Help requested - Carvin ValveMaster issue

[Runewalker]

Valvemaster (Actually a Stagemaster - 2X12)

It is hard to describe this problem, but I'll give it a shot.

On both channels (but more pronounced on the clean channel), it fires ups, then will play normally, then suddenly get muffled, at a lower out put but still hear-able. It is though the volume was turned down and the treble turned to zero, but the mid and bass turned to about 7-8.

Then just to D*ck with me, it goes back to normal for a while, then muffled, then back to normal, then muffled ......

The problem was worse when I first bought it used. The mid pot was nasty and scratchy, and would pop and crack, but also send the amp into its muffled state, between some of the pops and cracks. If the string of the guitar was struck hard the amp would crackle and get very loud then muffled.

I replace that pot, and went ahead in performed the Stage I Hasselbrock mods, but not Attitude and MastVol. That cured the crackly pot, but the amp would still have that weird problem with hard string strikes.

I replace the preamp tubes and that problem improved, but did not disappear.

When I run a modeler through the clean, if it goes muffled, I can turn up the output of the modeler, and it will drive it back to normal .... for a while ... then serendipitously return to muffled. Then I repeat the modeler surge and it goes back to normal ... etc.

This thing sound great but the back and forth muffling thing is making me want to kick in the speakers.

I am about to open it up to complete some more Hasselbrock mods .... and fix this prob, but am looking for some guidance.

Might it be the power amp tubes? I did not replace those.

Filter caps?

Any help is vastly appreciated.

[UnklMickey]

i''d suspect a coupling capacitor.

[sumgai]

Rune,

You're describing a classic heat-related issue. Tops on my list of suspects is the screen grid dropping resistor across the power tube's socket(s). Those tend to either get tired, and become cyclic as you describe, or just give up entirely, and take the tube(s) with them.

If you're brave, go out and snarf up a can of "Cold Spray", known by various names in different stores. It sprays a moderate refrigerant vapor that absorbs heat like the dickens. Apply a small amount to the suspected part. When said part has been rendered cool (don't touch! this is high voltage here!), it should start behaving itself right away. If there's no change, then that wasn't the problem.... move on to the next likely culprit.

Otherwise, have a quality tech check it out.

HTH


sumgai

[Runewalker]

SG: Welcome back, bud.

www.carvinmuseum.com/pdf/amps/112%20Nomad%20212%20Bel%20Air%20REV-E1.pdf

I've replaced and or added a few caps/resistors to this board, and it is pretty easy. Above is the Bel Air schematic, which is basically the same unit.

Where on the schmatic are the resistors you described named?

"Tops on my list of suspects is the screen grid dropping resistor across the power tube's socket(s)."

[sumgai]

rune,

runewalker said:

SG: Welcome back, bud.

Thanks, pardner, glad to be back!

runewalker said:

www.carvinmuseum.com/pdf/amps/112%20Nomad%20212%20Bel%20Air%20REV-E1.pdf

Where on the schmatic are the resistors you described named?

The diagram shown is broken across two pages, right where you need to look! Check out R62/63 and R66/67. These are 350Ω @5 watts each. That makes them pretty big, so they may not be physically located right on the tube sockets (them being 9-pin mini types). You might wanna look at them anyways, checking for discoloration. If nothing is obvious, then they are probably OK.

Still, heat is the likely culprit. Check closely anythng else that is located right around the tube sockets.

And of course, I'm making the massively presumptuous assumption that you've already exchanged the tubes with known good ones, right?

HTH


sumgai