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Old 27-04-21, 08:16 PM
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Kei Kei is offline
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Join Date: Apr 2013
Location: South Wales
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After someone mentioned trying to build a Mullard 5-20 using 807 output valves on DIYaudio, I got it into my head to look at whether it was possible using my spice simulation as the ideal output transformer for 6L6 based valves is 6.6K. (which is what the 5-20 happens to use) Turns out, based on my simulation, 6L6GC and it's 30W+ derivatives will work just fine in my amp. So on a whim, I bought some of the high dissipation (35W) 7851A's to try out.

With all of my output valves side by side, they look pretty similar to the EH 6CA7 on the far left, but the primary difference is that the cathode is quite a bit smaller. It does seem odd that it has a 35W dissipation limit whilst the rest are all 25W.

Fitted and running. Cathode biasing seems to work nicely. Plates sit at 428V and screens just below at 427V via the 43% ultra linear taps. Small bonus these have is the reduced heat output from the lower heater current (0.9A vs 1.5A) After a couple of hours listening, it sounds every bit as good as the other EL34's and their derivatives. I figure the ~20dB of negative feedback is minimizing any audible differences there may be between the various valves I've tried. A big added aesthetic bonus is that these have the blue glow on steroids compared to the rest.

I will admit that the valve bug bit hard once this amp was functional. I've ended up 6 quads of output valves (x24), 5 pairs of phase splitters (x10), 10 pairs of input valves (x20) and 2 pairs of rectifiers + a spare. (x5)

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