Amp crackling when it warms up

timhulio · Post by **timhulio** » Mon Sep 01, 2008 9:37 am

My bandmaster has started crackling continuously once it's warmed-up (after about 20 minutes).

Does this indicate any particular valve is screwed, or could it be any of them?

Post by **Doog** » Mon Sep 01, 2008 9:39 am

Is this during play, sitting there but on, on standby?

Post by **Mike** » Mon Sep 01, 2008 9:42 am

Hmm.

Could be either preamp or power amp valves. Have you got any spares lying around?

timhulio · Post by **timhulio** » Mon Sep 01, 2008 9:47 am

It does it while sitting there, doing nothing. The power valves are the two 6L6s right? Unfortunately I aint got any spares. Think it's worth buying some to check?

Post by **Mike** » Mon Sep 01, 2008 9:50 am

timhulio wrote:It does it while sitting there, doing nothing. The power valves are the two 6L6s right? Unfortunately I aint got any spares. Think it's worth buying some to check?

I would say so, as expensive as they are, it's always a good idea to have at least 2 spare preamp valves and a spare set of power amp valves lying around.

timhulio · Post by **timhulio** » Mon Sep 01, 2008 9:50 am

Ah the crackling happens on both channels. I wonder if these use the same preamp valves?

Post by **Mike** » Mon Sep 01, 2008 9:52 am

They'll definitely use the same power amp valves.

How many preamp tubes are in your bandmaster mate? I'll look for a schematic.

Post by **Mike** » Mon Sep 01, 2008 9:54 am

OK that was easy:

They have a 7025 each and then share the 12AT7 Phase Inverter. The 12AX7 is fine as that's vibrato only and wouldn't affect the Normal channel.

So it points to the 12AT7 or the two 6L6GC's.

I would get a replacement valve set anyway, it might sound boss if you replace them. There seems to be a 10K bias point, are there bias probe points on your amp?

timhulio · Post by **timhulio** » Mon Sep 01, 2008 9:56 am

I looked on the Fender Amp Field Guide and it reckons:

Normal: 7025
Vibrato: 7025 and Â½ 7025

Post by **Mike** » Mon Sep 01, 2008 10:01 am

Schematic says no.

There's probably a bunch of different circuits.

timhulio · Post by **timhulio** » Mon Sep 01, 2008 10:21 am

Can I swap the power valves without rebiasing? I'm not entirely sure what that entails!

Post by **Mike** » Mon Sep 01, 2008 10:23 am

You should probably be ok if you get some by the same manufacturer and a matched pair.

Justin J · Post by **Justin J** » Mon Sep 01, 2008 3:25 pm

Mike wrote:Schematic says no.

There's probably a bunch of different circuits.

doesn't he have a bandmaster reverb?

Post by **Mike** » Mon Sep 01, 2008 3:30 pm

That makes more sense. Bubbles to the rescue

LimpDickCheney · Post by **LimpDickCheney** » Mon Sep 01, 2008 9:21 pm

I would look at the rectifier tube also, since it is common to both channels. You have my Super Reverb in a head basically.

timhulio · Post by **timhulio** » Tue Sep 02, 2008 8:05 am

Thanks chaps. I'll try the power tubes first, since I've just ordered some. If that doesn't work I'll swap some more at random til it does. Luckily it shares all the other tubes with the Deluxe Reverb I've got.

timhulio · Post by **timhulio** » Thu Sep 04, 2008 8:59 pm

Oh noes. It's none of the tubes- I swapped em all: new power tubes, and swapped the others with those in the Deluxe Reverb, which I know are fine.

What else could it be? What do bad filter caps sound like?

Post by **Mike** » Thu Sep 04, 2008 9:01 pm

That, or a dirty jack would be my next bet. Drop LDC a pm to direct him back to this thread - guy is the Fender BIBLE.