My latest single ended amplifier. First built for some old EL-34 tubes I bought on Ebay but decided to bias the amplifier for a pair of Gold Lion KT-77's. Those old EL-34 tubes do not sound that good, tired. I have had the KT-77's a few years and they have not been used that much. Still in the process of biasing it for the KT-77's so more details soon. Right now I am running 47ma of current with 428V and for me that is a little too hot. Roughly 20 watts dissipation for a tube rated for 25 watts. There may still be a tiny bit of red spotting on the plate of one tube. Really hard to tell being that it is on the back side of the tube and it may just be a reflection but I would still like to see 17 or so watts of dissipation for long tube life.
Attachments
I found 4 new Electro-Harmonix EL34's for $110 plus shipping, which is a pretty good price. I hope they perform well & are reliable.
It is Tubelabs SSE biased for KT-77 tubes. I just this morning found the bias that I am satisfied with. Cathode to plate voltage is 439V. Running 42ma of current through the tubes for a plated dissipation of 18.4w from a rated 25w tube. Should give long tube life. With higher numbers yesterday I had a little over 7w at the speakers. I would expect some depreciation from that figure with lower current through tube and slightly lower C to P voltage. I have big horns so anything over 5 watts is more than enough for me. The case is completely built by me using solid cherry wood and hand cut dovetail joints. The aluminum is 1/8 plate cut and drilled for use making this amp. Hammond PS transformer and Hammond 125Ese output transformers. Nichicon capacitors and Dale resistors when available for correct ohms needed. Sounds really good.
Great, thank you!
So this schematic ( http://www.tubelab.com/images/AssemblyManualSimpleSE/Simple_SE_Amp_Sch_11-08.jpg )
Is the Ra of the transformer 5k?
Thanks
The original KT-77 offered 10,000 hours of service at 25 watts dissipation. ;-) IMO, you shouldn't have any problem pushing those higher to get more wattage and better linearity. I have found the Gold Lion KT-66s and KT-88s to be very robust. One or two of the KT-66s in my Williamson amps do show a slight bit of pink at some of the seams. This has not devolved into anything worse after many hours of use.