Help!

[viking]

I swapped the 6L6's with some JJ KT77's in my M&B Nomad 55 212.The results were very good but after a few weeks,the sound suddenly distorted (using a DS1)for an instant and from then on,the reverb somehow disappeared...Usually,the reverb was stronger with the clean channel and there was a slight bzzzzz (6 months old).Now,you could barely hear if there is any reverb with the same clean channel (no more bzzzz though..) but,curiously,you can still hear the reverb with the 2 high-gain channels.(It looked like some treble were lost too).So,i put the 6L6's back in but it seems to be similar about the lack of reverb..(i didn't test it extensively though,it was too late..).So,what do you think?Could it be the reverb's driver (see the bzzzzz)?In the manual,they don't show any.I'm thinking about changing the V5 12 AX7 ,it's the phase-inverter.Any help would be appreciated...

[hamfist]

If any one of the channels work OK, then it's definately NOT the phase inverter or the power tubes at fault.
 My first checks would be to try a known good tube in the reverb driver spot, and also to check all connections to the reverb tank.
  If none of that worked, I'd take it to a tech, or back to the shop you bought it from as it should still be under warranty.

Alan

[Henk]

I would put the 6L6's back before you return the amp, they will probably say you installed the other tubes wrong and bill you for the repair. A mate of mine had that even though he had the pre-amp tubes changed by a  friend with an electronics degree.

[LazyNinja]

If any one of the channels work OK, then it's definately NOT the phase inverter or the power tubes at fault.
 My first checks would be to try a known good tube in the reverb driver spot, and also to check all connections to the reverb tank.
  If none of that worked, I'd take it to a tech, or back to the shop you bought it from as it should still be under warranty.

Alan

I agree, try out a different preamp tube in the reverb driver spot. Reverb on my 2210 was temperamental until I changed the driver tube.

[viking]

If any one of the channels work OK, then it's definately NOT the phase inverter or the power tubes at fault.
My first checks would be to try a known good tube in the reverb driver spot, and also to check all connections to the reverb tank.

Thanks,guys but i didn't make myself clear here,i think.My intention is to change the reverb's driver but i don't know where it is...and because the phase inverter is often coupled with the reverb's driver (at least in vintage amps),i thought about changing the V5 12 AX7.On the tube chart,from the manual,there is no mention of any reverb's driver..btw,my amp is second hand,no shop involved.Does anybody know where is that reverb's driver? :?

[hamfist]

The manual can be found at http://www.mesaboogie.com/manuals/Nomad%20Manual.pdf

Tube positions are found on p.27 of the manual (which is p.31 of the document).

Alan

[viking]

Thanks,Alan! 8) But i already have the manual and the trouble is that they don't show any tube driver.Only the power tubes and the five 12AX7's.That's why i don't know where is the reverb's driver. :oops:

[hamfist]

Thanks,Alan! 8) But i already have the manual and the trouble is that they don't show any tube driver.Only the power tubes and the five 12AX7's.That's why i don't know where is the reverb's driver. :oops:

You're right Viking. My bad. I didn't look close enough.  I am assuming there is is NO reverb driver tube. All the gain stages of all 5 12ax7's seem to be spoken for, according to the manual. I'm guessing it must use a solid state circuit to drive the spring reverb tank.
   Therefore, going back to your original problem - if you've checked, and re-connected all the connections to the reverb tank, with no improvement, I think it's probably time to take the amp to a tech.

[shaman]

I think the reverb is tranny driven on these-might want to go to the boogie board forum and search for the nomad mod-schematics for a modification that can be performed by a tech-boogie board is a link on grailtone.com