Ferguson 396G Record-Player 3 valve amp!?

[McMichael488]

Hello all. I'm rather stuck!
Please can anyone help?
I've a Ferguson model 396G portable record player with 3 valve stereo amp.
I can't find its circuit diagram nor any referance to this model online.

Has anyone any experience of this Ferguson / Thorn amplifier please?
A circuit diagram would be immensely helpful!

It's a clever portable where the top & bottom 'covers' incorporating a speaker each, remove & prop-up like picture frames, providing wide stereophony.

I'd not heard of a 3 valve stereo amplifier before, but having researched it online there's instructions on building them so I accept they exist
Valve line up : ECC83, X2 EL84.

It's recently been serviced by someone prior to me obtaining it, as there's modern polyester caps inside.
One channel is almost inaudible with a dull distorted output & mains hum.
Both speakers fine, swapped pickup inputs; no change, then swapped the two EL84's, of which one was considerably hot compared to the other?!
Swapping them switched the good channel to the opposing speaker channel output indicating a faulty valve.
BUT..
Mains hum normally points to the H.T. caps but that would surely be equal on both channels?
Is it a combined fault / perhaps a cause & knock-on-effect where a valve is damaged by H.T. issue?
Or just a dodgy valve!

I noticed H.T. caps haven't been replaced yet they're the first I would replace myself although I'm still a novice.
Unless a very neat job of hiding new caps inside the old can has been done but I doubt it.

Any assistance would be much appreciated..I'm confused

I'll upload pictures.

Thanks all, Rickim

[Nuvistor]

The faulty EL84 could have a heater cathode leak putting 50HZ into the electron stream. Change the valve and check the operating voltages on both EL84 valves to confirm they are working correctly.
Confirm the grid couping capacitors to the EL84’s are in good order, if original paper type, replace.

[RojDW48]

Strictly speaking I suppose it's 2 single ended 2 stage amps on one chassis, with the ECC83 providing both first stages? (still getting to grips with the theory).

[julie_m]

I spy some modern-ish capacitors in there, including to the EL84 first grids, so it has been worked on sometime since the 1990s.

It appears still to have the original semiconductor HT rectifier -- possibly a full-wave type, which can cause excessive hum if one diode is failing. You would be able to see at once with an oscilloscope on the HT line; you will need a 10:1 or 100:1 probe. (A simple attenuator formed with a 10MΩ and 100kΩ resistor in series can be used, in the absence of anything better.) A modern silicon part can be substituted if it is faulty; but it will require a fairly high-power rated series resistor to compensate for the lower voltage drop, and you will have to experiment to find a suitable value.

[McMichael488]

Wow thanks people!

So basically.. possibly faulty cathode in valve hence extra heat.
Possible issue with solid state H.T. rectifier.
Possible leaky H.T. capacitors.

I hadn't realised a single valve could perform two pre-amp functions in one. Makes sense thanks!

No oscilloscope here sadly, far too basic.
I'm really a GPO phone buff that has leaked over to a few midget ADC sets, then a Stereogram & this portable record player.
Not even dared look at my inherited valved Quad stereo yet!
Only test equipment I have is old failthful ex-GPO multimeter but it's served me well so far.
I've a friend with oscilloscope, variable transformer & all sorts but he has limited time / patience for my vintage adventures.
I'll get some EL84's on't tinterweb ;-)

Very kind of you all thankyou

[Edward Huggins]

Originally Posted by RojDW48

Strictly speaking I suppose it's 2 single ended 2 stage amps on one chassis, with the ECC83 providing both first stages?

Yes, that's exactly what it is and quite capable of giving a very good sound.
This kind of circuit was common in many early stereo radiograms and can give over 4 watts per channel.

[RojDW48]

Send me a PM with address and I'll pop an ex-Russian military EL84 in the post for you for cost of postage - I have more than I need.

[Herald1360]

Originally Posted by McMichael488

I hadn't realised a single valve could perform two pre-amp functions in one. Makes sense thanks!

The ECC83 is two separate triodes in the same envelope. Apart from a possible slight increase in crosstalk, using each one in separate signal paths is fine.

