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

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Old 23rd Nov 2020, 3:32 pm   #1
percival007
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Default Amplifier Identification Help

Hello Folks,

I have been asked if I can help with the Amplifier in the attatched Pictures.
A chap has been tasked with repairing it and wondered if I could shed any light on things.
I have no idea what it is so thought of you good people.

The Amplifier Hums and has a poor Frequency Response.
The Phono Stage seems to have some very bad and peculiar Crosstalk. With the Balance in the Middle, there is almost a complete muting of the Monoaural content of the Music, ie. No Vocals at all hardly.

If not is forthcoming a Schematic will need to be drawn out but I said I would ask on here to see if anyone recognises it.

Thanks in advance,

James.
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Old 23rd Nov 2020, 4:01 pm   #2
GrimJosef
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Default Re: Amplifier Identification Help

What's the valve line-up (looks like EL34s in the power valve and a 12A?7 double-triode with the two halves wired in parallel on the input, but actual type numbers might help) ? First thing is to localise the fault(s) to the power amp or the pre-amp.

EDIT: I beg it's pardon. The input valve on the power amp looks like it has a 6.3V heater.

Cheers,

GJ
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Old 23rd Nov 2020, 4:23 pm   #3
Nickthedentist
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Default Re: Amplifier Identification Help

An ad for UEP here, on page 69 of the PDF: https://worldradiohistory.com/UK/Wir...ld-1947-07.pdf
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Old 23rd Nov 2020, 4:40 pm   #4
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Default Re: Amplifier Identification Help

Apologies, Valve Line-Up and fault description from the chap looking at it...........

''............Front end is EF86s followed by ECC83s into two pairs (stereo) of EL34s. I’ve swapped out the electrolytics in the PSU and the hum is now acceptable, although not perfect.
It’s got two probs:
1] the phono input (two more EF86s) is very strange. There is a huge amount of crosstalk as in either input fires up both channels to a degree and whatever addition is taking place is out of phase. So anything panned to the centre of the stereo image disappears - vocals in particular.
2] the frequency response (line input) is lacking in treble until you turn the treble up. ‘Flat’ isn’t very flat. I could get around this one by just re-zeroing - not a perfect solution............''

The Phono Stage fault is just there as the Amp works fine, in that resepct on a Line level I/P
The Frequency Response is just poor in the Power Amp Stage.

Thanks Nick for the Advert. Though that Amp is described as using Triodes whereas this is using EL34's.

James.
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Old 23rd Nov 2020, 5:01 pm   #5
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Default Re: Amplifier Identification Help

Yes, sorry, I didn't mean it was the same amp, just "exciting" to see a mention of this obscure brand online
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Old 23rd Nov 2020, 5:44 pm   #6
GrimJosef
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Default Re: Amplifier Identification Help

Quote:
Originally Posted by percival007 View Post
... Front end is EF86s followed by ECC83s into two pairs (stereo) of EL34s ...
Same as the Mullard 5-20 then http://www.r-type.org/articles/art-003d.htm.

If the hum is present in just one channel then that points to a fault in an audio component in that channel. If it's present in both channels then it points to something that they share in common - power supply, grounding, something radiating more hum than it should which is then being picked up in the wiring ...

100Hz hum tends to be an HT rail problem (failed smoothing, or something drawing too much current). 50Hz can be many, many things (half of a full-wave rectifier failed, grounding, heater-cathode leakage, transformer flux leakage ...).

Cheers,

GJ
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Old 23rd Nov 2020, 9:10 pm   #7
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Default Re: Amplifier Identification Help

Centre vocals not present - try reversing wires to one speaker as probably one sucking while the other blows...
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Old 23rd Nov 2020, 11:27 pm   #8
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Default Re: Amplifier Identification Help

The pictures are not great but it looks like the EL34's are triode wired? The yellow wire from the output appears to show a connection for -ve feedback via a 5K/47 divider which I assume is connected to the cathode of the first stage. I also can't see any cathode resistors for the EL34 so it may be fixed bias?

It does not look like a Mullard design so you would need to draw it out. I guess there is another chassis with the PSU? Have you done the usual DC checks?
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Old 25th Nov 2020, 6:37 pm   #9
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Default Re: Amplifier Identification Help

Thanks for all the replies folks.

Quote:
Originally Posted by buggies View Post
Centre vocals not present - try reversing wires to one speaker as probably one sucking while the other blows...
Fault only audible when using the Phono I/P. OK on Line Level I/P's.

Quote:
Originally Posted by PJL View Post
Have you done the usual DC checks?
Not sure which usual DC checks you mean I'm afraid.
Without a Schematic the chap looking at it isn't sure what to expect but now it's been suggested it's a Mullard 5-20 based design perhaps, at least we have that to go on.

Thanks again,

James.
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Old 25th Nov 2020, 7:03 pm   #10
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Default Re: Amplifier Identification Help

Quote:
Originally Posted by percival007 View Post
Thanks for all the replies folks.

Quote:
Originally Posted by buggies View Post
Centre vocals not present - try reversing wires to one speaker as probably one sucking while the other blows...
Fault only audible when using the Phono I/P. OK on Line Level I/P's.

James.
OK - in that case try a wire swap on one channel at the cartridge?
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