Decca AMG111 hum

[peter_sol]

Check that one side of the heater does go to ground.
Then hum may be being induceded from the mains transformer to the output transformer.
Try moving one or the other or turning them.

[vidjoman]

Post some pictures - someone may spot something that may cause the problem.

[Ambientnoise]

Another simple thought ! It’s not the physical arrangement whilst it is open for servicing is it ? The mains transformer or wiring can induce hum into the output transformer or even speaker in some cases. Similarly, lights and other near equipment can cause hum.

[Edward Huggins]

I'm quite familiar with this player, and it's well-designed amplifier, as I've just been working on one. Given that you've been very thorough so far, I would double check the soundness of the connections on those two small interconnect plugs/sockets into the amplifier again.

[AC/HL]

I know shorting the pentode grid removed the hum, but how about just disconnecting C5 and leaving the output stage idling?

[PaulR]

Thanks again

I have managed to reduce the hum quite a bit. When I replaced the valve holder I used screws and nuts to hold in the new one. The tab to which the earth of the heater is connected is under one of these. I had checked the earth continuity using an ohm meter and there was more or less no reading. I was having another look this morning and managed to tighten the nut a little more. This seems to have reduced the hum. It is difficult to say whether this is now at an acceptable level until I put it back together properly again. In the meantime here is a photograph of the underside.

Yes it is a nice amp, Edward and sounds good despite the hum. I have never noticed that the tone control makes much difference though. I have very fond memories of going with my parents to buy the original one and having Beatles and Gerry and the Pacemakers EPs to play on it - 55 years ago! It was eventually replaced by a Bush SRP 51.

[Herald1360]

Hmmm, not a very pretty sight!

I wonder if the problem is what looks like double grounding of the heater- the tag on the transformer looks to be connected to the HT- winding tag and it's not clear what exactly is going on round the ground tags by the valveholder.

I would suggest that separating the HT and LT windings at the transformer and using twisted pair wiring to the valve heater pins with just a ground link at the valve end might improve matters. If you're really up against it, balancing the heater supply to ground with a 100R or so pot across the heater supply and its wiper to ground (a humdinger circuit) and twiddling for lowest hum may help, though this is normally only necessary for very low signal level preamp circuits.

Also on the HT supply ensure that the HT winding is connected one side via surge limiter to half wave rectifier then rectifier output to reservoir+ then take HT+ directly from reservoir+ not anywhere further back towards the rectifier. On the other side, connect the winding directly to reservoir- then take HT- directly from the reservoir- to ground. (A can type reservoir may do this implicitly.)

Essentially don't include any common wiring in both the ripple current loop and the HT supply output loop, and likewise don't have any common current paths for either HT- and signal grounds and the heater current.

Many radio/amplifier designs have ignored this and may get away with it given high enough signal levels, but given that there is a hum problem here doing it all "just so" can only help.

[Edward Huggins]

This is one of the best sounding record players in it's price range I've ever worked on.
And it only has (especially for a Decca) a relatively small speaker. If you study the circuit diagram (available from this Forum) you'll see that the tone control should provide an effective tonal range. The associated capacitor may need to be replaced.

[PaulR]

Thank you Chris. I will give it a go.

[PaulR]

Edward - yes it does sound good. Although I was only 8 I remember my parents listening to several record players on the shop and going emphatically for this one. Have you ever had hum problems with them?

Chris - I have put in a twisted pair heater supply with the earth at the valve end. I have also removed "my" 32uf cap and wired the power supply as suggested. Unfortunately it still hums. The hum can be heard quite clearly in the room. I have the volume control at minimum. I previously could not get a trace of the ripple on the HT but I now can for some reason and I am attaching photographs of the result for the anodes of the triode and pentode. I am also attaching a photograph of the underside as it is now.

Thank you

[cathoderay57]

Hi Paul, this one is certainly posing a challenge. The only thing I can see from the image that strikes me as a possible concern is that it looks like the thick brown wire going left-right with twin (red and black) inner wires is mains and it is passing very close to (just below) a small brown polyester capacitor. If that cap is in the audio coupling circuitry then it is close enough to give quite good 50Hz pick-up. Other than that the best clues seem to be:
a) The hum pickup is not arising in the pentode stage.
b) The hum is originating in the triode stage either via HT ripple on the anode, or via heater supply.
c) When you disconnect the heater the hum disappears.

