Normal hum level for Philips 580A

[Tim]

Hi all.
I have just finished one of the above sets. All the paper(paxolin encased) and main smoothers have been replaced/re-stuffed. Generally I am pleased with the result but there is still a gentle 50 hz hum audible in a quiet room with the volume at minimum. It's not obtrusive when the radio is playing. I just wondered if this was normal given the age of the set.
It is at the same level irrespective of volume control setting, and is really only a problem on quiet passages when one is close to the set.
Anyone else have one of these who could advise?
Thanks
Tim

[yestertech]

I don't have one of these sets, but I can sympathise !
Of course as has offten been pointed out here, a cetain amount of hum was expected and the design vs price constraints in the early days of radio often guarantees some !
However, When one has spent time refurbishing a set with somewhat better quality components than originally fitted, it can be infuriating to be met with a less than perfect result.
At this point I will admit to trying to improve on the original design as necessary to achieve a more satisfying signal to noise ratio.
I have at various times rerouted mains wiring with dramatic results, used screened cable where none existed or even extra smoothing
( where rectifier specs permit ) - just so I can have the final satisfaction of a totally "quiet set"
In one modern clock radio I used to use for bedside listening, I was annoyed by the low level hum produced. Given that I never used the set at remotely high volumes, I simply inserted an attenuator in the speaker leads and used a higher volume setting. Finally, no hum - which was really pleasing !

Are you certain it's 50Hz hum ? If so , this would seem likely to be more "curable" than 100Hz. Is it still present with the OP valve grid grounded ? Preamp valve grid grounded ? etc.
It may even be worth removing the LT centre tap and fitting a "hum dinger" to see if any worthwhile improvement results ?
A question of time spent vs. annoyance factor at the end of the day I suppose.....

Andy

[Peter.N.]

If you dont mind cheating you could try adding some extra capacity to the smoothing, it would be worth a try just to satisfy yourself if that is the source of the hum, just bridge the main smoother with say 47 uf and see if it makes a difference.

[Al (astral highway)]

Originally Posted by Tim

It is at the same level irrespective of volume control setting, and is really only a problem on quiet passages when one is close to the set.

Hi Tim. What an annoying problem! But it seems to me that if the hum level is independent of the volume, then extra smoothing and so on is not going to make a difference. If hum originated in insufficiently-smoothed DC getting into the audio output stage, it would increase with volume, with the rest of the signal.

Is everything as it should be elsewhere in the set? As you increase the current through a transformer or a choke, it vibrates audibly more.

So if your standing current is more than it should be, the choke and/or main transformer might make more noise. That doesn't have to be anything nearly as drastic as a short circuit, especially if these components are physically small...

[Tim]

Thanks everyone. This is a 1935 set, using full wave rectification and two 32uF capacitors and a resistor for smoothing, so pretty good for those days!
The output valve is a PM24M(tested!) directly heated type. There is a grey RS electrolytic across the bias resistor of this valve, but it tests fine. Actually I think there is a "Hum dinger", which I might try twidling.
As I think the hum is 50 Hz, it's likely to be pick up. No wiring is screened and the owner wanted me to keep it as original as possible, so I'll have to conceal the co-ax in varnished fibreglass type insulation.
I suppose I could try earthing the volume control case as that sometimes helps.
Of course this level of hum may well be normal..............

All quoted voltages are close to published values(using AVO 7) so I don't think there is much amiss that way. Trader sheet No 26 says measurements were taken with a "high resistance voltmeter" an AVO 7 probably WAS in those days!

[Ian - G4JQT]

Check that the hum is not being picked up by wiring around the volume control/mains switch.

It may be worth moving these wires around if possible in case it's picking up hum from the mains wiring.

I had this problem on another Philips set. There was enough slack in the wiring to move the audio and mains wiring further apart to cure it - nearly.

Regards,

Ian

[PaulR]

Hum is the bane of my radio-meddling life! I think that with modern equipment we have got used to very minimal hum and we try to transfer this standard to very old sets.

The majority of my valve sets hum to some extent. I think that much of it was there originally, some is caused by slight movement of components and wires from their original positions when replacing components, and some is due to slight H/K leaks in aging valves.

I spent ages working on the amp chassis of an 1931 HMV radiogram which had the dreaded pitch block and a very poor circuit diagram. I was hoping for great things as it has 2xPX4s in p/p. When I finally switched it on I found that the sound quality is great, but it HUMS. Nothing I have tried stops it and I have come to the conclusion that it is as good as it gets.

I would accept the hum level you describe and listen in wonder to a set that was expected to last for, maybe, 10 years and is still doing its stuff 73 years later. Amazing when you think about it.

Paul

[PaulR]

Originally Posted by Peter.N.

If you dont mind cheating you could try adding some extra capacity to the smoothing, it would be worth a try just to satisfy yourself if that is the source of the hum, just bridge the main smoother with say 47 uf and see if it makes a difference.

But don't touch the terminals of the cap afterwards without discharging it. I did this, left the cap disconnected for a week, and could still produce a healthy spark from it!

[G8HQP Dave]

Does the hum definitely come from the loudspeaker, but start before the valves have warmed up? My Pye P45 does this. I assume it is caused by magnetic coupling between mains and output transformers via the steel chassis. Not a lot you can do about that!

Adding extra smoothing capacitance might help. Adding extra reservoir capacitance (at the rectifier) might make it better or worse - less HT hum voltage, more HT winding peak current.

[PJL]

I have a 588A I restored and remember being very impressed with it's performance but it was a difficult restoration as I had to rebuild all of the trimmers. Can't remember excessive hum but I'm not too fussy anyway. I think the speaker was a permanent magnet one? it has a choke filter, pentode output and centre tapped heater arrangement so should be OK.

Is it mechanical noise?

[Tim]

Hi PJL.
I had to rebuild the only original trimmer left on this one too( and re-assembled with nylon bolts.............) Speaker is PM, and has o/p xfmr mounted on it. Actually come to think of it, the o/p xfmr is quite near the mains tranny, and not at right angles to it. The 588a has 32uf-choke-32uf, 580a 32uf-resistor-32uf. I'll try it again and see if the hum happens instantly. Apart from that it works well, and the hum is not excessive.

[G8UWM-MildMartin]

1st thought:
It may be normal for a directly-heated o/p valve (PM24M) . but would that be 50Hz or 100Hz?
2nd thought:
Does reversing the mains input connector make any difference.
3rd thought:
Does earthing the chassis, if not already done, make any difference?

[Aerodyne]

There's no doubt in my mind that some modern capacitors are more sensitive to AC fields and therefore pick up mains hum which is then amplified, so it may be that the new couplers and decouplers are at the root of the problem.
It might be worth fitting NOS metal-cased types (such as Sprague made) for coupling around the volume control. Also ensure that the metal case of the volume control is making contact with the chassis.
-Tony

[PaulR]

That's very interesting Tony. It could well account for many of the hum problems encountered.