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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 6:42 am
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That brings up a question: This will be my first time working on a circuit with components this old. Is CRC Electronic Parts spray safe for these pots, jacks, etc. or is there something in it that will harm or break them down? Same question about ISO alcohol, which I usually use to clean the residue off of new solder joints.

I'm hoping to pull the chassis tonight. I'll photo document each step.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 7:00 am
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CRC (or similar products) is perfectly safe for use, as is isopropyl alcohol. Avoid contact with the grille cloth though -- the '68 weave does contain organic fibers (the pale blue stripe) which might be discolored.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 4:54 pm
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Arjay and all, I pulled the chassis, taking pictures all along the way. I took about 30 pictures, put them in a Dropbox and am sharing the public link here. Let me know what you think.

You'll see, the original filter caps have been replaced by Spragues. This is the first time I'm looking at this vintage circuit, so I'll need time to identify things immediately noticeable to you all.

Thanks for your time.

https://www.dropbox.com/sh/4twdqgl55tb2c1f/AABbeoC5WemMv15nd8eNA90Ma?dl=0

Best,
Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 6:16 pm
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The amp looks like an AB763 circuit. Has that style wiring to the bias pot. Has the two 220k-ohm G2G resistors in characteristic V-pattern on board.

One thing I did notice ---- someone took out the 1.5k-ohm grid stopper resistors off the 6L6GC sockets. Look at the layout from PDF files and you'll see two sets of resistors on each socket. The 470 ohm, 1-watt and the 1.5k-ohm, 1/2-watt resistor in a crisscross fashion.

The 470-ohm ones are there. The 1.5k-ohm are not. Wonder if someone was sleeping on the assembly line, the time the amp was out together?

{edit} I see what they did. Fender soldered two 1.5k-ohm resistors across each push-pull pair of 6L6GC signal grids. This is not good. Each 6L6GC should have its own grid stopper resistor. And that resistor SHOULD be soldered onto the socket to minimize the resistor's lead length. This is all about maintaining amp's stability and keep it from parasitic oscillations. I'd prolly fix that.

It would requiring soldering a 1.5k-ohm resistor from pin 1 to 5 of each socket. And rewiring the input signal line to pin 1 and jumpering pin 1 to pin 1 of each push-pull pair of 6L6GC tubes. I'll try to find photos, if you decide to do this. :)

http://ampwares.com/schematics/twin_reverb_ab763.pdf

Thanks! For the photos. Looks all original. I'll look at it later... gotta get ready for work, now.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 6:43 pm
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Built in the 33rd week of 1968. No way did it leave Fullerton as an AB763 circuit. And Steve's right about those 1.5KΩ grid stoppers -- that's one of the hallmarks of the AC568 revision. Apparently a previous owner tried to blackface the bias-supply circuit but neglected to backdate the grid-stopper arrangement. And some nefarious thief light-fingered all of the snowman knobs. The casters are not original to the cabinet and it appears that the speakers are later replacements.

With a bit of work and a handful of the correct parts it should be relatively easy to accurately blackface this chassis -- all the major components are there, they just need a slight reconfiguration.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 7:32 pm
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Great, thanks guys. So I have a decision to make, and wouldn't mind hearing your opinions. I have an option to pick up the amp or pass on it. If it was a true BF, I would jump at it and use to learn more about this circuit.

If I decide to keep it and want to finish converting it to a BF, I'm assuming I can just work my way through the layout diagram, schematic, and whatever pics I find online of the AB763.

My fear is that even when I'm all done I would still grab my '65 BFDR Reissue when I head out to a gig. Much lighter than my former 135w master volume SFTR, but still a load.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 7:36 pm
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I forgot to ask, what makes you think the Oxford speakers were later replacements? Did you spot something structural or is it the vintage?

Thanks,
Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 7:58 pm
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What do the speaker ID labels say?

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 4:21 am
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So, the label says on the first line 105379, second line 465 048, the third line is 12 T610B. I only know how to interpret that third line signifying that it's an Oxford. Do the first two lines help identify whether or not it was the original or a replacement?


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 7:18 am
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The "465 048" indicates that the speaker is an Oxford made in the 48th week of 1970.

The "105379" is probably a customer ID number that Oxford assigned to Fender.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 8:50 am
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Of course, the 0 would be 1970, since it couldn't be 1960! Sorry for the rookie question. Thanks.

While we're on the topic of speakers, if I complete the Blackface mod on this amp and want use it as a clean jazz amp with both my ES-335 and Tele (Yes, I love jazz on a Tele), what speakers would you load it with?


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 8:56 am
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If it were mine, no question......a vintage pair of JBL D120F's.

For a similar sound (rich, articulate, crisp high's, balanced low's, and ample power-handling capability) for the economy-minded, Weber's California Special would do in a pinch.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 9:12 am
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Great suggestions, thanks.

When I first read your recommendation of the JBL, I started to laugh because with my last Twin (a '79 135 watt version), a friend of mine gave me a pair of E120s he had laying around just to try out. It made the amp so heavy I simply couldn't lift it any longer. I know the D120s are much lighter, but my back still tightened up just reading JBL and 120!


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 10:01 am
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Quite right. The E-series ceramic JBL is significantly heavier than the alnico D and K speakers though the latter are no lightweights by any means. My two JBL-equipped Twin Reverbs are bicep-busters and seldom see duty anymore. One's a blackface re-issue (with K120s, about 95 lbs) and the other is a '78 ultra-linear (factory D120F's, close to 120 lbs). That's why I mentioned Weber's California Series. The pair will only add about fifteen pounds over the stock OEM Oxfords.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 10:55 am
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That's right, I forgot you had an ultra-linear. You helped me out a couple of years ago when I found mine in the back of a pawn shop. It was just too heavy and when I got my BF Deluxe Reissue, I simply stopped using it outside my practice room. When I put it up for sale, I had a guy drive from two hours away and paid me probably more than it was worth because he loved that model so much. (He was a lap-steel player.)


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