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In Reply to: RE: New K&K ST70 power transformer question posted by Cpwill on December 29, 2012 at 15:47:37
The new series PC board will support both the lower voltage "stock" power transformers and the new high voltage/current transformer. The specific goal of the new board was to accommodate a 550VDC supply with KT-120s running at 70mA each. The improvement in sound quality is pretty substantial. It's even more substantial if you fit a pair of Lundahl LL1682/PP outputs (for 4 ohms) or LL1663/PP (for 8 ohms). I just finished fitting my upgraded prototype new board unit with LL1682/PPs and before they are even broken in they are another big improvement over stock ST-70 transformers. The new Lundahl transformer covers for these outputs made that possible. A picture is attached showing this amp. The chassis is a Triode Electronics chassis, otherwise the parts are all K&K Audio sourced.
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
Kevin,The updated boards are on the way too? Great news.
Three quick questions about the new PCB configuration:
How tall are those heatsinks in the middle and how much space is reserved on the new PCB for the .1uF coupling capacitors? Output-wise, what are we looking at WPC?Thanks again.
Edits: 12/31/12
The heat sinks extend about 5.25" above the top of the chassis with the board mounted on the supplied 0.375" stand-offs.
The capacitor layout is 1.4" x 0.7". However, one capacitor can sit partially or completely above the other in the case where the capacitors are wider than this. It's also possible to stretch the length significantly beyond the layout lines by elevating the capacitors on spacers.
My early measurements of power output on the prototype got me 11 watts/ch at waveform clipping with the higher voltage K&K Audio power transformer and 65mA of bias on KT-120s. But I've mades some changes since then, so I need to remeasure.
Kevin Carter
K&K Audio
www.kandkaudio.com
Kevin
Looking at your picture of the prototype, the KT120s seem to be too close. Genalex advises that the centres of KT88 tube sockets be not less than 4inches apart and that pins 4 and 8 of each tube be in line. From page 5 of the Genalex KT88 data sheet. I would image that this would also apply in kind to the KT120 being a similar if somewhat bigger heat producer. I've seen plenty of new KT88 amps work with closer than 4 inch spacing, but the question of "for how long" usually answers in the "not for long" category.
There's a couple of ways of looking at this. There is the way that you advocate, which is to say that tube life will be significantly compromised, so that perhaps you shouldn't even try it. Or there is the way that I advocate, which is that the result is sonically superior and tube life isn't drastically shortened, so why wouldn't I do it?
It's for each of to decide what's more important. I can't change the output tube spacing on an ST-70 chassis, but I can change the tube to one that will allow more power output and, as a result, much improved dynamics. That seems like a fair trade-off for an unknown amount of tube life reduction.
Kevin Carter
K&K Audio
www.kandkaudio.com
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