Have you tried the previous suggestion of running the heater from a separate DC supply? If you want to send me one of your ECL82 valves via the post I would be happy to test it using my VCM163 for the cost of postage, unless there is anyone closer who could do the same? That would eliminate heater-cathode leakage although I accept it is unlikely that you have 3 bad ones. I tested all 4 of my spares and 3 were OK but one Bentley Acoustic Corporation ECL82 that looked new in box had quite poor heater/cathode isolation. Cheers, Jerry

[cathoderay57]

Just looked at the previous image. It appears that the poly cap and the 1k resistor beneath it are the negative feedback components connected directly to the bottom of the volume control. I would gently move the mains cable well away from them and see if the hum subsides.

[Edward Huggins]

Originally Posted by PaulR

Have you ever had hum problems with them?

No. There is better than average smoothing on this model. As I said the plug-in interconnects needed cleaning on mine. I'm quite surprised that after all you have done there is little improvement.

[PaulR]

I was surprised in view of the trouble taken with the smoothing, Edward. I don't know whether the ripple shown on the 'scope is reasonable or not, though. The caps I fitted were new but I wouldn't want to think that one wad duff.

That seems to summarise it Jerry.

I have already tried moving that mains cable about and it doesn't help. It is very close to the feedback components and doesn't seem to be a good design. I might try re-routing it.

I think I will try a separate DC supply for the heater as already suggested to confirm that is the problem. I am pretty sure, however, that the hum went when I disconnected it, but I am beginning to doubt myself now!!

I may well send you an ECL82 test if all else fails. I don't have another piece of equipment that uses them at the moment to pull one from.

Thank you

[julie_m]

Try probing the screen grid of the pentode section. It only takes a little ripple there to manifest as hum on the output. Ripple on the screen grid would go away when you cut the power.

[PaulR]

I think I see the mistake! I have connected the heater wires between one of the heater taps on the transformer and one tap of the HT. The HT connections are similarly wrong.

SWMBO is getting upset about the time I have spent with this now so remedial work will have to wait until tomorrow!!

[PaulR]

Sorting out the wiring didn't help unfortunately. I tried using a 6vDC wall wart for the heater supply as suggested earlier and that didn't help either. I am not sure why I thought that the hum went when I disconnected the heater but it doesn't now.

I cannot get the 'scope to lock onto any ripple at all on the screen grid of the pentode but I could resurrect an old analogue 'scope to look at it on that.

I have tried disconnecting the grid of the pentode as well as earthing it and both stop it. Earthing the grid of the triode does not help. It is the same with three different valves.

I think that we may have come to the end of the line with this one and admit defeat, at least for the moment. It is a shame as I was looking forward to re-living my childhood with it.

Many thanks for all the help and suggestions - it is good to talk!

[cathoderay57]

Hi Paul. Quite a saga. When you used the wall wart did you connect the negative lead of the supply to chassis? Most of these are 2-wire devices with no earth pin other than a plastic dummy pin so lack of earth would not have helped hum. My final 2 suggestions are 1) try a different type of coupling capacitor between triode anode and grid pentode. 2) did you disconnect the wires from the output transformer at any point and is it possible that the phase of the negative feedback has been reconnected 180 degrees out? Usually it howls when that happens though. Cheers, Jerry

[PaulR]

Hi Jerry

I attached the leads to the new twisted pair at the transformer end. One of them would have been earthed via the connection at the other end but I didn't look to see which it was.

I did try a different coupling capacitor, I even attempted to find which was the lead to the outside foil using the oscilloscope but that wasn't conclusive.

I haven't touched the connections at the output transformer but I did try disconnecting the feedback with no improvement.

I can't help feeling that it is something obvious that someone with more experience than me would see if it was in front of them but it is beyond me!

[cathoderay57]

Hi Paul, just looking again at the images... You appear to have replaced most of the resistors. Maybe it's a digital image thing but the colour bands look like odd values. For example R4 Triode anode resistor should be 180k brown grey yellow looks like brown grey black (18R). Also R5 680k blue grey yellow looks like blue grey black (68R). Surely just an image distortion? Cheers

Jerry