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Fixing a Fender Twin Silverface Amp

Discussion in 'Amp Tech Center' started by karateinthenow, Jan 2, 2014.

  1. karateinthenow

    karateinthenow TDPRI Member

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    Hi folks,

    My name is Allen, and I'm new to this forum, although I have used it for many things. Lots of great info and helpful folks.

    I was going to post my thoughts and questions on the amp that I want to fix. I did a lot of research, however I need other peoples opinions about this as well. It can sort of get confusing, and frustrating when trying to get info on vintage amps.

    I bought this amp used about a year ago. It is a Fender Twin Reverb Silverface amp. This amp was gigged pretty heavily I believe. I got a great deal on it.

    This amp was working great for me for the longest time until recently. I turned the power onto the amp, at this point the standby switch to turn on the amplifier was turned off. As soon as I flicked the standby switch to on, then all I heard was a popping sound. So it turned out that the fuse had blown, which sort of surprised me, but this fuse could have been in there a while. So I finally tracked down and ordered online, some 3A 250V slow blow fuses to replace the fuse of the same rating that was in the amp previously. However, again it powered on fine, but when I turned the standby switch to on, there was a loud hum sound and a large glow of light out the back of the amp. The fuse itself didn’t blow, but I am pretty sure that one of the power tubes on the input is gone, because the glass on the side of it is black. I am not sure as to what caused this, but I am hoping to fix this amp back to operating condition.

    Something must have finally failed in it. It still has all the old caps in it, which will have to be replaced. Just trying to figure out what it was.

    I have tried to date the thing, and the closest that I can get is that it was made in 1967. This is based off of the transformer number and cross referencing it to the fender database. (there are three transformers I could only get a number off of two of them though EIA-606-634 and EIA-606-701)

    http://www.fender.com/en-CA/support/articles/amplifier-product-dating/

    I know that you can use the tube chart that is posted on the inside of the amp housing, but I couldn’t find anything on what these numbers mean.
    However I can also find the date from the serial number that is punched into the metal chasis (A627164). This dates suggests a date of 1973.

    http://www.superiormusic.com/page195.htm

    I am still in the process of figuring out what circuit this thing used. I am finding it a bit tricky, because I am still not 100% sure that this amp is dated correctly. Even though the number on this suggests 1967, it could possibly be later. This amp might be just on the border of production dates.

    The circuit diagrams that I have found do not show a master volume on them, even though this silver face does come with a master volume switch.

    I believe that I am dealing with either the AA270 or the AA769 circuit. Have to find the differences between the two, they seem pretty similar.

    Since I have everything apart, I am going to blackface it switching it over to use the AB763 circuit.
     

    Attached Files:

  2. karateinthenow

    karateinthenow TDPRI Member

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    Here are some more pictures
     

    Attached Files:

  3. Ed Storer

    Ed Storer Tele-Holic Silver Supporter

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    The discoloration on the 3rd power tube from the left (last Sylvania) make me think that it was redplating (that's very bad). It is likely that the tube is shot.

    It also looks like your power tubes are 3 old stock Sylvania tubes and one that I couldn't identify. Power tubes for a Twin should be purchased as a matched quad.

    If you haven't ever adjusted the bias, then you should have all this done by a tech. If not proceed.

    Pull the power tubes and get a new set. Sovteks do reasonably well in an amp that isn't likely to be run at full volume very often. I like TAD's. Others on the forum endorse Winged =C= as the best of the current production 6L6's. Your tech will probably have a set he'll sell you.

    Have the Sylvania's tested. You can probably get a few bucks for a good used matched pair.

    Once you've replaced the power tubes, switch on with the back cover off and watch. If the fuse blows or the tubes show more than the normal glow, power off and take it to a tech. Hopefully the problem is as simple as a resistor gone bad.
     
  4. trancedental

    trancedental Tele-Meister

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    Looks like a 72 onwards MV to me, parts & heater wiring are from that era? There's a adjustable pot near the pre amp section on the back, which is not original, probably some sort of gain mod! :rolleyes:

    Get it to a tech, I would ask him to keep those Sylvania tubes & try to match them up with others to make a new set, if not make sure you get them back!!! :D

    Get the amp working first before changing the circuit the '70's silverface amps still sound great & some people prefer them to the earlier blackface circuits :eek:
     
  5. Pup Tentacle

    Pup Tentacle Tele-Holic

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  6. trancedental

    trancedental Tele-Meister

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    Never seen inside one of those, nor any of the higher powered silverfaces mid to late '70's, they are quite rare over this side of the pond. :cool:
     
  7. Andy B

    Andy B Friend of Leo's Silver Supporter

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  8. blues bondsman

    blues bondsman Tele-Afflicted

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    Can you get a close up picture of the bias board ? it's the small board near to the power transformer with a few diodes, one cap and two resistors.

    The cap looks funny from the picture
     
  9. jimbo735

    jimbo735 Tele-Afflicted

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    I have the similar amp I would highly recommend having a trusted tech give it the look over,
    Its a great amp, has a lot of potential.
    Matched tubes is a plus to you.
    Fixed bias mod is a plus, your choice on that.
    I up dated all the electrolytic caps.Bonus!
    Cheers!
     
  10. mherrcat

    mherrcat Tele-Holic

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    Just curious if you ever got your amp fixed. Looks like a 70's master-volume Twin Reverb to me. Not a bad amp.

