Thermistors in Acoustic 150 head

[disorder]

I'm repairing an Acoustic 150 head that blows fuses. Two of the four output transistors are shot (the first two in each row). Everything else seems to check out fine, however the two thermistors that sit after the transformer measure about 130-140 ohms when cold. I know they are supposed to be 100 ohms cold and decrease when hot. Does the extra 30 ohms warrant new thermistors? I measured these out of the circuit in a cold basement if that matters

[R.G.]

I'm repairing an Acoustic 150 head that blows fuses. Two of the four output transistors are shot (the first two in each row). Everything else seems to check out fine, however the two thermistors that sit after the transformer measure about 130-140 ohms when cold. I know they are supposed to be 100 ohms cold and decrease when hot. Does the extra 30 ohms warrant new thermistors? I measured these out of the circuit in a cold basement if that matters

Maybe.

This is an update of the same kind of output circuit that's in the Thomas Vox amplifiers. This circuit is very much like the Vox "Beatle" output stage. The bias is set by the 400/10W and the parallel combination of the 6.8R resistor in parallel with the thermistor. The idea is to trickle a little DC into the bases of the two output transistors. The right trickle is about 0.4 to 0.45V.

The thermistors start out high resistance, then decrease as they get hot. In the position they're in, they then decrease the DC bias by reducing the DC level to the bases. Higher value thermistors would make the thing tend to run hot.

However, I don't think this is all of your amp's problem, or even most of it. The Beatle, after all, has zero thermal compensation and - mostly! - gets away with it. I think that maybe the transistors are just marginal in the application. It's for sure that if one dies, it will another with it.

I would replace all four with 2SC5242 ($2.48 each, Digikey). These transistors are at least an order of magnitude more reliable in that circuit. For $10 you get better than new. The only real issues with this is that the 5242 is in a plastic TO-3 package, not a real TO-3. So you'll have to figure out how to mount the new transistors flat on the heat sinks with one bolt apiece. Remember to get insulating washers/compound, etc. I like Sil-Pads.

I would also
- remove the dead transistors and let it run on the remaining two good ones while making sure it works into, say, a 50 ohm load. Should work that way for testing. If it doesn't, you have other problems hidden in deeper layers of the onion.
- measure the actual values of the 400 and 6.8R resistors and correct as needed
- temporarily parallel the 6.8R with another 6.8R to 15R during bring up after putting in new transistors. This underbiases them, making the output run cold and causing crossover distortion, which is of no consequence until you have the thing putting out signal reliably. Then you can remove the extra paralleling resistors and rely on the thermistors while you monitor the line current under load as you bring it back up.

[gbono]

I found some replacement varistors here:

Disc & Chip Specifications - NTC Thermistors - PTC Thermistors

They might sample you some if you think you need them.