Weird tremolo effect going on with my amp...

[Deluge Of Sound]

Here's the situation, playing a Epiphone Casady into an 90's Sovtek MIG-100, through an Orange 1x15", there's a weird tremolo-type effect going on whenever I hit an A... its most prominent on the low A (I'm playing in standard tuning).

Now, I've run my backup head (an apparently immortal Peavey Mark VI) into the cab, and there's no wobble to be heard... I assume its a tube problem, but I'm not sure what to do about it, as this is the first problem I've had with this head in time that I've owned it. Am I due for a retubing? What exactly is going on?

Care to help a brother out?

[BassmanPaul]

Sounds to me like you have a filter cap on its way out. When this happens you can get all sorts of weird and wonderful symptoms. I'm guessing that your A is exciting the positive feedback that the cap is allowing making the amp oscillate at a low frequency.

Paul