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Silverface Fender Twin Reverb Low Volume before Distortion
Old 11th July 2006
  #31
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Hi
Probably a bad solder connection on the main circuit board. Some of these amps are not soldered well and the boards can flex. Bits of track and components with nearly 500 volts on them make fault finding fun. Circuits are available on the web.
If you do not feel 3000 percent confident then get someone else to sort it out. Shame Norwich is so far from Liverpool / Manchester.
Matt S
Old 13th July 2006
  #32
Gear Head
 
Marty Shields.'s Avatar
 

If the Master vol circuit has been removed,then its been fixed at a certain level, probubly with not much headroom! 12AT7 tubes have been fitted to the frontend to reduce the gain, and break up dirty at a certain level to sound tame like a jtm 45 rock amp! If you want the at7 tones, replace the MVC or change the resistor,and change the bias on the AT7's they will be less microphonic with the correct bias!
Marty Shields,
Martstone Amplifiers Australia.
Old 14th July 2006
  #33
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Marty, thanks for your post. I forgot to mention that I bought some GE 12AX7 NOS tubes that I'm currently using in the 7025 positions (instead of the 12AT7's that I had in there). Also as far as the Master Volume being "stuck" at a certain level, since I changed everything to match the blackface schematic (pre Master Volume), I didn't think that would be the issue. I guess I'll try some new power tubes next. Does anyone have any experience using the new Svetlana 6L6GC's? I can get a matched quad at Guitar center for $50. Are they really that much worse than the older Winged C's?

-Ben-
Old 5th November 2007
  #34
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Hey there Ben,
Did you ever find a solution to this issue? I have a similar occurance with a Silverface 135W Twin. Still has original layout. The Vibrato/Reverb channel gets distorted after about 10 minutes at low volumes and the Normal channel is ok, but doesn't sound as good as the Vibrato in general.
Thanks,
Ken
Old 5th November 2007
  #35
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hangman's Avatar
 

let me ask you this... does it just start breaking up really early? or is it also really quiet?
when you first turn it on, does it seem to function properly?

the distortion problem is ONLY on the vibrato/reverb channel?
Old 5th November 2007
  #36
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Like I said, after the amp has had a chance to warm up, it starts distorting when I play chords and gets gradually worse. Only on the vibrato channel. I've taken it in to 3 amp techs here in Chicago, spent good money with no fix (1 thought I was making it up!). I've also tried the following: different speakers/cab, different/removed tubes, different cables/guitars, all with no luck.
Sadly, the distortion coming out is NOT a good sound, and shouldn't be occurring at a 3 on channel vol and 4-5 on master with a 135W Fender Twin Reverb.
Old 5th November 2007
  #37
Gear Nut
 
SAnthony's Avatar
Check under the hood.......

Ben, If I am summarizing this all correctly. Your Twin was wired with speakers in series at 16 ohms and you have some other mods with 6L6s in the output stage. This is a 1977? What you might want to check is voltages on the 6L6s and values of the resistors in the output stage. These amps are often modified for different Output tubes and depending on who worked on it Someone may have just stuck 6L6s in it to comply with original schematic and not realized it could possibly have been modified for 6550s or EL34s..... This types of mods require resistor changes which alter the supply voltages to the tubes. Also you need to see if the output Transformer has been changed... Someone could have put a 16 ohm output transformer on it (or an 8 ohm trans) If the output transformer has ever been changed with a non stock transformer or suitable replacement this can cause it to distort at lower levels. Finally if all of these check out, You may just be incorrectly biased and you need to have the amp rebiased, especially if it was biased with a 16 ohm load..... Also have the speakers been changed? And if so can they handle 100+ watts RMS?
Hope this helps...
Take Care...
Shawn
Old 5th November 2007
  #38
Gear Nut
 
SAnthony's Avatar
135W distortion on vibrato

Ken, if you have someone nearby who is handy with a soldering iron, you might try recapping the Vibrato channel. In particular do a shotgun changout of all the electrolytic caps in the vibrato channel. If I remember correctly there are like 4 or 5- 25uf 25v caps in there change them... then with the amp unplugged check the sockets for the 12 AX7 and 12 AT 7s in the vibrato channel look for dirty or loose connections on the sockets when you insert the tube pins... sockets may need to be cleaned and a few contacts may need to be bent back into shape to insure a good connection... Hope this helps...
Shawn
Old 5th November 2007
  #39
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hangman's Avatar
 

Quote:
Originally Posted by kenbonner89 View Post
Like I said, after the amp has had a chance to warm up, it starts distorting when I play chords and gets gradually worse. Only on the vibrato channel. I've taken it in to 3 amp techs here in Chicago, spent good money with no fix (1 thought I was making it up!). I've also tried the following: different speakers/cab, different/removed tubes, different cables/guitars, all with no luck.
Sadly, the distortion coming out is NOT a good sound, and shouldn't be occurring at a 3 on channel vol and 4-5 on master with a 135W Fender Twin Reverb.

have the filter caps been replaced?
do you have a VOM?
Old 18th September 2012
  #40
Here for the gear
Hi, just for people like me trying to find some info for their not working silverface twin reverb: I had the same problem as described above when I tried to fix a twin reverb for a friend of mine, barely any sound, tubes were OK, and it didn't sound like the speakers were damaged. Turned out that the right speaker output caused the problem, switched the speaker jack to the left output, and the problem was solved! The syphtomes were exactly like described above, so I hope this helps other people in the future.
Old 18th September 2012
  #41
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Quote:
Originally Posted by ampguy-uk View Post
Dunno if I should start a new thread for this, but has any body had problems with a Fender "twin amp" (1990's) changing channel by itself, but only when being played?
Have the same issue here with mine.

Sent from my SCH-I510
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