Push-Pull Cathode Bias Resistors

[john_tracy]

I didn't realize this was an old thread until I got to the end here. I'll throw this out anyway. I have PPP mono-blocks that run 4 KT-120s off of a single cathode CCS. Yes matching tubes is a bitch. Yes the output is only 37W triode class A. However they sound great. Ran them for a while with a cathode resistor (problems with CCS oscillating) but the sound while good lacked the "magic" that I hear with a CCS. FWIW the output is IT connected with each pair connected to its own CT secondary. No issues with grid current. BTW the CCS seems to limit current very well. My output tubes were getting a little "long in the tooth" and recently one of them arced. After the arc cleared the amp kept playing, no damage (600V B+, 300mA total bias current).

 

[TonyTecson]

one advantage of separate cathode resistors is you can check tube matching right there.
measuring the cathode resistor drops you will know how far off from each other the tubes
are if at all so that you can put them side by side according to bias...

 

[john_tracy]

I have 1R resistors in series with each cathode for that and tip jacks on the amp for easy measurement.

 

[TonyTecson]

My output tubes were getting a little "long in the tooth" and recently one of them arced. After the arc cleared the amp kept playing, no damage (600V B+, 300mA total bias current).

plate pin 3 is very close to pin 2 so that arcover do indeed happen,
also happened to me with my el34 pp amp with only 420 volts of B+..

during cut-off, plate voltages can run 1200v in your case....
my fix for that is to use s heat shrink tube over the pin 3 terminals...
it is no wonder then that tv scanning tubes put the plate in farther
away to the filament pins....

 

[john_tracy]

The arc that I experienced was caused by insufficient space charge due to an aging tube. The arc occurred during a peak in the music.