp. Anyway, almost no improvement in the osc department.  To recap, the amp didnít always go parasitic, but as the mic audio was increased it would go when it wanted to. I then added 2 of the same 100 Ohms to the plates of the 810ís. There was a marginal improvement, in that the oscillations sometimes could be stopped by changing the grid bias. It also appeared that the oscillations didnít occur as often as before Ė although this couldnít be quantified.
I have a 1.5K, 10 W resistor, so in desperation this morning, I placed it across the grids of the 810s (which is affectively across the driver transformer). For a short test, No parasitic oscillations observed with a few yeah a loowss and moving the bias around. So it looks promising.

klc


Title: Re: This should be interesting
Post by: K8DI on November 30, 2021, 03:19:13 PM

I have a 1.5K, 10 W resistor, so in desperation this morning, I placed it across the grids of the 810s (which is affectively across the driver transformer).


This.

Tube amps run with no load (like a disconnected speaker) often oscillate.....


Class B modulator grid impedance goes from very high to kinda low when it starts drawing grid current. If it is fed by a transformer output tube amp, the previous stage is working into an essentially open circuit until grid current happens. Thus the need for a beefy driver to avoid distortion when suddenly the current jumps, and the practice of minimizing the jump by swamping the grids with a moderately low resistance so current is always flowing.  That's basically what you've done, added the swamping resistor. In my HB class B 812A modulator chassis, I have a 4K resistor across the driver transformer. One has to balance lower values meaning less of a glitch when drive gets to the level of bias (class B entered/current drawn/impedance drops like a stone) vs. higher values not using up driver power so the driver doesn't clip before the class B stage.


Ed


Title: Re: This should be interesting
Post by: Opcom on December 02, 2021, 12:05:42 AM
For 10 bucks you can get these on amazon...

https://www.amazon.com/gp/product/B087PRYGRT


Anything from China or amazon gets tested, especially cheap high performance rectifiers that should be expensive. Do they get hot when 2A DC is run through them for an hour? Had a bad Amazon Chinese diode experience before, parts turned out to be counterfeit. Solution is use old USA parts or buy new diodes from authorized distributor like digikey or equivalent.


Title: Re: This should be interesting
Post by: Opcom on December 02, 2021, 12:51:13 AM
klc:

Putting 100 Ohm resistors in a class AB2/B grids of a modulator is a poor thing to do because the peak grid current is quite high on those tubes and so you weaken the drive, create a grid leak sort of non-linear bias voltage but at audio frequencies. It's like a high level audio compressor effect.

When you drive the modulator hard, as with 180V peak to the grid, the grid can consume 250mA peak, and that means you lose drive voltage of 25V across the 100 Ohm resistor.
page 6 of https://frank.pocnet.net/sheets/049/8/810.pdf


Also, what you and Ed just posted about swamping/loading the 6CA7 speech amp stage with a resistor across the 810 grids. 100% agreement!!

I skulled 1.63K for the swamping resistor to consume 10W based on the 2KV plate supply class B audio data from the above data sheet, which seems to correlate with the Terman-Jones tradition of burning 1/2 of the driver power in a swamping load for class B modulators. YMMV. Bump it to a 20W part for margin though? -or measure later anyway and see.

(amusing because a 3-500Z modulator can consume ~700mA peak grid current so that swamping is more like 500 Ohms and I spent a couple months figuring out the class B driver transformer there was a terrible match for those low-Z grids, but would have been good for 810/304/250T/833s)

Looks like you got it.  8-)


Title: Re: This should be interesting
Post by: KB2WIG on January 11, 2022, 02:38:23 PM

Later than never Weekend update....

Drilled out Lexan for the 6A10 bridge rectifier. Decided to use the 23 uF oil can without the choke, hope the RF deck load keeps the  V happy. I'm going to mount the choke near the cap just in case it do get needed. Ring terminals, HV test leads bolted to stuff  inside a 6' rack to keep things safeish. The mod deck is ready to be stuffed inside.

RF land.... race to the Rally.... i've got two 813s that I liberated from an X Ray machine 34 years ago. These are the giney (?) pigs.  For now, need to source screen and bias supply. The bread slicer and link coupling look OK.

I'll post a topside of what I have now, later today, as I'm at work, and am screwing around on the interwebs.

Anyone have a mod reactor they are willing to sell?

KLC


Title: Re: This should be interesting
Post by: W7TFO on January 11, 2022, 03:43:43 PM
Howzabout a 30H 2A reactor for the shipping?

85139 origin.

73DG


Title: Re: This should be interesting
Post by: KB2WIG on January 11, 2022, 04:21:35 PM


Sounds FB, OM.

