Amp Trouble

[NLD09]

I've got a tube amp that's cuts in and out while I play. It sounded like it simply had a bad tube, but to be on the safe side I took it to an amp tech and had him do some work on it. He replaced a switch and resoldered a few things, changed the tubes, and I even had a new speaker installed (more to change the tone.)

Today he called me saying the amp was ready and fixed, but when I got there the problem still remained. He messed with the impedance switch on the back and I couldn't get the amp to cut out anymore.

Now i'm home and it's cutting again. The strange thing is the tubes. In standby they glow orange, and when I turn the standby switch on to play a blue glow joins the orange glow. However when the amp cuts out the blue glow cuts out with it. Any ideas before I take the amp back tomorrow?

 

[jimh]

I'd say you've got a biasing problem.

What make and model is the amp?
What are the tubes? EL34's, 6L6's etc?
Do you have any idea how they are biased by the circuit? or what the bias is set to?
:dontknow:

We'll figure it out 

 

[NLD09]

They're 6L6's. It's a carvin clubmaster from the valvemaster line. switchable bias so it can run on either EL34s or 6L6's.

I have no idea how they're biased in the circuit or how to tell.  I've got no experience with amps and was more than happy to take it to a tech.

 

[NLD09]

I understand that the blue glow only shows when voltage is applied to the tubes. What causes an interruption to the voltage being applied to the tubes?

 

[Alfang]

I bet the blue tube is a rectifier, did he change that one as well?

What's the model and brand of amp?

 

[-CB-]

The red orange glow, in standby are the heaters.

The blue glow is the gas-effect glow, in gassy tubes.  Some tubes have no blue, most have a little, some have a lot.  Generally, gassy tubes are... prone to noise, but some sound outrageously good - like any other tube.

If the amp cuts out clean, then there is a clean break in the B+ voltage someplace.

If it was a bias issue, and it lost bias (no negative voltage on the grids) then the blue would increase, and the plates would soon turn bright orange ... and the tube would do a core meltdown and die.  If bias were an issue, and you increased bias to full negative bias voltage, then the amp would in fact cut out.  The sound of that cutting out would be a little different, softer, than the B+ voltage being interrupted, which would be a sharper cut out, maybe even with a slight pop or crackle.

See if any preamp tubes are also glowing blue... and when it cuts out... look at those.  Take the covers off and look at them.  If one of those has a glow, and it quits glowing blue... then its a B+ problem for sure, since it would effect all the tubes.  Bias only effects the power tubes.

Spray all switches.

 

[mayfly]

If the amp cuts out clean, then there is a clean break in the B+ voltage someplace.

+1.  The blue being gone when it cuts out is a giveaway.  The B+ supply has a faulty connection somewhere.  Carefully manipulating the B+ wires while it's powered up should uncover the spot.  Note that the B+ line has lethal voltages on it - be careful in there!

Oh - is this a PCB amp?  that will make things harder to work on and find the fault.

 

[jimh]

Yeah, Or of course it might be the B+  :doh: Missed that one

:icon_thumright:

 

[NLD09]

The sound when it cuts out isn't sharp or immediate. It sounds more like a car that won't start. In fact it sounds almost exactly like that. The amp comes to life, then makes a "putt, putt" noise, the blue cuts out, and everything is quiet. Even the low buzz of the tubes is gone. If this were an old car i'd turn the ignition and put a little more gas in the carburetor this time.

I agree that it has to be a connection problem. If I tap the side of the amp or lean it forward it momentarily comes back to life. I'm bringing it back to the tech today and I'll have him check the B+ supply.

 

[-CB-]

Yeah, Or of course it might be the B+   :doh: Missed that one

:icon_thumright:

Well it doesn't HAVE to be, it CAN be bias, but the likelyhood of bias going super low is not very great.  More often, you lose bias, and the tubes go runaway.

I had a Princeton Reverb, and it varies the bias to warble the Vibrato.  The thing is - even with no signal, the bias is tickled... and the tubes flicker blue to black to blue to black... if they're at all even just a little gassy.  The single channel Fender amps Champ, Princeton, would vary the bias.  This was not workable on two channel amps where you wanted only one channel to be effected while the other wasn't.  In that case, Fender came up with the neon-tube/photocell combination "vibro-bug"

 

[mullyman]

Well, one thing is for sure, you DO NOT want to go tinkering around inside there all by yourself. Even if it's not plugged in you can get electrocuted. Leave it to the tech guy.
MULLY