A15MK2 issues been in storage a few years, now unstable Transients cause LOUD white noise

  • Hallo there, I had successfully built this wonderful amp a few years ago, it was spectacular and worked a blast! It had a small issue with one of the tubes but I put in V3 so it only affected the loop which I wasn't using.

    Unfortunately even though it's a "small" tube amp it was still spectacularly loud, and I had a baby so I had to put it into storage.

    Fast forward a few years, I just changed pickups in a guitar and want to hear it played through a proper tube amp so I pull it out of storage and plug it in.

    For a whole 5 minutes it works spectacularly well on both channels with and without boost, oddly enough even the loop works (with a little noise).
    The I tap one of the knobs and it makes a dust crackle/transient sound and the Ammp just looses it, it falls into a white noise crescendo into maximum volume. I had to flick the standby. I managed to get the amp to work a few more times, but it's always crackly and if I tap the chassis I can hear it, and more often than not the transient triggers the oscillation cascade.

    I haven't read up on my electronics for a while now so I'm a little rusty and don't remember the circuit particularly well, I had a spare 12AX7 and tried swapping V3 that was giving issues years ago, but it didn't change much.

    Any advice?

    Do you think this is going to be a simple fix, or am I in for a wild goose chase?


  • I used the return input and the poweramp is fine, and perfectly clean, so the issues are in the preamp. However this time before turning it on, I figured since it's not been used for a while I'd turn all the knobs and flick all the switches a few times to just give them a wee bit of a clean up. This seemed to make the amp playable, there's still some noise that shouldn't be there, and even plugging the footswitch in caught some interference. Also the fat switch really kicks when you flick it, is it normal for it to have such a big transient?

    I hope it wasn't just pot luck and that it doesn't go berserk on me again. Any resources on an amp cleaning/servicing routine I could do?

    Thanks again.

  • Hi Martin,
    The use of cleansing spray with pots is heavily discussed - I have read some heated arguments about this topic. Some say it works well for a while but in the long run it would cause severe damage by destroying the pots over time.
    Do you have any personal experience with this topic?
    A little off topic, but I just wanted to ask.
    Max <3

  • Their website says "WAY OF USE: Spray on surface and leave it work for about 10-15 seconds and then remove the filth. For heavy filth, repeat the operation" so it's meant to remove mechanically the filth after dissolving it. I'm not sure that it washes out the filth from inside a pot but you can give it a try...

  • Ahh yes that could just mean that gunk gets moved around instead of eliminated, which could be worse in the long run. In the mean time the amp actually seems to work normally now, changing the microphonic valve seems to have made the loop usable again, so there's that too. I'm still a little weary about it, since on first startup it often throws itself into oscillation, and touching the wrong switch or knob could trigger it, but so far so good!

  • Oh well, that didn't last very long, I think the culprit is a microphonic powertube. Which is annoying cause they're expencive.

    Tapping them gently with a pencil, one of the valves clicks while the other doesn't, so I guess that's what sends the amp into noise meltdown.

    While I'm asking, is the A15 mk2 compatible with powervalves other than the 6v6?

    Thanks again.

  • Thanks again for stopping me from doing something silly!

    My issue now is that I have difficulty figuring out what is actually causing the noise and malfunction, I still strongly suspect the powertubes, but I'm not entirely sure. Basically the whole darn unit is microphonic, even just tapping the chassis, with result in a corresponding sound through the speaker, all the valves will sound if tapped. Is this normal?

    To actually use the amp I basically tap it gently until it finds its equilibrium and hope it lasts.

    At the moment I switched out the loop valve and the inverting valve and put ECC82s instead of ECC83s, these seem to be a little quieter and dont tend to cause the amp the same kind of aggravation, and I can use it more, but it's not fixed and the tone is somewhat less "Alive".

  • I don't think it's all tubes, it's more likely that it's just one tube being very sensitive. Crackling noise is typically either a broken tube, soiled tube sockets, a faulty plate capacitor or a bad soldered connection that results in an unstable electrical connection.

  • Yup definitely sounds like I'm going to have to open it up. I'm told I can't just change a single powertube out, I have to get them paired, and rebias once their in.

    Thanks again, I will eventually get this thing working properly!

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