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Old 7th Nov 2018, 11:27 am   #41
vidjoman
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Default Re: Decca AMG111 hum

Most are 4 colour bands plus tolerance band.
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Old 9th Nov 2018, 2:30 pm   #42
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Default Re: Decca AMG111 hum

Sorry for the delay in responding, grandparent duties seem to be taking over more and more!

Thank you for the suggestion Jerry, but I think they are correct. The colours aren't easy to see as the colour rendering on my lamp is not very good.

The fault must be situated around the valve holder or the valve itself. I think that upgrading the heater wiring has helped which is why I thought that the hum had gone when I first disconnected it. It was just wishful thinking when I heard the reduction and I didn't listen to the residual hum. I can't help thinking that I must have made a mistake when I replaced the holder but I made copious notes and everything seems to follow the circuit diagram. As my daughter's My Little Pony book of many years ago said - "A mystery never to be explained"!
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Old 9th Nov 2018, 8:27 pm   #43
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Default Re: Decca AMG111 hum

Frustrating, isn't it? Knowing that something worked fine before but won't behave now
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Old 9th Nov 2018, 8:59 pm   #44
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Hi!

This might be a daft one, but you haven't mistakenly included the –HT from the bottom of the mains transformer HT secondary to chassis in the hearer or loudspeaker chassis return by any chance?

The lead from the bottom of the HT secondary should go directly to chassis–earth or metalwork directly at the reservoir cap (use one of the clip mounting bolts with a good shakeproof washer biting into the return tag), and the earthy point of the 6.3V supply should ideally go to chasis at the valve–holder only, and the speaker speech–coil secondary circuit earthed to chassis direct away from the reservoir cap or HT secondary connection point.

If you're really "nil desperandum", I am going "Oop North" to collect a Scope transformer next weekend, and I can easily call into Southport with a squigglegraph, a good contact–cooled rectifier and another couple of ECL82 valves, and would be delighted to assist – no charge!!

If you'd like me to call and give you a hand, please PM me!!

Chris Williams
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Old 9th Nov 2018, 11:22 pm   #45
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Default Re: Decca AMG111 hum

Hi!

To clarify my previous post, I've attached a skeleton circuit-diagram of a typical ECL82 pentode O/P stage as used in a record-player of this type showing the recommended earthing arrangements!

Also note that the volume control can introduce hum into the amplifier if it's case isn't bonded to earth, but this should be done by the screening of the lead connecting it to the pick-up and the ECL82 triode-grid circuit (pin 1).

The usual recommended way these volume controls were wired was with a short length of twin screened cable, typically with red and black inners, the red connecting to the top of the volume-control track (LH side tag as viewed from rear of control) and to the output-feed from the PU cartridge (tag-strip under deck), with the black inner connecting to the middle tag and to the ECL82 pin 1 circuit, and the chassis-end of the control (RH tag as viewed from rear of control) connected to the screened-cable braid/screen, with the braid returned to chassis-earth as close to Pin 1 as possible, and finally a short piece of bare wire soldered to the control and then the RH tag of the volume control.

It might also be worth bonding the chassis-end of the LS speech-coil secondary circuit to the speaker frame, (as shown in the attached diagram) as some loudspeakers are prone to pick up static-charges causing odd noises or a high-pitched hum if this isn't done!

Go thro' your amplifier wiring carefully, also masking sure the screen of the mains transformer is connected to chassis, again with a separate lead!

PM me if you'd like more help!

Chris Williams

PS!

Just a thought - I presume the amplifier has an ISOLATED mains transformer and not just a "heater" transformer? Looking at the photo provided would suggest it is, you didn't show the top of the chassis but I assume the mains I/P is wired on the top of the transformer.

If you have only a "Heater" transformer (there will be a lead connecting one side of the primary to chassis if this is the case) then you must bond the LS frame with a 2M2 resistor only for safety reasons!
Attached Files
File Type: pdf ECL82!!!.pdf (87.0 KB, 54 views)
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Last edited by Chris55000; 9th Nov 2018 at 11:38 pm.
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Old 10th Nov 2018, 7:04 pm   #46
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Default Re: Decca AMG111 hum

Thank you for all that, Chris.

I put it back into its case yesterday to listen to the overall hum but it is now wired much as suggested in your diagram except that the heater is earthed at the valve end and the speaker frame is not earthed. I cannot quite envisage how the screened wiring is done using double core flex, though.

Yes, it does have a "proper" transformer.

The case of the volume and tone controls are not directly bonded to the screen of the flex, but the hum did remain when I grounded the grid of the triode section. There is additional hum when the volume control is turned up and further bonding may help with that.

I will get it out again next week to have another look at it.

I will also PM you regarding your kind offer to call past.

Thank you
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Old 17th Nov 2018, 3:13 pm   #47
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The hum is now completely cured thanks to a visit from Chris.

It turned out that the smoothing was inadequate. He let me have a bridge rectifier and suggested that the outputs of this should have another 33uf capacitor across them. It is now so quiet that I initially thought that one of the speaker connections had come off!

Thanks Chris.
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Old 17th Nov 2018, 6:31 pm   #48
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Default Re: Decca AMG111 hum

Time to spin a copy of the Tremeloes "Silence is Golden"
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Old 17th Nov 2018, 9:12 pm   #49
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Sounds like the problem may have been an original (lack of) design feature!
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Old 17th Nov 2018, 11:02 pm   #50
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Yes it would seem so. At least it wasn't a mistake when I changed the valve socket.
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Old 21st Nov 2018, 7:33 pm   #51
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Default Re: Decca AMG111 hum

Hi!

M.G. Scroggie, writing under the Psuedonym "Cathode Ray" wrote two articles titled "Negative Feedback And Hum", in the first of these, he built an experimental switchable output stage. configurable as triode, pentode, with or without G2 smoothing, and with various confugurations of NFB, and fed in 30V rms in series with the +H.T. line to simulate badly smoothed supplies, then measured the output ripple from each test configuration.

(This experiment is readily repeatable with an EL84 valve, using it's normal 150R cathode bias resistor and a 22u capacitor)

These two articles are well worth a read (search "Negative Feedback & Hum" in the Wireless World page of A.R.H., and I would also like to draw Members' attention to an unusual method of hum cancellation using a Wheatstone Bridge technique, see W.W., Oct 1955!

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Old 28th Nov 2018, 2:09 pm   #52
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Default Re: Decca AMG111 hum

Some 5w wire wound 1k resistors arrived today and one has been fitted. The HT is fractionally below that shown on the sheet and the hum is vastly improved. I just need some cream and gold knobs for it now - see the parts wanted section.

Many thanks to all and particularly Chris.
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