negative bias voltage for dummies....me

[studiodunn]

I've now built and modded enough amps to realize that I know enough, but really nothing. I am beginning to get into the fine tuning of things and started thinking about the importance of the balance between -bias and plate voltages. I read allot of build comments that usually talk about the negative voltage range and then other builders talk about fine tuning the amp at the bias voltage. So I wont pretend that I can fully comprehend the deep fundamental of this, so I would ask: is there a simple calculation that determines what the bias voltage should be in relation to the plat voltage?

[martin manning]

See if this helps: http://www.valvewizard.co.uk/pp.html

[studiodunn]

Found this
https://www.vtadiy.com/loadline-calcula ... alculator/

[wpaulvogel]

It really only depends on how close your tubes are to the published data sheet. Hot tubes require more negative grid voltage than cold tubes.

[Stevem]

X2.
The stronger the tube test(s) the higher the negitive bias voltage will need to be, and keeping close track of the ma they idle at individually will tell you how the tubes are aging and how well the tubes are matched as you apply signal and rev them up to higher output.

[martin manning]

I would ask: is there a simple calculation that determines what the bias voltage should be in relation to the plat voltage?

Posts above are spot on. The grid bias voltage is adjusted to achieve an idle operating point that sounds good (gets the power stage out of crossover distortion) and will not destroy or unreasonably shorten the life of the tubes. This is most often gauged by the anode power dissipation at idle, which for pp amps is typically set at 60-70% of the data sheet maximum. The grid voltage required to make that happen varies with the plate and screen voltages, from one tube to the next, and as the tube ages. Accordingly there is no simple calculation for the grid bias voltage. Use plate dissipation to set the operating point.