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In Reply to: RE: Dual 5AR4 input capacitance posted by Michael Samra on September 17, 2016 at 14:44:17
Thanks, Michael! Yes, it looked to me that the voltage is high for those cans, so Authenticap make ones at higher voltage? BTW, this particular amp is a bit of a wreck, it is all there, but dirty and half in pieces as the owner tried to do a half-ass resto and gave up, that is where I am taking over. Do you know where or do you have any pcb-mount 9 pin sockets that have the ground pin on the side for the tube shield? The ones on the boards are pretty corroded. Also, I was talking to a friend at the meet and he really thinks the amp is shit because of the implementation of the paraphase inverter, what do you think? Thanks, Randy
Follow Ups:
Paraphase splitters do have issues. BFD. If the "iron" is decent, tweaking the signal path topology is easy enough.
Look at the schematic. Way more open loop gain is present than that required by modern digital sources. You degrade S/N performance by attenuating the driving signal at the I/Ps.
I suggest you switch to Mullard style circuitry, with an EF86/6267 voltage amplifier and a 12AY7 as the LTP. Please notice that no additional burden is being placed on the power trafo filament winding. Definitely use a 10M45S CCS in the LTP's tail.
Eli D.
Eli
Draw something up if you get time and maybe address that over voltage in the power supply you can.The power trafo does make a nice omelette cooker after it's on for 3 or 4 hours ..LOL
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Bringing the B+ rail voltage down is easy. Use 5R4s, instead of 5AR4s. Slightly "soften" B+ rise with a CL-90 between the rectifiers and the PSU filter.
Like the KB85, the original Mullard 5-20 employs self biased "finals". Replace R8 with a 10M45S CCS. McShane has New Sensor made 12AY7s.
BTW, Mullard's schematic does not show "death caps." and the KB85 does. The "death caps." must go and a proper, 3 wire, safety grounded, power cord installed.
Eli D.
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