Fender Twin Restoration help needed!

Discussion in 'Amps and Cabs' started by Rich M, Nov 19, 2005.


  1. Rich M

    Rich M Member

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    Hey, my brother-in-law dropped off his Fender twin today for me to get working. I'd like to surprise him and clean it up a bit. It is missing the back panels and last he remembered "didn't work". I'm a Marshall guy so I know squat about these, but to me it looks like a late 60's/ early 70's unit. It say CBS on the back panel, has three 12AX7 and two 12AT7, a fender logo with the double underscore and serial # B 106**. I have some questions/ favors to ask.

    1.) Can anyone send me a dimensional sketch of the back panel(s)? I have some 1/4" plywood I could cook some up with.
    2.) Can someone sell me some scrap tolex to cover them?

    3.) Are there mods I can do to improve the sound (blackface, mid boost, etc.) As I am quite ignorant on these things, I would be interested in how to make this amp more flexible without hacking it too bad.

    4.) Have people tried post-phase inverter master volumes on these? I really dig them in my Marshalls. Can they replace the standard MV?

    5.) One of the knobs is cracked, anybody want to sell me one without requiring me to "assume the position"?

    I'd appreciate any help!
     
  2. doctord02

    doctord02 Member

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    Just restore it as is _first_, unless you already know exactly what he wants out of an amp. Fender Twin guys generally dont want their amp to sound like a Marshall.

    If he wants a different sound, then have him sell the restored Twin and buy something else. Modding a 100 watt Silverface Twin is always a losing proposition, IMHO. From your description this sounds like a '72 hundred watt Silverface. That was when they added the master volume.

    Schematics are available at http://www.ampwares.com/ffg/
     
  3. Rich M

    Rich M Member

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    Thanks for the FB. I'm not really looking to make it sound like a Marshall, a cranked Fender isn't a bad thing :). I was just curious if there were things that could be done to make this amp achieve more tonal variation maybe with push pots, etc. Do "blackface" mods really kill the value? I'm still interested in folks 1st hand opinions on PPIMVs.

    Got the chassis out and tested the tubes - one weak 12AT7. Gonna start reforming the caps now - they look original w/ brown paper tube sleeves..
     
  4. Rich M

    Rich M Member

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    Got it on the bench working, but it sounds lik ass. What should the PT be biased at currentwise? Sounds better when I run it full bore into my Weber MASS, leading me to think the PPIMV might be a good thing.

    Can someone please provide me with back panel dimensions?!
     
  5. Wakarusa

    Wakarusa Member

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    These are from my 1971 Twin Reverb. Keep in mind that the dimensions are "finished", so you have to allow for the tolex thickness when you cut the panels.

    Lower panel is a plain 24.5" X 5" rectangle.

    Upper panel looks like this:

    [​IMG]
     
  6. Rich M

    Rich M Member

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    Wakarusa,
    Thanks so much! I can't wait to fix this up. Do you have a photo of the back of the amp to get the vertical positioning?

    I found one of the screen resistors is toast. I guess this would explain the poor sound. What would cause this? A faulty tube? If so, how can I figure out the bad one. I have an emissions tube tester only. I mixed the tubes up when I reformed caps. BTW, the resistors are 470 ohm. I've heard some say to up this value. What is the ramifications of this? Does it affect sound? power output?
     
  7. Wakarusa

    Wakarusa Member

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    Positioning is trivial. It goes all the way to the top (the 1.5" horizontal bits should butt up against the top of the cabinet).

    Sometimes screen resistors just get old (particularly when parked over a hot tube socket). They can also indicate an internal fault in the power tube. Your emission tester should be fine for this kind of fault (particularly if it tests for internal shorts).

    The primary purpose of the screen grid resistor is to limit screen grid current. In a fault condition it tends to act like a fuse :cool:. A 470 works fine in your twin - I recommend a 3W flameproof metal oxide type. Some folks sub a 1K, but it can kill off some of the high end shimmer. About the 3W rating: there isn't actually 3W worth of current/voltage here, but you have to derate component to account for the high temperatures. In a nutshell, the higher wattage resistor lasts a lot longer.
     
  8. Rich M

    Rich M Member

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    Thanks again. My tester has a shorts test, so I will go through them. After looking at the burn marks on filament wire and chassis shield plate in box, I can see why a flame proof would be nice:eek:
     
  9. Rich M

    Rich M Member

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    Todd,
    I assume in that drawing, the overall length is supposed to be 24.5"?

    Anybody got any scrap tolex they want to sell me to cover these?

    One last question, can you run these amps with only two output tubes to get 50W?
     
  10. Wakarusa

    Wakarusa Member

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    Yes on overall length. I did some poking thru the spare parts pile here and have a spare twin reverb upper back panel. Shoot me an e-mail or call if you want it.

    I'm not a fan of pulling a pair of the outputs in a twin.
     
  11. AL30

    AL30 Member

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    My current amp is a '72 Quad Reverb - if you don't know, these are twin reverbs in a 4 x 12" package (yes it's a combo - WHAT were they thinking??!!).

    Anyway, when I got it (aside from looking like it was run over by an elephant) it was stock. It was in need of some maintenance though. So here's what I did. First off I didn't want to do any mods to the circuit. No point in changing anything if I don't know what it sounds like to begin with.

    1) Cap job - filters and all electrolitics. Tightened things up quite a bit.

    2) Changed all the plate resistors. I had some snap/crackle and pops even after the cap job - these were the culprits.

    3) Changed hum balance pot back to a bias pot.

    4) Swapped out the speakers - Jensen re-issues. ALL of the speakers had holes in them so there was no getting around this issue. Bass response is very much improved.

    5) After a year of playing it Silverface I was sure I hated the ice-pick tone (some people like it) so I changed it to Blackface circuitry. The cleans are much fuller sounding to my ears. I also disconnected the Tremolo - adds a little gain - not much it's a Twin.

    The MV (no pull-boost) is still there, there were no supressor caps on the tubes. The basic maintenance (along with some fresh tubes) made a HUGE difference in the overall sound.

    AL
     
  12. Shea

    Shea Member

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    Was the MV was modded to get rid of the pull boost? I don't think they made a Quad Reverb without the pull boost.

    I like how Fenders sound dirty, so I like the results of a blackface job and changing the master volume to a post-pi one. The pre-pi master volume is completely useless in the stock circuit, IMO.

    It all depends on whether the long-range plan is to keep it or sell it. I wouldn't change anything if the brother-in-law plans to sell it eventually. And I also wouldn't change anything unless I first consulted with him about it and got his permission.

    Shea
     
  13. AL30

    AL30 Member

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    Nope, it's stock without the pull-boost. It's identical to a '72 twin circuit. The pull-boost didn't come into the circuitry until somewhere around 1976 - to the best of my knowledge.

    I always here people knocking the MV circuit. I haven't really had any problems with mine ( I do not have a non MV to compare it to though) but I'm not a "crank the pre-amp" player either. The MV is always set louder than the pre. Maybe that helps. Then again, maybe my ears are mush after years of 100 Watt Marshall abuse.
    :D

    AL
     

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