Question for The Tube Experts

[Paul Marossy]

Thanks for the help guys! I will crunch some numbers this weekend and give that "miller cap" a try. 

[jrc4558]

Thats a nice name for a tube pedal - "Miller Cap"

[Paul Marossy]

it's the sum of the Cgk, G*Cga, and G*Cgp, where Cgk is the grid to cathode capacitance, Cga is the grid to anode internal capacitance, Cgp is the external capacitance you may have added from grid to plate, and G is the voltage gain.

Doh! The data sheets I have offer no info on Cgk or Cga.  I guess I'm going to have to wing it... 

EDIT: I think I've just found some info on that.

[Paul Marossy]

OK, here's an update. The cap from anode to grid idea has to be scratched because the hiss is too low in the audio band - I think it's around 900Hz. It's not that bad, though, if I can totally eliminate any hum.

Which brings me to one last question. On the 12AT6, it's a duo-diode, high mu triode. In this circuit, the diodes are not connected to anything. Should they be connected to something? I'm wondering if these unconnected pins might be picking up hum from the transformers because they are just kind of "floating". Any insight on that? 

[Dai H.]

been a long time since I tried it, but I had some 6AV6 (a 6 volt, high gain version with same pin outs), and I grounded the diodes after seeing that in an old amp schematic. (6AV6 is like half a 12AX7 plus some diodes, *number*AT6 is lower gain.)

[Paul Marossy]

Thanks for that piece of info, Dai. Well, I guess it couldn't hurt to try grounding the diodes, could it? 

[brad]

The 12AJ6 used in Stephane's tube clipper pedal thingie is a triode/diode combo too.  I grounded the diodes after confirming it with an old radio schematic...

[Paul Marossy]

Alright then, I'll definitely try grounding the diodes. 

[R.G.]

In a tube, anything connected to the cathode cannot accept electrons. Anything more negative than the cathode repells them