Power Tube glowing extreme orange and crackling from speakers.

[usalabs]

At the moment I couldn't tell y'all what model his Peavey tube amp is, but as I'm the bands sound guy, the lead guitarist called me up and said his head is crackling and one of the tubes is glowing extreme orange, he shut it off immediately it happened, and when I got there, I asked a few diagnostic questions, and found that he was messing with a Line6 POD HD500 multi effects unit connected with the guitar in to the unit and the mono out to the head, and while messing around with the different effects, he wanted a loud lead/solo sound but while messing with the parameters the default output for each module is +22db and the overall master out was at +40db, it seems he had inadvertently drove the power tubes to far, and from my experience with old TV's using tubes, when a tube glows bright orange like that is means the bias has been altered, by a fried bias resistor, so I unplugged everything except the speakers, and set the unit to standby, then powered on the head, at first during warm-up just the usual filament glow was seen, then even with all volumes and controls to minimum and the unit on standby, I started hearing the crackle which slowly built up, not that loud, and at the same time I noticed one of the power tubes started to glow orange, then get brighter, so I powered it off immediately.

Luckily he has a Marshal solid state amp he can use temporarily until I can fix his Peavey head. I am a professional electronics engineer, but my profession was old tube TV's and radios, not audio equipment, such as PA's or Amps, sooo, at a guess without the head's model number, would I be correct is saying the bright orange glow along with an initial light blue faded glow near the heater filament, could be because of the bias not set due to a possible blown resistor? But I won't be doing anything until I get the schematic.

And when I get the model number, is the schematic available on this site?


ADDENDUM
I just called the guy, and it's a Peavey 3120 tube head, S/N available if needed.

[Classic30inCincy]

Install NEW Tubes

[usalabs]

Whether this would also help, I just found out earlier today, he was using 2x4 ohm cabs, with the head set to 4 ohms, but according to the user manual, the head's output is parallel, meaning if using 2 cabs of the same impedance, then the head would need to be switched to 1/2 of one of the cabs, so,, being that the head has a minimum setting of 4 ohms, that would need 2x8 ohm cabs not 2x4, as there is not a 2 ohm setting. Could this imbalance cause the tube(s) to overload and burn out with constant use of the imbalanced cabs, over time?

By using this formula the switch on the head can be set according to this, assuming the cabs are of equal impedance:-

Switch = 4,8,16 ohms
cab1 = impedance of CAB1
cab2 = impedance of CAB2

Switch = 1/((1/cab1)+(1/cab2))

so, if the cabs are 8 ohm, that would mean:-

1/((1/8)+(1/8))=4

basically means:-

1/8 = 0.125
0.125+0.125 = 0.25
1 divided by 0.25 = 4

So the head would need to be switched to 4 ohm, which is the minimum the head can be set to.

Also, I found out, that the HD500 effects unit output can exceed the head's max input, but that would blow the pre-amp not the output stage.

[Classic30inCincy]

Recommend New Tubes AND Correcting The Ohm Mismatch

[Pappy B]

I have encountered the "Red Hot" power tube in old radios and it has always been a bad resistor/capacitor. If you find a bad resister and it has a capacitor in parallel with it I would change it to.

[Enzo]

The red hot tube was carrying excess current. It could be from a leaky coupling cap to its grid, or more likely just a failed power tube itself. If the amp has four power tubes, then a failure of a coupling cap would usually make TWO tubes red plate.

If you play "too loud" a signal into the amp, all it does it overdrive the amp. You can;t really kill tubes with signal. it just clips.

You of course SHOULD match load impedance to amp setting, however, this is not a solid state amp, so we can't blow up an output transistor from excess current. Usually being off "one notch" on output impedance won;t hurt an amp. If you run too far off an impedance load, it usually sounds bad and has less power. It doesnl;t usuall;y just up and kill a tube.

COnsider that any tube can die at any time, so it was bound to be that he was doing SOMETHING at the time the tube failed. Doesn't always mean it was the reason for the failure.