Pedro Vecino 5/21/2016 4:20 AM
Annoying noise in Dual rectifier 3 Channels (with solution).
I had a Dual Rectifier three channels with a very unpleasant noise (stable and low frequency) dependent of gain and volume settings. The area of the rectifying diodes in the heaters line looked bad and after extracting them I have found that the board was blackened.
At some connection points I found low resistances (between 5M and 13M) and thought it would be the source of the noise. Diodes are well.
Contrary to exposed in the schematic are two 12AX7 that feed with DC (600mA + switching circuit) and the diodes generate too much heat dissipating in poor form.


The solution is to extract the PCB, cut the lines in that area and bypass connections aerially.


And install a new bridge. I used 3A FRED diodes to better dissipate heat and also serves to obtain 5.9V instead of the typical 5,7VDC (approximately) installed with rigid wire on sockets pins to support firmly the structure.


Finally itīs necessary retain the positive side for the channel switching circuit also aerially (it was taken direct from the positive side of the rectifier bridge). Itīs connected from the positive side of the 1N4148 diode associated with the marked relay until the end of the 22R resistor (+ 5,9VDC). It could also be taken directly from the positive on the socket but in this way does not interfere with welding.


That is all. I hope this can help someone. Sorry for my poor english.
Leo_Gnardo 5/21/2016 6:11 AM
Excellent work Pedro!

I'm not looking forward to seeing Dual Rectifiers from now on. Removing/replacing the PC board = long day in the no fun zone.
Pedro Vecino 5/23/2016 9:14 AM
Thanks, Leo. I said a prayer before connecting (always helps)
drewl 5/23/2016 12:31 PM
I've replaced those diodes in a few Mesa's.

I try not to wait until the board is burnt, sort of preventive maintenance and keep them up off the board to prevent heat damage.
Jazz P Bass 5/23/2016 3:50 PM
One of the few coherent things SGM ever passed along as far as Mesa amps: keep all resistors up off of any traces that pass under them.
The Dude 5/23/2016 4:42 PM
Quote Originally Posted by Jazz P Bass View Post
One of the few coherent things SGM ever passed along as far as Mesa amps: keep all resistors up off of any traces that pass under them.
Most likely plagiarized from someone else's post.
Justin Thomas 5/23/2016 8:50 PM
I still maintain that they sound great. Until they break. Don't bring it to me! That's a serious case of ten pound of $#!+ in a five-pound bag...

Seriously, though, I really respect guys who can work on such a tiny scale and in such cramped spaces. Nice, creative, and elegant solution, Pedro!

drewl 5/23/2016 9:22 PM
You want to see tiny?

Hahaha... I've got some microscopic surface mount stuff that's a real pain to work on!
xjbear 7/29/2018 4:50 PM
Had to rebuild a burnt one lately and used a 4 leg rec brick on top of the socket. Thank God for the photos you uploaded here because the traces ended up all nuked beyond recognition. Thanks again man!