+ 1 to 2 DCV on control grids of EL34's Matchless Clubman 35

[punkykatt]

Hello all, I always thought the power tube control grids should read  0v on cathode biased amps. (original reading was 3.19vdc)  I replaced the two .047 coupling caps post PI and when the amp is on for 15 to 20 minutes the grids creep up to  +1 to 2 vdc. I tried another set of EL34's, did the same.  MPD (-) screen grid (I)  is 112% to 114%.  Is this the norm for these HOT biased amps? Thanks in advance for any input.

[sluckey]

Measure the resistance between pin 5 and ground for each EL34.

[tubeswell]

Have you installed any other components between output tube grids and ground that we are unaware of because there's no schematic (e.g. Cg-k, which might be leaking)?

[punkykatt]

Steve, pin 5 on one tube reads 270r to ground the other pin 5 reads 273r to ground. This amp appears to be stock except two PS filter caps were changed from 33uf/450v to 22uf/450v. All resistor read within spec in the amp except the EF86 330k plate load R drifted to 448k I replaced that with a 2w metal oxide 330k. All the E caps tested good with the ESR meter.  Before I did the clean and service I put my bias probes on the pwr tubes mA around .087 and .089 they drifted to .102 and .104 after about 30 mins. and went back down. After the clean & svc the pwr tubes have been holding steady. The + volts on pin 5 after it warms up puzzles me. Those new coupling caps should be taking care of that.
any ideas?????Thanks for posting Guy's

[PRR]

> pin 5 on one tube reads 270r to ground

Re-read that. It can't be hundreds of Ohms or you would complain it hardly plays.

[punkykatt]

Sorry my bad, too many logs in the fire at one time. That was pin 8.   I will measure pin FIVE (5)to ground tomorrow. Dang, I gotta slow down and smell the roses.

[punkykatt]

Ok, one meter lead to chassis ground and the other meter lead to pin 5.  Master volume pot effects readings on both tubes.  full CCW = 128.2k,  full CW = 208k.  The control grid R's are 10k on this amp not 1.5k .

[pdf64]

If an EL34 is removed, does the voltage at terminal 5 of its socket remain at 1-2V, or drop to 0?
Replace the EL34.
How about if the valve in the phase splitter socket is removed (don’t rely on a type 3 master volume to kill the audio)?

[punkykatt]

It takes about 20 to 30 min for the voltage to climb that high, i will try that. Thx

[punkykatt]

Pulling one tube does not change the voltage on pin 5.

[punkykatt]

Pulling the PI tube makes no difference.

[pdf64]

It seems that the replacement coupling caps may also be bad. What happens if you unsolder and lift one of their legs?

[punkykatt]

Got another project to tend to right now. I will try that later this afternoon . Thx

[tubeswell]

Have you installed any other components between output tube grids and ground that we are unaware of because there's no schematic (e.g. Cg-k, which might be leaking)?

Of course I meant 'no as-built schematic'. Did you put Cg-k on the output tube grids by any chance?

[punkykatt]

Have you installed any other components between output tube grids and ground that we are unaware of because there's no schematic (e.g. Cg-k, which might be leaking)?

Tubeswell:  No. It ended up being defective new coupling caps (how do you like that? Go figure) Mallory.   uf tested perfect on the meter, when the amp warmed up they started leaking DCV. What can you trust . Thanks all for your input, much appreciated!!!

Of course I meant 'no as-built schematic'. Did you put Cg-k on the output tube grids by any chance?