Driver stage load resistor

[Klimon]

Hi,

I'm looking for a little advice on how to proceed when determining the working point of a triode signal tube in a grounded cathode, RC coupled driver stage. Few things I know:

1) Don't get too close to zero grid volts

2) The steeper the loadline the less gain

3) A certain amount of current needs to be run through the tube to make it sound good, some tubes sound warmer when run at higher currents, others sound thin (e.g. 6c45)

4) Is it, apart from the mentioned variables a good idea (when it seems to give the best specs for the tube in that circuit) to use high or low value load resistors or should one stay away from extremes here?

To make it concrete: I'm searching for a working point for a 2c51 driving a 6ck4 output stage (needing about 54Vpp voltage swing). Max. B+ available for the 2c51 is 250v

-------The common option (seen others using it this way): 250v B+ / 20k load / working point 6,5mA @ 120V

------- Another one, which would make using led-biasing possible (other posts here suggest not to use led-biasing unless about 10mA is drawn): 250v B+ / 12k load / 9,5mA @ 140v

Gain is about 55 in both cases.

I welcome any tutorial!

Simon

[vax9000]

I am sold to Tubelab.com 's constant current source (CCS) load plus power drive idea. You can find it at tubelab.com

[Dave Cigna]

Originally posted by Klimon
-------The common option (seen others using it this way): 250v B+ / 20k load / working point 6,5mA @ 120V

------- Another one, which would make using led-biasing possible (other posts here suggest not to use led-biasing unless about 10mA is drawn): 250v B+ / 12k load / 9,5mA @ 140v

Once you have the amp up and running it won't take much to try it both ways ....

Of course it would be possible to perform some kind of distortion analysis, perhaps by building the two version and connecting them to a spectrum analyzer. Or you could have faith in some tube models and perform a simulation by computer. Then choose the version with the lowest THD or the nicest harmonic profile.

Or we could just say that the higher load will produce lower distortion, so it must be the superior circuit.

In truth, there will be a certain synergy between the driver and the output tube. Finding the way they work well together is where the art lies. It tends to be a subjective process. If that bothers you then connect the entire amp to a spectrum analyzer, not just individual stages. But my advice is to do it by ear first.

-- Dave

[Klimon]

Cheers!! Up to the next stage

Simon