Mark IV weirdness, got any ideas?

[woodbutcher65]

I've had a Mark IV widebody combo for a few months now and I haven't really had a lot of time playing it yet, because it's doing something that I can't believe is normal. I'll try to explain it clearly.

I'm in the lead channel with the drive turned up most of the way. I'm getting a good tone. BUT...if I have my guitar volume full up and I strum hard, it causes bright, harsh harmonics to come out that are considerably louder than anything else. It's like I've got a second amplifier connected and it's set louder, with a trebly tone, and it is connected thru a gate. When I strum hard enough, the gate opens and the second amp goes to work.

I thought it might be leakage from another channel, which would be affected by the master volumes and output volume. But no, not in this case. The behavior happens at any set volume level

The Mark IV is in very good condition and was recently serviced. I did a full preamp tube swap and that had no effect. While the filter caps are original, I know the symptoms of bad filter capacitors and this checks none of those boxes. I'll recap the amp anyway, some time soon, but it's not critical and can be left for another time.

 

[rarebitusa]

interesting... perhaps isolation of the preamp vs power amp to divide & conquer. Can you recreate it with only using the Fx Send?

 

[woodbutcher65]

I just checked that by plugging the outputs of the Mark IV into one of my Marshalls.

FX send, same results but without EQ, since the FX loop is pre-EQ in a Mark IV.
Slave out or Satellite out, I get the full tone of the Mark IV.

In all instances the behavior is the same as originally noted. So it's happening in the preamp prior to the FX send.

I will redefine the oddity as a matter of the tone getting very brighter, unmanageably so, if the guitar volume level/input level is too high.

The problem is that I like a highly saturated tone out of the Mark IV and don't want to keep the guitar volume below 7.

 

[woodbutcher65]

Did some more testing and trying to isolate it to a specific channel. I figured that with separate master and presence controls for each channel, if it's a bleed-thru problem I should be able to detect it.

It happens in R2 and in lead. There's not enough available gain to trigger the problem in R1, if it's even there.

So, it's fairly early in the circuit.

I guess I'll have to try to trigger the problem using a signal generator and start probing with my scope.

 

[user.39921]

Look into your LDRs and switching matrix.
It's possible if a LDR is misbehaving to leak signal. I had problems w/ bleed from channels because of bad transistors in the switching matrix (the 2N6426).

Broken LDRs can be fixed, as you probably know. I rolled new ones and replaced a few in my MKIV and they turned out with better specs than the original VTLs.

I am looking at the schematics right now and another cause may be your coupling caps between the stages (0.47uF Orange Drops).

BTW. Test the clean channel w/ some kind of overdrive or distortion pedal in front to see if the problem can be replicated on the clean ch.

 

[rarebitusa]

I guess I'll have to try to trigger the problem using a signal generator and start probing with my scope.

Let the fun begin :lol:

 

[El Rey]

Circumvent the rampant ridiculous nonsense and pathological guessing.

Ask Mike Bendinelli at Mesa. Besides Randal Smith, no one KNOWS more about them, or serviced as many of them. PERIOD!!!!!

Hyperbole intended!

Send him an email.

[email protected]

 

[woodbutcher65]

Eventually I got back to this and soon had it figured out. It's basically a behavior that seems to be normal for a Mark IV when you've got the output level turned down almost to nothing. The amp isn't really within its intended operating range when set to bedroom practice at night volume levels. Turn it up just a little bit more and it clears right up and acts right. Although this behavior is somewhat sensitive to preamp tube quality as well.

Interestingly, switching the reverb on while in the lead channel with lots of gain brings back the same unpleasant added twang that was bothering me at really low volume. The reverb circuit's tone is bright and jangly, and doesn't really work well for me at least in the lead channel.

I might take a look at taming the reverb circuit's equalization. Or just use a digital reverb/delay in the effects loop instead.