should early 60s radios hum?

[chokesrule]

my recently up and running saba villingen is being spoiled by a back ground hum... am I expecting too much out of this radio to have no hum? I replaced all the capacitors with quality polyprops and electrolytics just left the original polystyrene and ceramic...It hums most on medium and long wave.....Alan

[Steve_P]

How much hum, and is it AC or AC/DC circuitry? The AC/DC circuitry does tend to hum a bit sometimes, it must be said.

But that is rather more expensive, Saba sets were as a rule. Has the smoothing can been changed? Does this hum vary with the volume control?

It could be a touch of heater-cathode leakage in a valve.

Cheers,

Steve P.

[paulsherwin]

I've never encountered a valve domestic radio that has absolutely no hum. There's a lot of 50Hz AC floating around inside a valve radio and it's virtually impossible to prevent some of it turning up at the speaker given the cost constraints involved. Even professional audio gear was prone to hum - listening to a vinyl record made during the valve era on high quality equipment will reveal some hum in the noise floor (this is normally removed as part of CD remastering so won't be heard on a CD reissue.)

That said, there shouldn't be obvious hum present when a radio is in normal operation.

[murphyv310]

In a word not really, there is usually slight perceptible hum but thats all.
I have about a dozen radio's and none of them hum bar a KB Rhapsody mains/battery and its pick up from the DAF96.
There are many types of hum that you can hear on a valve radio, the usual culprits are open circuit or low value (lost capacitance) smoothers, smoothing chokes with shorted turns, full wave rectifiers with one section not working, heater cathode shorts or leaks in valves, which can be any valve especially in AC/DC sets. Also ON/OFF volume controls not properly earthed can pick up hum, poor screening on audio leads. O/C caps on the heater line (decouplers).
Modulation hum also can be a problem, this manifested as a hum when tuned into a station this is often caused by either an O/C cap across the mains switch or the caps across the heaters.
Certainly yours sounds like modulation hum which is more common on AM, perhaps the mains filter cap (usually across the mains just after the ON/OFF switch) was removed prior to you getting the set, you could try a 0.1uf X type cap rated at 275vac across it and see if it helps.

[chokesrule]

my hum does not vary with the volume...if I completely turn the volume down , this is when the hum is most audible.... i mean that as you turn the volume up: the music drowns out the hum.....but at comfortable listening levels it is still very audible and annoying......I have changed "all the caps" ...the valve in the tuner module is an ecc85 and I think I can straight swap it for a russian 6n1p which I have in my parts bin but I want to double/triple/quadruple check this: I am new to valves...but as there are only 4 other valves I might try getting hold of replacements...
Paul, I have just built a valve output stage for a cd player and yes I get a very faint amount of hum from it but the radio gives much much more....I think I will leave it turned off until my diagram arrives and check everthing systematically ; go thru all these pointers you have all given me....i have put 100k bleeder resistors on the high voltage caps :glad I have done that now!!! Yes, Trevor I just had a look and there is no mains filter cap across the mains.. you might have it here as somebody before me has worked on the radio.....on my way down to the high street as soon as I can! I will let you know how I have got on Thanks Alan

[DAVEHALL]

Something for the experts to ponder over .Sadly at the time I had little/no electronics knowledge , so I can't describe the component.From memory it was a wire wound pot across /somewhere in the supply to the heaters.
Getting fed up with the cost of batteries for my portable ,as a young lad, I bought a HT/LT power unit. ( 90v + 1.5 heater) ,from the mains. One day i found I had hum problems ,and by playing around in the heater section ,found a dry joint to the pot .Soldered up it cured problem . Bit of reading suggested that this was known as a "humdinger" -but can't find any mention of this on line .Variation of the pot only optimised hum ,it didn't seem to change heater volts . ( or so my cheapo meter told me )

As said -one for the old timers .

