Fender Twin Reverb problem

[Randy Brown]

I recently bought this '70 Twin (I posted other details about it previously). It sounds GREAT. I recently had it retubed (all power tubes replaced, only the 2 preamp tubes that were deemed "bad" by the tech were replaced).

Problem: It occasionally makes a popping noise while I'm playing. The pop is about as loud as the volume, but not as loud as the pop that happens when I unplug a guitar with the amp on or something like that. It is so intermittent that I can't reproduce the sound when I'm trying to diagnose it, which makes it really tough to figure out. It only seems to happen when I'm working the volume pedal - e.g. when cranking the volume pedal to sustain long notes (but not every time). Unfortunately this makes it very hard to show the guy at the shop.

I've done a little basic troubleshooting:

• Happens on both channels.
  Happens on either input of either channel.
  Happens with Reverb disconnected or connected.
  Doesn't happen with nothing plugged in (as far as I know).
  I've tried 3 different volume pedals (2 hiltons, 1 regular pot pedal).
  I've tried replacing all cords.
  I've tried 2 different pedal steel guitars, and it happens on both.
  I've pulled each preamp tube and replaced them with some new ones I had laying around, to try to isolate it, but as far as I can tell it isn't a preamp tube.
  I've tried going direct from the guitar to the amp, but I haven't heard it pop (only seems to happen when working the volume pedal).
  I haven't heard it while playing Tele (but again, I don't use a volume pedal with the tele).
  Once it starts happening, it will usually happen repeatedly for a while but it usually quits again.

It did this before the retube as well, so i don't believe it is something the tech did. I had hoped the retube would fix it. I told the guy at the shop about it before the retube. He even played it for several minutes (spanish guitar) and it never happened.

I believe it has something to do with the higher output of the steel pickups in combination with the volume pedal. The odd thing is, I have never heard it while plugged directly in (although just because I haven't heard it, doesn't mean it won't happen).

Are there any other things I can check before taking it back to the shop? Any input is greatly appreciated.

[Pete Burak]

From what I'm reading here, you have isolated it to your volume pedal(s)... Is that right?
It doesn't do it when plugged in directly from a Guitar or Steel?

[Randy Brown]

Pete thanks for the response. So far I've only heard the noise while using a volume pedal, but I've used 3 different volume pedals, and I've changed all the cords out, and it still happens. Also I don't hear the noise when playing those vol pedals through my other amps, so I'm pretty sure the volume pedals are not the problem.

[Ken Fox]

We discovered the Peterson Strobe tuner will do that on the tube amps I have built. Nothing to do with the amp, just a problem with the tuner

[Gary Chiappetta]

Dittos to Ken. I have to disconnect my Peterson when I use the Twin Reverb.

[Mark Lavelle]

Dittos to Ken. I have to disconnect my Peterson when I use the Twin Reverb.

Has anyone contacted Peterson about this?

[Randy Brown]

Correction - This evening it was happening and I unplugged everything from the amp and left the amp on, and it did pop a few times even with nothing plugged in. Does that change the diagnosis?

Also Ken, just curious - does the Peterson problem happen when the tuner is engaged or not?

Thanks for the help, guys!

[Dennis Wallis]

My '78 Twin has done that on occasion when not used for some time . (I hardly ever turn it on anymore) I thought it was the caps becoming dried out since they are so old .

[Ken Fox]

Tuner popped even with the tuner function off.

A fun read for trouble shooting Fender amps here:

http://acruhl.freeshell.org/mga/main/troubleshoot.html

[James Morehead]

I use a Petersen Stroboflip with both my Vibrosonic and Twin(which Ken recapped for me) and I have no issues. I leave it on, and in my signal chain--no issues whatsoever. (My tuner is modded for true bypass)

Ken, you did such a great job on my amps, that I forget they are on, they are so quiet. I keep my volume on 10(my volume is controled with my volume pedal, and "10" opens up those tubes!!).

[Tony Prior]

well certainly not calling myself an expert but a realist with knowledge and experience..and schooling... Practicality says the issue is in or around the final gain stage or output stage....seeing it happens regardless of channel,both channel inputs feed the same preamp tube input by the way , the only difference is a 1 meg R on the 2nd...supposedly a 10db cut in gain...

But...if it is the amp and not an outside source...

I would start by peeking at the power supply, high DC. Insure the integrity/stability of the HI DC and the rectifier circuit.

Did you say that this amp DID have a full cap replacement ? If so, go back and look at the solder joints...If not..well...

Also check the 4x6L6 tube sockets as mentioned in Ken's link..

Another thing to check with old Fender amps is the grounds ( thin unshielded wires) that run from the PTP bds to the strip under the POTS..I call this POT ROW. I think these are very suspect and recently ran across two amps where these "really large and bad" solder joints at the ground strip were indeed poor and causing issues.

I would start here , and with my own restores, I address these items each time.

Loud popping is not a good thing and should be repaired before it takes out a speaker which it will if it continues.

good luck

t

[Paul Arntson]

I second what Tony said:
"...
I would start by peeking at the power supply, high DC. Insure the integrity/stability of the HI DC and the rectifier circuit. ..."

I have had that happen once or twice where the high voltage circuit had found/made a carbon track to ground and was arcing. Hard to find.

Also a cracked resistor or solder joint can do that too.
Good luck and please post the solution when you find it. We would all gain info from knowing what it was.
Have your tech go through it and do the "chopstick test" looking for cracks.

[Justin Richert]

Hi,
I would bet that the 100k resistors in the preamp need to be changed out. I have had to do this with a few old fenders to get rid of the popping-cracking sound. Hope that helps. good luck jr

[David Britton]

Some good advice so far....Here's a few other things to try:

Unplug all preamp tubes. Turn amp on and listen. If it pops, then you know the problem occurs in the Power amp and related circuitry. If it doesn't pop, plug in the preamp tubes one at a time beginning with the Phase inverter(turning amp off in between, of course). Do this until it starts popping, thus finding which stage the popping is occuring.
What I'm leaning towards, and has already been mentioned, is arcing. Most likely candidates are worn out plate resistors. Another possibility is arcing at the recitifiers. I think this is a S.S. rectifier, right? If so, the simple route is to just replace them all with appropriate Zener diodes.

How competent inside an amp are you? Do you have any test equipment, i.e. Multimeter?