Motorola 114 Car Radio

[Old Sparky]

I'll be replacing the 560 ohm and others with metal film types anyway, Jeffrey, unless there's any reason (and i don't know of one) why these shouldn't be used in a typical complementary symmetrical audio amplifier circuit employing silicon transistors.

[ppppenguin]

The only engineering reasons I can think of for using carbon composition resistors these days are for their surge current capability and possibly their voltage rating. If you need higher voltage capability then you can usually use 1W or 2W film resistors or VR37/VR68 series high voltage parts.

I'm not sure why you would use carbon composition instead of wirewound for surge current capability. WW are probably more expensive and their failure characteristics may not suit some circuits. They might hang on for too long under fault conditions. That's true for 100R and suchlike. For much higher values, such as the 68K linked by Mark, you just don't get high surge currents. I'm left wondering why you would ever want to use such a part.

[Terry]

The price is a bit of a shock. The RS parts are over 20p each. 0.25W 1% metal film parts are usually under 2p each.

Yes, I noticed the high prices too!

When I started work in 1960, we bought Erie 20% resistors in dozens, neatly packed in cardboard packets with cellophane windows, for 4d each - just under 2p, the price you quoted for modern 1% components.

Using the Bank of England's inflation calculator, 4d in 1960 would have been 7s 6½d - ~38p in 2018!

[Mark Hennessy]

That series goes up to 22M. They are used in some audio applications, such as guitar amps.

[Terry]

When I started work in 1960, we bought Erie 20% resistors in dozens, neatly packed in cardboard packets with cellophane windows, for 4d each - just under 2p, the price you quoted for modern 1% components.

Using the Bank of England's inflation calculator, 4d in 1960 would have been 7s 6½d - ~38p in 2018!

Actually there was an error there, the modern day prices would be 9s 0½d (45½p) - I forgot to add the 20% VAT!

Of course, there was no VAT in 1960 and its predecessor, Purchase Tax was levied on complete receivers, CRTs and valves but not on other individual components.

Valves designated as industrial types were tax exempt which led to the strange anomaly that the EL95, widely used in Grundig tape recorders, was the only tax exempt valve we ever stocked or sold!

PT went up and down like a yo-yo - it was every chancellor's favourite tax knob to twiddle with and hardly a budget went by when it didn't change: 25%, 50%, then back to 25% and back again until one chancellor found a new game to play and the 50% went down to 45% and so it went on.

It wasn't an accumulator tax like VAT, it was levied on the wholesale price and that was that. One of our wholesalers sent us a look-up table which they updated after every budget so it was case of pick up valve and read price: 12s 6d plus PT. Look at table on wall: Price = 12/6 PT=2/1. Add both together and charge customer 14/7.

Oh happy days!

[Amie]

Remembering a conversation with dad.

When I once asked why he was replacing so many resistors in one of his last restoration projects, his answer was that although the guts of the Pam TV he was working on was in near pristine condition, it had been stored unused most of its life, and that compo resistors would drift in value more than if ran at near full ratings. He wasn't just doing it fort the sake of it either, some were factors out

The output transistors 2R2 1/2W composition  emitter resistors in a little practice amplifier I repaired recently one had turned to charcoal the other O/C, so I replaced them with 1/2W carbon film types that failed almost immediately, every day a school day

Thanks,
Amie.

[Terry]

The output transistors 2R2 1/2W composition  emitter resistors in a little practice amplifier I repaired recently one had turned to charcoal the other O/C, so I replaced them with 1/2W carbon film types that failed almost immediately, every day a school day

Oh dear, it sounds as if you replaced the resistors that had been damaged by the original fault but omitted to find and rectify the fault itself.

[Amie]

Yup.

One OC 81 dead short....

(Series push pull )

Amie

[Mike Watterson]

Composition resistors are made of a mix of clay and charcoal/soot, hence go off badly when damp and with age.

If the push pull is transformerless and both same polarity, it's "totem pole", if NPN and PNP then "complementary push pull".

There are series type amplifiers. RF cascode is one type. It's quite interesting, used valves then transistors. Practically obsolete now with single GaAs transistors with 1V supply, 0.2dB noise and up to 200 GHz.

[Amie]

Thanks Mike.

Transformer series drive?
OC81D single ended driver 2×OC81 "totem pole" transformerless output. Split battery supply, no big electrolytic for speaker coupling.

Cheers,
Amie