Amp Fizzing Out? Shorted Tubes?

[NDRU]

Today my F30 started sounding fizzy & quiet. I put it on standby for a few minutes and it was fine.

Could this be the tubes shorting out, as described in the manual? The fuse was not blown. The power tubes are pretty new, not making any noises when tapped.

I have a show tomorrow, I'm wondering if I should bring a different amp. Like I say, it worked fine, but it's disconcerting when the amp does something funny.

It that a one time thing or a sign of bigger problems around the corner?

 

[AJ_Newkirk]

My MarkIII did the same thing last week and it ended up being a defective jack in the speaker cabinet. If you are using a head i'd first check the cable and the cabinet wiring/components.

In my case it was a very easy fix...but would have been certain death to my MarkIII had I not caught it.

 

[NDRU]

That's a thought, since I was messing around with using an external cab recently, I might have plugged the internal speaker in incorrectly.

I only wish I didn't have to drive across town to look at it right now!

 

[NDRU]

I don't think it's the speaker. I'm leaning more towards the preamp tubes.

How do I tap the tubes to listen for microphonics? They have covers, and if I remember correctly, the amp won't turn on if the covers aren't on. If I tap the covers lightly with a pencil eraser will that accomplish the same thing?

 

[AJ_Newkirk]

Since you were playing around with an external speaker, first try plugging a cable into the external speaker jacks a few times..to make sure a jack isn't giving you trouble.

If it isn't a jack issue, try tapping on the preamp tubes as you mentioned. Not sure how effective it would be to tap on the tube covers, but i tend to think a bad tube would at least make a slightly louder noise than a good one even with the cover on (?). My amp comes on fine when I remove the preamp covers. Please exercise caution when doing this!

 

[NDRU]

I was wrong about the tube covers, it came on fine without them.

I wasn't able to duplicate the problem, for better or worse. So either everything's fine of it's waiting till my big solo in front of the record execs to blow up.

 

[NDRU]

It happened again today, and I noticed that the Tube was glowing pretty bright. I turned it to standby and it was fine, but I think maybe these tubes don't agree with the amp.