Crate BV 6212 Problem?

[Jazz P Bass]

Just to clarify the purpose of the balance pot.
If there is any imbalance inside the transformer itself, winding to winding, a balance pot may help with ac heater hum.
If there is an imbalance, using two resistors of the exact same value will not help (the imbalance).
Thus was born the hum balance circuit.
The potentiometer sets the two resistor values (whatever value that may end up being).

[robvb68]

Thanks for the info guys. In checking the pot using the manner Enzo described above by pulling the power tubes and checking the pot using J34 and J35, the reading I get is 140 ohm across the terminals, and then on each side to ground I get roughly 100-140-100 ohms turning the pot, which is not anywhere near the numbers above (and seems messed up anyway since it should go Low-Hi through turning it not Low-Hi-Low). So this, and the fact that this is a repeat issue, points me to the pot. Right after this was fixed before, the amp had another issue on the power tube side, so maybe it fried the pot too. I am OK with replacing the pot itself it there are other benefits as Jazz P Bass points out. I imagine I can get the pot through Crate.

[Jazz P Bass]

I may be mistaken but the pot may be fine.
The way to test the pot is from the middle leg to either end.
In the exact middle position both legs, with respect to the middle, will read the same.
As you turn the pot, one leg will go high as the other leg goes low.
So in the end, instead of two 100 ohm resistors you end up with something like a 120 & an 80.
That is how the imbalance gets balanced out.
To actually set the balance pot, with the amp warmed up, you turn the pot until the output (ie: at the speaker) hum is at a minimum.
That is correct. At a minimum. It is not a cure all.
You can simply use your ears or you can hook up a meter at the speaker terminal to read Volts ac.
This is assuming no input at the signal jack.

[Enzo]

Did you remove all the tubes AND disconnect the 6v wires from the transformer?

If you have a 100 ohm resistor from the pot wiper to ground, then at either end of the pot, the wiper will create a 100 ohm reading to ground. If the pot is centered, and let us say the pot is a 200 ohm pot, then from the center there should be about 100 ohms to either end. BUT, if the transformer is still connected - it has a VERY low resistance - then it is like the two ends of the pot art wired together. SO with centered wiper, we see essentially two 100 ohm resistances in parallel - about 50 ohms. Now when you turn the control to the center, either end - and remember the ends are tied together through the transformer winding - you get that 50 ohms plus the 100 ohm resistor, 150 ohms. Pots are notoriously not accurate in value, so your 40 ohms is close enough to 50.

And if you did disconnect the transformer, you mentioned pulling the power tubes. But you also have to pull the small tubes. Tube heaters have very low resistance as well, especially if several are in parallel, so they can do the same thing the transformer winding does.

SO for a 200 ohm balance pot with a 100 ohm tail resistor from wiper to ground, I'd say your 100-140-100 sounds pretty good.

[robvb68]

Enzo, thanks for the clarification for pulling ALL the tubes. I did pull the 6V wires before, but only the power tubes. After I pulled all the pre-amp tubes and re-checked, the pots tests as it should the way you originally mapped it out. About 200 across the terminals and then from 100-300 on either leg to ground. So maybe I am back to the drawing board, even though this has the exact same symptoms that this did when it was "professionally" fixed before (that was when I was told the problem at that time was in the hum balance control). I have a VERY noticeable hum on channel 1, not really noticeable so much on channel two, but the hum is there whether I have a guitar plugged in or nothing plugged in, no change. The controls don't seem to have much effect on the hum (vol at 1 or 10, tone, etc.), other than as i mentioned switching channels seems to mostly get rid of it, but that was also one of the original symptoms. The hum balance has effect, but even adjusted at it's quietest it still has a real noticeable hum indicating an issue somewhere.

[soundguruman]

hahahahahahahahahahahhhhh!!!!
hehehehehehhohohohohhehehehehehehehhoho.

[Steve Conner]

What?! :-O

JPB: The purpose of the hum balance pot is to correct imbalance in the tube heaters, not the transformer. You're supposed to adjust it for minimum hum whenever you replace a preamp tube.

[Jazz P Bass]

Oops.
I could have sworn it was for the tranny.
Thanks for the clarification.
JG.