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Post subject: 1968 Fender Super Reverb trying to figure out rectifier mod
Posted: Fri Oct 04, 2013 4:18 pm
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I was given a 1968 super reverb from a Guitar Center in California, it was shipped to Boston, MA for my birthday. Upon receiving it, I opened it up and verified that it is a "blackface" ab763 cloth wiring and all, I also discovered that the rectifier tube had been removed and bypassed, and one of the transformers has been replaced. The reason for this post is that there is a "hum" for lack of a better word from the amp, and I would like to find out the best way to diagnose and repair it. I would assume that putting the rectifier tube back in, would be the best place to start, but am unsure. This is my first tube amp, and I would love any tips and tricks to getting the most out of it.

The link is pictures of the set up and internal components as they currently are. I can take more if there are any questions, or anything that needs to be closer up.

http://s1114.photobucket.com/user/ejtuthill/slideshow/1968%20fender%20super%20reverb

Edit: Flipping the ground switch, can reduce but not eliminate the "hum" entirely.


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:29 pm
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wroche1, welcome to the Forum!

The OEM GZ34/5AR4 rectifier has been replaced by a solid-state, four diode full-wave bridge. The wiring & soldering doesn't look too clean, so some of the hum maybe from the mod. But more likely, the power supply filter caps need replacing. Didn't see a photo of them, under the doghouse.

BTW... is there a wya to stop the slide show? Really, couldn't concentrate on the wiring or components. Thanks! :mrgreen:


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:38 pm
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BMW2002Ti wrote:
BTW... is there a wya to stop the slide show? Really, couldn't concentrate on the wiring or components. Thanks!


Use the "pause" button, Steve.

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:42 pm
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Arjay, thanks!

Now, who puts the "pause" button on the lower left of the screen? :lol:


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:44 pm
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For the lunacy of others I am blameless.

:mrgreen:

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:50 pm
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Arjay, that PT looks like it's been re-wound. Lead wires are not the OEM colors of a Schumacher. By, the looks of the lacquer-based sealant coming out of the port-hole --- this tranny may have been cooked.

wroche1, do you have a receipt for any work done to the amp? Like tranny work?


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:56 pm
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BMW2002Ti wrote:
Arjay, that PT looks like it's been re-wound. Lead wires are not the OEM colors of a Schumacher. By, the looks of the lacquer-based sealant coming out of the port-hole --- this tranny may have been cooked.


My conclusion as well.

I wonder who did the re-wind......?

Arjay

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 5:59 pm
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Actually just got the amp friday so the amps history is a mystery it was purchased from guitar center in sacremento and shipped to me. Mangaged to score this for less than any RI super reverb they where selling due to it looks a little ragged (score one for me lol). By ragged i mean the drip edge is rough and the chassis and the cloth have a dent in the middle from what i can only assume is carrying this beast speaker side to ones leg. I noticed the wire colors where off, that and the newer wiring lead me to believe it had been replaced at some point in its life. Also if anyone can chime in what would the best way to clean up inside the chassis.

And are these blackface circuit 68 SR's commonplace? From what i have read this amp with the ab763 and the cts alnicos are desireable ;)


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 7:28 pm
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wroche1, this is what I'd start with:

1.) Pull & open up the PT bell housing and look at the re-winding. If it looks like a work rewind --- you can keep it and test it later. If not, opt for a new or good used proper Schumacher. Or a nice Mercury Magnetic (or some such aftermarket PT).

2.) Revert the amp back to tube rectifier. Either GZ34/5AR4 or 5U4GB.

3.) Replace all electrolytic caps. In the power rail (under the doghouse) check the resistors, also. Actually, just replace them with proper value, 2-watt metal oxide ones. That includes the two 220k-ohm ones under the big caps.

4.) I'd also replace the electrolytic bypass caps in the gain stages.

5.) Check tubes. Replace weak ones.

6.) Ramp up amp. Take B+ measurements. Have speakers connected, zero volume, no device input.

7.) Now, if all is ok... progress forward until you have good tones.


Good luck! :mrgreen: :D :mrgreen:


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 7:49 pm
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I would add to the excellent advice given by BMW2002Ti, replace the carbon-comp screen-grid resistors with good quality flame-proof. The screen-grid resistors are the two big resistors on the output tube sockets (470 ohm).

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 7:53 pm
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Was this amp shipped from the Alta Arden GC in Sacramento?

Arjay

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 8:04 pm
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id need to check the shipping info but im sure it shipped from the Sacramento store. Lol interesting question tbh? Btw thanks for all the electrical info i will keep this bookmarked and take notes. I really meant that in the more dumbed down sense of cleaning up the large amounts of grime in there while not hurting anything. The information is tremendously useful. Other than fixing the rectifier and looking into the power issues i do plan on installing a 3 prong cord in the near future. Thanks again guys, the info is invaluable.


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 10:06 pm
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If it's the same amp I saw there back in July of this year (asking price, $850 IIRC), it was in pretty sad shape. Redeemable though, if you're knowledgeable and skilled in both vintage electronics and cosmetic restorations.

Arjay

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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Fri Oct 04, 2013 10:43 pm
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" Cleaning up the large amounts of grime in there while not hurting anything. "

Not hurting anything is key. An overall cleaning can lead to a large number of boo-boos that can drive you donkey-bonkers --- trying to find them.

Good luck! :mrgreen:


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Post subject: Re: 1968 Fender Super Reverb trying to figure out rectifier
Posted: Sat Oct 05, 2013 7:25 am
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BMW2002Ti wrote:
" Cleaning up the large amounts of grime in there while not hurting anything. "

Not hurting anything is key. An overall cleaning can lead to a large number of boo-boos that can drive you donkey-bonkers --- trying to find them.

Good luck! :mrgreen:


+1

Tread cautiously.

Arjay

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