A brief diagnosis question?

[rockstarzusa]

Hi all, I have a Divided by 13 FTR-37 amplifier that emits a very faint rumble sound that causes the volume to drop or spike significantly when playing. Because the amp is relatively young and has been well cared for I am not tending to think of caps etc, but perhaps a tube issue. The amp contains 6 preamp tubes, 1 x 5879 1 X 12AT7, 4 X 12AX7. I have replaced the 4 12AX7's to begin with, the sound still persists. I don't have a spare 5879 or 12AT7 to test at this moment. The valves are 6V6's, and I have not got any spares of these at this moment either.

There are no associated hums or pops or interference with the amp, just the slight rumble, and volume spikes and surges. Cables, guitars etc have all been eliminated as potential issue.

I am taking the amp to a tech tomorrow, but just wanted to see if this sounds like a common particular issue for some of the more savvy amp techs here?

Thanks in advance

[AJ_Newkirk]

Sounds like a tube issue to me. I've had this happen to a few of my amps, usually when the power tubes are shot.

[SatelliteAmps]

+1 Tubes are almost always first to suspect.

[pir8matt]

Rectifier tube maybe? I had a DC-30 that was doing this, and it went out entirely on me at one point - I tapped the rectifier tube and it came back on, but then finally died completely.

Cheap enough to replace and find out, I suppose.

Is it a consistent issue though? I find that the FTR37 (I have one) is very finicky about wall voltage. I've had mine brown out on me at venues that had shaky electricals. I keep saying Im going to buy a voltage regulator but havent been gigging enough to justify it.

[Blue Strat]

I suspect a bad tube/tube socket connection.

Start by cleaning all tube pins and sockets as per the FAQ on my website, under Resources.

[donnyjaguar]

Agreed on the output tubes being the culprit. The symptom could also be caused by sub-audible oscillation. Most likely caused by a whipped tube. Unless someone has upped the value on the coupling capacitors or otherwise bastardized the bias circuit (if its a fixed bias affair).

[rockstarzusa]

Thanks guys, well my tech seemed to find tw of the 12AX7's were quite microphonic in nature... so next week I'm taking it in for a bit of a service. He said the socket was the possible culprit (I say possible because the problem never occured while there with him)