Ashdown needs a diagnosis... phasey sound?

[ishouldbeking]

My Ashdown ABM 500 EVO II is acting a little funky lately. It started a couple months back with isolated volume drops when I first powered up the amp. Normally, I flip the switch, the volume gradually fades in and I'm good to go. But occasionally it wouldn't fade in, and the input would barely register, so I'd usually fiddle a few things and before I knew it, it was fine. This evolved into a situation where I'd be playing, usually only a minute or two after powering up, and the volume would drop way down for a moment, before stabilizing.

I asked about this before, and some folks suggested I replace the preamp tube. Did that, swapped in a JJ 12ax7, and I haven't noticed the volume dropping per se, but now it's occasionally making this strange sound shortly after powering up... I can only describe it as "phasey". Say I'm playing 8th notes, the tone will gradually phase in and out, almost seeming to go out of tune, but then resolving back to where it started. It reminds me vaguely of a crappy phaser pedal, minus the funk. It seems to stop doing it after a few minutes, and it only happens maybe every 4th or 5th time I power it on, but it's disconcerting, to say the least.

Any idea what could be causing this?

[OxJohnson]

How fast does it "phase" (quick or slow and droney..)?

What did you "fiddle" with before when you could make it go away?

Have you popped the top on it yet to see if there is anything obviously wrong with it?

Ox.

[ishouldbeking]

Originally Posted by OxJohnson

How fast does it "phase" (quick or slow and droney..)?

What did you "fiddle" with before when you could make it go away?

Have you popped the top on it yet to see if there is anything obviously wrong with it?

Ox.

The phase is slow enough to hear it phasing in and out, or back and forth, if that makes sense... as opposed to taking an EHX Small Stone phaser and turning the rate way up where everything is super warbly. It's almost like the amp is breathing, and when it hits a certain point it almost sounds out of tune. I've never really heard anything like it, other than playing with modulation pedals. Luckily it's somewhat subtle... not everyone in my band noticed it, and for lack of a better option I just played along with my band, and by the time we finished our first song it seemed to have stopped.

Re: the fiddling... it was when the volume first started dropping, and I hadn't determined it was an amp issue. My first thought was that it was one of my pedals, or a power supply, something like that. However, I plugged straight into the amp at one point, and the volume was still dropping. I have considered it might be cruddy power at my rehearsal space, in which case there's probably not much to be done short of buying a voltage regulator... but I'm not sure what kind of weird sounds I should expect from insufficient or dirty power.

When I swapped the preamp tube I had to pop the top, and everything seemed all right. Nothing burned or melted, no smoke or char marks... actually I was surprised to see it was dust free (guess those fans keep it clean as well as cool!). Given, my knowledge of electronics and amp guts in particular is practically nil, so I might fail to recognize something rotten inside an amp, but it all looked the way I expected. I couldn't believe how many bits and pieces they fit in there, honestly.

[craig.p]

Could be the new tube is bad, just in a different way than the original. What happens if you disable "valve drive?"

I assume you've already proved your instrument cord and speaker cord to be good.

[anderbass]

Next time its acting up try plugging your bass directly into the amps Effects return jack. Bypassing the preamp section like this will help you narrow the problem down to the amps poweramp or preamp sections. (note: it wont be as loud this way)

If the problem seems to be in the preamp section you could you could further confirm this by plugging your Effects Send jack (preamp output) into any other amp.