DIY build, power section issues.

[eurekaiv]

I'm pretty good at screwing stuff up but this is the first build I've had any significant power supply trouble with and I've done about 7 or 8 amps from scratch now, so it's time for a second set of eyes. Thanks in advance for looking.

The amp is a plexi style pre-amp mated to a cathode biased EL34 power section. It works but it's not loud (like a plexi should be) and this makes sense as because I'm only getting ~225v at the plates. I'd expect ~370v based on the specs of the amp I gutted to build this (a Calrad AP-35) and if I yank the tubes I measure over 450v, which seems about right. I've tried disconnecting power from the pre-amp sections (marked with the red X), swapped in different tubes and tried a solid state rectifier. I also tried a different power transformer. I do not have an appropriate spare output transformer to swap but bench measurements looked ok before I put this thing together. I've looked over my wiring so many times my head hurts thinking about it. Anyone see something I've missed as a potential problem area, or something in my schematic that would cause this voltage drop?

[shooter]

I'm only getting ~225v at the plates

what the current draw?


Vk/Rk = Itube

[eurekaiv]

From memory I had 10.5V on the cathodes and I have 240ohm cathode resistors so that works out to about 44ma.

[PRR]

Are you running on a lamp-limiter?

[eurekaiv]

Yes, on a limiter and it lights up about like any amp... an early rush and then goes dark and comes back up to a moderate glow.

[sluckey]

Plug straight into the wall. The current limiter has served it's purpose and is no longer needed. Now check voltages.

[eurekaiv]

Yep... all better. Thanks for pointing out something I should have figured out on my own, per usual.

A couple minor issues remain... I built this based on the Matchless Clubman output section but if I'm doing the math correctly, that thing is biased at around 135% dissipation. Also, the Calrad AP-35 that was in this chassis before I rebuilt this had a 130Ω cathode resistor but shared with both tubes so the plate dissipation was basically the same as the Clubman, and the same as what I'm measuring... about 135%. I've got 23.5v at the cathode off 408v at the plates. Grids measure ~368v which seems like it could be a little low but these bias numbers look kinda crazy. So I think I must be missing something here.

Also, my presence control is basically not doing anything. Should it behave in this cathode biased power circuit, as it would in a plexi style adjustable bias output section, or will I need to tweak it (or maybe shift to the Matchless style brilliant control) to get it to properly function?

[shooter]

I'd go back to 2 R's, dial it back to 80% - 100%


my math on your #'s (assuming 1 130 for 2 tubes) is 38watts a tube

[eurekaiv]

I copied the clubman's 1 resistor per cathode, so I have 2 240Ω resistors (that are measuring 220) because that's what I could get that was high enough wattage that wasn't too big to fit in the chassis. I didn't re-use any of the old amp outside of the transformers and chassis.

[eurekaiv]

I upped the resistance on my cathodes and basically all my voltages have increased proportionally. I now have 431v on the plates, 397v on the grids and 28.5v on the cathodes with 350Ω cathode resistors and my plate dissipation is identical. It also sounds a bit like an improperly biased fuzz pedal since making this change. How would I go about getting this back under control?