microphonic tube detector!

[jb's 52]

Hehe...

Kind of a joke but check this out. My amp started developing this
crunchy and scratchy sound. Not too loud but persistent. I let it
go for a while but finally got sick of it this weekend.

To look for the problem pre-amp tube, I tried the "pencil test."
This didn't work. So I rolled pre-amp tubes, one at a time, but
that didn't help me narrow it down either.

So, I started messing around with the switches in the back.
When I put the amp in "Hard Bypass" the old high pitch squeal
kicked in. BAM!

Now I knew I had a bad pre-amp tube for sure. I did the
"pencil test" again and found V3, V4, and V5 all to be
microphonic! V3 was the biggest culprit and much louder
than the other two and quicker to squeal. Anyhow, I replaced
all three and now the amp is dead quiet.

It's the first time I've found three bad tubes at once. But, it's
close to a 4 year old combo and also the first I need to roll
around everywhere. I'm thinking the rattling over asphalt finally
got to them.

tldr; Use Hard Bypass to expose potentially microphonic tubes.

 

[knotts]

I'm thinking the rattling over asphalt finally
got to them.

I pulled the guts out of my 2x12 and built my own head shell because of this. I am pretty sure I spent enough money in tubes to justify it.

 

[BrownieD2W]

Why are you rolling it everywhere? I'd never roll an Amp around without a Travel Case or something. Without a case I just pick it up like a piece of luggage.

 

[jb's 52]

Why are you rolling it everywhere? I'd never roll an Amp around without a Travel Case or something. Without a case I just pick it up like a piece of luggage.

Sorry, I didn't mean to imply that I am without an ATA case.
It's a good one too, 4" wheels as well. Thinking I may need
shocks and larger tires!

 

[BrownieD2W]

Why are you rolling it everywhere? I'd never roll an Amp around without a Travel Case or something. Without a case I just pick it up like a piece of luggage.

Sorry, I didn't mean to imply that I am without an ATA case.
It's a good one too, 4" wheels as well. Thinking I may need
shocks and larger tires!

Ahh ok, that makes more sense, I could still see it being rough on asphalt, in those circumstances I still prefer carrying it myself.

You mention larger tires though, you might try a quick fix at a hardware store. Look for some literal air-filled tires with fittings that work for your case, and don't keep them at 100% full, maybe 85% so the tires don't transfer as much of the shock to the amp inside. Another thing I have seen a few old-timers do is whenever they move their amps, even in cases, they stuff a super relaxed but soft blanket in the back of their amps, also helps keep any condensation out of the electronics when moving it over long distances or in cold weather.