Car radio fuse rating and type?

[Station X]

I've just repaired a mid 1960's car radio. It's mono, transistorised, with a push pull output stage and has a specified output of 8W. It can be pushed to 10W if you don't mind a bit of distortion.

The manufacturer's service sheet says that the in line 32 mm x 6.3mm glass fuse should be rated at 5A. This sounds a bit high to me, but possibly represents the lowest value automotive fuses available at the time of manufacture when most cars had just a couple of 45A fuses serving everything that was fused.

I'm happy to stick with a 5A fuse, but what speed of fuse should I use? T, F or FF?

Thanks.

[kellys_eye]

Recall that the fuse protects the supply wiring, not the equipment itself so perhaps select according to what your particular wiring situation is although I suspect any car wiring is rated higher than 5A anyway. Standard 'blade' fuses don't seem to be anything special so the type (T, FF ect) might be irrelevant

[Station X]

Originally Posted by kellys_eye

Standard 'blade' fuses don't seem to be anything special so the type (T, FF ect) might be irrelevant

Automotive blade fuses seem to be available in slo blo, normal and fast acting types.

[Herald1360]

FF probably isn't a good idea, but anything else will be fine. As already said, it's only there to protect the car wiring- the trannies in the radio will protect the fuse!

[Nickthedentist]

8-10W is a pretty impressive output for a mid-60s car radio, I have to say.

[Herald1360]

FF probably isn't a good idea, but anything else will be fine. As already said, it's only there to protect the car wiring- the trannies in the radio will protect the fuse!

[paulsherwin]

Not worth worrying about. Car radio line fuses blow because of some catastrophic event, usually a short circuit unrelated to the radio itself.

[Radio Wrangler]

5A was the norm back in the day, and the fuses were just straight tinned copper wire, no sped modification either way. By the time they'd blown, the damage had been done, anyway. The fuse just stopped a wire melting in the harness - very nasty and very expensive.

David

[Herald1360]

Originally Posted by Nickthedentist

8-10W is a pretty impressive output for a mid-60s car radio, I have to say.

Yes! Does it get there by using transformer output or multiple speakers in parallel?

About 5W is the limit for a standard complementary output stage on 14V supply and a 4ohm speaker. A bridged output would just double that.

['LIVEWIRE?']

As a matter of interest, what make & model is the radio? Most mid 60s ones of which I know ( Radiomobile, Motorola, Philips, Pye, Ekco, Elpico, & Blaupunkt ) had amplifiers capable of producing 5 - 6 watts into a 3 or 4 ohm load, maybe 7 or 8 watts into two 4 ohm speakers in parallell. The in-line fuses fitted by Radiomobile & Motorola were always 1.5Amp 1.5" (32mm) Glass types, and the average current drawn by a typical push-pull o/p stage at max. o/p would be no more than about 0.8A @ 14v., IIRC.

[Station X]

The radio is a Pye 2060 Major and according to the manufacturer's service sheet produces 8 watts into a 3 ohm load when powered from a 12V supply. I has a push pull output stage and drives two speakers in parallel. I've measured the output as 10 watts using a dummy load/power meter.

It has a 3.5mm microphone socket and inserting a plug into this connects a mic across the volume control disconnecting earlier stages. I have no idea why this socket was provided, but I intend to try using if for an MP3 player.

The 5A inline fuse protects a mere 6 inches of wire. A short prior to the inline fuse will have to blow a 35A fuse and the wire certainly won't stand that current for long.

I don't have any 5A 32mm in stock and while searching for them online I came across this little gem which is repeated on several websites.:-

British Fuses (Lucas Type)

The LUCAS type (also an old fuse) is of similar appearance to a Glass Fuse, but that is where the similarity ends.

Available in two different lengths, 1" (inch) or 1-1/4" (inch), the information stamped onto the Lucas fuse can be confusing. Basically, the figure displayed is that of twice the continuous amp rating that the specific electrical system should be using.

Therefore, you should divide the ("stamped") figure by 2, which will give you the protected circuit continuous amperage.

I think I'll just fit the smallest fuse which doesn't blow.

Thanks for all your help.

[Herald1360]

Alles ist klar......

It's a standard p-p PNP TO3 TX type output stage, so the O/P TX ratio will be arranged to present a low enough impedance load to the trannies to extract that much power from the low rail voltage. In effect it will be a step up rather than the more usual step down TX.

With that much grunt, I imagine the mic input would be used in something like a coach installation, so the driver/guide could address the passengers. The radio would also be capable of being of some use in a noisy large vehicle. I could have done with one back in my 400E Thames days

[Hybrid tellies]

Originally Posted by 'LIVEWIRE?'

As a matter of interest, what make & model is the radio? Most mid 60s ones of which I know ( Radiomobile, Motorola, Philips, Pye, Ekco, Elpico, & Blaupunkt ) had amplifiers capable of producing 5 - 6 watts into a 3 or 4 ohm load, maybe 7 or 8 watts into two 4 ohm speakers in parallell. The in-line fuses fitted by Radiomobile & Motorola were always 1.5Amp 1.5" (32mm) Glass types, and the average current drawn by a typical push-pull o/p stage at max. o/p would be no more than about 0.8A @ 14v., IIRC.

We always use to add an in line fuse holder with either a 32mm or 20mm fuse rated at 1.5A. I saw the results of not fitting a fuse on such a radio when a bodged repair went wrong and the whole radio and feed cable started to smoke and then catch fire. The 30-40A fuse in the car's fuse box remained intact.

[emeritus]

My understanding is that it has long been the peculiar practice in the automotive industry for fuses to be marked with the current at which they will blow, rather than with the continuous current carrying capacity that is used by everyone else. I guess this is consistent with the "divide by two" Lucas fuse advice.

['LIVEWIRE?']

AFAIK, the in-line fuses fitted by all manufacturers during the 1960s & early 70s was either 1.5 or 2 Amp, though some later Blaupuknt models were supplied with a 3.15A x 20mm fuse in 'clip-in' holder on the back of the radio. This being the case, 1.5 or 2A in-line fuse seems a good choice for 12 volt radios of that era. 6 volt ones would probably need a 3 Amp fuse