Low voltage on cathodes of EL84 output tubes

[peteb]

Now that is a surprising result. I would assume the valve rectifier would take some load off the PT, not cause more current and heat.

the rectifier tube is a load to the power supply.

 

[Lowerleftcoast]

this is push pull, not single ended.

Correct... push pull but it is not fixed bias.

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.

 

[2L man]

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]

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]

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

 

[tubedude]

You could just get an appropriate transformer for the demand.

 

[zook]

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.