    That is a "hum balance" pot and it is original. Some people replace it with two resistors to ground (not sure the value; 100k?) on the heater circuit at the pilot lamp.

    The two power tubes on the left actually look like 6V6 tubes, what with the smaller size and the bottom getters. Seems like this in itself would cause some real problems.

    The power tube on the far right looks like a Sovtek 6L6; can just make out part of the name and type. Looks like "6L6GB" or maybe "6L6WGB."

    It's possible all this amp needs is four new matched 6L6GC power tubes and a re-biasing. The "Winged-C" tubes mentioned are good in this amp. I have a set in a 90's Vibrasonic; basically a TR with a 15" speaker.
     
  11. Ike286

    Ike286 Tele-Holic

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    The resistors some people use instead of hte the balance pot are two 100 ohm resistors not 100k, but 100. And they are standard in MANY amps. They are used basically as a virtual center tap for the heater winding, if the PT doesn't have a center tap there. If adjusted properly, the hum balance is supposed to work pretty well. Also, if your amp has the pull boost, I'd rid of it, and disconnect the master volume while you are in there. Otherwise, just blackface it. It should sound great! :)
     
  12. Commodore 64

    Commodore 64 Friend of Leo's

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    Might just need some new tubes. Winged C's are pretty expensive these days because most believe they aren't being made any more.

    I just messed about with a lot of tubes for one of my amps. I ended up with a pair of JJ 6l6GC. I tested Tung Sol (USA) 5881s, some Groove Tube 6l6 and a pair of 7581.

    To me, it looks like you have a Sylvania 5881, a Sovtek 6l6GB and two Sylvania 6l6GC in there. The 5881 looks to have given up the ghost, which seems logical to me since (I think) they would prefer to run at a lower plate dissipation than the 6l6s in there.

    In your position, I'd order a quad of JJ 6l6 and see if that was all it needed. If one of them fries, you aren't out a ton of dough.
     
  13. Ike286

    Ike286 Tele-Holic

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    I know that many times people recommend 5881's for the 135W twins due to 5881's being able to handle more plate voltage, now I'm not sure about that, but that's what's been said, so I'd say 5881's should be fine in that amp, being how its the regular 100W amp. And JJ's are a good choice, good all around, not too expensive. :) Winged C's I can say are really nice, I only have experience with their el34's but those are nice for sure.
     
  14. Commodore 64

    Commodore 64 Friend of Leo's

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    You may be right. I shouldn't spew unless I'm sure, I'm a journeyman. A 6l6GB and 5881 max out at about 23w. I think 6l6GC are about 30. That is something different than the plate voltage though, so you are probably right.
     
  15. Ike286

    Ike286 Tele-Holic

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    I'm not sure either, I just remember people saying that a lot. And I think it was basically that 5881's were military 6l6's basically, so they were built TOUGH. That way they take it better. Not sure, it's been a while, but if you look up tubes for 135W twins on google it should take you to several forums that will probably say that.
     
  16. Ike286

    Ike286 Tele-Holic

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    Then again, you say they max at 23W and 30W for the 6l6GC, but what about when run ultralinear.... We need some smarter people on here. LOL
     
  17. Wally

    Wally Telefied Ad Free Member

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    Hello, Allen....that could be made into a good amp. You don't tell us what your experience is. I agree with the observations that Jimbo made above in that this amp deserves and demands some experience. The schematic that Andy linked us to is the correct starting point for understanding what you have there. IF you have looked at the schematic and understand it, if you have worked on some amps, and if you understand the safety aspects of the work; then let's hear the questions and get that amp going! IF you don't have any experience, then imho it is time to start studying some books and schematics if you want to work on them. Meanwhile, you could get an experienced tech to get it going for you...and learn a bit simply by being an aware part of the equation......or ou could put the amp in a closet and gain some knowledge and experience until you are ready to tackle that rather complex amp.

    I'm not trying to butt in with unwanted advice. I am just curious about the situation and sitting here digesting my lunch. Good luck with it! There are some really interesting things that can make that amp into a more versatile and functional beast, ime.
     
  18. firemedic

    firemedic Tele-Meister

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    The discoloration on the 3rd tube looks like a getter flash, if it's still shiny it should be a good tube, everything else being equal. You have what looks like 4 cool old stock power tubes, individually. Whatever they are.
    I would get 4 "matched" (meaning, kind of close in current draw) 6L6GCs; 5881's are equivalent to 6L6GB tubes and I personally would not use them in a high voltage amp such as a SFTR. As far as ideal tube brands, there is no shortage of strong opinions on these forums.
    And +1 on the sage advice, to proceed very carefully or hand over the job to someone who will.
     
  19. karateinthenow

    karateinthenow TDPRI Member

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    Thanks everyone

    I haven't been on here for a while...lol..., but I am going to take all your ideas and try to get this amp back up again. Thank you to everyone who posted.
     
  20. SamClemons

    SamClemons Poster Extraordinaire

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    From your description, I seriously doubt tubes are the problem. I would take it to a good amp technician. You stand the chance of blowing even more stuff if you keep messing with it. If nothing else, a decent technician will have some old tubes to try in there (substitute) and see if that is the problem. Caps, blown grid resistor, transformer problems, are more likely.
     
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