mesa oscillation

[drewl]

Anyone have problems with Boogie F100's oscillating, i think I did but I can't remember.
Only on high gain channel, it's not the tubes.
I can change the freq. of oscillation by moving some of the wires around.
The way it's laid out I'm not surprised.

[drewl]

With the gain and master both almost dimed with no input it will squeal.
even into a load w/no speaker.
Turning the speaker switch off kills the squeal....tried swapping OT leads for the heck of it, a little worse.
tried disconnecting NFB, still oscillated but at different freq.
V1-A- input comes from a relay with a short wire running over the socket to pin 7.

I wrapped wire around it and grounded one end like a shield and it's alot better, still has oscillation with gain and master both full up.
Think I'll try shielding wire from input jack to pc board, and maybe a real shielded wire on V1-b

[drewl]

Damn this thing to hell!
Anyway, I shielded the input wires to the first two stages and the feedback went away.......Until I installed the chassis back in the combo.
it feedsback like crazy with the internal speakers, but not with an external cab.

You can tap the cab lightly and hear the preamp tubes ringing.
The manual says this is normal.....even with some dead quiet mullards I subbed in this sucker rings like a bell.

Any ideas?

[Dave Curtis, dB AudioTech]

Damn this thing to hell!
Anyway, I shielded the input wires to the first two stages and the feedback went away.......Until I installed the chassis back in the combo.
it feedsback like crazy with the internal speakers, but not with an external cab.

You can tap the cab lightly and hear the preamp tubes ringing.
The manual says this is normal.....even with some dead quiet mullards I subbed in this sucker rings like a bell.

Any ideas?

Microphonics. ALL tubes are microphonic to some degree. Do you normally play with the gain and master almost all the way up? Many Mesas like the F100 have ferocious amounts of gain, and some of the manuals state somewhere that it's enough to cause the amp to oscillate with extreme settings.

Get first stage preamp tubes from a reputable dealer that screens for excess microphonics and knows what amp they'll be going into.

-DC

[Mars Amp Repair]

FYI,
Richard Duvall @ Mesa puts it like this...We design the amps to have as much gain as the customer can get in trouble with.

Basically what he seems to be saying is yes the amp has enough gain to where you can get weird reactions from it if you're gonna dime everything!

Is the feedback well into the audible range or is the amp really trying to amplify some ultrtasonic oscillation. IOW, what you're hearing are the sidebands of the main feedback signal which is much higher. You'd need a scope to see this.

If your feedback is really ultrasonic, that's when moving wires around really seems to help. I have no answer for that, tho.

Just as an aside, have you cleaned & tightened the input jack? that's the main chassis ground for the Mesa's. It is critical.

Also where you shielded the input wires, you need to be certain to only ground one end of those shields or you can create loops that will make the situation worse.

RE tubes; Mesa suggests using they Russian Mesa's in the preamp stages & the Chinese Mesa's in the cathode follower stages. The Russian's are less microphonic & the Chinese tubes seem to have a stronger Cathode/filament insulation to withstand the large potential you can find in PI's & cathode follower ckts.

As of late I have had big issues finding quiet anything Mesa tubes. I'm ending up using JJ's in the high-gain preamp stages after going through 4 or 5 new russian or Chinese Mesa 12AX7's.


Just some thoughts, glen

[drewl]

Thanks,
yes I saw that in the manual and was going to mention it to the owner.
It definitely sounds like feedback that's being amplified, as the neg. feedback runs very close to the high gain stage that is oscillating.

After some work I was able to get it to where you could have the gain and master almost all the way up, but when I put the chassis back in the combo that went to heck and now it feedsback with the gain only about half way up.

I tried tightenening the input but will check that again for a proper ground.

[Mark Black]

Just a thought - make sure the "tension screw" in the middle of the chassis is firmly contacting the cabinet. I've never asked Mesa, but it seems like that is there to dampen chassis vibration across the center.