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Post subject: Showman Reverb reversion to BIAS pot
Posted: Tue Jan 20, 2015 7:27 pm
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OK. Gonna redo Jerry's amp from BALANCE pot to BIAS pot. Since there doesn't seem to be a recipe for the Showman Reverb, will follow conversion of the SF Twin Reverb to BF Twin Reverb.

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Resistors and balance pot wiring of AA768 circuit. Two 0.1mfd phase inverter coupling caps pulled out of the way for clarity.

Image


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Bias pot replaced and re-wired to AB763 TR specs. See above photo for old wiring (Balance pot).


Image

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AB763 TR wiring of resistors. Balanced 220k-ohm grid-to-ground replace imbalanced 68k-ohms. 100k and 82K replace the 47k-ohms plate loading resistor on the phase inverter.


Image


Will test voltages and move onto suspect output tranny.


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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Tue Jan 20, 2015 8:14 pm
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Aspiring Musician
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Looking good! :D


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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Wed Jan 21, 2015 7:46 am
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There is something still very wrong with the Showman Reverb. Voltages are all ok (main PSU, bias circuit, screen grids --- with no tubes in). The PT, OPT (output tranny), choke, sockets, all electrolytic caps, diodes, and coupling caps from PI to 6L6GC's have all been changed.

All is cool, with power on, in standby and only the four 6L6GC inserted. No other tubes. When you hit the standby switch --- one 6L6GC (V9) 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 from V9 and left it empty. Hit the standby and 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.

Has anyone here worked on a SF era TR or Showman and had to add the snubber caps, cathode-tied resistors, and bypass caps paralleling those resistor back to the amp to make it stable? The amp seemed to have an AA768 circuit, originally.

I had no phase inverter tube inserted --- so I don't think the issue is out-of-phase global feedback.

This amp did work fine, after I'd replaced everything but the OPT. Then, the amp went unstable and blew a fuse (proper amps) ---- about 10 minutes into a session. It has blown fuses and had ballistic 6L6GC issues ever since the last blown fuse.


Thanks, for any input.


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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Fri Jan 23, 2015 5:08 pm
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Aspiring Musician
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Steve,
I have a early 70's bandmaster reverb TFL5000D and also did the balance to bias pot conversion. I remember my amp having the ceramic caps on the output tubes tied to ground and removing them. I haven't had any issues with the amp but I remember someone saying they were there to keep the output tubes from running away. You may be on the right track with installing them.
Good luck and please keep us updated. This is valuable information.

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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Fri Jan 23, 2015 7:18 pm
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Here's the YouTube link that Jerry sent to me. You can see this is NOT a subtle thing. And subbing tubes into the same socket = same result.


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https://www.youtube.com/watch?v=zF38odd ... e=youtu.be


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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Sun Jan 25, 2015 9:46 pm
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It appears to be only happening to that one position, indicating something wrong with that spot only. It must be something in the wiring of that particular circuit, maybe even the socket itself. :idea:

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Post subject: Re: Showman Reverb reversion to BIAS pot
Posted: Mon Jan 26, 2015 12:25 am
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Thanks, for the reply. I have replaced all sockets twice and the screen and grid stopper resistors twice.

The other three 6L6GC sockets do not exhibit than kind of pyrotechnics, but their bias current is unstable. Hit the standby and the readings start at 20mA, then 30mA, then 40mA... 60mA.... 70mA... still rising... I usually hit the switch at 50-60mA. Takes about 15-20 seconds the mA to rise pass 60mA. Plate voltage on the anodes is around 475VDC.

Gonna try adding the cathode-tied resistor and snubber cap back to the sockets. Bypass caps on those cathode-tied wire-wound sandbox resistors, too. See what happens.


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