Low voltage on cathodes of EL84 output tubes

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,546
Age
62
Location
Finland
Pete!

Cathode biasing limit power supply sag-effect because increased current increase cathode bias voltage, which restrict tube conduct, which lower the current and power suoply does not sag much.

Bypass capacitor can keep bias voltage low only short time when tubes pass high current. Perhaps this could be called ”cathode bias sag-effect”?
 

peteb

Friend of Leo's
Joined
Apr 25, 2003
Posts
4,950
Location
Cascadia
There will not be the voltage swing in a push pull cathode biased amp that will be experienced in a push pull fixed bias amp.
Maybe less, but it is still there. Only class A single ended amps have no power supply sag.

the idea behind the push pull cathode biased amp is, that increased current cools the bias.

increased current results on power supply sag.

only class A runs constant current and no power supply sag.

it is true that most class AB amps operate in class A at low volume and class AB at higher volumes.

just my thoughts.
 

peteb

Friend of Leo's
Joined
Apr 25, 2003
Posts
4,950
Location
Cascadia
B
Bypass capacitor can keep bias voltage low only short time when tubes pass high current.

ypass capacitor can keep bias voltage low only short time when tubes pass high current. Perhaps this could be called ”cathode bias sag-effect”?

cathode sag is sometimes called squish.

The cathode capacitor allows the AC signal to pass thru the tube, while keeping the signal off of the cathode, which would be negative feedback. The lack of negative feedback, allows for more signal to pass thru the tube. As the tube only passes current in one direction, increased signal current thru the tube, adds to the DC demand on the power supply.

Just my thoughts




the flow of
 

zook

Friend of Leo's
Joined
Aug 6, 2003
Posts
2,938
Location
Cochise, AZ
The AO44 power transformer should work just fine. I'd guess there's something going on with the circuit. The original 6GW8s that the PT drove are very near EL84s in performance.

Check all your components for values.
 




New Posts

Top