The gear switching I’ve been doing on this rainy afternoon has been putting the 6AS7 “Purpleheart” SET in the “finished” column of the ledger and getting to work on the upcoming 6L6 SE-UL amp. And I’ve already run into a delay.
When looking for the materials to get the new amp going, I quickly learned that I needed to get new output transformers and a main filter choke. So I made an Edcor order and they returned my new iron in record time. I talked about it in mid-April while discussing the SET. But what I didn’t order was a power transformer.
When initially doing the design, I was going through my parts closet and happened upon a power transformer which I purchased almost five years ago for another project. Since that project had not taken shape, and I had already used the power choke for that project in my new 6AS7 SET, I decided to use the old power transformer in the new 6L6 SE-UL. The only issue was that the secondary voltage was a little high; 800 VCT. But I figured that, since I wanted a 400V B+ for the 6l6 SE-UL, I could make it work.
Well, today I have thrown in the towel on using that transformer and have ordered an XPWR178 with a 660VCT secondary. This transformer, using a 5U4GB rectifier, should come fairly close to supplying the required B+. And the best part is I can safely build the power supply using standard 500V filter capacitors which are readily available. The other transformer was going to require some esoteric parts in the power supply.
I have a top layout almost complete for the amp (albeit with the wrong power transformer) but I am going to hold off doing any more physical layout work until I have the iron in hand. I can however start work on choosing chassis materials. I have some really nice spalted alder that could provide a really striking look for the amp. I’ll have to spend some more time on that aspect of the design.
As always, questions and comments are welcome.
Pingback: The Next Amp | Cascade Tubes
800 VCT might be too much for 6L6, but you had mentioned doing KT88 testing. Would your spare transformer have enough current for that tube?
I think that with the big transformer I could reasonably get about 500v B+ at full load with a 5U4GB rectifier. The unloaded voltage would be closer to 600v or higher. This level of B+ requires some rather expensive components and special layout to reduce corona effects which add noise to the amplifier. This could easily give 8W out of a KT88 in SE triode strapped operation.
However, the problems associated with reaching this voltage don’t necessarily seem worth the expense and complexity. As I discussed back in December in the post “Talking About Sets”, an alternative design with the B+ set lower may make much more sense.
In the upcoming 6L6 SE-UL the B+ will be set at approximately 400V. With the KT-88 in SE-UL mode in this circuit I measured the bias at Vp≈350v, Ip≈73mA, and Is≈3.1mA. I have estimated the KT88 power out at this operating point at around 7W. So similar to the 6L6GC. For final values we’ll have to wait for testing.