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Fender Twin Reverb (1971) Reverb noise/Power tube recommedations?
Old 25th January 2007
  #1
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Fender Twin Reverb (1971) Reverb noise/Power tube recommedations?

Hi,

1.) I'm having a problem with the reverb on my 71 Fender Twin Reverb.
When it's on I hear a lot of noise, it almost sounds like there's an electrical storm going on in my amp. It's not really loud, but loud enough to cause concern and be annoying.
I tinker with this amp a lot, and wonder if I may have done something to cause this.
I tried changed the reverb driver tube (AT7), but it still does it.
Can someone help me out with this?

2.) Does it matter if I have Sovtek 6L6WXT's on side, and 5881WXT's on the other side?

3.) Can anyone recommend the best tubes for this amp? I read that JJ tubes are the best, but that was from the reseller.

Thanks!
Will
Old 25th January 2007
  #2
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paully's Avatar
 

Will,

Get a matched set of quality output tubes and have a shop install / re-bias them, otherwise you risk damaging the amp. According to the schemo:

http://www.schematicheaven.com/fende...c568_schem.pdf

turning the reverb knob off should remove any noise generated in the reverb path proir to the summing section (V4 a). Removing V3 will give you a better chance of isolating noise in the reverb path. These preamp tubes generally last for many years with no problems. Sounds like your problem is in the hi-power (output tube) section. Those mismatched tubes are a no-no. I wouldn't turn it on again until it's completely checked out by a qualified tech.

Good Luck, Paul
Old 25th January 2007
  #3
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Hi Paul,

Thanks for your advice. I should mention that the noise is only present when the reverb is on. Perhaps it's caused by poor contact.
I don't need matched tubes because I've modded my amp to allow each tube to be individually biased. But I do want to install tubes of all the same type.
I don't understand how the problem could be in the output stage when the noise disappears when I turn the reverb off.

Will
Old 26th January 2007
  #4
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paully's Avatar
 

Quote:
Originally Posted by wthiessen View Post
... the noise disappears when I turn the reverb off...
>>>>>>>>>>>>>>>>
Apparently you know your way around inside an amp. The only way to find something like this is to trace it out. At the least, you'll need a scope. If the schemo that I posted (dated 1968 I believe) is an acurate discription of your 1971 and you don't have a scope, start by pointing up all solder joints and recinching tube socket pins. Use a good cleaner on the reverb pot. If no luck, then start replacing listed components in the reverb path starting with V3 and V4 (easiest first). I or someone else would be happy to help with the process, assuming that you know the dangers within these high voltage amps. Something as simple as a noisy resistor can make a ton of noise.

Paul
Old 26th January 2007
  #5
Gear Addict
 

1) I thinks its a huge problem that you have different types of tubes on both sides- there no way the amp is properly biased. Best 6l6s for reasonable amount of dough are SED (the original winged c svetlanas) or Jan Phillips 6L6wgb- neither set of tubes should break the bank- and it would be huge improvment.

2) As for the reverb problem, check the reverb recovery tube as well as the At7 driver you already checked out. The problem may also be one of two old ceramic coupling caps in the reverb recovery section.

sometimes a good approach is to grab a couple tested ax7s and At7s and start swapping tubes until you find the problem. If that does nt help, Check the reverb driver output tranny and move on to testing caps and resistance values inside. Only proceed to this step if you are comfortable draining the charge inside the amp befoe you start poking around inside th chassis.

Hope this helps

Cheers
Old 26th January 2007
  #6
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paully's Avatar
 

Quote:
Originally Posted by 1979 View Post
1) I thinks its a huge problem that you have different types of tubes on both sides- there no way the amp is properly biased.
>>>>>>>>>>>>>>>
Ya, that one made me stop and wonder, but I'd already warned him to get it to a tech and install matched tubes & re-bias. Oh well..

Paul
Old 27th January 2007
  #7
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Ok, I got a set of JJ 6L6-GC tubes and biased them to idle at around 15W. I still have to take a look at the reverb section, trying what you guys have suggested.
I am comfortable working inside my amp, I've done so many times and know how to discharge the voltages.
Thanks for the great response.

Will
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