Champ 12 what made this resistor burn up?

[Randall]

Red knob Champ 12 came in for a loud hum and no reverb. I replaced the four 47uF filter caps, and the hum stopped. I noticed R31 on the cathode of the 6L6 was burnt up to around 750 ohms, so I replaced it along with the two 220uF/35v caps, C17 and C19. The two 390 ohm 1 watts R 32 and 33 look OK.

So my question is, what would make that resistor burn up like that? The tube checks out good, and the C17 although it did show as a bit leaky, it wasn't all that much.

https://ampwares.com/schematics/champ_12.pdf

The reverb turned out to be an open plate resistor on V2b.

[Helmholtz]

Measure the voltage across R31 to find its dissipation.
A shorting C17 would strongly increase R31 dissipation. This might happen only at elevated temperature.

[Randall]

The amp is done and awaiting pick up by customer. But, you are likely correct Helmholtz. Makes sense.

[eschertron]

I'd ask the owner if they replaced the power tube before giving up and bringing it to you?

[Jon Snell]

For R31 to change value due to heat is likely a faulty output valve due to age. Nothing more sinister.

[vintagekiki]

So my question is, what would make that resistor burn up like that?

Maybe pins in tube socket (pin5) have loosened due to work (age), or on tube socket there is cold solder.
Leaky is a relative term. If C17 leaky, it behaves as a resistor connected in parallel with R32, value of cathode resistor R31+R32 decreases, which leads to decrease cathode voltage, and thus increase anode current.

[Helmholtz]

For R31 to change value due to heat is likely a faulty output valve due to age. Nothing more sinister.

Tube current flows through R31 (220R/2W) and R32 (390R/1W). Larger resistance means more dissipation.
If the problem was caused by a bad tube/excessive cathode current, R32 is more likely to show signs of overheating.

[vintagekiki]

About R31/R32

R31+R32 = 610 ohm
Uk = 27 VDC
Ik = 0.044A

P₂₂₀ = 0.42W
P₃₉₀ = 0.75W
P₆₁₀ = 1.18W

[Randall]

I did wonder about the tube, and will ask of it's prior status when he arrives today. I also did retention the tube sockets, which were a little loose, and spray cleaned them. Hopefully that helped.

[Jon Snell]

Tube current flows through R31 (220R/2W) and R32 (390R/1W). Larger resistance means more dissipation.
If the problem was caused by a bad tube/excessive cathode current, R32 is more likely to show signs of overheating.

R32 and R33 are effectively in parallel, except for a couple of volts through the LEDs, which makes say 200 Ohms ≈ (or somewhere near that figure), at a 2W rating. They will dissipate less than a single 390 Ohm resistor but it makes no odds, the fault has been rectified and a new 6V6 should keep it going for a long time.

[Helmholtz]

R32 and R33 are effectively in parallel, except for a couple of volts through the LEDs, which makes say 200 Ohms ≈ (or somewhere near that figure), at a 2W rating.

You're right (I overlooked the DC path through R33). But still means that all 3 resistors will reach their individual dissipation limits at roughly the same tube current.
Of course if R31 decides to increase its resistance (maybe because it's mounted in a hotter place), that will relieve the other two.

I would still replace C17, especially if there was no problem with a bad 6V6.

[Randall]

To re-cap, I did replace C17 and C19, as well as R31. Also, the tube had been in the amp for a long time, and showed no sign of trouble on my tester. I did not think to measure current draw while it was here however. I regret that. I replaced R31 with a 4 watt equivalent (2 paralleled 2 watters). We shall see.