A stereo amplifier with only two valves is common enough- use a pair of ECL8x devices.

[McMichael488]

Just an update to you all.
I'd much like to thank RojDW40 for donating a spare EL84 for me to test.
It seems his diagnosis of faulty cathode was the main cause of my problem.
It's replacement provided full Stereo & no aggressive mains hum on one channel !!

I've a few more tweaks to do like improve shielding for hum at high volumes (concluded by silence upon shorting out pickup inputs) & my tone control only works within the first fraction of a turn.

As the pickup cartridge is a late 1970's replacement (so I've been advised) I'm told there may be impedance imput mismatch with tone control.

My primary use for this amp will rarely be the record deck, if at all, but more so a line-in amplifier connected to my phone / pc's line out jack.

I've tried this & got excellent reproduction, however the tone control now has zero effect in this instance.
I'm guessing this is due to a complete input impedance mismatch issue, that is yet beyond my comprehension, other than something to do with the capacitors or variable resistors used in tone control?!

The adventure continues!

Thanks again to all whom shared their thoughts & spared their time.

[Edward Huggins]

The flip-under replacement cartridge now fitted into the Garrard autochanger should be an excellent match to the Ferguson's amplifier. However, there is likely to be an impedence mismatch with your Mobile/PC and this may result in low and thin output. In short, you are trying to marry modern technology with old - and it's not always that straightforward.

[McMichael488]

Thanks there for your advice

The tone control has some issue when using just the pickup as it makes no effect on sound output except only in the first 1/8th of a turn.
As this set was serviced before my owning it I wonder if the new Wima capacitors in the tone control are not exactly matches to whatever they replaced.

Sadly I've no circuit diagram still as it's not on the instant downloads list. Also a Ferguson 396 tractor was also built by the tractor manufacturers of the same name so that has made searching for the circuit diagram rather more difficult!

Yes I know I'm merging technologies & it's not that simple but I'm trying to learn as I go along.
I'm far from the first to connect modern devices to gram input & my personal view is doing this makes a lovely vintage appliance considerably more useful usable & enjoyable having this totally reversible addition, as I've only a fraction of my music collection only on vinyl discs

I'd very much like to understand the best or better ways to configure a line in input that will improve / match my amp?
If this involves additional components either vintage or new then I am keen to pursue this.
I'm also keen to improve the amplifiers tone control for both pick up & especially for line in, even if I had to add a selector switch to change values between input modes. I'm all up for retrofitting improvements that do not drastically alter the device but only improve its usability in the modern world. I'd even add bass & treble control but sadly don't yet understand all this enough to work out the best route.

I'd like to mention that this appliance was repeatedly slashed in price as it didn't sell over many weeks & would likely have been binned in the end had I not bought it.

I hope to give it a new lease of life & combined improved function whilst still keeping it at core a Valved Amplifier

Thanks again.

PS. Does anyone know any good books they can recommend to me for understanding & servicing vintage valved circuitry please?
It would be good to have some reference books at hand rather than endlessly asking you all questions & taking your time over things I could self teach.
Only a few years ago I didn't understand the circuit of a Bakelite 300 series telephone but after reading up, cross referencing & following its circuit diagrams I now know it almost off by heart, so I'd like a good all round reference source for this

[McMichael488]

Just been reading other articles related to 3 valve simple stereo amplifiers & issues over gain. Assuming this set originally had a crystal pickup might that now be why the tone control is mismatched if it had a later lower ouput pickup?

Anyway something that has alarmed me is a 'live' neutral chassis!!
I'm well aware of this as a common feature on AC / DC radios. Both my McMichael 488 & Bush DAC 10 are so.
But I didn't think by the 60's they were still doing that? !
My Ferguson has a mains transformer but I noticed many components soldered to chassis & this entire appliance has NO EARTH connection on its original mains input!

Is my chassis live? If so it's extremely close to my metal record deck & that's way to risky for my liking. One can treat a bakelite radio with cautious respect over live grub screws in control knobs but I'm not so keen on an un-earthed metal record deck!!

