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Post subject: Re: Showman Reverb revisited
Posted: Tue Nov 25, 2014 7:25 pm
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Rock Star
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Ok, fixed. The 22mfd/500VDC filter cap that supplies the phase inverter failed. Interesting, as this is the second F&T 22mfd/500VDC which has failed on me in the last two years. A new one failed on a DR.

I replaced the last two 22mfd caps with Sprague 20mfd/500VDC ones. All other caps were replaced one-by-one to rule out failures. Starting with the first 80mfd/450VDC cap.


No more hum. Thank goodness! :yeah:

-----

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Post subject: Re: Showman Reverb revisited
Posted: Wed Nov 26, 2014 1:26 am
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Aspiring Musician
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I knew you would find the problem in short order. You work is truly amazing. You are a magician with a soldering iron :mrgreen:

I am very grateful.

Jerry

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Post subject: Re: Showman Reverb revisited
Posted: Wed Nov 26, 2014 2:19 am
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Rock Star
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Jerry, you are welcome.

These things can be infinitely frustrating (esp hum) when nothing works. Then, the most rewarding, when it is finally fixed.

I stepped the idle bias current down. This amp has hum control pot. So, you need to play with the set line-to-ground resistor to get the bias voltage into the correct ballpark. The original 15k-ohm resistor soldered to the balance-hum pot was changed to 27k-ohm. This got the idle bias ranging from 35-40mA per 6L6GC. The tubes definitely aged at different rates --- from their original "matched" values.

You need to move the tubes around to get the best matched pairing. Then, adjust the pot for lowest hum. Hum changes as the amp warms up. So, I adjusted the pot after about 5 minutes. Then, again at 20 minutes.

As the tubes age, you can re-adjust the pot to get the lowest hum.

Hopefully, the amp will be more reliable this time. But, thank goodness for correct 2-1/2 amp slo-blow fuses! :D


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Post subject: Re: Showman Reverb revisited
Posted: Wed Nov 26, 2014 11:07 am
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Aspiring Musician
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BMW2002Ti wrote:
... As the tubes age, you can re-adjust the pot to get the lowest hum. ...


Hum Control Pot. Is this the pot you're talking about ? (circled in red in the photo).

Image

Thanks very much for you expert work BMW2002Ti. Your attention to detail and knowledge of these old amps is unsurpassed!

Again, Thanks very much!

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 revisited
Posted: Wed Nov 26, 2014 12:30 pm
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Rock Star
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Yes, it's the hum pot.

:)


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Post subject: Re: Showman Reverb revisited
Posted: Wed Nov 26, 2014 1:32 pm
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Rock Star
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BMW2002Ti wrote:
Ok, fixed. The 22mfd/500VDC filter cap that supplies the phase inverter failed. Interesting, as this is the second F&T 22mfd/500VDC which has failed on me in the last two years. A new one failed on a DR.



Wow Beemer...you are tuff...two whole caps in two years? Thems some high standards!
:mrgreen:
T2

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Post subject: Re: Showman Reverb revisited
Posted: Wed Nov 26, 2014 4:50 pm
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Rock Star
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LOL! Two fail new caps out of many, many changed. This is the second time I have had F&T 22mfd/500VDC cap fail, prematurely.

I'm wondering if the voltage rating needs to be a bit higher in some of these amps. That push near 475VDC, and God knows what during transients.

I'm going to try those $$$ Sprague ATOM 20mfd/600VDC in the critical position(s), next time. See if that helps.


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