EL34 Push-Pull: big temperature differences between the 4 tubes - problematic?

[BrassTeacher]

BrassTeacher,

Because you used 1 Ohm between cathodes . . .
A good DMM is needed.
Most analog meters do not read that low.

I mV difference between cathode voltage, is 1 mA difference current.
Probably OK for most push pull output transformers.

5 mV difference between cathode voltage, is 5 mA difference current.
NOT good for some push pull output transformers.

Thank you, I am aware of all this. That's why there are 1 Ohm resistors there, so I can use my very good DMM to measure the voltage drop across them, which is far more accurate than the original scheme.

 

[Gruesome]

Wait a minute, brassteacher is not the OP. Neither is Chris Hornbeck.

Any news from dvdr (the OP)?

 

[dvdr]

Any news from dvdr

Here‘s an update:

• one of the tubes was completely out of spec for this matched quad. I was targeting 35mA bias current (only one potentiometer per pair) so its partnering tube ran with 9mA less! Yes: NINE! No wonder the huge temperature differences.
• I swapped tube positions to rule out the amp - this very tube still was 9mA off in any position/combination
• I put in a spare quad: matching now is within 0.3mA difference: 35.0, 35.0, 34.7, 35.0
• Front position V4 still seemed to be a bit (not that much now) cooler, so I got a cheap „laser thermometer“. Temperatures are within spec (max 250°C afaik) running at around 160-180 measured at the sides, but the tips were way cooler. Maybe, that’s what I was experiencing right above the tubes - rest is convection and distribution of hot air…

Thanks again for your input!