"Balancing" bias in two output tubes. (and another thing.)

[Erlend_G]

I also read- that in some older amplifiers (was it Fender?)

the bias balancing pot, was called something like the "hum canceling control"-

because the amplfier would start to hum, if the tubes were greatly differently biased.

..my Blackstar is the quiestest amp I've ever owned, included Hi-Fi equipment. You can crank EVERY control, but there isn't even a hiss. It's dead quiet.

[dystrust]

Without knowing more about how your amp is designed it's impossible to say how it *should* be set. The claim you read about a richer tone with slightly mismatched tubes is widely held; in fact it's so widely held that a lot of amps deliberately mis-balance their phase inverters to achieve the effect. In amps like that a balanced phase inverter is a waste of money.

If I were biasing your amp I'd try to balance the tubes reasonably close which would insure the most even wear.

[beaubrummels]

Thus the hand in pocket rule. 500 volts of AC will fry you. 500 volts of DC makes stuff explode.

Sent from my SM-A115A using Tapatalk

Additionally, my understanding is "While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal." which would include >250v plate voltages.

(FWIW I just read and saw film of the Edison elephant experiment which said it was 6600 volts, but terrible story either way.)

[Demanic]

Additionally, my understanding is "While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal." which would include >250v plate voltages.

(FWIW I just read and saw film of the Edison elephant experiment which said it was 6600 volts, but terrible story either way.)

Oh, I obviously miss recalled the voltage.
I was basing what I said on the fact that AC causes you to get hung up on the conductor.

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[Jacew]

I would open the amp first UNPLUGGED to check what kind of bias control is there. Egnater is nice to provide mA values printed on the board next to pots.

Blackstar is usually pretty smart about these things so it might be quite simple.

I've only used balanced/paired output tubes myself, so can't say about that.

[Gtrjunior]

I think a bias probe and volt meter or modern bias tool is worth the investment if you want to do this going forward.

They are not expensive.

I almost never use my Oscope anymore -just not needed.

100%.
You absolutely need a devise that will show you readings. You can’t just fiddle with the bias knob and hope for the best.

[Gtrjunior]

I realize that this isn’t your exact amp. This is the same Blackstar that I own. But this is how to bias it.
I own 4 tube amps and the bias procedure is exactly the same for all of them.

There is a mathematical formula that is used to calculate what your bias setting should be in YOUR SPECIFIC AMP.
There is no universal bias setting.

https://youtu.be/PtvsRn9H29Q