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Post subject: 1968 SF Twin Reverb AB763
Posted: Tue Mar 21, 2017 4:52 pm
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Hello all, I just got my hands on an early 1968 SF Twin Reverb with the BF AB763. I believe I have correctly identified it as a transition model containing the blackface circuit with the silverface cosmetics. It has the AB763 sticker, vertical lines on the SF panel, etc. It works pretty well and sounds good. A little "mushy" but loud and clear.

This weekend I'll pull the chassis to see what's going on. Once I do, I'd like to post pictures and have you all who are much smarter than I take a look to see if you spot any mods. In the meantime, do you know of any written or web resources that discuss this amp - perhaps with pictures? I've found a few short articles and a couple of references to this model in my tube amp books, but I'm wondering if there is something I'm missing. Thanks in advance.

Best,
Skip B.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Mar 21, 2017 6:21 pm
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Is this a drip-edge Twin Reverb? The AB763 circuit has a bias pot and is obvious from photo of layout. It has two 220k-ohm grid-to-ground resistors in a vee-pattern, on the board. They attach to grid of 6L6GC tubes.

Later silverface circuits (AA769 and AA270) had balance pot and 68k-ohm grid-to-ground resistors, not placed in a vee-pattern on the circuit board.

Photos would definitely be nice! :)


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Mar 21, 2017 6:34 pm
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Steve, Steve......the OP's amp is a TWIN Reverb, not a DR! You remember, don't you? Two twelves, four 6L6's, 85 watt output.

To determine your amp's pedigree you'll need to remove the chassis and look for a build date stamped inside, adjacent to the power tranny.

The Twin Reverb's AB763 circuit was superceded by the AC568 revision in May of 1968 so if your amp was built no later than the 17th week of that year it is indeed an AB763. Ignore the circuit designation on the tube chart because FMI/CBS never bothered to update them as the new circuits were developed -- they just kept using the blackface charts.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Mar 21, 2017 7:09 pm
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Arjay, I started to answer DR, but correct the post to TR.

I forgot the AC568 circuit. First of the TR to have balance pot and 68k-ohm grid-to-ground resistors.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Mar 21, 2017 7:41 pm
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Thanks for the replies. As I said, it has the AB763 sticker, vertical lines on the silver face, aluminum edge around the grill clothe, etc. It has a bias pot on the bottom of the chassis (not a hum balance). The serial number puts it in 1968 and all of the transformer and choke date codes are late-1967. Again, I'm pretty certain it's one of the SF ASB763 from pre-May 1968.

Once I pull the chassis later this week, I would like to post some pics to see if you guys notice any mods. This will be my first time poking around the AB763, so I won't readily notice components that have swapped out....but you eagle-eyes will!

From external inspection, the tubes are in good shape. (They're fairly new Fender Groove Tubes.) The preamp tubes are a mixed bag, but none of them look original. The speakers are the original Oxfords 12T6s. The one thing I noticed is somewhere along the line a new three prong power cord was added. I'll post some pics to this thread ASAP.

Thanks again for the responses.

Best,
Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 12:25 am
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Seems like you do have a AB763 circuit TR. Looking forward to seeing photos of the circuit. For some reason, I've never restored any later SF TR amps back to AB763. Have done several Showmans.

I hope the previous owner did not modify the front end with any those "hot-rod" mods that were so popular in the 1980-90s.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 1:52 am
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I'll reserve a definitive circuit assessment until I see the chassis pics. Be sure to pull the doghouse off and check out the filter-cap array. If they're the original Mallory's, plan on replacing them with F&T's of the appropriate values. Dittos with the electrolytics in the bias supply and all cathode followers. Examine the 470Ω screen grid resistors on each of the four power-tube sockets for evidence of thermal damage and replace as necessary (metal oxide types are preferred).

The best thing about the OEM Oxford 12T6's is how the amp's tone improves when you replace them. The Eminence Legend is a great upgrade as is the Kendrick Blackface. Expect a modest volume kick from these modern speakers as they're much more efficient than the vintage Oxford's.

The Twin Reverb is the epitome of loud/clean so plan on using pedals for any dirt/OD or serious distortion requirements.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 5:10 am
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Thanks to you both for your input. I've had several TRs over the years and am familiar with their qualities. My hunch is that no significant mods have been done. It could be that the amp was bought new from the folks I got it from and that it's been sitting in a closet most of its life.

Thanks for the tips on the caps. I have to say though, I disagree with you about the Oxford 12T6. As a jazz player I love them, especially in a SF TR, so they won't be going anywhere, provided they're in good shape. Remember, I'm a jazz player - clean headroom without being brittle is my goal.

I'll shoot some pictures soon. I teach college and midterm grades are due this week. Taking the amp apart is my reward for getting my grading done and grades posted!

Quick question: I'm more familiar with posting on The Gear Page. On this site, what's the best way to post pictures that are large enough for people to be able to see them clearly?

Thanks,
Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 5:46 am
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Most of us rely on Photobucket for posting images. Upload your pics there then use the site-provided "IMG" link assigned to each photo to insert them into the body of your post.

If jazz is your bag, then the Oxford's may suit you. That's about all they're good for.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 7:38 pm
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Arjay, I just posted a filter cap question about a 2002 Hot Rod Deluxe on the modern amp board. I didn't want to mess up this thread by changing the topic, but you've always given me good advice, so take a look at my question if you have a minute. Thanks. Skip B.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 8:03 pm
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Yeah, I saw it and responded. Wait for Steve (BMW2002ti) to weigh in -- he'll have some additional advice regarding the HRD amps as well.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 8:14 pm
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Great, thanks. I created a Photobucket account tonight, so I'll drop some pics of the '68 TR into it soon. I might try to pull the chassis tomorrow night.

Tonight, I cranked it up to play through it for a little while, and notice that when I lightly bang on the top of the amp it crackles pretty loudly and both channels cut in and out with nothing plugged in. Eventually, I can tap it and make it stay on. I'm sure that's just one of many problems I'll find.

Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Wed Mar 22, 2017 10:30 pm
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Two possible/probable causes:

One, solder joints that have fractured with age.

Two, carbon-comp resistors that have reached and likely exceeded their design service life.

Both of these phenomena are relatively common with amps of this age but with a careful inspection, you should be able to isolate the problem areas and effect the appropriate remedy(s).

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 12:21 am
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SkipBurz wrote:
Tonight, I cranked it up to play through it for a little while, and notice that when I lightly bang on the top of the amp it crackles pretty loudly and both channels cut in and out with nothing plugged in. Eventually, I can tap it and make it stay on. I'm sure that's just one of many problems I'll find.

Skip


You can try a simple thing. Apply some good contact cleaner/lubricant onto the tube pins and sockets. Also, the input and speaker jacks. Work it in, a little, to help remove any contact surface oxidation. I like Deoxit D100 for this job. You only need to use a little. Remove any excess.

Can't wait for those photos!

:)

https://www.amazon.com/Booster-Electric ... eoxit+d100


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Thu Mar 23, 2017 1:00 am
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SkipBurz wrote:
...and both channels cut in and out with nothing plugged in.


BMW2002Ti wrote:
You can try a simple thing. Apply some good contact cleaner/lubricant onto the tube pins and sockets. Also, the input and speaker jacks. Work it in, a little, to help remove any contact surface oxidation. I like Deoxit D100 for this job. You only need to use a little. Remove any excess.


A waste of time.

Arjay

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