5F2a, built and at the starting gate

[dig22]

If your DVMM has survived this far, this could be caused by a bad power tube or a leaking coupling cap from the #6 plate lug of the preamp tube.

I used some really old oil caps because I thought they'd be interesting, not leaky. So I'll try some different tubes and swap out the .02 caps. It's embarrassing I don't really know the function of of each part. So dumb question #89: are both .02 caps coupling caps? I have the schematics and layout but is there a document -- a cheat sheet -- that lists out the function of each part and section? Would have been helpful to have one in the beginning but it's never too late to learn.
-d22

[tubeswell]

are both .02 caps coupling caps?

Yup. They provide AC-only coupling from the plate of the previous stage to the grid of the next stage so that the signal from the previous stage's plate can get to the grid. But they do this by blocking DC so that the grid isn't receiving any direct current from the previous stage's plate. If the grid receives any direct current from the plate, it will affect the biasing of the following tube, which causes problems.

There are ways of doing DC coupling between stages, but I won't go into that now because it'll only complicate things.

I have the schematics and layout but is there a document -- a cheat sheet -- that lists out the function of each part and section? Would have been helpful to have one in the beginning but it's never too late to learn.
-d22

Tube Terminology for Dummies

also this site is excellent (but more advanced)

How to design valve guitar amplifiers

[dig22]

I'm back, and baffled: my grids ain't great

Howdy All,
I took Jhow's suggestion and got a real Switchcraft 12A jack. Low and behold, it works when wired correctly.
I replaced the scratchy volume pot and swapped out both .02 caps with .022s. I also swapped out power & preamp tubes with JJ's.
With all above mentioned done, the amp sounds amazing. I can't get over it. I can't stop playing.

Voltage-wise, all appears well but for grid 1 on 6v6 and 12ax7. This has me concerned. I rechecked the new connections and they seem alright.
Here are all voltages:

6V6
pin3: 343 VDC
pin4: 314
pin5: 30ish grid 1
pin8: 18

12AX7
pin1: 178
pin2: 66 grid 1
pin3: 1.4
pin6: 164
pin7: 0
pin8: 1.4

All ideas and suggestions welcomed.

[RWood]

a leaking coupling cap from the #6 plate lug of the preamp tube.

I vote for this as the reason why you are getting DC voltage on your power tube's grid. Change it out for a new one, not a vintage one, and see what you get.

RWood

[dig22]

I vote for this as the reason why you are getting DC voltage on your power tube's grid. Change it out for a new one, not a vintage one, and see what you get.

RWood

It was a new cap. The vintage had a much lower voltage though still not zero. I'll swap it out with another new cap tomorrow. While I'm at it, I'm trying to remember whether or not I read somewhere to use shileded wire from 68k resitors to input jacks to reduce noise. Does that sound right? If so is it worth the effort. -JG