Twin Reverb 15 Popping/crackling on startup

[stratomaster]

Hello,

I have a Fender Twin Reverb amp that pops then sort of crackles on startup. The crackling (sounds kind of like touching the tip of a cable to the input jack of a guitar when the other end is connected to a live amp) dies out after about 10 seconds or so. When I look at the back of the amp, the power tubes seem to be glowing blue and become a brighter blue in sync with the crackling sounds (even with all controls turned down in both channels). They stop glowing blue once the sound dies out. Any ideas on what could cause this?

[Phostenix]

Do you have any other tubes to swap in to test?

[melvin]

I thought a Twin was called a Twin because of the two speakers...what gives?

I suppose you know the blue glow is normal. You should also see it when you play through the amp, especially at higher volumes. It is in sync with signals being amplified because the brightness of the glow is proportional to current.

It is common for the power tubes to draw more current at power up and then quickly stabalize. If it lasted 10 seconds I would be concerned about your bias supply. But the bright glow probably just appears prolonged by the amplified crackle originating elsewhere in the amp.

If you power up the amp without the phase inverter tube, it may give us a clue. If it still crackles it unlikely in the preamp.

[stratomaster]

I will swap in some tubes and pull the PI and report back. Thanks.

[stratomaster]

Turns out I have 2 sources of crackle/static. One was a bad power amp tube (JJs are now on my ban list--I've had a 6L6 and a 6V6 go bad all within the month). This went away when I pulled the PI and changed tubes. When I put the PI back, I have an abrupt crackle/static that is very short in duration. I'm going to be swapping preamp tubes to try to rule them out. Any other ideas (plate resistors, bypass caps, etc.)??

[stratomaster]

Wow! Bad PI and Bad 6L6. Amp back to functioning sans crackle at start-up. I'm going to give Ruby's another go.

[Phostenix]

[melvin]

i would withhold judgment until you've cleaned and retensioned the sockets. just to be sure...

[stratomaster]

Full cleaning and re-tensioning is part of my re-tubing ritual. I've had JJs go bad pretty much a week out of the box within a month of each other in two different amps. Could be a bad retailer (not thorough testing), or just bad quality. I've had more JJs go bad than any other tube brand. They are now on the ban list and I will be giving TADs and Ruby's another go, for 6L6s that is.

[Lee_ranaldo]

it has to be something related with a capacitor charging.
maybe it's either a bad carbon comp resistor.

My guess is that the bias cap or the power caps have leakage.

I always change the 1.5K and 470 Ohm resistors with 1% film types for hiss free operation.

[melvin]

so, no more crackling? excellent.

just curious, what are you typically biasing the jj's at, current-wise when retubing? and what are the JJ's doing when they go bad?

I've always found JJ's to be solid, but have you tried SED's? A bit expensive, but they sound fabulous in a Twin.

[Rayme]

I had this same problem on a twin awhile back and spent alot of time on this, and still had the crackle and pop. Finally ended up isolating the output jack with neoprene washers and it has been so quiet since. Don't know if this was a fix or not but it is still working without all that noise.

[stratomaster]

I bias all my tubes to no more than 63% dissipation at idle regardless of the amp or brand of tube--unless the tubes sound too flabby. I'll tend to reduce it then. This means about 41mA per tube (using a bias probe with a 1 ohm resistor) in the Twin. It means the actual plate current is a few mA less (accounting for screen current) meaning even less idle dissipation.

I've had 3 sets of JJ power tubes all fail in a different way. The first pair was 6L6s in my Super Reverb. After about a year one of them quit conducting (no glow) and caused the amp to produce a very nasty tone. The second set was a pair of 6V6s that sounded great in my Deluxe, but one of them soon (after about a week) developed terrible filament rattle that could be heard as a ringing on the low notes and as a screeching on high notes. The third set was a matched quad for my Twin. Again after about a week one developed some sort of intermittent operation that caused a loud pop and static sound for a few seconds after start-up and a rustling sound thereafter.