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31

Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by sluckey on July 25, 2026, 09:07:56 am »

The actual voltage on the grid is not that important. It's the voltage ***BETWEEN*** the grid and cathode that's important. Go read some Merlin pages.

So what's the fix for his problem that you describe?

The fix is to revert back to the original bootstrap bias circuit used in the original PR.

32

Tube Amp Building - Tweaks - Repairs / Re: Help with 1987 build

« Last post by shooter on July 25, 2026, 09:06:11 am »

socket wired wrong?


ohm socket pins (tube side) back to the parts they should be soldered to, ohms near zero?

33

Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by wsscott on July 25, 2026, 08:44:19 am »

Sluckey-Thanks.  I had thought that the grids on the preamp tubes were supposed to be at or near 0 VDC, and maybe similarly for the cathodes.  So seeing these values of 42.8VDC on the grid and 56VDC on the cathode has gotten me confused.

I know I'm missing a lot about operation and theory, but is it different for the grids and cathode on the phase inverter part of the circuit?  Or is it something else that's going on?

So what's the fix for his problem that you describe?

34

Tube Amp Building - Tweaks - Repairs / Help with 1987 build

« Last post by ltkrepp on July 25, 2026, 08:29:37 am »

Hi all,
I'm hoping someone will see or help point me in a direction to solve an issue I'm having with a Marshall 1987 build. All voltages are correct until I plug a 12ax into V1. Once a tube is in that spot, voltage drops to roughly 29v on V1. Assuming the tube is running away drawing a load of current. I've checked the cathode resistors (820 and 2.7K ohm) and they're good. V1 cahode bias cap (330uf) is in the right direction and is good. The 10k and 8.2k dropping resistors all read as they should.  All the grid connections and connections to the cliff jack appear correct. Tube socket pins, etc all look good. Tubes in all other positions V2-V5 do not have this issue. I've subbed other 12ax7's into the slot known to be good and they all exhibit the problem. Attached is a photo of the V! preamp section. Any help/suggestions/ideas would be greatly appreciated.

35

Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by sluckey on July 25, 2026, 08:14:00 am »

Tubeswell-how did you calculate that number? I’d like to learn something. Thanks

It's a simple voltage divider that can be solved with Ohm's Law. You have 250V applied across a 3.3M in series with a 680K. Solve for the voltage dropped across the 680K and you have the grid voltage.

R_t = 3,300,000 + 680K = 3,980,000Ω
I_t = 250/3,980,000 = .000063A
Voltage across the 680K is .000063 X 680,000 = 42.8V

There is no grid current so no voltage is dropped across the 1M or 470K so that 42.8V will also appear on the grid.

36

Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by wsscott on July 25, 2026, 06:06:38 am »

Tubeswell-how did you calculate that number? I’d like to learn something. Thanks

37

Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by tubeswell on July 24, 2026, 11:48:47 pm »

Thank you, Sir. I tried this idea I found on TDPRI for a fixed bias cathodyne phase inverter and forgot to document it in the schematic. Could be part of the problem...

https://www.tdpri.com/threads/fender-cathodyne-phase-inverters.755580/post-7796034

^that^ cathodyne grid is set at 42.5V, which will significantly reduce clean output voltage and cause early clipping (the cathode voltage will not stay at 56V - it will automatically drop to follow the grid and this will cause clipping in this stage as the amp is cranked) because it will muck up the B load-line.

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Tube Amp Building - Tweaks - Repairs / Re: Why is this Princeton style amp not loud?

« Last post by Sonny ReVerb on July 24, 2026, 07:40:49 pm »

Thank you, Sir. I tried this idea I found on TDPRI for a fixed bias cathodyne phase inverter and forgot to document it in the schematic. Could be part of the problem...

https://www.tdpri.com/threads/fender-cathodyne-phase-inverters.755580/post-7796034

39

Tube Amp Building - Tweaks - Repairs / Re: Single tube Reverb headache

« Last post by Willabe on July 24, 2026, 06:41:00 pm »

I thought shielding would avoid possible noise issues when sending and returning rev signals to the front panel reverb pots. Well, apparently not. I guess I underestimated just how finicky those lines are.

(Not saying I think it's the reverb placement or wire dress.)

The reverb tanks output signal is the lowest/smallest ac signal in the amp. It's even less then the pickup output.

So it is easily overcome by stray interference from what's around those reverb signal wires.   

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Tube Amp Building - Tweaks - Repairs / Re: 1968 Fender Vibrochamp AA764 Resto

« Last post by shaun on July 24, 2026, 03:30:32 pm »

Beautiful!