Can a dual triode use a common cathode resistor?

bluegti · 3rd March 2010, 09:38 PM

I'm working on the layout of my circuit and noticed that its getting crowded on the tube socket. Then I thought, "I wonder if I can use a common cathode resistor."

I'm using half of a 6SN7 as the driver for each channel in a stereo amp. The cathodes of each triode are pins 3 and 6. Can I connect pin 3 to pin 6, then use a common cathode resistor and bypass cap (see attachment)? If so, do I use the same value resistor (e.g. 1.6K) or would I need to double or halve it?

BTW. This is the first amp I've ever "designed" by myself. Admittedly, there are other very similar circuits out there, but I am looking at the grid curves and doing the math myself so I'm not looking for suggestions of "better" circuits to use. I just want to know if I can do this or is it a really bad idea.

Thanks!

OneyedK · 3rd March 2010, 10:02 PM

Yes, you can, but use a very good decoupling cap, otherwise your channel separation will suffer...

Same cathode voltage > double cathode resistor current > 1/2 resistor value. (800 Ohm in your case)

pointy · 3rd March 2010, 10:02 PM

thats not crowded.......................this is crowded (see link)

Eli Duttman · 3rd March 2010, 11:30 PM

Double the bypass cap. value and halve the resistance value. The remark about a high quality part in the bypass cap. position is correct. That cap. is now a common path to ground. Use nothing of a grade less than a Nichicon KZ.

If possible, switch to a LED stack for bias.

bluegti · 3rd March 2010, 11:40 PM

Quote:

Originally Posted by OneyedK

Yes, you can, but use a very good decoupling cap, otherwise your channel separation will suffer...

Same cathode voltage > double cathode resistor current > 1/2 resistor value. (800 Ohm in your case)

By "very good decoupling cap", are you referring to the capacitor in my diagram that is in parallel with the cathode resistor? I thought this was called a "bypass cap".

Or, are you referring to the caps going between the plates of the driver (pins 2 and 5) and the grids of the power stages? I thought these were called "coupling caps".

Can you suggest a "very good" cap?

Thanks!

alexg · 3rd March 2010, 11:59 PM

The Zen amp (SE EL84) have a common cathode resistor for the power tube and it sounds good.

3rd March 2010, 11:30 PM	#4
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	Double the bypass cap. value and halve the resistance value. The remark about a high quality part in the bypass cap. position is correct. That cap. is now a common path to ground. Use nothing of a grade less than a Nichicon KZ. If possible, switch to a LED stack for bias. __________________ Eli D.

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3rd March 2010, 10:02 PM	#2
OneyedK diyAudio Member Join Date: Oct 2007	Yes, you can, but use a very good decoupling cap, otherwise your channel separation will suffer... Same cathode voltage > double cathode resistor current > 1/2 resistor value. (800 Ohm in your case)

3rd March 2010, 11:59 PM	#6
alexg diyAudio Member Join Date: Jan 2005 Location: Philippines	The Zen amp (SE EL84) have a common cathode resistor for the power tube and it sounds good.

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