sound city 120 mk iv bias

[pontiacpete]

I'm working on one of these and this time, unlike the past, I did research the forum for previous posts on this amp. The amp here has the 6 el34s and the 2 bias pots in parallel. I have a bias supply of -41v. The tubes a running very hot at 60ma. Not knowing much about this circuit I started increasing the value of the 10k resistor that splits the 2 bias pots and goes to ground. By doing this I was able to bring one side down to the 38ma but the other 3 tubes remained in the 60sma. Can any one here help me sort this out?
I read here about re-building the bias supply to increase the neg voltage. Is that the only option?
Or do I need to add a resistor of some value in parallel to the 100uf bias cap?
Thanks in advance,
pete

[Alex R]

A resistor parallel to that cap will dump some of the bias voltage to ground and make the tubes hotter. I'd replace the cap if it's an original though, to take it out of the equation. The 10K resistor to ground was the right one to increase to try to get more -ve bias voltage. It may be that the tubes one side are just pulling more current and you need to balance them out or replace whichever one seems to want to draw too much.

What I'd do as a first step is to take out the power tubes and check the pin5 voltages, to see if they are in the right ball park. Check the resistors on the cathodes (can't read the values - and that's if yours has them) are on spec more or less. And lift the power tube end of those two .047 coupling caps, and see if they are passing any DC (say, a volt or more), and if so, replace them.

[pontiacpete]

The bias cap has been replaced, the .047s have been changed out. The cathode resistors are right on spec. 60 ohms for the 3 tubes that are pulling more current and 20 ohms on the other 3. I checked all the grid resistors and replaced several that were too high from the 10k ohms.
Perhaps the tubes? I haven't tried replacing them, I'm waiting for a set to arrive.
I just pulled the tubes, on the side there's -45v on pin 5 of each and -32v on the other set.
Not sure why this is so,, how can this be?
Could the bias pot the that side have that effect? I measured it and it seems to be doing the same thing the other does, about 5K

[Ampzone]

I had a SC 120 that I added a voltage doubler to. It was too hot stock for a couple sets of tubes I tried. The doubler worked fine. As for the difference your seeing on each side that is odd. See what gives with a second set of tubes.

[Alex R]

That difference in the voltages is pretty indicative. Check resistances to ground, recheck the trimmers, are the 100K resistors ok?

[pontiacpete]

I checked the trim pot on low side and somehow, ended up breaking it. Don't know if it was bad or not. Might not have been. The only trimpot on hand was a 47K.(I have a bunch of the blue ones that I've used in effect pedals but don't know if they would work) However, now the negative voltage is equal on both side, but not enough to bring the current down. I think I'm going to have to build the voltage doubler or try a larger resistor than the 10K, so far I'm up to 1meg and still the current is up around 75ma
I was thinking I would try this one from a previous post.

[Ampzone]

The coolest I could get the draw to was 41ma with the PT. The SC PT is uber enemic in the bias. Way too hot for some modern tubes. The doubler fixed that in a snap.

[Amp Kat]

The doubler will work like a champ and is used quite often in these situations.

[pontiacpete]

Thanks for the great help to all, I installed the voltage doubler above and wound up using the cermet pot on hand, it seems to be working. The data sheet says it is good for up to .5 w and up to 300v. I guess it will survive..(?) I paralleled the 10k resistor that goes to ground with another 10k , this enabled me to dial in 33ma on both sides of 6 tubes. Both sides do not have equal negative voltage, side with the 10k cermet has -72 and the other -91. I think this is because the cermet is not so good. I have a few and tried another and got -56v. Not sure why the difference, perhaps the quality. I order some better trimmers and probably will wait and install them to see if I can balance the neg bias voltage.

I have one remaining question: I used 100uf/100v for all the caps. I suspect this value is needed in the 2 in totem, but is it necessary to have 100uf for the other 2 that are on the wipers of the trimmers?