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In Reply to: RE: Now it's back to its bad ways. posted by Tre' on January 12, 2017 at 16:52:27
Hi Tre, i could not find the schematic in my files but it is a very basic type. The filament tx is reversed wired to a .8A 6.3v tap, the 120v is FWBR and filtered by a choke about 25H x 715dcr. this voltage is filtered by 50uf x 120v then dropped by 5K 3/4w mf then 50uf x 120v then dropped by 10k 3/4w mf then 100uf x 100v to the 50k AB 2W carbon pot. The wiper is 50k 1/2w cc connected to the power tube grid stopper. The wiper also has a 50uf x 63v connected to it.
Well, i did not have a single 50uf x 120v cap so i series connected 2 63v x 100uf caps, and not remembering that the metal can is "hot" sometimes they were sitting on the chassis. I guess i better change them out. thanks for the help! cheers, Dak
Follow Ups:
"The wiper is 50k 1/2w cc connected to the power tube grid stopper"Did you mean the wiper of the pot connects to the bottom of the grid resistor?
BTW I think you said there is -60 volts feeding the pots?
two 50k ohm pots in parallel to ground is a 25k ohm load.
60/25000=2.4ma. of current. That should be the total load current on the bias supply as long as the output tubes are never driven to grid current (overload).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/13/17
I changed them out and the amp worked fine at the shop for almost an hour everything stable with nice audio. Brought the amp home hooked it up was stable for about 10 minutes then the kt90 bias current started to climb and i shut it down when the meter was at 2.0.
So i unhooked the amp and put my 2 chassis amp in place with the same kt90 power tubes. this amp is the same circuit with fixed bias. I hooked everything up and turned it on. Adjusted the bias to .90vdc or 90ma and everything is good. No bias drift the meter is showing no problems. The sound is very nice.
Regarding you picture. That is exactly how i have it with the grid resistor being 50k. The actual bias voltage before the pot is -51vdc. The bad amp is SEUL with Edcor outputs the 2 chassis amp is SE pentode with Electraprint outputs.
I dunno, the fact that it seems to work at the shop and not at home is really pissing me off. cheers.
Edits: 01/13/17
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