Why Only Automatic/Semi-Automatic Bias?

[PsychedelicFish]

Hi guys,
So I don't ask for help often, because I usually like to figure things out for myself, but I was looking at the datasheet for the EL41 and read that it should only be used with automatic or semi-automatic bias.

1) What is semi-automatic bias?
2) Is there any good reason for this?

I ask this because I want to try out a little guitar amp with them (yes, I know guitar amp threads should be in the instruments forum, but this is more of a general valve question) and have found that with other amps I prefer the distortion from a fixed bias power section by far.

Thanks,
Edward

 

[Matt BH]

By automatic bias they mean cathode bias. By semi automatic bias they mean part cathode part grid bias.

Not sure it's the best tube for your purpose. Do you have a bunch of them or have you a pair from an old radio?

Cheers
Matt.

 

[PsychedelicFish]

I have a couple of them lying around, and I have sockets for them. I just want to knock something together and see how it sounds.

Still though, is there any good reason not to use fixed bias with them?

 

[kevinkr]

It definitely is not going to win any awards for linearity. Cathode bias tends to stabilize operating point (via dc negative feedback mechanism), and this may be helpful in types where large variations in transconductance with relatively small changes in grid bias occur (possibly as the thing heats up as well) - the concern here probably being limited plate and screen dissipation. I'd heed the advice or use another tube like the EL84/6BQ5 which is suitable for fixed bias operation.

 

[DF96]

The EL41 has a reputation for developing grid current and then going into thermal runaway. By specifying at least some automatic bias the datasheet is implicitly admitting this problem. If you really want to use fixed bias then use a much smaller grid resistor.

Better to sell them to someone who has a 1950s radio to keep working, and buy something more suitable with the money. EL41 are fairly rare these days so if in good condition can fetch a good price.