Mesa 3 ch DR clean channel squeal

[GainFreak]

Hi,

I changed the power tubes in a friend's amp and when testing it I noticed that the clean channel starts squealing on higher Gain, Master, Presence and Treble settings (if one any of them is higher set). I didn't play the amp before changing the power tubes but I don't think that's the problem.
The other thing I noticed is when I switch the Standby OFF I get a loud pop again when clean channel is active. When ch 2 and 3 are active the pop is insignificant but on the clean channel is loud.
I tried different first two preamp tubes but the problem persists. No similar problem on ch 2 and 3.
I would appreciate any ideas or comments.

[TimmyP1955]

I find it to be fairly normal for many Mesas to pop when coming out of standby or the first time that a channel is switched.

[GainFreak]

I didn't mean when switching channels. It just pops loudly when clean channel is on. When I turn the Gain pot to zero then the pop is not audible.
No such problem when either ch 2 or 3 are active.

[Pedro Vecino]

The first channel in clean mode should not squeal. But in pushed mode with gain and treble settings at maximum, yes.
Their tendency to do depends on the units and also the model and condition of the first 12AX7.
First of all, check that the amp does a distinction between the two modes. After that, use another tube in the first socket.
You can also discard the fourth 12Ax7 disabling the loop (loop, final master and Solo) and even remove the tube to try it in the first socket.

[wizard333]

Tubes can cause both those issues. Popping off standby can be due to a variety of things, but most often it seems to be that the craptastic quality of preamp tubes nowadays causes some of them to make a huge pop coming off standby. One other thing I see fairly commonly with modern crappy tubes: scratchy noise when the volume pot on the guitar is adjusted, sounds just like a bad pot but it is the tube grid instead. How do you know? Happens on any guitar plugged in when the same guitars don't cause the issue on other amps.

Try switching which slots your preamp tubes are in, and if that doesn't work, try known good tubes.

[GainFreak]

Few observations:

1/ Changing first two tubes doesn't solve the problem. You still get that honky whistle when you rotate the Gain pot which however disappears when the pot is maxed which effectively eliminates the 120pf cap across the wiper and the high pass filter it makes with the pot.
2/ The squeal also disappears when the effects loop is ON. When it's off the clean channel sounds brighter as well. This is maybe due to those two SA22 TVS diodes at the loop output and input stages as they have a capacitance of 1.2nF each which I assume are killing high frequency content and thus preventing the oscillation. When the loop is on I can max all pots in both clean and pushed modes. Also it looks like the Presence pot (which is grounding a 2n2 capacitor parallel to the Master) has most influence on that problem.

It would be good if anyone has a DR by hand to try those thing on their amp and report back.

Any other comments would be appreciated.

[Pedro Vecino]

I can not associate the existence of 120pf capacitor in the gain pot to that effect. His effect is very light, just to create a natural transition (more presence) when using lower settings.
The loop of the amplifier (the block loop/master/solo) is not the most respectful of the original signal. Generates a slight loss of bass and the transformation of the harmonic content providing it's the own compression of the tube selected in V4, but does not generate restrictions in the treble. You will find the most similar behavior equating the volume with the loop activated through the master at twelve o'clock.
In such circumstances (if you have clearly that is not from the preamp tubes) could be some problem in the switching circuit. The first thing I would check is the bypass FET over the 470K resistor previous to V2A.

[GainFreak]

In such circumstances (if you have clearly that is not from the preamp tubes) could be some problem in the switching circuit. The first thing I would check is the bypass FET over the 470K resistor previous to V2A.

This is rev E board and it has some differences compared to the schematic that circulates in the net. I also have a drawing of an older version PCB and it looks like that FET is now used to switch the V2a 15uF cathode bypass capacitor. A relay is bypassing the 470k grid resistor in this one.
Without doubt the clean channel sounds clearly brighter with the loop OFF (with the same channel settings, Output max, listening levels made equal by using the Send pot).