Why do some radios hum as they warm up?

[PaulR]

I have a couple of sets that start to hum as they are almost warmed up. The hum disappears when they "come on" completely. Can somebody explain why this happens, please?

Thanks

Paul

[G6Tanuki]

Various reasons!

Two I can think of are if the output valve's filament gets up to full temperature before the rectifier's then the HT will have a greater amount of ripple on it than when the rectifier's up to full emission, and in some radios the speaker-transformer has a small 'hum bucking' winding on the primary whose effect at cancelling the hum will only be as intended when the HT currents drawn by the output-stage and the rest-of-the-radio have stabilised fully.

[MarcoBerti]

Another reason is audio feedback, if present, does not work properly until the correct current is flowing in the AF valves.

[PaulR]

Thanks for the replies.

One set that does this to an extent as an AC/DC Philips Music Maid clock radio and the other is a Marconi 559. The latter had been got at and the original connections to the smoothing capacitors had been altered. It also has a PM speaker in place of the original mains energised one. The energising coil has been retained to provide smoothing. This set hums quite a lot as it warms up but it is more or less OK when it properly comes on. It isn't really a problem but I don't imagine it did this originally.

Paul

[Radio Wrangler]

Choke input filters need a certain minimum DC current to stop them trying to charge to peak voltage, and having the rectifiers go into cut-off over larger parts of the cycle.

David

[AndiiT]

Originally Posted by PaulR

...One set that does this to an extent as an AC/DC........

I have a Murphy AC/DC set which hums quite a bit until it's fully warmed up (usually not much more than about a minute to be fair) and I always thought that this sort of hum is just an idiosyncrasy of the AC/DC design.

Regards

Andrew

[dseymo1]

Someone has to say it - they're trying to remember the words (sorry).

[Leon Crampin]

Some of the better designed sets made by the likes of Murphy and Philips had hum cancelling primary windings on the output transformer. This circuit is only effective when equal and opposite ampere turns of hum current are present in the two primary sections. This will only be the case when all of the valves are fully heated.

Generally, with an indirectly heated rectifier, the ripple voltage will be greater until full emission is reached, which may be a little later than the output valve. This is because many of the later rectifiers (6X5, EZ80 etc) have heater cathode insulation designed to withstand full HT voltage - enabling a single heater winding to be used for all valves. The thicker h-k insulation usually results in a longer heating time. TV boost diodes are a good example of this.

Leon.

[bikerhifinut]

If someones already said this apologies.
I have noticed this effect on Valve power amplifiers, the cause in that case is likely induced hum from the adjacent power transformer into the output transformers, at least that's what was explained to me.

Andy.

[paulsherwin]

Originally Posted by PaulR

This set hums quite a lot as it warms up but it is more or less OK when it properly comes on. It isn't really a problem but I don't imagine it did this originally.

Don't you believe it! Both manufacturers and customers had a very relaxed attitude to hum in the valve era and wouldn't have seen a bit of hum in the warmup phase as a problem. Many people seem to have liked some hum as it gave them feedback that something was happening.

[PaulR]

Thanks for the replies, everyone.

Paul

[kellys_eye]

There are some people who actively associated the hum with 'good times' and Elektor magazine, some years ago, showed a construction article that recreated this 'switch-on hum effect' to add to domestic TRANSISTORISED receivers!

[jamesperrett]

Some transistor gear hums for a second or two after power up as it settles down - my old Pye 1557 record player and the Dynatron HFC101 are two that come to mind. I would guess that this is down to the reservoir capacitor charging time.

[PE9ZZ_JO22KI]

Hell, my old Labgear PC speakers and subwoofer buzz for a second when I switch them on. I'm sure that when I feed them with DC that's over. But I should replace the set anyway, it's noisy.

Tjerk, 9ZZ

[avocollector]

I'd say that switch-on hum was also possibly a sign of the smoothing caps in the power supply pi filter after the mains transformer/rectifier getting a bit weary/leaky before reforming completely.