Sunn 300T popping thermistor

[Jerry]

I've got a Sunn 300T amp here that has popped the thermistor off the board. The thermistor, TH1 on the schematic, says 2.5 ohms, this one reads 2.6 ohms. This is the second time in 3 months that this has happened. It happens while the person's playing and nothing else is blown in the circuit. A friend who's been doing this for a long time suggested taking out the thermistor for now and sourcing a part with a higher rating. Any ideas or opinions, I'm listening.

[Enzo]

And if a fuse kept blowing, would we source a higher rated fuse?

What is the steady current draw from the mains without signal but in operation? Is it elevated? I'd be looking for something that occasionally spikes the current in the amp.

[Jerry]

Sure wouldn't Enzo. The thermistor is back in. The amp is drawing close to 3A idling with no input.

[Enzo]

OK, that is something, 360 watts of idle. ABout 75-80 watts for heaters, and depending upon bias 100-150 watts of power tube dissipation. The big transformer will waste a few watts. A few watts burnt in dropping voltages and Vregs. And the solid state stuff uses a few watts too, say 20.

I guess I can see 360, seems a little high, but not outrageous.

The thermo is rated 8A, and the amp is rated 900W draw, so that is just within. Although I wonder if the amp gets to the full 900 watts draw.

Are there any other clues? DO any of the fuses blow when this happens? Are there any sonic anomalies when it does? Does it explode at power up or in the middle of use?

There are three tech bulletins, but only one seems apt. It blows mains fuses, but what the heck, it could pop the thermo instead. check it out:

In some amps an end cap of R326 can arc to a trace under it. The end farthest from XV3. Cure is to raise that end of the resistor enough to prevent arcs.

If you are interested, send me an email with Sunn 300T service bulletins as the subject line. send to:
tmenzo at msn dot com

I was going to figure the wattage available through teh B+ winding befoer the fuse blows, but there is no HV fuse. SO any momentary short on the B+ can take out the line fuse or your therm. In my book, that points to bad power tube, bad flyback diode, or arcing OT, in that order. Of course it may have nothing to do with B+.

Of course the heaters are not fused either, so something shorting that could get you. are the two littel 100 ohm false center tap resistors OK? Not that they would do this, but if they are damaged, it is a clue.

And after all that, I suppose if would not kill us to install a 10 amp thermo.

[Jerry]

When I had the amp apart there was some heat showing under the 330K resistors that are talked about in one of the TN bulletins. I didn't have 2W, I did have 1/2W so I put those in and elevated them per instructions. The output tube board checked out fine for components and one of the filament fastons showed some heat stress but still making a solid connection. The amp has been blowing while the owner's been playing. He said he uses different cabs but always sets the impedance switch correctly and has new Monster spkr cables. Enzo, thanks for your time!