save bias supply

[txstrat]

Hi all,

I've built a decent amount of amps so far and have used different kinds of bias methods. Mostly it was fixed bias with a trim pot inside the amp, where the wiper provided the negaive voltage to the grids. I know that if the wiper fails (burns up) the grids wouldn't get any negative voltage and the power tubes might blow (actually I believe they will blow).
In my current build I want to go "save" and use the 10k pot I have (with a screwdriver slot) as a variable resistor (i.e. if the wiper fails, the bias voltage would be highest - negative).
What values would you guys recommend for a voltage divider circuit, whit the pot (I already have) at 10K and a separate separate 50V winding on the PT.

Thanks

[tubeswell]

Hiya txstrat

When I wire the bias pot as a variable resistor, I short the wiper to one side of the pot, so that if the wiper fails, there is still a leg for the voltage divider to stand on

[txstrat]

That's what I'm gonna do (thanks to your advice some threads before ).
What values do the other resistors in your build's bias'es have? I'm asking cause different values work under different power conditions. And I don't wanna use 5W resistors for the bias supply.

[tubeswell]

Whatever I need to get the voltage divider giving a good bias voltage range. Usually something in the 10s of ks on each side of the knee of the divider, e.g.; for a divider that gives -31V off a -69V* supply, I'd use say 82k as the numerator, and a 20k trimpot in series with a 56k resistor for the leg. This would give -28V with the pot cut and -33V with the pot dimed, and if the wiper failed it'd default to -33V (the way I'd have the pot wired). If you want a bigger range, use a 50k trimpot in series with a 39k-47k resistor for the leg - and the 82k for the 'numerator' part of the divider.

*50VAC rectifies to about -69VDC with a half-wave reverse-biased diode and about 8uF filtering

[txstrat]

I need about -50v for my 6L6 tubes. Think I'm gonna use 10K as numerator and 10k pot with 18k for the leg. That gives me a range around -45v to -53v. Should be enough and the resistors are not in big stress.

Thanks for your time TW.

[MWJB]

There is usually very little current in a fixed bias supply, half watt resistors are usually plenty good enough for the voltage divider in amps with a dedicated PT tap. Fender have used 1/4W in the past.

E.g. 53v*53v=2809
2809/28,000ohms = 0.1W dissipation.

45v*45v=2025
2025/10,000ohms = 0.2W dissipation.