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Hello Mr. Heran,
Following up on the topic of using low output tetrodes and pentodes in the KT120 monoblock amps, I have to admit that impedance matching is something I didn't think about. Fortunately, the Edcor CXPP70-3.5K output transformer has 4- and 8-ohm taps. If the 8-ohm tap is optimal for driving an 8-ohm speaker with a KT120 tube (with turns ratio estimated to be 20.9), then the 4-ohm tap (turns ratio about 30) should work to reflect the 8-ohm load across the transformer as 7.2 kOhm impedance seen by the output tube, in this case 6L6 type, for instance. So no worries, so long as the speakers truly are 8 ohms impedance. My VMPS RM30s are 7.2 ohm in the mid range ribbons (purely resistive), while the impedance of the three bass drivers is 10 ohm resistance, but with some inductive reactance. The tweeter ribbon impedance is nominally 8 ohm - again purely resistive. So I think that at least in my case at least, switching to the 4-ohm tap will do the job when using 6L6, EL34, KT66, etc.
We are on the same page regarding the rise in B+ voltage when the anode current is decreased for 6L6 tube. As I am using an unregulated power supply, the idea is to add a SPDT/off center position switch to allow three values of resistance between the first two capacitors in the B+ supply. This should allow the output tube B+ to stay reasonably close to the target 450V. The driver tube B+ voltage may change as well, but I would rather not add another switch and resistors to adjust this as well. We'll see if this becomes necessary. The KT120 tube is really spectacular with very low noise, seemingly flat response, openness, reliability, etc. The motivation for moving to different tube types is driven by nothing more than curiosity.
I installed a timer relay in the circuit to avoid the no/low current, high-voltage on the driver tube at startup. (I recall that you discussed this issue somewhere, as well as in the last post). The idea is to give about 25 sec for the tubes to heat up, as the current flows near instantaneously from the SMPS when the amp is switched on. You will argue that 25 sec is cutting it close, and I agree. I would have preferred closer to a minute, but 25 sec relays are what I had on hand. They seem to work fine.
You enlighten me on the topic of getting a symmetrical PS from a single power supply and a DC-DC converter. I may have to question you more closely if the issue crops up.
As always, many thanks for your helpful suggestions.
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