5F2a voltage: My Grids Ain't Good

[dig22]

Verify ohms readings at the grounded end of components, with meter black lead clipped to main filter ground, meter should show "default" reading, perhaps a third to half an ohm?

all read: .5 ohms

What readings did you get when checking ohmage from grounded eyelets (grounded end of 1.5K resistors at 12AX7 pin 3 & pin 8) to main filter ground point/AC cord ground? There is a discrepancy between cathode voltage & voltage dropped accross the V1a plate resistor.

pin3: .5 ohms
pin8: .9 ohms

[dig22]

Well, even if that 0.5R at pin 3 cathode resistor is meter default (check by pressing black & red probes together, deduct reading from that 0.5 to give "actual" reading) you still have a good half an ohm at pin 8 cathode. Remake this ground. Make life easier for yourself, add a buss wire to the pot backs & input jacks, ground to this rather than the brass plate.

will do.

[dig22]

Your board looks "scratched" & silvery, any ideas why this might be? There seems to also be considerable solder "splatter" around the tube sockets, it'll only take a few minutes to change these.

The lighting for the photograph highlights every ugly detail - things you cant see with your eyes. I was shocked (surprise-wise, not voltage -wise) myself when I saw pics. If you could see the actual board, you'd see a clean black board.

The splatter you see is chasis metal poking through paint removed from dremeling screw hole burrs.

[dig22]

Somehow or other you have a DC voltage at pin 5 of the 6V6. I think you need to look for bad wiring or short circuiting. Are you absolutely certain that the black wire is going to the junction of the 220k and the .02uF coupling cap on the back of the main board?

yes.

A lot of this started when swapping out the coupling caps and changing wire on the 12ax7 pin2 to shielded wire. I'm thinking I should put it back the way it was. Should I bother?

[tubeswell]

Unsolder the 'tail' end only of the 0.022uF coupling cap going to the 6V6 grid, and disconnect that end from the rest of the circuit by lifting it away from the eyelet on the main board. Then attach one lead of your VDC meter to that lifted end of the coupling cap, and the other end to the amp's chassis. Now switch the amp on and see if there is any DC on the end of the coupling cap (using the Vmeters 'ones' of volts DC setting). If there isn't any DC, then you are getting the DC reading on pin 5 from somewhere else, possibly from a short on the socket pins or something.

[dig22]

Well....I was so frustrated this weekend, I built a new board with new parts. And now pin 5 of the 6v6 is hovering around 21 VDC. Pin 1 of tthe 12ax7 is .7 VDC. So that's an improvement. But since it's all new parts, I'll try replacing the socket next and report back.

In the meantime, the new board is quiet quiet quiet quiet and sounds great, but to my ear, it lacks a certain snap the first board had. The first board -- you've seen it with the old style resistors. The new board is a thick resin material designed for circuit boards. Someone gave it to me a long years ago but I don't know what it's called. I used newer resitors because it's all I had on hand. And the the two 16@450 are a bit smaller. Lastly, my good soldering iron finally quit and I re-soldered with a Radio Shack 40 Watt soldering iron that just barely melts the solder so there is some serious ugly going on regarding soldering.
Will all the things above change the sound that much? Thanks again Tubeswell.

[tubeswell]

Hmmmm....

Are you absolutely definitely sure you're measuring Pin 5 (and not some other pin)?

And, if so, have you tried another 6V6 in there?

[loudthud]

Dig22, does your meter auto-range? Does a little "m" or "M" appear on the screen when you measure the grid voltages? Could there be a bad decimal point and/or leading zeros? What reading do you get if you connect the black lead to pin 8 of the 6V6 and the red lead to pin 5?