6V6GTA plate dissipation in cathode bias push pull amp

[pchilson]

I've resoldered all connections from the intensity control pot to the tubes. I've cleaned and tightened the pot and the pot mounting. Without the jumper straight to ground I get the same readings as before. If I attach the jumper straight to ground I get the above posted readings again so there is a bad ground there. With the connections cleaned, tightened and resoldered would this indicate the pot itself as bad? Its a 1 meg pot, would I want a linear taper pot?

[pchilson]

I am measuring 17 vdc coming off the .1F cap leading to the intensity pot. Would that be a no no?

[MWJB]

There shouldn't be whole volts coming off that .1 cap (replace it). Do you gave the footswitch? Bias should be measured with the trem switched off (not just turned down) & intensity set wiper to ground (check that turning the intensity pot to ground gives meter default ohms/continuity at the junction of your grid load resistors), yes, looks like a bad pot.

With 380vdc on the plates & a 256ohm cathode resistor 16W dissipation seems to be ball park.

[pchilson]

I want to report success. I really appreciate all who participated in helping me sort this one out. I will not claim all is as need be at this point but the high idle dissipation problem has been solved for now. Many thanks to Bruce for getting me looking in the right area. It turns out the ground on the intensity pot was ok and the pot itself is ok. The problem was with the .1uF cap badly leaking. I replaced it and my voltage drop across the cathode is the same now as it was with the clip lead straight to ground. Also, I reran the measurements and calculations and was still a bit high about 18 IPD. I measured the wall socket and I had 127 VAC at the outlet. I have a bucking transformer that I built and plugged in and ran the measurements at 115 VAC (more like what the amp was designed to see) and I calculated 14.01 IPD.

Now I wonder about all the caps in the amp at 52 years of age. I have at this point replaced all the electrolytics and the one .1uF. I don't have the means to test the caps. Can I assume that if I find any significant vdc on the outlet side of the cap it is bad?

What are your thoughts on Mallorys as replacements?

On the filter caps, whats your thoughts on 40uF on the first stage as opposed to 20uF as the 5Y3GT specs call for?

Any other hints, tips or suggestions you deem necessary?

Thanks again for all of your help guys. I really appreciate it.

[MWJB]

40uf is fine with a 5Y3, safe as houses.

Mallorys will be fine. A good way to test caps for leakage is to lift the end of the cap farthest from the plate resistor (to stop the circuit loading the voltage), carefully (triple check that there's no risk of a short) connect a voltmeter set to lowest dc volts from the free end of the cap to ground, power up & look for anything showing more than 0.25vdc. However, leaky caps in the preamp will usually sound very dull & pull down plate voltage. It's quite unusual to find lots of leaky caps in the same amp, if I suspect a cap I usually change it as it takes the same amount of time, or less, as the cap test. If the amp sounds OK, it probably is.

[pchilson]

Hi guys. Back again. Since any of the schematics for the M-12 don't indicate any voltages I was in to do some measuring and I about sh*t when I measuerd the 6V6 plates at 500vdc and the screens a few volts less. Now when we were working through the initial problem and came to a conclusion the last measurements were coming in at 350ish. I have been tweaking and replacing some components but have been using stock/same values. This shocked me. What could cause such an increase in voltage?

[MWJB]

Disconnected 6v6 cathode resistor?

[pchilson]

I've checked all the wiring for continuity and its still all connected and still measuring 256R.

[pchilson]

Sorry, false alarm guys! The 9v battery in my meter was only putting out 6v and it was giving crazy readings. I measured the wall voltage at 156 vac!!!
I put in a new battery and the readings came down significantly although they are on the high side. I have a bucking transformer that I can use to get voltage readings closer to what the amp was designed to see and they are still on the high side of where they should be. I have been going through the amp checking for drifted carbon comps and finding several so far. I have a new 20/20/20/20 cap can and have installed all new mallory caps. I'm using some older Sprague 30Ds for the other electrolytics till I find what I like for cap values.

[ThermionicScott]

I hate when voltmeters give wonky readings.

What are the voltages and dissipation now? Even though the 300R cathode resistor only brought down the cathode voltage slightly, note that your actual cathode current went down considerably, from 134mA to 108mA. (Actually, I'd recheck that too -- switching to the 300R resistor should have made the cathode voltage go up a little, but still at a lower current than before...)

- Scott

P.S. Whoops, slightly-old thread...

[pchilson]

I hate when voltmeters give wonky readings.

What are the voltages and dissipation now? Even though the 300R cathode resistor only brought down the cathode voltage slightly, note that your actual cathode current went down considerably, from 134mA to 108mA. (Actually, I'd recheck that too -- switching to the 300R resistor should have made the cathode voltage go up a little, but still at a lower current than before...)

- Scott



P.S. Whoops, slightly-old thread...

Thanks for your input Scott. I had actually gone back to stock setup and then sold the amp. Thread is essentially dead.