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Changing the BALANCE (hum) pot to BIAS pot.
Original resistor setup. Unbalanced 68K-ohm grid-to-ground resistor on 6L6GC's. 47k-ohm anode plate loading resistors on phase inverter. Two lines coming off balance pot.
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Balanced 220k-ohm grid-to-ground resistors. 82K-ohm and 100k-ohm anode tied resistors. One line off bias pot.
Follow Ups:
I do a lot of these. Just finished a complete resto-mod on a 71 Super Reverb. The customer was delighted with the resulting tone.
Edits: 01/21/15
Just sub in 12AX7s here, and rescale the voltages and this looks very familiar, doesn't it?
I'm actually building a JFET preamp version of a showman input stage dubbed the "FET2B" (ala Alembic's tubes F2B preamp)- a simple pair of J201s; first stage to get some gain (and act as an impedance match/buffer), a traditional Fender-style tone stack (with the obligatory bass shift added and presence boost) and the final J201 for an output. Not meant to drive a long length of cable or amps with lowish input Impedances, but enough for the tone I'm after in a studio setting for my bass guitar.
Dman
Analog Junkie
Be interested in how it turns out.There is something still very wrong with the Showman Reverb. Voltages are all ok (main PSU, bias circuit, screen grids --- with no tubes in). PT, OPT, choke, sockets, electrolytic caps, diodes, and coupling caps from PI to 6L6GC's have all been changed.
All is cool, with power on, in standby. Only the four 6L6GC inserted. No other tubes. When you hit the standby switch --- one tube immediately goes ballistic. Big bright blue spark inside the tube. Speaker sounds like high voltage buzz. Doesn't matter which 6L6GC I use in that socket. Same results.
I pulled the 6L6GC form the bad socket. The other 6L6GC's idle bias currents never settle down. 40mA then 50mA, then 60mA, still climbing, as I shut the amp down.
Seems like some drastic parasitic oscillations. Guess that I may need to add snubber caps and other "fixes" to the amp.
Edits: 01/21/15 01/21/15 01/21/15
That way I just have to bring a pedal-board along to the studio rather than my whole rig...
Dman
Analog Junkie
With the 6L6 tubes out, do you measure the negative bias voltage on pin 5 of each of the sockets? The grid stopper could have failed open. It really sounds like a bias problem. Alternatively, it could be the tube itself.
A wild guess here, as for some reason, my work compy won't let me see your photos (the Internal Internet Gestapo must be at it again)-
Worse case could be that the tube sockets themselves have pins shorted due to arcing causing... Had that problem on an old Traynor (Now Yorkville) YBA-1. The power tubes would light up, and I'd get sound for the first 15-20 seconds, then an arc flash (both in the tube AND at the socket pins), and the mains fuse would go bye-bye. Checked over everything I could think of- finally asked my Tube Guru/Friend, and he suggested replacing the sockets. I did, and it worked. Never had a problem after that. Sold it later with a clean bill of health to a friend (who's still rockin out with it!).
Dman
Analog Junkie
Here's the links, to the photos. Hope this works:
Bias pot and AB763 (blackface) wiring:
http://i1191.photobucket.com/albums/z461/BMW2002Ti/AB763biasandPIloadingresistors_zps56fa5912.jpg
Screen and grid resistors:
http://i1191.photobucket.com/albums/z461/BMW2002Ti/DSC00206_zps22b3508d.jpg
Dentist (blah) appointment this afternoon, so my speech should be more "flubbery" than my writing. I'll take a look this evening...
Dman
Analog Junkie
Cool. Thanks!
Bias supply is steady -52VDC. Tapping circuit board and wires = stays at -52VDC. Varies from -50 to -51VDC on each socket's pin 5. No tubes inserted.Grid stopper, screen resistors, and all sockets have been replaced twice. I am using Mills ww resistors, now. Voltage on the screen is steady 465VDC. Plate voltage = 472VDC. I used a 1k-ohm, 5-watt screen resistor.
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Edits: 01/21/15 01/21/15
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