[peetleetech]

Somewhat chastised I will try to add a relavent comment. I have been looking at this particular sort of problem on valve sets for a little time now. My first thought was that if AC is applied to the filaments then there must be hum inherent in the valve. Then I thought that there must be feedback to cancel the hum out but as almost every set and circuit diagram I have looked at shows AC on the heaters. I am stuck. Will the performance be better if one places DC on the filaments? If it is a 6.3 VAC would the DC voltage be 5V then?

[Steve_P]

Dave - try looking at this thread: https://www.vintage-radio.net/forum/...ead.php?t=7469

Peetleetech - Try here: http://www.kpsec.freeuk.com/acdc.htm

Explains it better than I could.

Cheers,

Steve P.

[Boom]

Originally Posted by peetleetech

My first thought was that if AC is applied to the filaments then there must be hum inherent in the valve.

This is something that I have often wondered. I suspect that the reason it has not - to my knowledge of course - been done is that in ye olden days it would have been all but impossible to design a domestic low voltage supply into a radio that could feed the heaters with enough rectified d.c. from a.c. mains. These days it would of course be a fairly easy thing to do so.

[howard]

I restored a Saba Schwarzwald a few months ago and that didn't hum once overhauled. I also have a 1961 Nordmende Elektra and that doesn't hum either. Both are AC mains sets like your Saba Villingen with selenium rectifiers. The Villingen may not have a 240v option, if not have you dropped the mains voltage down to 220v ? Have you replaced the 2 x 50uf smoothing capacitors ? If not, try replacing them temporarily with new 50uf 450v electrolytics. Check the rectifier as well. Finally try swapping the valves with known good ones.

Howard

[murphyv310]

OK a bit of simple theory is required, Battery valves have a "filament" a simple wire loop that is fed with DC it is also the cathode and to work properly needs a smooth supply, preferably a battery as the loop will act as a grid and the valve will amplify the "ripple" from an AC supply or a poor AC mains derived DC supply.

AC valves have a "heater" they are totally isolated from the cathode and under normal circumstances are not effected by the AC that is used to feed the heaters, But!! if there is a heater to cathode leak that can cause hum, though it is not always the valve, valve bases can leak too, heater supplies can leak across bases to cathodes and grids inducing hum. There is no point trying to go to DC as a good set really should not hum, if you have noticeable hum you have a fault, I would never modify a circuit I would find the culprit!

[paulsherwin]

If you have a hum which doesn't vary with the volume control, and which is louder than a very faint background hum when the volume control is at minimum, the culprits are likely to be:

1. Bad HT smoothing capacitors
2. Heater/cathode leak in the output or audio preamp valves
3. Bad HT rectifier

There are other possible causes, but those are by far the most likely. The smoothing caps may still be responsible even if you have changed them, especially if you've used a new old stock period capacitor.

[yestertech]

Interesting thread

I'm a bit puzzled by the conflicting statements about the hum being unaffected by the volume control but being worse on AM. Maybe there's 2 problems here ?

Assuming all the components are performing OK , then sometimes a little "re-engineering" may be required to bring the hum to a level acceptable for 21st century ears ! I have done this in a number of sets, and it's usually down to rerouting mains leads, replacing ordinary cables with screened cables or occasionally firming up dodgy earthing arrangements.
Making sets to a price usually meant accepting some performance shortcuts, and it appears very few people noticed or even minded a certain amount of 'hum'

Have we ascertained if this is 50Hz or 100Hz hum ?

I don't know this particular set, but if it follows the usual pattern, there will be an EABC80 audio amp followed by an EL84 output stage. With the volume at minimum, both of these are working at full gain, so any problems here will be independent of the volume setting.
The simplest way to split the problem in half is to earth g1 of the EL84 and see if the hum is drastically reduced.( there will still be some, probably 100Hz , remaining )
Assuming this is the case,( my hunch ) then suspect :

EABC80
decoupling cap ( if fitted ) to the EABC80 anode load
wiring to/from volume control
routing of mains cable ( is it too near the volume control audio leads ?)
Earthing points in and around that area.
screening plate/earthing tag on base of receiver cabinet ( if fitted )

If hum is at 100Hz, then it's probably down to the smoothing/reservoir caps, probably 2 x 50 uF Don't fit anything less, as this value is really necessary
Ensure good contact between the cans and their earth point.

