Feedback/oscillation in FX loop 2Ch Dual Recto

Having a bit of an issue with the FX loop on my 2 Channel DR. I noticed today that the signal from the send to my TC Nova system was low. Then when turning up the channel volumes the amp produces a screeching feedback sound but no signal. It starts from around 9 o'clock and then stops just when the channel volume is maxed, then the guitar signal comes back.

Tapping the preamp tubes doesn't reveal any obvious microphonics, but switching the FX loop OOC fixes the issue, so I'm confident something in the loop circuit is the cause.

Also *ahem* I did the parallel to serial mod on it which might explain things. But it WAS working fine with the mod, so wondering if it could be something else. Any ideas?

Which valve is the loop driver? I could try to change that first.

O.K. so I am not having issues that are exactly the same as you, but I too, performed this mod to my Tremoverb and have since returned it to factory spec...but to no evail.

My problem is that the delays from my Strymon and GMajor have just no real definition in the hi-gain channel.

In fact I would say that the normal channel is not up to snuff either.

It is a simple mod and I can't imagine this having any effect on the mother board or anything else in the circuit, but maybe it does.

I would gladly take it to a certified Mesa tech and pay to have this fixed.

I have since rolled tubes, but would love to know what valve in the Tremoverb is responsible, and of course I would roll again, hoping it would work.

I think the valve is V4 in the Tremoverb. I do use a 12AT7 in V6, but do not know if that would have any effect.

Any help on this would really be appreciated.

Thanks

V4 is the FX loop on both amps.

For TremoJem - On my 2 ch (factory serial loop) I have to run the loop level knob on the back wide open and set the gain on my Strymon slightly lower than unity to keep from having huge volume jumps when I engage/bypass the delay. Not sure if that'll clear your problem up since your loop is different, but it helped me get some power back into my sound.

For clarity on high gain cut the bottom end out of the repeats.

Swap V4 with V5 for testing. V3 and V4 are cathode follower sockets, they will kill weak tubes, like spiral filament Tung Sol or EHX, etc...Use Chinese or JJ Tesla in those spots.

vick1000 said:

Swap V4 with V5 for testing. V3 and V4 are cathode follower sockets, they will kill weak tubes, like spiral filament Tung Sol or EHX, etc...Use Chinese or JJ Tesla in those spots.

Click to expand...

I don't want to take the chassis out ATM if I can help it, so looking from the back of the amp how are the preamp tubes numbered? I assume V1 is the furthest to the left with the metal shield over it, followed by V1-4 to the right and the one on its own nearest the power tubes is the PI (V5)?

The noise is like tuning a radio, the frequency of the 'feedback' changes are you turn the channel volume.

Tried swapping V4 and V5 as you said, but no dice (all are Mesa branded)

Next I tried swapping each tube individually with a Chinese 12ax7. No change.

To reiterate, if I turn the loop off both channel volumes work without any issue, no squelchy feedback noise. On another noe I notice that the amp is very noisy. There's a lot of hiss *independent* of the gain setting. I would normally expect the hiss to decrease when the gain is reduced, but it doesn't.

What's my next move? All I can think to do is to reinstate the parallel loop.

To troubleshoot the radio noise, I would use knowingly good tubes and start at the beginning, by swapping one at a time.

If this does not work, you might want to consider cleaning the pots, but I highly doubt that is your problem.

My normal PM for my amps are to remove all tubes and use DeOxit to clean all tube sockets and pots.

I then use compressed air to blow out the excess cleaner and wipe the chassis dry.

Be sure to bleed the caps before you go inside the amp!

The Mesa tech told me to power up the amp for a minute and then just unplug the cord.

I then carefully checked the caps with my MM and they had no voltage.

I will say that for me, one time I actually had two tubes that were bad...that required me to completely swap out all preamp tubes in the amp and then check each preamp tube that I removed in V1, and sure enough I found two bad Tung-Sol tubes. These were actually NEW and I returned them to Tube Depot. I had ordered something like six of them and four were bad, but Tube Depot took good care of me. I think this speaks to the quality of tubes out there today.

Lucky for me, I have a stock of tubes to do this type of thing.

Good luck.

When I got the amp a few weeks ago I did a basic clean: Deoxit on all the pots and tube socket pins.

I'm familiar with what a bad preamp tube sounds like, it doesn't sound like that to me, but this is the first mesa I've had so its all new in that regard.

I've also changed all the power tubes (and tried EL34s, with the correct bias switch setting) and its set to silicon diodes so its not the rectifier tubes.

It can't be OT damage because with the loop disengaged the squealing effect doesn't happen.

Well, you hit all the points.

SD to rule out Rectifier tubes.

New Power tubes.

I am not sure what to think...

What about the send level?

How does this affect the problem. If you are serial then the mix pot is out of the picture.

This is perplexing.

TremoJem said:

Well, you hit all the points.

SD to rule out Rectifier tubes.

New Power tubes.

I am not sure what to think...

What about the send level?

How does this affect the problem. If you are serial then the mix pot is out of the picture.

This is perplexing.

Click to expand...

I felt certain it was the FX loop driver tube..is there one?

It turns out the feedback was not from the recto but from the nova system, because I had maxed the input gain to compensate. Never had this before, but I think the send from the recto was low, or maybe it was a dodgy lead.

All ok now. phew. probably could do with a new set of preamp tubes anyway.

Good for you...glad you worked it out.

What kind of tubes are in it now? If they're Mesa 12AX7s do they also say Russian, Russian 2, Chinese? Or do they just say 12AX7 with no extra labelling?

I saw this was fixed, but I wanted to add.

I also have the Nova. Be sure to use the Line Input and not the Drive Input. In the Level menu, see if the gain is set to auto gain by turning off the Advanced Settings. That way, it will balance itself to be at unity and not accidentally become an additional driver stage. When it's automatic, as the input gain increases, it reduces the dB out. Every 6 units of gain equals one reduction in the level out (gain 1 = 20dB, Gain 24 = 2dB). If there's an imbalance between the loop on and off, you can turn Advanced back on and go a few units above normal, but keep the above in mind.

For my DR, I had to reduce the Send a little to the Nova to have the volumes match when using the loop switch on the amp's footswitch. That's why I eventually set it to always on, switched to midi, and just changed effects and channels at the Nova.

Number one rule of trouble shooting, eliminate possible variables, go guitar>chord>amp>cab. And spares of each.