Directly heated cathode help

[sneakers563]

Hi all,
I'm starting a push-pull guitar amp project using the 6397, a subminiature "power" pentode with a directly heated cathode. The datasheet can be found here, for anyone interested. The filament has a center tap, and I would like use cathode bias. This is my first project using a directly heated cathode, and I was hoping someone could take a look at the picture I've attached and let me know if it looks reasonable.

6397_cathode.png

As you can (hopefully) see, I'd be powering the two filaments in parallel from a LM317 providing 2.5 volts. The two filament center taps are tied together to ground through a bypassed cathode resistor.

1) Are there any problems with biasing the tubes in this way? Specifically, is the ground scheme correct? Normally I would reference the bottom rail directly to ground, but that seems like it would short the cathode resistor. I believe this way that the only current flowing through the cathode resistor from the plates, and the bias voltage is serving simply as a reference for the filament supply.

2) I will need a 6.3V supply (I'd prefer DC again) to supply the indirectly heated tubes in the preamplifier section of the amp. Do I need to use a second transformer winding, or can simply connect a 2nd bridge rectifier to the transformer secondary and go from there? I would normally reference that supply to signal ground as well, either directly or by elevating it, but I don't know whether the addition of a 2nd path to signal ground will cause problems or is even necessary.

Thanks for any help.

[stiltamp]

Have a look at this page:

http://www.emissionlabs.com/Articles/AP ... index.html

[sneakers563]

Thanks, that is helpful. I hadn't seen that page before. It seems that I have it right in terms of cathode bias.

I do still wonder about whether it's ok to run the 6.3 supply for the other tubes from the same transformer winding, or if I should use a different one.

[stiltamp]

Here one more about:
https://diyaudioprojects.com/mirror/mem ... umbal.html
I think that in the first circuit in this topic, you can use the heating coil for multiple stages, but I am not sure.

[bepone]

use separate winding and you are sure that there wont be any problems

or , for the test if you like to do, use 6.3VAC heater line but dont ground any heater point and see if LM317 application and cathode bias grounding will be enough to remove all the heater humm from the amp (maybe)