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KB2WIG
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« Reply #25 on: November 09, 2021, 06:38:29 PM »

 This was a pain
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KB2WIG
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« Reply #26 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

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K8DI
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« Reply #27 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 vs. 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

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KB2WIG
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« Reply #28 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
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w9jsw
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« Reply #29 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
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KB2WIG
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« Reply #30 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
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KB2WIG
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« Reply #31 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 810’s. I’ve been led to believe that this isn’t the best way to do things on a Class B amp. Anyway, almost no improvement in the osc department.  To recap, the amp didn’t 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 810’s. There was a marginal improvement, in that the oscillations sometimes could be stopped by changing the grid bias. It also appeared that the oscillations didn’t occur as often as before – although this couldn’t 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
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K8DI
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« Reply #32 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
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« Reply #33 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.
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« Reply #34 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-)
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KB2WIG
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« Reply #35 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
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W7TFO
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IN A TRIODE NO ONE CAN HEAR YOUR SCREEN


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« Reply #36 on: January 11, 2022, 03:43:43 PM »

Howzabout a 30H 2A reactor for the shipping?

85139 origin.

73DG
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KB2WIG
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« Reply #37 on: January 11, 2022, 04:21:35 PM »



Sounds FB, OM.

Check yer PM,

klc
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KB2WIG
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« Reply #38 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


* 813 RF Deck carcass.jpg (3487.44 KB, 4032x3024 - viewed 239 times.)
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« Reply #39 on: January 11, 2022, 10:39:34 PM »

Perfect!
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« Reply #40 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.
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« Reply #41 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.

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w9jsw
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« Reply #42 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
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KB2WIG
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« Reply #43 on: January 12, 2022, 04:49:20 PM »



Them's the berries.
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w9jsw
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« Reply #44 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/
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« Reply #45 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' Huh


ALSO, thank you guys for the support.


klc

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KB2WIG
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« Reply #46 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



* Cheep and effective.jpg (2814.04 KB, 4032x3024 - viewed 171 times.)
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