Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Banjan73 on May 29, 2026, 02:41:59 pm
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Hello!
Refering to my Ampeg SB-12 thread, i have done some measurements with multimeter and scope.
Well. Truth is I am no scope expert, and I have one question:
What is going on here?
I have measured the PI (cathodyne). Cathode side seems normal, but on the anode side, I have this strange graph.
Attached is pictures of both of them + combined.
Anyone keen to explain whats happening with that strange one?
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The bottom two traces are frequency doubling - You have bias shift of the cathodyne when overdriven - Add a grid stopper to the cathodyne, 270KΩ and up seems to work well, anything over 1MΩ will shave off more highs than you probably want. I typically use 330K-470KΩ for a stopper.
--Pete
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Thanks!
Easy fix. I will do that.
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DL helped me with the same problem, before n after shots;
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Nice!
I'll see into that tomorrow!
(Btw... AI came up with the same conclusion. But much of that is actually based on you guys)..
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But much of that is actually based on you guys
:laugh:
if AI was human it would be arrested for plagiarism!!
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This effect is described very well by Merlin here: https://www.valvewizard.co.uk/cathodyne.html
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But much of that is actually based on you guys
:laugh:
if AI was human it would be arrested for plagiarism!!
Absolutely!
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This effect is described very well by Merlin here: https://www.valvewizard.co.uk/cathodyne.html
I have actually read that article in the past. Forgot all about it yesterday.
Anyway, 470k is soldered. Time to test!
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So.
PI seems fine now.
However a new scope question rises (sorry. I am a novice regarding scope).
I am testing the output here (over the dummy load). But the sinus curve looks a little bit odd.
I have attached two pictures. One is just before clipping at the top/bottom.
The other is at clipping. I have now soldered in a feedback loop to stabilize the thing a bit.
Any analysis on those curves?
Calculations showes 32 watt if I go with the first "clean" curve. A bit dissapointing.
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Hehe.
AI says I need to increase grid stoppers on EL34 from 3,3k to about 47k. And decrease coupling capacitors from 100nF to about 22nF.
Blocking distortion?
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Calculations showes 32 watt if I go with the first "clean" curve. A bit dissapointing.
RMS or peak?
calculations have little to do with the sound the audience hears through an efficient 4/12 cab :icon_biggrin:
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Well. Measured about 16VDC @ 1000hz over a dummy of 8 ohms.
With multimeter. So I guess RMS..
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the sinus curve looks a little bit odd.
I have attached two pictures. One is just before clipping at the top/bottom.
The other is at clipping. I have now soldered in a feedback loop to stabilize the thing a bit.
Any analysis on those curves?
Calculations showes 32 watt if I go with the first "clean" curve. A bit dissapointing.
That waveform makes me think either the bias is a bit cold, or the HT supply nodes for OT CT and screen grid are sagging somewhat under load.
What are the idle conditions, especially voltages at the OT CT and screen grid supply nodes, and Ia or Ik?
What happens to those voltages when it's putting out its max sine wave, at the onset of clipping?
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Yes, what he said ^^^.
Your first image shows lots of distortion, without clipping. Second is highly clipped.
I'm not at all sure what the cause is.
Do measure your voltages, and confirm your biasing. These are basics. When I am tweaking a build, I keep track of voltages after each change.
Then put your scope to work by looking at your signal after each stage -- attach your lead immediately after the coupling cap following each triode.
You should get a good idea about what each stage is doing. When you get an unexpected behavior, then you'll know where to focus your efforts.
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Sorry.
Some shit was going on in my head.
Measurements was off course 16VAC.
I will give you more info tomorrow.
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At 0 clipping, you shouldn't see any crossover distortion, and you should have a nice clean wave with basically the same properties as the input.
Maybe your preamp is driving too hard.
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^^^ what he said
this is input jack PI-IN speaker DL, measured "max" by adjusting preamp knobs (including TS, switches, etal) for max "clean" at DL
then if something is going sideways, i have a pretty good idea of the stage causing it, ie; "when i flip the boost switch, the waveform goes to sheet"
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Thanks.
I realize I have some work to do here, then. Lot of things to check.
Will be back with more info.
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Some updates.
It turned out that AI wad a little bit "off" when it came to my negative feedback loop. (The schematics will be updated in the Ampeg thread).
Now it is corrected.
Since the OT has a rating of 40W, I have tried to dial in about that value.
Attached is my newest measurements.
Seems like clean wattage is about 32-35 watt. Crossover distortion is manifesting itself at 40Watt-ish. Clipping occurs at 43Watt.
Last picture is at full volume. 72W clipping. Poor OT.
I think this setting will work. What do you think? Should I investigate more regarding the crossover stuff?
(I know that you would like more measurements, though😉).
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Hehe.
I see now that you wished for more spesific measurments. Will do that, but not today.
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BTW.
The bias IS cold, with purpose. Its set to about 22mA. Think I will try to increase it a little bit. But then I am getting concerned about my OT. So I may restrict the screen a little bit moe to reduce the output power.
Or do I need to do that?
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what's it sound like?
at some point ya gotta quit the geek and just play it like you stole it, swap out speakers, pedals, other guitarists.....
Then you're better informed on any "Mods"
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Think ill need to make some recordings. But then again, I dont use soundcloud or anything, and thins site dont allow large files.
I have just tested it briefly.
But I believe those spikes are due to the cold bias.
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Some mess.
Two threads about "almost" the same. Well..
Some updates.
I have changed the grid stoppers on thise EL34s from 68k to 220k. I also adjusted bias from 22mA to 27mA.
And.. the crossover distortion i almost gone.(see attached picture).
Wattage is now 42W.
And as requested I did some extra measurments.
Anode at idle: 749V. Just before clipping: 666V🤘.
Screen at idle: 340V. Just before clipping: 269V.
The amp has been tested a bit, and sounds pretty powerful. On thursday I will test it fully with a band.
Think I am getting there now..
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OK, well, I've never built an amp with those kind of voltages.
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OK, well, I've never built an amp with those kind of voltages.
Me neither😉
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OK, well, I've never built an amp with those kind of voltages.
With these voltages and the James tone stack, reminds me of the Univox 1236. Univox had a number of models with similar voltages.