Tube DIY Asylum

RE: Class A push-pull- something to consider

50.4.127.98

PakProtector

In Reply to: RE: Class A push-pull- something to consider posted by Ralph on December 13, 2024 at 11:57:04

Ralph, I don't suppose you could post a marked up version of what EV actually built, re. the driver/screen grid resistor, could you?

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

Follow Ups:

If you can find a copy of the schematic - Ralph 08:49:29 12/17/24 (2)
The plate resistor of the driver is also the screen resistor of the associated output tube. The coupling cap coming off of the plate of the driver is driving G1 of the power tube.

The schematic shows the driver plate resistor as tied to the screen and the screens then tied to the plate of the opposite power tube. Both versions will work.

In my amp and also in the schematic the driver plate resistor is a 12K 2 Watt device. The schematic as shown was glued to the bottom panel.
- PDF Schematic - Triode_Kingdom 09:18:55 12/25/24 (1)
  Here's a PDF of the user manual that includes EV's description of the circuit and a schematic. It's a PDF, so it can be rotated by most browsers. What I found most interesting about this is the statement that the output transformer is 1/4 the usual impedance.
  - A 20C Circlotron (Open in New Window)
  - Yes- that might be the advantage of a Circlotron - Ralph 08:58:47 12/26/24 (0)
    -With the lower turns ratio the output transformer has wider bandwidth due to lower distributed capacitance. You don't even need it to be a transformer- it can work as an autoformer too (but you have to be more careful about DC Offsets).
    
    The original Circlotron patents employed a bootstrapped driver circuit like you see in the schematic. But I think that is higher distortion and you can get into issues with blocking distortion and preventing thermal runaway of the output tubes. EV ran into a problem with this and so reduced the output tube grid resistor value to 100K and also increased the value of the coupling cap to maintain the timing constant.

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