help with bias circuit mod

[g1]

Just to step back a bit on this and look at the big picture in context (since you mentioned going as far as a separate bias transformer):
The marshall type circuit with the pot configured as a rheostat is pretty much a fail-safe design.
Any glitch or failure with the pot will cause the circuit to default to coldest bias setting, no harm done to any tubes, transformer, etc.
I understand wanting to use a reliable part, but also want you to know there is no real worry of serious repercussions.

[mikepukmel]

Just to step back a bit on this and look at the big picture in context (since you mentioned going as far as a separate bias transformer):
The marshall type circuit with the pot configured as a rheostat is pretty much a fail-safe design.
Any glitch or failure with the pot will cause the circuit to default to coldest bias setting, no harm done to any tubes, transformer, etc.
I understand wanting to use a reliable part, but also want you to know there is no real worry of serious repercussions.

That's fantastic,. thanks didn't know that. Nice design feature.

[mikepukmel]

To use a 10k pot in place of the 22k for the circuit in the OP why not just use resistors of half the value?
i.e. R29 91k, R30 7k5, R31 28k and VRx 10k
That should give the same range as the original circuit (about -38 to -50V)

Thanks, Dave, will do.!

[66 Kicks]

Thanks, Dave, will do.!

It's probably not of any concern, but Dave's version will have around twice the ripple as my version.

[Dave H]

It's probably not of any concern, but Dave's version will have around twice the ripple as my version.

Yes, I thought about that. Designing it now you could use higher value smaller modern capacitors.

[mikepukmel]

It's probably not of any concern, but Dave's version will have around twice the ripple as my version.

What's wrong with ripple? I thought Ripple was good?

https://www.youtube.com/watch?v=ciR7Fq2tqJ0

[mikepukmel]

Just to step back a bit on this and look at the big picture in context (since you mentioned going as far as a separate bias transformer):
The marshall type circuit with the pot configured as a rheostat is pretty much a fail-safe design.
Any glitch or failure with the pot will cause the circuit to default to coldest bias setting, no harm done to any tubes, transformer, etc.
I understand wanting to use a reliable part, but also want you to know there is no real worry of serious repercussions.

Its a character flaw. I get part way through something, and start worrying. Geez, what about the power rating on that resistor, ... I wonder if those screws are strong enough to hold up that railing; should I have put new pins in the brakes when I re-did them; should have bought a better reverb tank; oh boy that bias circuit looks like trouble; I bet that solder joint won't withstand an earthquake ...

[mikepukmel]

Its a character flaw. I get part way through something, and start worrying. Geez, what about the power rating on that resistor, ... I wonder if those screws are strong enough to hold up that railing; should I have put new pins in the brakes when I re-did them; should have bought a better reverb tank; oh boy that bias circuit looks like trouble; I bet that solder joint won't withstand an earthquake ...

Anyway, back to the problem at hand: if you all are saying that the little cheap trimmer pot is probably OK, and if it does fail it won't toast the output tubes, and of course, its going to be a whole other task to order more parts and wait for that to get the amp built, maybe I should just use the kit parts, get the amp built, and if there is a problem with this teeny pot, replace it later.

[g1]

For super reliability, you could adjust the bias, measure the pot resistance, then replace the pot with a fixed resistor.
Then write your own "white paper" about the dangers of adjustable bias, lol.

[mikepukmel]

For super reliability, you could adjust the bias, measure the pot resistance, then replace the pot with a fixed resistor.
Then write your own "white paper" about the dangers of adjustable bias, lol.

Oh I like it I like it! Can I work in some of those really complicated formulas with the squiggly math symbols?