Fender 'Twin Amp' Reverb Feedback

grappagreen

Hey All,

Hope all is well.

I grabbed the above from here some time ago. It's a killer amp sonically and also probably going to kill the planet due to the black hole level gravitational pull induced by it's weight..

Back to the point. As soon as I pop the reverb level above 1 the reverb circuit starts to feedback. It's frustrating me to the point that I'd like to fix it if poss. Anything I can look at before I have to hire a team of body builders to help me transport it to someone who knows what they are doing?

Any help appreciated.

Si

mani

Sounds like it's self oscillating - try tightening the screws holding the reverb tank down, should help.  I had a similar issue on an old TM60 amp.

ICBM

mani said:

Sounds like it's self oscillating - try tightening the screws holding the reverb tank down, should help.  I had a similar issue on an old TM60 amp.

Not with a Fender, it should be in a cloth bag which is only loosely attached to the cabinet. Where is the tank bag positioned? If it's touching the speakers or mounted in the wrong place that could do it.

Another possibility is that the original tank failed and has been replaced with one of the wrong spec. (I think these amps are new enough to be fitted with the notoriously shit modern Accutronics tanks.)

grappagreen

Thanks chaps.

I'll whip the tank out tonight and take a gander..

@ICBM - do you know if these use a 4AB3C1B tank? In light of your comment on Accutronics quality do you have any recommendation on something better if I end up replacing it?

Many thanks,

Si

ICBM

grappagreen said:

@ICBM - do you know if these use a 4AB3C1B tank? In light of your comment on Accutronics quality do you have any recommendation on something better if I end up replacing it?

Yes, I'm pretty sure that's the right one. The Ruby and TAD ones (which are almost certainly the same) are better - they don't have the plastic jumper blocks which cause all the failures on the Accutronics.

The faults and resulting warranty issues with the Accutronics ones are so common that they lost their biggest single customer - none other than Fender - because of it... Fender now use Ruby tanks.

grappagreen

ICBM said:

grappagreen said:

@ICBM - do you know if these use a 4AB3C1B tank? In light of your comment on Accutronics quality do you have any recommendation on something better if I end up replacing it?

Yes, I'm pretty sure that's the right one. The Ruby and TAD ones (which are almost certainly the same) are better - they don't have the plastic jumper blocks which cause all the failures on the Accutronics.

The faults and resulting warranty issues with the Accutronics ones are so common that they lost their biggest single customer - none other than Fender - because of it... Fender now use Ruby tanks.

Wow.. many thanks as always!

Si

grappagreen

ICBM said:

grappagreen said:

@ICBM - do you know if these use a 4AB3C1B tank? In light of your comment on Accutronics quality do you have any recommendation on something better if I end up replacing it?

Yes, I'm pretty sure that's the right one. The Ruby and TAD ones (which are almost certainly the same) are better - they don't have the plastic jumper blocks which cause all the failures on the Accutronics.

So an update after last night's fun..

Pulled the reverb tank out and looks like it's already been replaced with the correct TAD version. Checked it all and visual inspection doesn't indicate anything amiss. Did note however that it looked like it was plugged in the wrong way (red into white) but turned out to be a red herring as the lead must be connected the wrong way at the amp outputs.

Left the reverb out of the amp (on the cardboard) and tested it and it still I still get feedback. Interestingly the threshold of the feedback i.e. reverb knob setting seems to correlate with the volume of the amp; turn up the amp on the clean channel and the onset of feedback seems earlier.

Is it worth me swapping the driver/recovery valves and seeing if it makes a difference?

As an aside I checked the bias and it was really cold 40mV so set it to the recommended 80mV and balanced it. So easy to do on with the way Fender designed this.

The clean and orange modes on the amp really do sound fantastic.. shame it's so sodding heavy or I'd take it out and gig it. Oh and it's a little on the loud side

Si

ICBM

grappagreen said:

Pulled the reverb tank out and looks like it's already been replaced with the correct TAD version. Checked it all and visual inspection doesn't indicate anything amiss. Did note however that it looked like it was plugged in the wrong way (red into white) but turned out to be a red herring as the lead must be connected the wrong way at the amp outputs.

Left the reverb out of the amp (on the cardboard) and tested it and it still I still get feedback. Interestingly the threshold of the feedback i.e. reverb knob setting seems to correlate with the volume of the amp; turn up the amp on the clean channel and the onset of feedback seems earlier.

Is it worth me swapping the driver/recovery valves and seeing if it makes a difference?

As an aside I checked the bias and it was really cold 40mV so set it to the recommended 80mV and balanced it. So easy to do on with the way Fender designed this.

The clean and orange modes on the amp really do sound fantastic.. shame it's so sodding heavy or I'd take it out and gig it. Oh and it's a little on the loud side

The crunch mode on those is one of my favourite amp sounds.

The feedback will be from the speakers to the reverb springs so it should be volume-dependent, yes. If it’s feeding back even with the tank outside the amp, I’m wondering if there’s too much gain in the recovery circuit - that could be caused by something like the cathode cap being shorted. This is rare but it can happen… the amp could be up to 30 years old now. (I know, that’s scary!)

grappagreen

ICBM said:

grappagreen said:

Pulled the reverb tank out and looks like it's already been replaced with the correct TAD version. Checked it all and visual inspection doesn't indicate anything amiss. Did note however that it looked like it was plugged in the wrong way (red into white) but turned out to be a red herring as the lead must be connected the wrong way at the amp outputs.

Left the reverb out of the amp (on the cardboard) and tested it and it still I still get feedback. Interestingly the threshold of the feedback i.e. reverb knob setting seems to correlate with the volume of the amp; turn up the amp on the clean channel and the onset of feedback seems earlier.

Is it worth me swapping the driver/recovery valves and seeing if it makes a difference?

As an aside I checked the bias and it was really cold 40mV so set it to the recommended 80mV and balanced it. So easy to do on with the way Fender designed this.

The clean and orange modes on the amp really do sound fantastic.. shame it's so sodding heavy or I'd take it out and gig it. Oh and it's a little on the loud side

The crunch mode on those is one of my favourite amp sounds.

The feedback will be from the speakers to the reverb springs so it should be volume-dependent, yes. If it’s feeding back even with the tank outside the amp, I’m wondering if there’s too much gain in the recovery circuit - that could be caused by something like the cathode cap being shorted. This is rare but it can happen… the amp could be up to 30 years old now. (I know, that’s scary!)

Thanks John.

I'll arrange for my local place to take a look. Yes, hadn't considered the fact that the amp is potentially that old! Is it worth getting some wider recapping done as I can't see me ever moving this on and they'll probably bury me with it?

Si

ICBM

grappagreen said:

I'll arrange for my local place to take a look. Yes, hadn't considered the fact that the amp is potentially that old! Is it worth getting some wider recapping done as I can't see me ever moving this on and they'll probably bury me with it?

Probably a good idea to at least have it checked - although modern caps tend to last longer than they did when vintage amps were first getting to 20-30 years old and it was good preventative maintenance to replace them even if they hadn't already failed. That said, my '73 Bassman has its original caps and there's no sign of trouble, so... I'll leave it until there is!

Burying you with it should be straightforward - attach the coffin to it and drop it from about 20 feet up.