OC3 Tube Voltage RegulatorTo Drop Voltage To EL37 Screen Grids

[Mike Sulzer]

Very interesting! So a cost effective method of supplying the screen also happens to reduce a nasty problem in push pull designs. I wonder if "all resistor" is optimum, or if maybe there is some combination of series zener and resistor that would do better.

Using an EL34 with good screen current model, I get 3.7W for the screen dissipation fed from a constant 320V. The peak screen current at 30W out was 46mA so a 2.6K resistor off the 440 rail was substituted. The power out was about the same but the screen dissipation went up to 4.5W. I did not adjust the bias current between sims so the EL34 was baking at 30W idle with the resistor. If I reduced the idle current to be the same as the voltage drop case, the screen dissipation went down to 3.6W.

The most notable thing was the crossover distortion in the constant voltage case was significantly higher:

Constant V: 1kHz:22.6dB, 2kHz 3dB, 3kHz -20.9 db
2.6k + 175 cathode: 1KHz: 23.5dB, 2kHz -10.7dB, 3kHz -25dB
2.6k + 240 cathode: 1KHz: 20.1dB, 2kHz -11.9dB, 3kHz -25dB

[R.G.]

Great! Now we just have to find a vict... er, volunteer to go mess about with their amp's screen supplies!

[trobbins]

The output transformer is from the Stancor's Williamson stable of offerings, so would have been purposefully chosen for its bass end performance. There were other PP impedance models available than 6.6k, but perhaps the combination of 6.6kPP and a 300V nominal screen, and an acceptable EL37 idle current for design max dissipation, gave the best loadline to get in to the knee. The EL37 seems to have a beefier than normal screen rating (voltage and power dissipation).

It's likely that the OC3 max current rating (for continuous regulator operation) can be somewhat exceeded for this application, where pronounced peaking in OC3 operating current would only occur during half-cycle signal peaks into overdrive, and the amp may not have been purposefully or typically set up to easily overdrive.