Please could you look at my pictures & let me know.
And I hate to repeat but I'm desperate for a circuit diagram should anyone have one in a book of service manuals for Ferguson / Thorn please?

Thanks all

[ms660]

So far as I can make out it doesn't look like a live chassis design, the mains transformer so far as I can see has two secondaries, neither of which appear to be centre tapped, that would suggest full wave bridge rectification for the HT by the rectifier that's fixed to the chassis in the photo.

Lawrence.

[Boater Sam]

Simple to find out. Ohm meter between chassis and live wire and then/also neutral wire.
If it reads infinite i.e.open circuit on both it is just a non earthed floating chassis.

If so, putting an earth on it may put a bit of stress on the transformer insulation but I would try it, normally its OK.

[Edward Huggins]

Your player has a mains isolated chassis so is not AC/DC - i.e. the chassis will be safe to work on. It is quite specifically designed to work with a crystal cartridge and so this will have no effect on the range of the tone control. The fact that the tone control has little variation is liable to be the capacitor across the tone potentiometer being at fault or has the wrong value. I have had, and repaired, one of these and they do sound very good in their own right. In as far as you are trying to use it with in conjunction with modern digital devices, I really cannot comment on, other than that you may be trying to expect too much of something it was never designed to achieve. I think it is unlikely to meet your expectations in this matter, so just use it for what it was originally designed for.

[McMichael488]

Thankyou Lawrence & Sam for the reassurance.
Yes it is full wave rectified by a resin contained (selenium?) rectangular thing clamped to the chassis. One leg (presumably Negative) is connected to chassis. The other connected to one of the big smoothing caps etc. etc.
So the chassis must be a common negative. The speaker outputstage also connect to this common negative.
All very well unless the rectifier or something fries & connects AC to the lot!!

[McMichael488]

I'd just like to point out that I meant to thank & credit member Nuvistor for his correct prognosis of a faulty leaky cathode on this set which was resolved by kind donation of replacement EL84 by RojDW48

[1100 man]

Hi,
Having looked through the red 'radio & TV servicing' books from 1959 to 1965, there is no mention of a 396G. There is a 394, 395 and 397, none of which is remotely similar to your amp. The only Ferguson circuit I could find that even uses EL84 and ECC83 was for a 658RG. This is stereo but uses a single output transformer driven by each EL84. It is described as 'phantom push pull' which is a new one on me!
Each channel uses half of an ECC83 to drive each EL84. There is a further stage of pre- amplification provided by a second ECC83. This might be to cater for a lower output pickup than was fitted to your player.
The pickup signal is fed, via a selector switch, to the top end of each volume pot. The 'tape recorder' input is also fed via the selector to the same point.
The circuit also incorporates bass & treble controls which you might be able to add to your unit.
Whether the output stages, V6 a&b and V7, V8 are in any way similar to your unit remains to be seen.
I've posted the circuit below in case it might help. It is printed very small in the 1961/62 red book so is not easy to photograph. It's strange that the actual circuit is so hard to track down.
This is for the 658RG
All the best
Nick

[Nuvistor]

I found this description in a Radio Constructer Magazine, it describes a Thorn radio gram with an ECC83 and 2 EL84 valves, mono radio in push pull and stereo single ended using seperate output transformers. It’s in the “ In your workshop” article.

An interesting circuit, could it be the same as the 396G?
http://www.americanradiohistory.com/...RC-1966-03.pdf

[McMichael488]

Thanks Frank. I am going to draw a circuit diagram from the actual circuit with aid of pin-out diagrams.
It will probably be a good way to learn & understand how these things are put together in the process.

I might see if it's possible to have two pre-amp valves instead of one, so I can double pre-amp the input for line-in from an MP3 player if the power supply transformer can heat and extra valve & I can figure out how to modify it all.
The tone control needs adjustment anyway & as no one wanted it & it was nearly binned I've no concerns over exact conservation in this instance.

I've bought several books on valve radio / amplifier repair & so I can underderstand them better without asking too many questions on here.

Thanks everyone for your help