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Post subject: Fender Twin Reverb Rebuild (lots of Pics)
Posted: Wed Mar 31, 2010 8:59 pm
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Hi, Ive started to restore this Fender Twin Reverb and need to determine which circuit diagram I should use to check the circuit.

Serial Number: A977419 (Engraved on the metal back has a serial number sticker inside but missing the last digit as it torn)

Image

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Thanks for any help

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Last edited by Stephen_James on Wed Apr 07, 2010 2:07 am, edited 1 time in total.

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Posted: Wed Mar 31, 2010 9:35 pm
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Hi there Steve, I'm going to guess it's a '76 or so, but you'll have to date the transformers and pots to get a better idea. Schematic would be the 100watt master volume. From the photos, I see that you have the second generation Silverface amp. Those blue caps were an upgrade and should be very good, if they test out within specs. The serial number can't be used for dating these amps, as far as I know. Art

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Posted: Thu Apr 01, 2010 12:47 am
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Thanks Art for replying, I should have included back plate picture

Image

from what iv'e been told as it was made not long after CBS took over. Dad had already figured out which diagram it was. CBS was written extremely small on the 135Watts RMS diagram hence my confusion as the one with CBS written in big writing was 100watts.

Both the transformers (Audio and Power) missing. The condition it was in is as in the first picture + the cabinet which is in good condition i will post a picture soon.

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Posted: Thu Apr 01, 2010 12:57 am
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Forgot to ask how do you date them from the pots? (speed pot was missing)

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Posted: Thu Apr 01, 2010 1:05 am
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This is a late '77 or '78 135-watt "ultra-linear" Twin Reverb chassis. The schematic may be found here...

http://www.schematicheaven.com/fenderam ... _schem.pdf

You may find OEM replacement transformers for it on Ebay but be prepared -- they are EXPENSIVE. Equivalents from Mercury Magnetics or Heyboer will probably not be any cheaper (though they are routinely available as production items). A word of caution....no Fender amp ever pushed 6L6 power tubes as hard as this chassis, with plate voltages approaching 500 volts. Do not scrimp on output tubes if you expect them to survive for long. I've owned one of these '78 Twin Reverbs since the day it was new and the hardiest 6L6s I've found are the JAN blackplate RCAs. They sound great and correctly-biased, they'll last.

HTH

Arjay


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Posted: Thu Apr 01, 2010 2:42 pm
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Thanks,

I'm probably going to take the audio transformer out of a music man (that got converted to 6L6's) and will get converted that back to EL34's it checks out on the specs has been run at 120watts and is within specs (i think its a fender one if i remember correctly).


the chassis was also missing the 500Volt capacitors so far I have 450 Volt ones or do you think they won't be enough? (had trubble finding 500Volt ones :( ) . Also probably going to put a choke in as well.

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Posted: Thu Apr 01, 2010 4:40 pm
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Ah yes, the mighty ultra linear. Steve, I get a lot of components from Antique Electronics Supply(www.tubesandmore.com). I'm leaning towards the F&T caps right now, Sprague TVA's seem to have had some QC problems lately. The 450's would probably be okay, but don't give you much of a safety margin. Maybe Arjay, can advise you on that, since he owns one those amps. Art

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Posted: Thu Apr 01, 2010 10:49 pm
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I didn't see anything rated above 450 VDC at the AES site either. Mouser may have something...

http://www.mouser.com/

The 450's will probably work but I suspect their service life may be shortened, especially if the amp is driven hard for sustained periods. The only other tube Fender chassis I've ever seen pushed to its maximum component design limits like the UL Twin Reverb was the 400PS.

HTH

Arjay


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Posted: Mon Apr 05, 2010 9:09 pm
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Thanks, Mouser do have capacitors 500V capacitors good to know theres still places where you can get them. Came accross 2x 100uF @ 500V capacitors when stripping down a ampeg amplifier for its Transformers (Audio is multi tapped and can run 6L6's).

My progress so far
Power Transformer mounted and connected to mains, tube heater line connected to transformer and tested with valves.

Connected Power transformer to bridge (however was blowing fuses a dead diode was the cause have replacements ready to go in)

re-Made Vibrato input's,
connected Line out and speaker out and external to main circuit

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Posted: Mon Apr 05, 2010 9:24 pm
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That's swell, Stephen! Glad you found some suitable caps.

Post more pics as this "rescue" progresses. That amp deserves to "roar" once more.

Arjay


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Posted: Wed Apr 07, 2010 1:18 am
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Vibrato Inputs (now connected to V2 via shielded cable

Image

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Fender Blacktop Telecaster '10 (Black Maple)
Fender Bassman 100 (Rola 12UX)
Fender Twin Reverb '77 Ultralinear


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Posted: Wed Apr 07, 2010 1:51 am
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Bridge circuit rebuilt, got 458V across one capacitor. Glad i went with 500V capacitors :).

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Fender Blacktop Telecaster '10 (Black Maple)
Fender Bassman 100 (Rola 12UX)
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Posted: Wed Apr 07, 2010 1:57 am
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Choke, Black insulation will go on the choke cables.

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Fender Blacktop Telecaster '10 (Black Maple)
Fender Bassman 100 (Rola 12UX)
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Posted: Wed Apr 07, 2010 2:06 am
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the 16uF capacitors (gray ones) where tested at about 19uF i laughed.

Image

Just need to mount & connect up the audio transformer
build a -52V circuit
then testing can begin :)

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Posted: Wed Apr 07, 2010 6:55 pm
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Yeah Steve, looks like you're making good progress. I was curious about the Elna electrolytics, they're from Mouser, right? Just wondering, if you know the country of origin? Art

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