Troubleshooting a Peavey MX VTX head

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timthetortoise

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Okay, so here's the deal. My band's guitarist just got an MX VTX head. We tried it out and it seemed alright, except the sound was a bit muddy. Checked the tubes, and they were old GEs. So obviously needed to replace those. Figured it'd need a cap job too so added that to my list. So we're going through practice, everything's fine, and then bam! Fuse blows. There wasn't a loud pop or anything like that so I figured not a big deal.
So I bring the amp home, get the caps all replaced, put some new tubes in. Today I bring it up and power it on. I let it warm up and take it out of standby. What greets my ears is a loud hum accompanied by intense blue in one of the tube pairs (it's a quad). So I figured it was a grounding issue. Went through and double/triple checked every cap I touched. Traced grounds, checked for leaks, checked voltages, continuity, the whole nine yards. Everything checked out fine. So I figured I better not test on new tubes in case the overload would damage them. Put the old tubes back in and power on. And guess what! One of them didn't heat up! So now I'm a little bit worried about what could have gone south with that tube. I checked the voltages on the tube board while powered on, and the voltages on two pins (I believe it's the grid side but don't know much about pin arrangements yet) are significantly less than that of the other side.
So I traced the input for that side's pins and it led to one of two power transistors.
So my question is: would a faulty transistor do something like this? I was set on the problem being a ground or power issue in the preamp, but when I pulled the pair from the troublesome sockets, the amp ran fine. I'm not an amp tech by any means but would like to be able to resolve issues like this myself, so any guidance other than "take it to a tech" would be appreciated.
 
Hm, alright then!
Well, I found a newsgroup post from 2 years ago outlining the same problem and apparently it is the transistor. So, time to find a good replacement.

Edit: Just ordered 4 MJ15003s from Allied. Hopefully they'll work well!
 
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So I fixed the previous problem, now here's another weird issue going on. If the high or pre pots are turned up past halfway at the same time, or if the high is turned past halfway with the post around 1/4 of the way up, a high-pitched squealing sound comes in. What are some possible causes of this?! Help on this would be appreciated because I'm clueless!
 
Hey, did you get this issue resolved? What purpose did that transistor serve? These amps seem to be an unusual hybrid design.

I've got one at home. I bypassed the preamp, and use only the power amp. One of my first DIY audio electronics projects was building a driver ciruit for the reverb tank, before realizing it was easier to just trace the existing one and bring out the input/output to the front panel jacks. Now I run the reverb in the effects loop of a TubeWorks preamp. Nice tone and stunningly powerful. The only thing left to try is bypassing the solid state phase splitter and installing either a 12AX7 or 7199 to do the job.
 
By the way, I love ISIS. Been going to see them play for many years, back to when they were opening for Dillinger Escape Plan and Candiria. I think the Mosquito Control EP had just come out. Anyway, they just keep getting better and better.
 
timthetortoise said:
So I fixed the previous problem, now here's another weird issue going on. If the high or pre pots are turned up past halfway at the same time, or if the high is turned past halfway with the post around 1/4 of the way up, a high-pitched squealing sound comes in. What are some possible causes of this?! Help on this would be appreciated because I'm clueless!
If the "high-pitched squealing sound" occurs constantly at whatever post setting, without a guitar plugged into it, try replacing the first preamp tube.
If this doesn't fix it the problem may be caused by the way various wires have
been routed during any previous repairs or mods. Open the chasis, power it ON, and adjust your post to create the squeal. Now carefully begin moving bundled wires to and fro using a wooden stick to see if you can affect or eliminate the offending "squeal". If you can eliminate it, make note of "where" you re-positioned the wires and tie them off there. If you can alter the squeal using this method, work backwords from there all the way back to the input jacks--checking for shorts, poor grounding, etc. If, by chance, you're able to alter the squeal by gently moving a single wire, power down and replace it with a shielded wire with the shield connected ONLY at one end. Isolate the other end of the shield with a piece of thick heat shrink. Hope this helps.
 
Ah man, just found this for some reason.
I ended up replacing the two output transistors and it worked like a charm.
As for the reverb tank, I took it out and never really got a chance to rework it (though I was going to do much along the lines of what you did). Wouldn't recommend converting to a tube PI, as that would be asking for a meltdown if the tube were to go (I'm a Musicman HD130 owner, so I know what can go horribly bad with a tube PI).

OH YEAH!
I also replaced all of the power tubes and did a full cap job on the thing. I had some trouble with the fan, so I pulled that out and now it works great (though the guy whose amp it is decided to get a Laney head so he hasn't used the VTX in awhile).
 
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