Hum in power amp

[mykey]

it's really hard to say because what you are telling me
does not pan out.
are the tubes new? they should be, you may just have a bad tube.
60 ma and no red plate? seems like you are reading wrong with your
meter, have the meter installed wrong, have a bad meter.
your tubes are out of balance from what you are saying and you
are better off starting with fresh new tubes.
you grounded the grids? that shuts off the bias voltage, and grounding
the grids really does not affirm anything.
It sounds like you have a bad tube and you don't want to buy new ones. you are determined to fix the amp and nothing may have been wrong in the first place (besides one bad tube or 1/2 bad primary output transformer.)
OK i found your schematic, this is an ultra linear output stage. you have a bad tube, a bad screen grid resistor, a bad transformer...choose one of the above. you have 10 ma difference? (I don't have a clue how you are measuring that) that's why it hums, it's out of balance. you adjusted the bias pot and now it's 6 ma out of balance? I rather suspect that you have been fighting one of the problems I listed. If you can't test the winding, replace the tubes (with new, not used tubes), test the screen grid resistors, it's time to take it to someone who knows how.

[Rattler66]

Let me clarify:

I have 2 sets of tubes...Groove Tubes KT66-HP (rating of 4)
Shuguang KT-66
both sets have less than 10 hours time being used....the first set GT had some rattle in it and hummed so i got the second set in and they hummed also. I don't mind getting another set but it seems odd that 2 sets from different places would be bad and have the same problem. (but it is possible, I'm just trying to rule out any other possibilty.)

i have completely rebuilt the amp with all new parts and as i built it i tested each part as far as i could to verify values etc...

i don't know about the OT. I had a custom built JTM45 Radio Spares clone from Heyboer made, so I had to assume it's fine (how can I test that?)

the way i measured the bias was i put a 1ohm resistor from cathode to ground on both tubes. Then i set my meter to mV and measure it should be close enough given any discrepancy in the resistor. when i installed the first set i got a difference of 10mA (36mV vs. 46mV on meter). I swapped sockets with the same tubes and it dropped to about 5-6mA difference (my V4 position is always higher). The same happened with the second set but it stayed at about the 5-6mA difference. Now as I adjust the sweep in the bias it would hum louder then minimize up to a point then start to hum again. The point where it hummed the least was on tube measuring (56mV on meter and the other high 40's). If this indeed is the bias then i would expect redplating that's what is wierd to me.

what i meant by grounding the grids is i ran a .1u cap in series from the grid to ground so any AC coming out of the PI would be grounded, so my bias was still intact.

i'm wondering if my OT is misbalanced (how do i check that), i'll go with your suggestion and get a new set of tubes (i'll have to make triple sure they are balanced because i'd hate to be out a third set). this was why i asked about independent bias for each tube so i could balance them out.

thanks

[Rattler66]

Dual Bias

would this work? separate controls

[mykey]

if you have rebuilt the amp with all new parts, the only thing left IS the
output transformer which more likely has a shorted winding...
with the amp ON and the tubes installed
what is the voltage at the center tap?
what is the voltage at each of two plates (pin three) output tubes?
what is the voltage at each screen grid (pin four)
when you find the spot where there is no voltage drop between the center tap
and the plate or screen grid, you have located the shorted winding
and the reason why the amp runs out of balance.
it is not customary to rebuild an amp to eliminate every possible cause
of failure, but you have learned a great deal about tube amps by doing so
which is a good lesson in tube amps.
the more tube amps you work on the faster you will troubleshoot.
until one day you become an expert which is also a good thing.

look at the schematic of a fender twin reverb to learn about making
a grid balance control and a heater hum balance control
that is an easier method than two separate bias controls
and it does work very well for balancing mismatched tubes.
I admire your determination and you will make a fine tube
amp tech someday, we need lots more tube amp techs.

[Rattler66]

thanks mykey, i am studying the fender schem for grid balance. it seems like it doesn't have bias control though, is there a way of having both? (should have two trim pots together the output of the first(bias control) into the second (grid balance)?

also the heater hum balance control, is that just running a 100 ohm res to ground from each heater leg?

thanks again

also i'll check on ot mismatch

[mykey]

the bias schematic you drew won't work that way but you do have the right idea, it's just the way you have it, the two grid voltages are not separated. (the controls will affect each other) but yes two bias controls will work to balance it. or you can have a voltage adjust and a balance adjust, I think I would rather have voltage and balance...
I will post a link to that balance control for you this weekend,
but I think there is still another bug and you should check all 5 of those voltages I mentioned...I think your going to find that one side of the transformer primary is not dropping voltage across the winding same as the other side. now that may be that some of the windings are bad on one side or that the transformer was not wound exactly equal in the first place.
the thing is that two 100 ohm to ground in the heater circuit is not quite as good as an adjustment pot but you can use resistors, especially if they are 1%. the balance is not as good with 5-10-20% resistors. But the problem here is not as much heater hum as a current imbalance between the output tubes.
so I have to go to college tomorrow (they make electricians take on going education) but this weekend i will look at that again.

[Rattler66]

thanks so much for the help, i really appreciate it.

here are the voltages

Center Tap = 482v

Tube1 pin3 = 475 Tube2 pin3 = 476
Tube1 pin4 = 470 Tube2 pin4 = 473

i have tube 1 at about 47+ and tube 2 about 55+ and the hum is pretty much all gone. it seems too high to me i'm dissipating almost 25watts

also can i separate the 220k resistor and put a trim pot in there as a balance and what value should i use. thanks

[Rattler66]

mixed 2 tubes from different manuf.

alright, put in 1 GT KT66-HP and 1 Shuguang KT66 and they matched within 1.5ma i also got the bias down to 36ma dissipating 17 watts. the hum is almost non existent as you have to be close to the speaker to hear it. i think that's it, so i'm satisfied.

i'll play it for a while and check back. thanks for the help, i still would like those circuit links if possible. thanks