Good work!
Quote:
Originally Posted by Gabe001
Any comments and suggestions welcome.
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I have three:
The position of the UL tap! Unknown, though you can determine it by 'scoping the signal at the screen tap and comparing with the signal at the anode.
I designed a low-power UL amplifier a couple of years ago (using 1p24b Russian rod pentodes) - it became clear to me that the farther away from HT the taps are, the better. But with greater signal on the screen, the grid drive requirements increase (as you say). The limiting point comes when, to get sufficient downward swing on the anode to hit the valve's bottoming 'knee' the driving signal has to peak at 0V - you're then in danger of grid current and consequent loading on the first stage.
And... open-loop performance! The overall frequency response looks praiseworthy, but what is it like open-loop? When feedback straightens the frequency response, intermediate points can see larger-than-expected signal levels. So, for instance, of your output transformer is 3db down at 20Hz, and your actual response relative to 1kHz is 2db up at 20Hz (as it looks from your sweep), the EL34 grid must be seeing signal level 5db up at 20Hz... that's 1.8x normal. It's easy to unwittingly overload intermediate stages without realising it, if you expect full power over the whole frequency range.
And the last? On/off switch at the back, when it could so easily have been on the front! But maybe that's just me! (It's a smart-looking piece of equipment!)