So I’ve been thinking about my next amplifier project. And I thought I had made up my mind. Then things changed… again.

So, those readers with a good memory will recall that back in May of last year (i.e. 2022), when I was just getting going on the 6L6 “Spalted Alder” amp, I was finalizing the electrical design and had just received the new output transformers. This is when I realized that the power transformer I was going to use, an Edcor XPWR 011-120 with an 800VCT high voltage secondary, was just too high a voltage for that project. I wrote about it in the blog post “Switching Gears is Hard on the Clutch“. So, the big Edcor went back on the shelf.

Well, that power transformer was originally purchased to support a build of an 807 triode strapped amplifier. So I was thinking that I’d dust off my notes from that old project and bring it to life. So I purchased some 630V filter capacitors for the high voltage power supply and went back to look at the 807 operating points. That’s when I realized why it was on the shelf. The 807, in triode mode, has an absolute limit on plate voltage of 400V. With bias and OT loss, the max B+ is about 450V.

I calculated that with the 800VCT transformer the high voltage would be in the neighborhood of 550VDC. So after getting special high voltage components, and carefully laying out the power supply to avoid corona issues, I’d have to burn off about 100V (and about 10W of power) just to get back to my maximum B+. Frankly it seemed like a waste of the big power transformer (and the $150 USD it costs before shipping). Then I had another idea.

Instead of using the 807, I’ve been playing with the idea of a triode strapped KT88 for several years. I had revisited the KT88 Triode design in a post “Talking about SETs” and concluded that the added cost of the high voltage B+ supply might not be worth it. But since I now have all the special high voltage components required for the supply in hand, it was time to revisit the design.

Based on my calculation of approximately 550VDC using the big power transformer, I generated a new load line design between those discussed in the “Talking about SETs” post. The big 13W design from that post required a B+ of about 630DVC. This is more than I can get out of the XPWR011. So I reworking the design yielding the following load line design.

This is a single ended design, cathode biased at V_p=475vdc, I_p=66mA, P_d=31.4W, and E_g=-60vdc. This is nominally a 10W/channel design using a 547VDC B+ voltage. Each channel’s power tube draws only 66mA so there is lots of current margin in the power supply design. The other nice thing about this design is that there is significant leeway for the B+ to go up or down. If it comes out a little low it should still produce over 7 or 8 watts. But if the B+ voltage comes out a little high, even by as much as 50V, the stage will still be operating within tube specifications. This is a great characteristic for people attempting to build this with other power transformers.

I measured the Edcor transformer’s high voltage secondary at no load. With my local mains voltage of 122vac, the high voltage was just about 850VCT. So the maximum no-load dc voltage is just about 600V. With these measurements, I am fairly confident that my power supply will fall around the 550VDC number with the ≈135mA load and a GZ-34 rectifier. I also now know that the 630V electrolytic filter capacitors I have should easily withstand the amp being powered up with no power or signal tubes installed. This is an important safety point.

I’m currently planning on using the same 6SL7 driver stage that I used in the 6L6 “Spalted Alder” amp. This puts the amplifier’s sensitivity at ≈1.0v-RMS (≈1.42v-peak) for full power. It also allows the use of a 6SN7 driver tube for use with a preamp or high output voltage source.

I currently have another project and an optimization study in the queue, but these should go quickly. I’m hoping to get to actually build a chassis for this project right after the first of the year.

As always, questions and comments are welcome.