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This should be interesting




 
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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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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
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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