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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 25th Jun 2019, 7:21 pm   #1
captainpugwash
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Default Mullard 3-3 amplifier problem.

Good evening,
My amp has been out of service for a little while and the acquisition of a pre-amp has prompted me to get it going again.
It runs silently with no input but when a phono lead is plugged in there’s is a slight hum and when I touch the centre pin of the lead, there is a squealing noise and motor-boating rather than the usual buzz. The amp is not connected to the pre-amp.
This on both channels.
Any help would be greatly appreciated.
Kind regards,
David
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Old 25th Jun 2019, 7:41 pm   #2
GrimJosef
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Default Re: Mullard 3-3 amplifier problem.

Are you saying that something has definitely changed i.e. that it never hummed when you used to plug the same phono lead in and that it just buzzed when you used to touch the lead's centre pin ?

I can imagine that the amp's current behaviour isn't abnormal. If it's built to the 'control-less' circuit then the EF86's grid leak resistor will be 10Mohm and the sensitivity will be just 100mV in for full output. That circuit will be very prone to picking up hum from an open-input phono lead. When you touch the centre pin of the lead you will be acting as an aerial, picking up whatever electromagnetic noise is present in your living room and coupling it into the amp. Again I can imagine that that will be sufficient to upset your amp badly. You may find that the response depends strongly on what other equipment is switched on nearby (lights, TV, etc) and, in particular, on whether your other hand is holding the grounded outer of the phono plug.

Does the amp still sound fine when connected to a music source ? If so then I wouldn't worry too much. If you find it hums when you add a pre-amp then I'm afraid you may have discovered the wonderful world (more like purgatory, in reality) of ground loops. I take it the pre-amp is one with relatively little gain. The 3-3 is already more than sensitive enough for most modern sources.

Cheers,

GJ
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Old 25th Jun 2019, 9:14 pm   #3
Biggles
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Default Re: Mullard 3-3 amplifier problem.

I have found this happens from time to time with various amps I have been working on. I wouldn't worry about it too much unless the sound is affected while playing music from a matched source. It could be down to stray feedback from the speaker leads back to the input. Touching the input is normally a good way of testing for buzzing to quickly check the amp (I suspect we all do it), but it introduces all sorts of extraneous noise, and is not the normal connection with screen the amp is designed to accept.
Alan.
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Old 25th Jun 2019, 11:42 pm   #4
Refugee
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Default Re: Mullard 3-3 amplifier problem.

I have had amplifiers that pick up little snippets of any strong radio stations as well as the hum when the input is touched.
The hum overloads the first stage of the amplifier on the peaks causing the first stage to work like a detector diode.
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Old 26th Jun 2019, 8:47 am   #5
captainpugwash
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Default Re: Mullard 3-3 amplifier problem.

Good morning,
Many thanks for the replies. The amp was originally fed via a small QED passive preamp and I don’t remember any problems.
It was only when I started to set up my latest charity shop find, a Cambridge C500 preamp, that I noticed this phenomenon.
I note all your comments about pickup and ground loops and vicinity of other interference sources. The amp is now actually in a different place in the house which may explain that.
It does play fine, I was just concerned about any underlying problems which there don't seem to be thanks to your comments.
The background hum is more of an annoyance than anything else and now slightly more apparent with the Cambridge installed.
Many thanks again and kind regards,
David
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Old 26th Jun 2019, 9:05 am   #6
Edward Huggins
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Default Re: Mullard 3-3 amplifier problem.

Are you using this with seperate pre-amps because your source is now a MM cartridge?
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Old 26th Jun 2019, 9:16 am   #7
bikerhifinut
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Default Re: Mullard 3-3 amplifier problem.

Hi david,
Just to add to GJ's post in particular, I have a couple of "shorting" phono plugs, basically just soldered the terminals across, and I use them when firing up new/unknown amps on the inputs. This eliminates any extarneous sources of hum and noise and will onot harm the input of a valve amp, nor should it affect any properly desiggned silicon power amps etc as a passive pot pre will effectively be S/C at zero volume.
If you get anything other than hum induced via AC heaters etc then you know the amp is unstable. That said I built an EL84 SE amp and with carefull heater wiring its quieter than my P-P amps!
Cheers Andy,
PS hope your tuner collection is going strong.
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Old 26th Jun 2019, 9:18 am   #8
bikerhifinut
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Default Re: Mullard 3-3 amplifier problem.

Oh a quick extra, have you tried the C500 with a modern SS power amp or even into the aux/radio input of an integrated just to rule out any issues with it? Things like that often go to the charity shops because of minor faults that a less knowledgable owner can't or wont get repaired.

A.
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Old 26th Jun 2019, 11:18 am   #9
captainpugwash
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Default Re: Mullard 3-3 amplifier problem.

…"Good morning Edward and Andy,

The amp was initially connected via a passive preamp to a CD player but when I saw the Cambridge, priced at a bargain £2.99, I thought it would allow me to use a turntable and give better connectivity.
Unfortunately, I discovered that the Cambridge was missing the phono module so that put an end to that.
It just needed the power supply caps changing as they were oozing but trying it through another amp it is hum free.

Andy, the tuner collection is plodding along, mostly mono-it’s the future. The Heathkit has had a lick of paint and I have a new fascia to fit so it should look very smart and it is working well too !!

Kind regards,
David
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