Class A Push Pull - output transformer specs

[abbey road d enfer]

What can I say, comparing some clean stuff, the Class A 260v version is 1.2dB louder than the Class AB 325v version.

Class A has more gain than class AB, but less max output power. That's because in class A the output tubes are in the middle of their slope, when in class AB their are at the beginning.
It's easily noticeable on an amp with variable bias. Often, people tune them for maximum output when strumming their guitar, which tends to be at the maximum of the bias range. They often red-plate the tubes.

 

[CJ]

Get a dummy load and hook it to a 1/4" jack,

feed in a sine wave into amp and measure voltage across dummy load.

I have a table of values at work up on the wall,

5 volts = 12 watts,

25 volts = 50 watts

etc

Class A might have different harmonic content than AB, I don't know, I ain't no rocket scientist, I just stick in different components til it sounds good.

Don't forget that fixed bias class a might sound different than cathode bias class a. Maybe some compression on the self bias? Maybe some compression with a 5U4? Maybe some compression due to transformer DCR?

I don't know, what kind of weird amp did Hubert Sumlin use?

 

[CJ]

dummy load, stereo in case i get a weird amp in like a JC 120,

100 w at 8 ohms each, jacks on the outside are for a speaker load, Altec 15" on each side,

i can parallel those resistors if checking a 200 watt pig amp

transformer for isolating the output signal for the scope, sometimes the amp does not like a scope ground connected to the chassis, (oscillation)

V-AC for power and milliamp meter for doing a death bias, (shunting B+ to OPT primary)

 

[trobbins]

It's also worth going back to the loadlines in post #22 and appreciating what options you may have. The last plot shows an output transformer PP impedance of circa 4k5 ohm, but the low distortion section is likely for plate voltage swing between 75V and 400V, beyond which the peak of the waveform becomes increasingly more distorted.

To avoid that in vintage hi-fi amps, the PP impedance is increased to say 10k and the idle current reduced, and then the low distortion voltage swing extends to perhaps 50V to 450V, such that the loadline doesn't reach cut-off with zero current, and the total loadline effectively stays under the max dissipation curve you want to apply. One aspect to appreciate is that the actual loadline is not straight, but will extend out like an ellipse, so margin above and below the loadline allows for lower distortion operation.

So apart from tube selection, the next phase of 'design' perhaps is the OPT impedance you have on hand, and from that you can start to define the idle V and I point that achieves as much linear option in the tube curves before approaching tube regions where screen current rise starts to become a player.

 

[systemtruck]

It’s also worth going back to the loadlines in post #22 and appreciating what options you may have. The last plot shows an output transformer PP impedance of circa 4k5 ohm, but the low distortion section is likely for plate voltage swing between 75V and 400V, beyond which the peak of the waveform becomes increasingly more distorted.

To avoid that in vintage hi-fi amps, the PP impedance is increased to say 10k and the idle current reduced, and then the low distortion voltage swing extends to perhaps 50V to 450V, such that the loadline doesn't reach cut-off with zero current, and the total loadline effectively stays under the max dissipation curve you want to apply. One aspect to appreciate is that the actual loadline is not straight, but will extend out like an ellipse, so margin above and below the loadline allows for lower distortion operation.

So apart from tube selection, the next phase of 'design' perhaps is the OPT impedance you have on hand, and from that you can start to define the idle V and I point that achieves as much linear option in the tube curves before approaching tube regions where screen current rise starts to become a player.

 

[Wavelength]

That absolutely makes sense.. Yes. It outside of the typical tube amp topology. That 60-70% threshold is where the 2nd harmonic really starts to creep in and create the typical single ended distortion profile. But if you can do as you do and use a low Z driver and supply current to the grid then a single ended amp extends it's range quite profoundly.. but with that cost of a a grid choke. I like where you're going.. except for that 3-legged fuse of a LM317.

I use the LM317 as a current source. Also have done fixed with zeners or chained LEDS.