Turtlerock Forum

What is this?

Its on an old AWA germanium mic pre, but on what seems to be an output driver (impedance?) board rather than a gain board. The metal film and carbon resistors show the scale.

In circuit terms it seems to be part of some zoebel like filter structure. It lies between the signal path and ground, ahead of any transistors, in series with a 680R resistor and both in parallel with a 10K resistor from the same point in the signal path to ground.

I'm guessing it is a small inductor but I have no idea if this is right or how to obtain a value for the component. I've not seen one of these before!

Any help?

Thanks for looking, everyone, but no suggestions? I think it is too small to be an inductor - a simple bead perhaps?

A wire-wound resistor?

Yeah, a wire wound resistor would be my guess too.

Don't think it would be an inductor in the front end of the preamp.

Unsolder one end and measure the resistance.

Where's Alastair Reynolds when we need him.

There's a good chance Alastair would have a schematic for it.

Thanks for the ideas - my thought is that a resistor in series with another resistor and in parallel to a third makes an odd resistor network to ground. You would simply use one resistor of the correct value instead, yes? The circuit as I read it is that the 10K is the input impedance, and the 680R in series with this nooble is doing something else.

However, getting the multimeter over it is not a bad idea ....

could it be some kind of cap? If it's in a zobel network that would make sense.

Given its probably in a piece of ex broadcast gear it may well be a small RF choke to prevent oscillation in the following transistor. What piece of AWA kit is it in ?
As for "where am I" Greg, my account was off for months so I couldn't reply to anyone, (now fixed) and I've been in Europe & the US playing with radio stuff. In fact , a lot of time at the Gates Transmitter factory in Quincy Illinois ( home of the original Sta-level) playing with big transmitters , they make up to 2 megawatt AM , although I only got to play with a 200 Kilowatt one. For reference, commercial AM in Aus is 5-10KW typical, ABC is 50KW in cap cities, so we're talking monster transmitters here.

Thanks Alastair

Its in an AWA BAP3 mic pre, which had a rats nest of a power supply to create -37V, massive in and out trannies (that sound fantastic and give unique character), 2 amp cards separated by a 10K volume pot, and then a card with this part before the output trannie.

So the input to this card is the output of the 2nd post-gain amp card. An RF bead is the most intriguing idea with best fit to the circuit, it certainly does not look like any cap, modern or vintage, I can find.

I've got a method to work out inductance so I'll work through that and see if it gives a sensible answer.

Thanks again everyone.

Well I was wrong about it being a RF choke
But
I can tell you what it is and what it does.
It is a 4k7 thermistor, part no Philips B8-320-07P.
Its purpose is to control the bias on the driver stage in order to maintain linear operation as ambient temperature changes.
It will be on the base of a 2N217 pnp transistor, driving a push-pull pair 2N217 & 2N647 for the output drive of a transformer.
AWA tended to use standard building blocks, so this will be similar to a BAR-1 probably.

Thank you very much Alastair for the time and effort to chase this down for me. Your part description matches the circuit exactly.

So there we go, guys, its a thermistor - which is a resistor whose value is (deliberately) temperature dependent.

Pretty sure I would NOT have guessed that by myself!

Thanks again for all input. Alastair, do you have any guide on the input transformer leads - I know the ones that were connected but I'd love to know what the others do. Its marked 1XC61069, and I have primary in as Brown(+), Blue (-) and White (Ground), secondary out as Black (+) and Green (Gnd), but there are also orange, grey and yellow that are not connected, which I assume to be different primary windings that offer different impedances.

Regards

Hi Chris,
On those input transformers , there are 2 ways of connecting for different standard mic impedances.
Primary, starting from the top is
blue
orange
Grey
Yellow
Brown

Secondary is (from the top)
Green
Black

The white wire is the electrostatic screen

Blue & Brown is for 150-200 ohm
Orange & Yellow is for 30-50 ohm
Grey is the centre tap

On a BAR-1, Grey, White & Black are all tied to gnd. Green goes to the base of the first transistor via 100uF electro

Cheers
Alastair

You deserve every one of those valves in your endorsement, Alastair.

Thanks champ.

I just came across a very similar part in an old Univox J6 synthesizer. It is listed as a "brimistor" on the parts list - Brimar's proprietary name for their thermistors. At first I thought it was a burnt out wire-wound resistor until I remembered this thread!

This is one of those threads that show how unique this forum is on the WWW, and the rare standard of excellence of the endorsed audio professionals. A true gold mine of info. where's the Australian flag when you need one ha ha !

AMAZING!
Totally agree.