Help with adding balance pot

[Rattler66]

Hey Guys, this is the schem for my amp, the only difference is i added 1ohm resistors at the cathodes to read bias and the bias values are slightly different to compensate for my secondary on PT. The amp as is is finally working great but there is a slight hum that is present, this hum varies with the bias. i have it at a minimum now, but am hoping if there is a way to add a balance pot to it without losing my bias adjustment, thanks

I just realized something tonight, I am not using the bias tap on my PT secondary. I am tapping off the Secondary, so I have a bigger resistor to drop it down to where I can use it, can the bigger voltage source induce more hum than if I used the bias tap (~50vac bias tap voltage)? And if so can increasing the bias ckt caps help? thanks

[Rick Erickson]

The hum is probably just being amplified by the power amp circuit, which changes gain as you adjust the bias control. They are not necessarily related. Find the source of the hum and fix it and I bet you will be able to set the bias where you want it, without a balance control. This is most likely a ground loop problem in the preamp, or perhaps a noisy tube.

RE

[Rattler66]

thanks Rick, is it possible that I'm picking it up through the bias ckt, since it's present even withe all the preamp tubes out. thanks

i really wish i had a scope, it would make things so much easier

[Robert M. Martinelli]

Ever thought about getting a surplus o-scope?

Hi,
If you like to tinker like I do, and you wish to make things easier to yourself, have you ever thought about surplus oscilloscopes? They can still work great without hurting your wallet; I use an old HP 182 ( dual trace, 50 Mhz ) which I purchased from a surplus shop in 2002, it was dismissed from the German Ministry of Communications, totally refurbished and calibrated ( and also had the CRT changed ), I paid it as low as 200 Euros, ( and they included the original manual, the schematics and a spare probe too.... ) and it' s still going strong.
Hope this helps
Regards
Bob

[Enzo]

When you don't have a scope, and I agree anyone who is remotely serious about this stuff ought to have one, use your volt meter. Set it for AC volts, then it ignores the DC in a circuit nad just measures any signal or ripple. Want to see where the hum comes from with the PI tube removed? Measure the bias at the grid pins of the outputs. On DC it should read whatever your bias is, and on AC it should measure about zero. You get some AC there, and it is likely ripple on the supply.

You can trace signal through the amp that way too. Cant tell what it sounds like, but you can trace for its presence.

[Rattler66]

Thanks guys, Hey Enzo which meter would you recommend because mine never seems to be able to capture ac ripple. i think it's too old and too plain.

Vox thanks for idea on surplus scopes, i see one on webers site for about265.00. i gotta look for something better.

[Enzo]

Try a search here, we have discussed meter shopping before.

You won't go wrong with a FLuke product, but you shoudl be able to find a gangbuster meter for under $200, and frankly should find something reasonable for half that.

Ther are really nice analog meters - they still make SImpson 260, which was once the standard meter of all technicians - the ones with a moving needle, but the littel cheap ones like that are almost useless. They tell you hgih voltage is there or not more or less. A digital meter is what you want.

[TheTinMan]

Thanks guys, Hey Enzo which meter would you recommend because mine never seems to be able to capture ac ripple. i think it's too old and too plain.

Vox thanks for idea on surplus scopes, i see one on webers site for about265.00. i gotta look for something better.

Testequipmentdepot.com has great prices on oscilloscopes. Same one as on Weber's site is a bit less and IIRC shipping was "free".

ON TOPIC: it's a lot easier to set up a separate bias adjustment for each tube than a balance setup. Your circuit should filter the bias supply better than most Fenders given the two filter caps. Have you tried just swapping the two power tubes to see if that increases or decreases the noise? Might give you some idea of whether or not a balance control would help.

Chip

[Rattler66]

I think the problem isn't balance, I think it is what Rick said, amplifying the hum. the tubes are about 2ma difference. i'm looking into a scope and meter. hopefully i can check this weekend, thanks again guys