KB KR010

What is R25?

The thermistor is likely a higher value at 20 C

I get my NTC and PTC mixed up. But it should reduce in resistance with increase in temperature to combat any thermal runaway in transistors.

it is a thermister
i have a dwaw full of NOS ones trouble is i dont know which one are which but if you can work out the size i can send you one for the cost of postage.
rob t

TH1 is Thermistor, R25 is a resistor that may help in deciding what to get.

Put a pot x10 value of R25 approx in place of Thermistor and if you know the recommended Bias current and have a thermometer you can vary the temperature and see what resistance the pot needs to be reduced to for the same bias. Use Fridge first (should be 5C) and distant blow heater for above ambient and measure at 10C 15C 20C 25C and 30C.

Then you know what is the best thermistor to get. Fidelity used a Germanium transistor in that circuit with no parallel resistor as a thermistor as obviously it has the same temperature coefficient. I think base to collector.

Hi
Sorry I should have put a bit more down I was in a hurry as had to go out.
Whats left of the thermistor measures about 4.5k R25 is 150R.
Mike

Then the Thermistor can't be much more than 1.5K cold (10C or 15C) or it would have little effect.

It's in parallel with 150 Ohms and at 30C maybe has to be between 50 to 200 Ohms.

Hi
Sorry I should have put a bit more down I was in a hurry as had to go out.
Whats left of the thermistor measures about 4.5k R25 is 150R.
Mike

Maybe it still works Mike, what happens to the resistance if you blow a hair dryer on it?

Eddie

Hi
Yes it does work and the value drops if you warm it up. I have put the set back together for now but I just wanted to know a little more and learn about these devices. I have seen them before, but never had to replace one.
I have also seen a transistor used for a similar purpose as Terry mentioned but not fully under stood the principle of its function.
thanks
Mike

I assume the output transistors are germanium and hence more prone to thermal runaway. Two or three diodes were a common way of keeping the bias stable because they would warm up at the same rate as the output transistors and reduce the bias to the base of the transistors. Obviously the method wasn't that great since I do remember replacing them along with the output transistors and emitter resistors. We were taught to put a loop in the end of the diodes before soldering them in, this being just sufficient to act as a heat sink to prevent the glass seals cracking.

Eddie

Hi Mike, like you, I would imagine, I did a trawl for data on the KS56W and came up empty handed, although I did find a suggestion that it might be Siemens ...

However, a thought came to me in the night ...

The chances are that other KB sets used a similar output stage design. As you have such a vast range of these sets, why not look through a few service sheets for something with similar values in the bias network?

If you find a reasonable match it should be possible to identify it by measuring its resistance at 25°C (or near miss).

I would also measure it again at 20°C or slightly lower.

From the 25°C figure you might be able to find a suitable Mullard type number from one of these lists:

http://electrojumble.org.uk/DATA3/Mulla ... -Plate.pdf

http://electrojumble.org.uk/DATA3/Mulla ... istors.pdf

If Rob is prepared to measure the resistance of the thermistors in his drawer, the 20°C figure would probably make a reasonable comparison easier ...

the problem is looking at the photo the OP does not have one to measure as it looks like only one end of its there .i could if you like send a selection of what i think it is but a photo of a compleat one would help.
rob t

here are a few likely suspects
these range from 1k2 2k1 and 2m1 at 70'f
rob t

Don't they normally get passed off as ceramic capacitors?

Hi Terry
Yes good idea. I am sure I have seen these in sets before. (and still whole in one piece)
This one is still partly working but it is so fragile every time I go near it a little more falls away. I have had it out of circuit 2 time now and it is still measuring a few K. So off to check on a few circuits and photos of circuit boards.
Rob, the ones you have look quite large and are more like the types I have seen in valve gear in the past.
Ref is right the type I have is more like a ceramic capacitor, in fact I thought I had some see previous thread but they measure infinity on the resistance scale, the construction and material is very similar looking to the thermister in this thread.. (I was the one that wrote 20p on the component)

Mike

I would have a look at a few trader sheets from the slightly more "posh" sets.
They had one of these in them.

No, as it says quite clearly, that is a Germanium diode.

I think we've already established very clearly that the wanted item is a Thermistor ...

They used to work well.
If a manufacturer bought all the PNP transistors at the same time the diodes would be made from reject PNPs from the same batch as far as practical and the thermal tracking would most likely be spot on.
They often had the B-C leads welded externally to make them into diodes.
I said have a look at another trader sheet as I have forgotten if there was a series resistor or not.
They went across the bases of the output transistors with or without a series resistor.

Then the Thermistor can't be much more than 1.5K cold (10C or 15C) or it would have little effect.

It's in parallel with 150 Ohms and at 30C maybe has to be between 50 to 200 Ohms.

Yes agreed. Going along those lines, KS56 could mean 56 ohms which seems a bit low. Maybe it's 560 ohms which is a bit higher than Mike suggests but in the same ball-park of max 1.5K

Rich

I have a KB PP31/1 (another story) which has KS19W thermistors fitted across a 150R resistors (one for each OP trasnsistor). The KS19W measures 290R at 20 deg C (room temp) and 190R at about 35 deg C (under my arm)
So I am assuming the KS56W is 560R and that Rich is correct, it makes sence that it would be higher nominal resistance as it is across a 150R resistor.
Mike

Sounds reasonable that it's 560 Ohm at the higher temperature so about 860 Ohms cold.

That's pretty much what you need to look for. It doesn't have to be exact.