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Post subject: Showman Reverb
Posted: Tue Jan 06, 2015 3:57 am
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Going to attempt to fix Jerry's 1968 drip-edge Showman Reverb, for the third time. Now, the symptom is a runaway V8. Replacing this 6L6GC does not change the outcome. Still thermal runaway.

I believe this is an AC568 circuit. Hard to know for sure, as I'm having a hard time finding this schematic.

Going to replace the OPT. And prolly go to an adjustable BIAS control. As there are no Showman Reverbs that came OEM with a BIAS pot, I presume reverting to the blackface Twin Reverb AB763 style bias circuit will work. The BALANCE pot adjusts V8 and V7. V9 and V10 bias are not adjustable.

Has anyone ever done this mod to a 1968 Showman Reverb?


I'll start this project after I get back from the mountains. Clearing my brain of lead solder fumes, for the fix.

I believe that much of these "hidden" problems was a consequence of the previous owner running a 20-amp fuse. As I fix one part, another part's borderline operation comes to bear. So far: new PT, new PSU diodes, new PSU and bias caps. New rail resistors, new heater line wiring, new choke, new standby switch, new screen and grid stopper resistors. New heater line resistors.


Thanks!


Showman Reverb AA768. NB: Jerry's amp is labeled TFL5000D. But, lacks the 5mfd/25VDC non-polar caps hanging on the cathodes of the 6L6GC's. Leads me to believe he has the earlier AC568 circuit. If anyone has the link to this schematic, I would appreciate it. :mrgreen: :D :mrgreen:

http://bmamps.com/Schematics/fender/sho ... _schem.pdf



Twin Reverb AB763:

http://thevintagesound.com/ffg/schem/tw ... _schem.gif


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Post subject: Re: Showman Reverb
Posted: Tue Jan 06, 2015 3:49 pm
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I found a couple of AC568 circuits:

Image

http://www.jmallard.com/www_images/Dual-Showman-AC568-Layout.pdf

http://www.jmallard.com/www_images/Dual-Showman-AC568-Schematic.pdf

These don't seem like the correct schematics because there is no Reverb and not enough tubes.

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Post subject: Re: Showman Reverb
Posted: Tue Jan 06, 2015 3:52 pm
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I always thought the circuit for my amp was an AA768, also identified by Arjay back in my original showman repair post, "Your amp was apparently built in the 51st week of 1968, which would make it the AA768 circuit. The amp has the 2000pfd snubber caps tying the power-tube grids to ground, which is a hallmark of this revision."

http://www.jmallard.com/www_images/Showman-Reverb-AA768-Schematic.pdf

http://www.jmallard.com/www_images/Showman-Reverb-AA768-Layout.pdf

Of course, I can't read schematics - but this is the only one that correctly refers to the amp's name - "Fender Showman Reverb", not a Dual Showman.

HTH

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: Showman Reverb
Posted: Tue Jan 06, 2015 9:52 pm
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Jerry,

Your AC568 circuit schematic posted is a no-reverb amp. Your amp had no 5mfd/25VDC bypass caps on the cathode of the 6L6GC's. Just the 150-ohm ,7-watt sandbox resistors. Since, someone posted this supposed "AC568" Showman Reverb circuit (on another Fender amp board), I thought that I would look at the schematic and compare it to your amp. Before modifying the BIAS supply. Sometimes, there are slight mods to the BIAS supply and PI sections within these same-year circuits.

Note how Ampware references this AC568 Showman Reverb? Though it has no specific schematic for it.

http://ampwares.com/amplifiers/fender-s ... an-reverb/


Since I can't find a copy of the AC568 --- I'll just go with the AA768. I was really hoping that someone had already reverted a SF Showman back to BF style BIAS adjustable circuit. As of now, I will follow the usual SF Twin Reverb to AB763 style BIAS control.

Anyhow, a more stable BIAS adjustable supply (to all 6L6GC's) and that new OPT "should" do the trick. Knock on wood. :lol:


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Post subject: Re: Showman Reverb
Posted: Wed Jan 07, 2015 12:10 am
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BMW2002Tii,

Thanks for all the info. I knew the AC568 schematic I posted was not the correct one because it doesn't have enough tubes and no reverb, but I thought I'd throw it up anyhow.

I have no doubt you will repair the amp and I am knocking on wood :mrgreen:

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: Showman Reverb
Posted: Wed Jan 07, 2015 1:21 am
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Well, it looks like this maybe the only mod of a Showman Reverb from BALANCE pot to BIAS pot bias circuit, posted on the Net.

;)


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Post subject: Re: Showman Reverb
Posted: Wed Jan 07, 2015 12:19 pm
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Kool!! I'm looking forward to the end result.

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: Showman Reverb
Posted: Fri Jan 09, 2015 11:30 pm
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Hey! Thanks, to Ian & Brad... old account is resurrected!


:mrgreen: :D :mrgreen:


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Post subject: Re: Showman Reverb
Posted: Fri Jan 09, 2015 11:39 pm
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Great! I knew it would eventually get fixed.

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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