Check yer PM,

klc


Title: Re: This should be interesting
Post by: KB2WIG on January 11, 2022, 06:50:31 PM



As threatened, top side of the RF Deck. To the right of the cap is where the TVL Base sits.

klc


Title: Re: This should be interesting
Post by: Opcom on January 11, 2022, 10:39:34 PM
Perfect!


Title: Re: This should be interesting
Post by: w9jsw on January 12, 2022, 06:35:24 AM
Are you using a fixed HV supply? If so, I would suggest a screen dropping resistor. I did that on the suggestion of Chuck K1KW. Just so much easier.

I am running a pair of 100W WW 50K in parallel and get a reliable 75ma of screen current with 30ma of grid drive current on the 813s I have at 2KV. If you go with a screen supply you then must also devise a clamp so that your supply does not run away with the screens if the HV suddenly leaves town. No need with the dropping resistor.

Yes it can generate heat. I shut off the HV on RX so heat is only when I talk.

Also, on the 813 grid bias, I am using unregulated 75V with a 3.5k GRL. 2K Swamp. Tom's 813 schematic said 7K but I could not get enough negative bias with that value.


Title: Re: This should be interesting
Post by: KB2WIG on January 12, 2022, 04:08:45 PM
J,

The HV is a 1/2 amp 2135 V Petrr Dahl transformer I got for $20 at a fest. I'll be using it for the 810 Modulator and 813 RF deck. I ran the mod HV off of it, along with some 1 A diodes in a test. I'll find out if there is enough juice for both

Yes, a screen drop would be easier, but I dont know how much the Dahl can give to the cause, therefore a screen supply. I shudda bought a " tetrode board ".    Just need to figure a fail safe fer the screems.

 The  original design  looks like its out of Late 50s West Coast Handbook (Orr), and I also think maybe the RCA Transmiting Tube manual.  I ,have used the Another 813 thread for inspiration; we'll discuss my jealousy at another time.... ..



Some background.... I started this thing around 2006/7, and didn't get back to it around October of last year. The Thing was a  disassemble transmitter along with a rack and a hand drawn schematic. It looks like I was sold extra parts also. I 've been finding things I didn't know I had. Back to the story.



Title: Re: This should be interesting
Post by: w9jsw on January 12, 2022, 04:18:11 PM
Screen R will take a bit in excess of 100 watts so you are probably right. Not enough gas in the PS.

How about this for the screens...

http://www.nd2x.com/K5JL.html


Title: Re: This should be interesting
Post by: KB2WIG on January 12, 2022, 04:49:20 PM


Them's the berries.


Title: Re: This should be interesting
Post by: w9jsw on January 15, 2022, 07:01:35 AM
Apparently, there are some issues with the article I suggested. As I am using a screen dropping R, I have not tried to use these approaches.

https://forums.qrz.com/index.php?threads/tetrode-protection.792475/

https://forums.qrz.com/index.php?threads/screen-protection-circuit.795219/


Title: Re: This should be interesting
Post by: KB2WIG on February 01, 2022, 01:19:42 PM
Another UpDate.....

I wanted to get this together for Rally Weekend, but thats not gunna happen.

My search for Fe has ended, and I'll have enough ceramic standoffs to start things off. I'll try John's opto-isolator screen protection suggstion; Mouser is my friend.

Phill, AC0OB, has a ckt utilizing a Antek AS-1T275 toroid for an 813 screen/ bias, and I'll try that. These things (Antek) look like fun, and the price as delivered is a lot cheeper than a Hammond transformer.

For now,  I'll try to get this going on 160 & 75. This should be fun, as the coils have to be rebuilt, the 'plastic' is warped, and I have to figure out why I have more breadslicers than I think were needed. (Maybee they wanted to change to a pi output??)

Do I try to stuff all this into a 6' rack, or into one thats  around 4' ???


ALSO, thank you guys for the support.


klc



Title: Re: This should be interesting
Post by: KB2WIG on February 17, 2022, 02:13:53 PM

One more once.

Mouser delivered, the Antek transformer showed up, and the 10V filament transformer was welcomed to it's new home. So, the plan fer the weekend is to assemble the screen & bias supply. I'll take Phil's idea, and make the supply so that it can be used for other projects.

As of now, everything will have to run off of a single 120 V ckt.  The HV and screen can be wired for 240V when it becomes available.... Cu prices are insane!


The Mod transformer protection thread currently running is interesting.

Emptied the 6' rack of the plastic bag stash, and began stuffing the HV power supply parts on the rack floor.

klc

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