Newb Mark 111 help

[shush]

Just got a mark 111 blue stripe combo 1988, changed all the tubes for mesa tubes and I have microphonic feedback in lead mode !! if i gently tap the treble control it disappears but then comes abck, do you think my amp needs a service ?

 

[Tuna141]

Although you said you changed them already, I'd still guess that it's a preamp tube going microphonic. Try swapping them around to see if it goes away.

Another idea is a faulty reverb (runaway). Are you running your reverb high? Try the amp with the reverb off to see if it makes a difference.

 

[shush]

Ok settings as follows :
Volume 6 pulled
Treble 5
Bass 6 pulled
Middle 5
Master anywhere between 1 and 3
Lead Drive 7 pulled
Lead Master anywhere between 2 and 4
No reverb
Presence 6-7
Eq not quite classic v sort of an abbreviated version, if that makes sense.
Tried it without effects in loop, still the same, havent got round to swaping preamp tubes around, although the problem was there before I changed them for new ones. I figured it was possibly preamp tube problem thats why I changed em. Thanks so far

 

[MrMarkIII]

Be sure to change pre-amp tubes one at a time. Change the first tube, then test the sound. If the problem persists, put the original tube back in and move on to the next tube. The first pre-amp tube is the one closest to the input jack. You might find it useful to: 1) Remove the speaker, if it's a combo. 2) Put the amp upside down on the handle, on a desk or bench that's at least waist-high. 3) Get a household fan to blow on the tubes so you don't burn fingers. 4) Flip it to "standby" before pulling anything from its socket. 5) Now you have a nice 12" hole to reach in and switch tubes around to your heart's content, with no pesky power tubes, etc., in your way.

 

[shush]

Ok changed em one at a time, still the same :evil: do you think it could be a loose connection or something ?

 

[Tuna141]

Hummm....

*** CAUTION : IF YOU DO NOT HAVE ANY ELECTRONICS EXPERIENCE, I SUGGEST YOU BRING IT TO A TECH - THERE ARE LETHAL VOLTAGES IN THE AMP !!!


What I would do now is take the amp out of the chassis and start poking around a little until I located the part or portion of the circuitry that is making the amp oscillate. I'd try tapping various components (start with the treble control) and various places on the PCB to see if I heard a difference. I'd use an insulated (plastic or wooden) object and start tapping and gently moving wires around. This may help identify a flaky component or cold solder joint.

While I had the amp open, I'd also look for evidence of damage to the circuitry, cold solder joints, caps that looked like they were bulging or leaking, I'd blow out the dust, clean the tube and audio jacks/sockets, make sure heat sinks and other fasteners are all secure, etc.

*** CAUTION : IF YOU DO NOT HAVE ANY ELECTRONICS EXPERIENCE, I SUGGEST YOU BRING IT TO A TECH - THERE ARE LETHAL VOLTAGES IN THE AMP !!!

Good luck.

 

[Paul Secondino]

Ok changed em one at a time, still the same :evil: do you think it could be a loose connection or something ?

Since this is only happening in lead mode, I would strongly recommend trying a different guitar. it's entirely possible that your guitar is the culprit.Your gain settings are reasonable so it shouldn't be feeding back big time like that.

Worst case scenario, if you are certain that it isn't your guitar , your best bet is to send it back to Mesa by getting a RA# first. They have my Mark 3 at the moment and they work much faster than most techs who don't know the amps.

Good luck though.

 

[shush]

Well how stupid was I !! read the manual and changed a mesa tube V3 for a Marshall one and hey presto, problem solved, thanks guys for all your input, if I hadn't have been so 'bullish' I should have taken into account what I was told on this thread