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Old 29th March 2009, 03:52 PM   #1
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Default blackfacing a twin

I've got a twin from around 1974 with master volume. I've been looking around to try to find schematics and discussions pertaining to the various mods that could turn it into a blackface-ish amp.
So far I haven't found anything concrete so I was wondering if anybody in here has any links to some relevant material ?
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Old 29th March 2009, 04:21 PM   #2
llwhtt is online now llwhtt  United States
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Find the schematics for both of the amps, the one you want to copy is the AB763. Most of the differences are centered around the phase inverter tube and possibly the bias circuit. Yours probably has non adjustable fixed bias with balance. The Blackface has the more common bias adjust. Leave the master volume full up and pushed in if it has that feature, and you should have a Blackface. Check out "Schematic Heaven" for the schematics. It's easy to do with just a few parts and tools.

Craig
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Old 29th March 2009, 09:12 PM   #3
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Quote:
Originally posted by llwhtt
Find the schematics for both of the amps, the one you want to copy is the AB763. Most of the differences are centered around the phase inverter tube and possibly the bias circuit. Yours probably has non adjustable fixed bias with balance. The Blackface has the more common bias adjust. Leave the master volume full up and pushed in if it has that feature, and you should have a Blackface. Check out "Schematic Heaven" for the schematics. It's easy to do with just a few parts and tools.

Craig
Thanks Craig. You're right about the amp being fixed bias. I was planning on taking out the master volume as I've heard some say that it does make a difference (if only slight though).
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Old 29th March 2009, 10:30 PM   #4
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Find all your info and schematics here:
http://www.thevintagesound.com/ffg/
Cheers,
Ian
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Old 30th March 2009, 12:38 AM   #5
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Blackfacing a Twin so far removed from the AB763 is not entirely possible without changing the trannies, but you can certainly make it better by going as far as you can without the tranny swap. CBS screwed up a lot of things and paid for it...literally as well as figuratively. Aside from the PI and bias sections previously mentioned, the lead dress was also messed up. Notice the wires coiled around others? That was to stop parasitic oscillations from poor lead dress. They ran wires over the board that should have been under...like Leo did. Also, the caps are garbage compared to the blue molded caps Leo used, so I always change them out to Orange Drops or Mallory 150s.
And DEFINITELY ditch the MV...and if it's a 135 watt model...youre better off ditching the whole amp IMO...
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Old 30th March 2009, 12:55 AM   #6
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What's interesting about this subject of "Blackfacing" is how many amps with the silver face cosmetics are actually identical to the black face versions. I have a Twin Reverb amp that I think is about a 1969, no master volume, JBL D speakers and AB763 tube chart, layout etc. The amp sounds great for just about any type of music.

I just did some work on a Super Reverb for a friend and his amp was about a 1973 with the master volume and the pull distortion. The master volume pot was broken so I removed it and did away with the switchable distortion. I set it up with a new set of output tubes(New Sensor Tung Sol 6L6GC) and it sounds nice. His amp didn't have the small capacitors in the phase inverter section that were added to many of the amps after the CBS change over. Question is were there different "levels" of silver faced? I've heard that some amps like the Deluxe Reverb were not changed in any way other than cosmetics is this true?
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Old 30th March 2009, 01:19 AM   #7
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There were no across the board changes, so to speak. Matter of fact the early CBS SF amps sold so poorly that some of the changes were reversed...lots of complaints. That's what you get when you let MIT grads design amps (CBS) instead of musicians (Leo). It seems like the Super Reverb was the earliest amp to be ruined...they beat up the Twin Reverb and the Princeton Reverb pretty badly too. The bias/balance thing was devestating to tone as was those supression caps...the tiny ceramic disc caps on the output tubes. Fortunately, a lot can be removed. I have simply snipped off the supression caps...sometimes it works, sometimes the amp oscillates. Then you just experiment until you find the smallest value that stops it. I use silvered mica caps in place of ceramic discs whenever I can. Ceramics don't belong in guitar amps IMO.
I have a 1969 Twin Reverb that I BF'd several years ago, and to be honest, I can't remember what caps were on the board. I do remember changing the PI...and it sounds pretty darn good. I also have a 1969 Vibrolux..it had the blue molded caps...same changes and it sounds PHENOMENAL !!!
I'd better close it up now...get me talkin' amps and I may never shut up!!!
Looking at the Deluxe Reverb, there were a lot of changes: The AB868, A1270, A1172 were CBS changes, and the awful pull boost from 77-79....
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Old 30th March 2009, 02:11 AM   #8
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lots of info here if you dig around:
http://tone-lizard.com/Table_Of_Contents.htm
__________________
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 30th March 2009, 02:34 AM   #9
PB2 is offline PB2  United States
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I believe that the black face was pure pentode mode output stage and the later ones were ultralinear.
Might try removing the ultralinear taps, and set it up like a black face.

Pete B.
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Old 30th March 2009, 03:04 AM   #10
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I guess the new guys after the takeover just overthought the circuit design. In my opinion a simple is better approach is what works in a guitar amp anyway. Leo Fender and others knew that but they mustn't have told that to the CBS management.

So were the later master volume, pull boost, ultra linear circuits and other things also part of the whole Silverface outlook or where those changes market driven?

Another point of interest, blackface does NOT mean pre CBS. There are a lot of blackface amps that were made after CBS took over. Do any of those amps have the changes, I always wondered about that one.

Pre CBS amps should say Fender Electric on the panel as does my 1964 Princeton Reverb which is a fine amp by the way.
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