Loss of gain on retube

[Meson Boogie]

I just retubed the power section of my amp and on playing I couldn't get more than a breaking up clean tone.
The retubing might well be irrelevant however, as I think it must be a preamp tube problem, what with about 95% of my gain coming from the preamp. Also, if I put the gain control over about 5 I get some nasty high pitched feedback, and knocking on the preamp creates an audible sound through the speaker, which it didn't do before. I've tried tapping on each individual preamp tube though, and I don't get microphonics from that.
I've also tried swapping the preamp tubes about, but I can't seem to get rid of the problem. I should also mention that it occasionally returns to normal, and this might be linked to turning up the preamp output level, but that could have just been coincidental.

Does this sound like a simple preamp retube is in order?
Many thanks.

 

[stokes]

It would help to know what amp you are talking about.Knocking on the preamp tubes and getting a sound is not uncommon and not necessarily an indication of a problem.All preamp tubes are microphonic to some degree.If the amp had no problems before and all you did was change the power tubes then it is most likely a problem with the power tubes you put in there.You describe a "breaking up clean tone".What does that mean?Is your clean channel breaking up where it should be clean?Or are you not getting enough distortion from your lead channel,if this amp has one.

 

[Meson Boogie]

Sorry, I did leave out some info there.
I'm running a studio preamp and 395 simul class.
Basically, once I put the new powertubes in (6l6 str440) I lost all gain on the lead channel, which usually has heaps more than anyone could need. When I would normally be getting a high gain lead tone, I have almost clean.
I immediately thought it was a problem with the new tubes, since it happened the moment I fired it up after changing them, but I reverted back to the originals and after a while they started doing it too. I also tried a different new pair of tubes and it was no better. I really can't see how I could have damaged the power amp with a simple tube change, and the strange behaviour of the gain control made me wonder if it had just been a coincidence that a preamp tube failed at that moment.
But I'm pretty clueless, I'm fortunate enough to have never had an amp fail on me in any way before.

 

[stokes]

How is your clean channel?Are you having just a loss of overdrive or is there also a loss of power?Try the clean channel and if you have good clean power you can most likely discount the power amp as the problem and focus on the preamp.Do you have another preamp or power amp you can try.Or an amp with a pre out/ power amp in?I am leaving for a few days,so I wont be around to reply again until monday or tuesday.

 

[Meson Boogie]

The clean channel seemed fine at the time, and there was no loss of volume at all. The feedback sound does occur on the clean channel, but it is very faint, as it seems preportional to the amount of gain (while the gain is actually working, that is).

When I last investigated it, I couldn't get it to reproduce the loss of gain, although the feedback sound was still occuring. I had a look around inside the preamp and I can't see any issues. However, I found a solder lug at the input jack that when touched produced the high squeely feedback sound. If I touch a metal surface while touching the lug, the squeeling sound changes pitch, so I'd venture it's a grounding issue (unless this is normal?). I've checked all the grounding and everything and that seems fine though, and I've played with the ground lift switch and that had no effect. The wire coming from the solder lug in question appears to go straight into the V1 tube, but I'd have to remove the board to be certain.

I do have some new preamp tubes on their way now to try a swap. It wont hurt to have some spares hanging about if they aren't the problem, as they're pretty old anyway.

 

[stokes]

Sounds like it could be a microphonic issue with V1.The squealing would be louder in the lead channel since you have extra gain stages in the lead mode which would cause the squeal to be amplified more than in the clean mode.

 

[Meson Boogie]

Just as an update, I retubed the preamp and the problem persisted for about 10 seconds, stopped, and hasn't occured again after lots of use.
In the past few days I confirmed it was the preamp as you suggested, and found that it even made an audible high pitched ringing noise when on standby (without the poweramp turned on/plugged in), ie. it wasn't just something in the signal but a physical, ringing, vibration in the preamp that was being picked up and amplified.
Is there a chance the problem might return? The retube seemed to fix it, but as I said the ringing persisted for a few seconds when I powered it up with the new tubes. This lasted about the time it takes for the tubes to heat up, if that means anything.

 

[Meson Boogie]

I came across this:
http://music-electronics-forum.com/t7477/
Sounds very similar to my problem. Since there are no obviously burnt out resistors, I guess it could just be a bad solder joint somewhere. I'd take the board out and look but that would be a lot of hassle, and since it's working now I guess I'll just leave it and have a look if it plays up again.