Weird amp problem

Adam_MD

So my Mesa has started making weird noises. Once it's loud enough and has gotten up to temp you can hear a slightly delayed sound after the note you've just played.

It happens on both channels and has an almost digital tone about it. I've tried to record it tonight using my phone but you can't hear it on the recording.

I replaced the power valves a few weeks ago. Anyone have any ideas what could cause this? Sorry for the vagueness but I'm not sure how else to describe it.

SunDevil

I'm no expert, but is it valve rectified? ..thinking it might be ghost noting due to a lack of power supply filtering (and valve rectified amps are limited in the amount of filtering they can have) and so would be more prone to a filter cap going South?

ICBM

Did it do it with the old power valves?

If not, put them back in and see if it still does.

Adam_MD

Yes the noise was there before I changed the power valves and hasn't changed. Ghost note is actually a good description of the problem. You only start to hear it when you're really driving the amp. it happens on both channels and at all power settings.

ICBM

Could be a preamp valve. What model is it? Does it have reverb?

Adam_MD

Mesa Ta-15 no reverb. According to the manual

V1 A+B parallel input stages for channel 1
V2 A input stage channel 2
V2 B 2nd gain stage for channel 2
V3 A+B tone control driver channels 1 and 2
V4 A+B driver phase inverter

http://mesaboogie.com/media/User Manuals/TA15_110210.pdf

ICBM

If it's a valve problem at all it has to be V1, 3 or 4 if it does it on the clean channel, so if you don't have a spare 12AX7 you can try V2 in any of the other positions to see if it fixes it. If it doesn't, it's unlikely to be a valve problem.

Adam_MD

Thanks I'll order a new 12ax7 and see if I can find one which is causing the problem.

Adam_MD

Well I swopped V2 and V3 around and the weird noise has stopped. I have 2 new 12ax7s on route from hotrox so I have a spare. Thanks @ICBM

ICBM

Keep the noisy one too but mark it as such - you never know when something like that can get you out of a fix even if it's not ideal or permanent. It might even be perfectly OK in some positions, eg the phase inverter.