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[Formaldehyde]

I've got his amp that has been driving me nuts for about a year now. It's a 50W class AB valve amp built onto a PCB. It's very overdesigned with op amps and all kinds of unnecessary *Poop* in it. The op amps are for the effects loop, the reverb drive (even though the reverb circuit HAS a 12AT valve in it) and there is an op amp on the 4-ohm feedback loop. But the preamp and power amp are all valve.

There are three problems I'm having. I don't know if they're all related but we'll see.

1) Intermittent problem with the amp cutting out in the overdrive channel. I don't know if it affects the clean channel at all, I haven't noticed a drop in that but there could be, I don't use it enough because this amp doesn't do clean all that well. Anyway, it's like the amp loses its balls. It does lose a bit of volume, but even if you crank it up more to compensate for that it just doesn't have that bottom end bite that it normally has. I call it that kick in the guts feeling, because when you use palm muting you can really feel it through your entire body. The amp just doesn't sound as nice when it's in it's failed state either.

2) White noise in the clean channel. I'm not sure if this comes in at the same time as the problem with the distorted channel but I'll try to find out. Anyway, after some time playing I'll occasionally get a white noise distortion that comes over my clean notes. The noise is only there when I play a note, and the amp goes silent again once I've muted the guitar. If I turn the amp on standby then turn it back on I get a nice clean sound again.

3) High pitch squeel in the effects loop. It seems no matter what I put in there, I have to keep the gain down otherwise I get this high pitch squeel that sounds kind of like microphonic feedback but it doesn't go away if I turn the master volume down on the amp. I have a pod XT that sounds pretty decent when hooked up to the effects loop on the amp (sounds *Holy Cow Pud, Batman* when used front end) but whenever I try to use distortion on the pod I get this sqeel. Is this normal, I'm not all that experienced with using effects loops.

What I have tried:

- Taking it to an amp technician. He charged me $200 for a good *Golly Gee* up the *Ay Matey, Watch 'Yer Language Or Ye Be Swabbin' The Deck*. He didn't do anything. I'm not trusting amp techs again.

- Asking the manufacturer for help. They told me to take it to an authorised technician. I looked around everywhere, asked dealers who kept sending me to their amp technicians who knew nothing about the amps. I asked the manufacturer to send me some schematics or if I could buy a service manual. They said no, only an authorised technician could have these, I asked them where I could find one of these elusive technicians, they never got back to me.

- Replacing and biasing the output tubes.

- Replacing the preamp tubes. Admittedly the tubes I replaced them WITH were old tubes, but I'm almost certain they were good. I replaced them in an old preamp unit I bought years ago because I thought they were dead but it turns out the preamp unit was just a lousy peice of *Poop*.

- Got my hands on the schematics. Checked the B+ rail, it's a good 30V higher than it should be. it seems to be stable, but every time I read it it's somewhere between 460 and 480. I may possibly have found out why though.

- Attacked the op amp on the 4-ohm feedback with freeze spray while the distorted channel was playing up.

- Attacked the solder joints for the valve sockets with freeze spray. This made the amp spring back to life for a moment, so I took the amp board out and resoldered the valve sockets into place, adding a little more solder to it. It was some shoddy soldering in the first place. I probably should have removed the old solder but it seemed to grab on OK when I added some fresh fluxed solder to the mix. I also resoldered the grid resistors.

- When I was freeze spraying everything, I noticed a some arcing coming from one of the grid resistors and an adjacent track (which I didn't bother to track down, might have been a smart idea). It could have been the freeze spray that did it (water condensation), but fail to see how water condensation could cause a conductive pathway when the track is coated, so the coating on the track must have already been breached so I'm wondering if this has been going on for a while. The room was dark when I noticed this, normally I have the amp well lit when I'm working on it but I have automatic lights in my room and they turned off because my system sucks. While soldering everything else I dug a channel between the grid resistor and the hole in the track, my logic being that an incidental short had caused a charcoal pathway between the resistor and the track.

- When I retried the amp after going this it seemed to work great. I put it back together and when I started it up again it was quiter than it should be and didn't have that lower end kick. It did spring back to life a few minutes later and I haven't heard it go down again since (it's only been a day though). This could possibly be explained by the fact that the reverb tank has been disconnected for about 10 months now and thus the reverb circuit hasn't been used. Dry caps maybe?

- Today I noticed the clean channel had that white noise problem again. I couldn't really crank it up to find out if the distorted channel was giving me that kick in the guts thing though, it was pretty early in the morning.

I'll add on to this post as I find out more, but if you have any ideas please share.

[Formaldehyde]

Update: The white noise thing doesn't have anything to do with the distorted channel cutting out.

[greenblob]

sounds like youve done all the obvious things.. if you changed the tubes you can rule them out and move on..
my best guess from what you have described is a dry/failing capacitor.. what is the amp and how old is it?
really hard to be sure without sitting down with the thing and testing it (and not knowing what amp it is!) but a failing cap(acitor) fits most of your symptoms.. if you found poor solder joints on your valve bases the pcb pads/tracks may also be an issue.. but if the sound is fading in and out, odd hiss's and crackles w/swelling in volume etc, its most likely a cap.. have had to replace a few on old gear i have as they just simply tend to fail (dry out) over time.. not the easiest things to find, but start with the biggest first (physical size - google how to test em). All valve amps have caps.. err .. actually.. all amps have caps... I may well be wrong with my diagnosis.. but it fits..
hope it helps..

[greenblob]

just remembered.. an obvious thing to try is twisting all the pots (volume, gain, eq.. all of them) from zero to full and back a dozen times or so... you may just have a grubby pot... this is by far the most common amp repair issue ive been faced with.. and has different symptoms on many amps..