Brimistor Advice Required

[paul1962]

Hello everyone,

I have an issue with what appears to be a Ferguson 3002, badged as a Marconiphone, as seen in the Record Player section of the forum.

According to the schematic, the Brimistor is a CZ8A type. No matter how hard I search, I can't find one anywhere for love nor money.

I can find some CZ1A Brimistors though and was wondering if I could utilise this in place of the CZ8A.
It is used as a thermal current limiter feeding the heater supplies of the usual UY85/ UL84 valve arrangement, common in most valve players of it's era.

I understand it's not original fitment, but with availability purposes, I can't see any other option apart from modern resin types (which have around 600 ohms less at the 20 degree temperature standard)

Any help gratefully received.

Kind Regards

[trobbins]

The 1960 STC CZ1A and CZ8A datasheets indicate that 2x CZ1A in parallel would be similar to a single CZ8A for the resistance vs temp plot, and also somewhat for the resistance vs current plot near 100mA. Perhaps may need to added some series resistance to adjust the heater current to nominal 100mA at nominal mains voltage once stabilised.

[dazzlevision]

From a 1959 STC Brimistors data booklet:

[Station X]

Looking at some Dansette diagrams, for players with a similar UY85 and UL84 valve line up, with the heaters fed from a tap on the motor, they don't even have a series resistor yet alone a Brimistor.

If you can't find a Brimistor substitute you could use a series resistor at the expense of heater life. 200R at 3 Watts might be a good starting point.

[paul1962]

Hello Graham,

I have to say, I have been checking a few schematics in the hopes I may find one with the same Brimister and have found the same situation as you have explained. It seems that almost, this circuit has been over engineered and very rare to find a thermistor fitted anywhere. Almost every circuit using the same valves has a straight through power supply from the motor tap.

One would suspect that fitting a series resistor, as you suggest, would still be a better option than most Manufacturers circuits and would have better regulation to the heaters.

A good call I feel and thank you for the suggestion.

Originally Posted by Station X

Looking at some Dansette diagrams, for players with a similar UY85 and UL84 valve line up, with the heaters fed from a tap on the motor, they don't even have a series resistor yet alone a Brimistor.

If you can't find a Brimistor substitute you could use a series resistor at the expense of heater life. 200R at 3 Watts might be a good starting point.

[G6Tanuki]

I have happily replaced failed Brimistors in 100mA series heater chains by a simple resistor, and have several Eddystone 840 type communications receivers in frequent use that have not had any valve failures in well over a decade.

I suspect that the Brimistor is more necessary in situations where you have either very few valves, or where there is a mix of valves intended for series heater service mixed with some not so designed, I am thinking here of the old tellies that might have an EB91/6AL5 double diode in the chain, which lit up like a lamp bulb because of the low thermal inertia of the filament/cathode compared to other valves in the chain.

[paul1962]

Some very reassuring information there G6Tanuki and thank you for your insight.

Originally Posted by G6Tanuki

I have happily replaced failed Brimistors in 100mA series heater chains by a simple resistor, and have several Eddystone 840 type communications receivers in frequent use that have not had any valve failures in well over a decade.

I suspect that the Brimistor is more necessary in situations where you have either very few valves, or where there is a mix of valves intended for series heater service mixed with some not so designed, I am thinking here of the old tellies that might have an EB91/6AL5 double diode in the chain, which lit up like a lamp bulb because of the low thermal inertia of the filament/cathode compared to other valves in the chain.

[kalee20]

If it's fed from a motor tap, then I'd second G6Tanuki's advice. The resistance of the motor winding is sufficient to provide adequate surge relief.

Strictly, as there isn't the 'hot' resistance of the Brimistor in circuit, you may want to add a bit of equivalent resistance to compensate (the motor tap voltage will have been designed I guess with this in mind). So measure the valve heater voltages after it has been on for 10 minutes, just to check (the UY85 is 38V, UL85 is 45V).

As to surge limitation - these valves have robust heaters, though to set your mind at rest just look at the heaters after switching on from cold in a darkened room. You shouldn't see any momentary overbright regions through the glass.

[PJL]

This is the other thread relating to this repair where the Brimistor appears to be overheating https://www.vintage-radio.net/forum/showthread.php?t=221882&page=2.

What I am not too sure about is the impact of the part motor winding on the AC waveform so voltages should be measured with a true RMS meter.

I have just noticed that the voltages on the brimistor measured at 95/113V which exceeds the 83V heater chain. Assuming it is running at 100mA, an alternative resistor would need to drop 113-83V or 30V at 0.1A so it would need to be 300 ohms and would dissipate 3W so is going to get hot.
The alternative would be to add a 100 ohm resistor in the heater chain which should bring it closer to the 83V.