The voltage checks all came out as expected, so the amplifier was put into use for an extended listening session. The most succinct description I can give is that the amplifier is clean, very fast, neutral in color, and musically powerful.
Here it is on my work desk during its first listening session.
After a couple of hours with various types of music, I can find no faults or weaknesses. The amp took everything I could throw at it and performed admirably.
It will take a few days to get the full technical testing of the amplifier completed. But for now I am simply enjoying its power and clarity. This is a very fine sounding amplifier.
As always, questions and comments are welcome.
Hi Matt
Yes I connected pins 1 and 8 when I built it. I guess some of of the new-production EL34’s have the suppressor grid connected internally to the cathode, but I connected the pins just in case.
I may try KT88’s at some point. I went with the 6L6GC and EL34 because the cost is a fraction of the cost of the 88’s. But if you try them in your amp and it makes an appreciable difference, I might just spring for them.
I like JJ tubes. I was looking at the pinout for their KT88 and it shows Pin 1 as no connection, but for certain I would investigate that before plugging them in, removing the pins 1-8 jumpers if necessary
Matt
Glad you got it finished and operational!
I am enjoying my build of your design every day. Very pleasing to my ears.
Just for fun, I plugged in a set of EL34’s yesterday. The sound is a little different, not better, not worse, just their own “personality” so to speak. If you ever decide to do your optimization on the EL34, it may be very interesting to see how much it differs from the 6L6. Thanks again for the great design!
Obviously you connected pin 1 to pin 8 at the power tube sockets. Since pin 1 is an internal screen (not screen grid) in most KT88, I would be interested in hearing how KT88s sound in your amp.
I found one brand of new-production KT88 that connects pin 1 io a metal ring around the base of the tube, but no mention of an internal screen inside the tube. But I haven’t taken the plunge and ordered any yet.
I’d be careful of that Pin 1 connection. There were conflicting standards back in the 50s and 60s and now it’s anyone’s guess what manufacturers are doing. One thought would be to implement a switch for the EL34s.
The switch is a good idea. I’m actually quite satisfied with both the 6,L6GC and the EL34 – so I doubt I’ll go with the KT88 in amp.. I’ll build a second amp in the future. I have another pair of the Edcor output transformers and a power transformer is on order. I’ll likely implement the switch idea that you suggested.
I’m curious, what would be the purpose of an internal screen in a power tube like the KT88? I’ve seen them in dual triode tubes like the 6,CG7
Some manufacturers use them to shield from external electromagnetic fields. Like the example you found with pin 1 tied to the metal shield abound the tube base. Other tubes use an internal shield for the same purpose.