Andy

[GMB]

Oh, it has an EL84 output does it?
Well I would check that out immediately...

One way to get bad hum is if something is drawing too much HT current, as a result of internal leakage raising the grid bias level.

I think this starts with a leaky coupling capacitor doing the same, with the increased current causing the EL84 cathode to evaporate and deposit all over the place, thus causing the valve itself to leak from the screen or anode into the grid. Valves like this look like there is more than just getter on the envelope.

A quick check is to measure the voltage drop across the cathode resistor and with the set switched off check it's resistance as the overload usually results in that drifting too - and the bypass capacitor is usually damaged as well.

Every radio and amplifier with an EL84 output stage that I have encountered has suffered this problem to some extent.

[howard]

Originally Posted by GMB

Oh, it has an EL84 output does it?

Not the Saba Villingen radio, from 1960 onwards the valve complement is ....

ECC85 ECH81 EBF89 ECL82 EM84

Howard

[Steve_P]

Much the same applies with the ECL82 and the over sensitive ECL86. But check the power supply side first. It obviously has a selenium rectifier which will probably want changing at some point.

Cheers,

Steve P.

[G8HQP Dave]

AC heaters need not hum, provided they are wired carefully. The thermal capacity of the cathode smooths out the 100Hz pulses of heat from the 50Hz supply. Old radios do not always have careful heater wiring, but fortunately they don't have really low level audio signals either so they can get away with it.

One thing I have noticed with both broadcast and communications receivers is that they don't always think about grounding in the way a modern audio designer would. There is often ample scope for heater current or reservoir charging pulses to find their own way around the chassis, polluting signal circuits as they go. For example, it seems quite common for radios to use the chassis as the return conductor for the heater current. In audio there would always be a twisted pair so that both magnetic and electrostatic induction are reduced.

[Mach One]

I can't see it mentioned in this thread before: Induced hum from the mains transformer to the audio output transformer. This would obviously be 50Hz and would manifest itself from switch on - i.e. before the valves warm up, although I have had sets where the character of the hum changes when they do warm up.

[Billy T]

Originally Posted by setsappeal

I don't know this particular set, but if it follows the usual pattern, there will be an EABC80 audio amp followed by an EL84 output stage. With the volume at minimum, both of these are working at full gain, so any problems here will be independent of the volume setting.
Andy

This is news to me and I'd be interested in knowing the theory behind it. AFAIK an EABC80 and EL84 would not be gain controlled, i.e. there is no AVC (as distinct from AGC in the RF & IF stages) so the volume control is not a gain control at all, it simply varies the level of the audio input to the EL84. It flows then, that the EABC80 and EL84 operate at a constant gain (amplification) level in accordance with the parameters set by the designer.

If the problem is independent of the volume control setting, then the cause is in usually the audio stages only, typically either ripple on the HT, heater cathode leakage, or a ground current injected via cathode or grid. The only other common cause is humbucking windings reversed or disconnected on an EM speaker, or a hum-cancelling winding on the primary of the output transformer (a favourite of Philips) that is reverse connected or bypassed. Of course you can never exclude the wild-card option of a wiring error of some sort, either recent or ex-factory.

I'd pull the EABC80 and see if it goes quiet, that will reduce the options considerably.

Cheers

Billy

Edit: Coupling between power transformer and output transformer is relatively rare, but you can check that very quickly by cutting bottom and top out of an old tin can, flattening it, then inserting it by hand between the two transformers as a shield. Keep it closer to the AF transformer, but field levels will be low and you should hear a difference immediately. If it doesn't change, forget that idea for now.

[hkn102l]

Only a thought: do you have any other Mains operated radios that you could try in the same mains socket? Does this one display the same symptoms? If it does suspect all the mains devices that are connected to the supply(i.e switched mode power supplies, televisions, cheap thermostatically controlled soldering irons, touch controlled lamps etc.etc)??
There is so much mains borne interference in most domestic situations!
cheers

John