Mullard ECL86 amplifier.

[greg_simons]

Just had this donated to me by an old friend, he built it from a kit in the very early sixties. I would like to know if possible who out of a myriad of outfits may have sold it then. Quick tests reveal no real problems at least on the primary side of the mains transformer and it powers up with heaters and pilot light. Next step will be replacing all those old capacitors etc.
As you can see from the pictures the input sockets are unusual and I confess have not seen anything like them before, possibly there was a lot of war surplus stuff about at the time that was readily available.

Should anyone have a spare plug let me know.

Will post more in time.

Greg.

[Herald1360]

Looks like an old Amphenol 97 series job.


http://www.ebay.com/itm/like/2921262...244&rmvSB=true

[Edward Huggins]

I don't recall Sterns, Laskys, Premier, RCS et al - ever offering a Kit like this.

This looks to me like a home brew. As to the circuit, if the 1st stage is an EF86, it may be some kind of lower powered Mullard 5-10 as the ECL86 (from 1962) was designed to emulate the ECC83 and EL84. From the blue shroud, they look like Parmeko transformers. Would be worth repairing though, just think what a valve 8 watts per channel stereo amp would cost new!

[greg_simons]

Hi Edward,
the transformers are made by rf gilson, the chassis seems to have been pre cut and drilled for them, it looks very professional and precise, too good to be a home job although not impossible, there is a published design by mullard for this configuration which uses an ef86 voltage amplifier first stage and the triodes of both ecl86's as the phase splitter.
greg.

[G6Tanuki]

The profusion of "RS" grey-jacketed capacitors makes me think this is either a homebrew or limited-production 'kit' offering.

Without access to a schematic it's difficult to tell whether it fits into the "really rather fun and worthwhile recommissioning" or "built-down-to-the-lowest-price" class.

[pmmunro]

I would agree that the use of Radiospares grey capacitors would suggest a home build project rather than a kit.

The, rather thin, leads broke off the solid dielectric version of these capacitors very readily, but what was/is their service record apart from that? They may not need to be changed.

PMM

[greg_simons]

http://www.r-type.org/articles/art-003f.htm
the above web address seems to illustrate a remarkably similar layout.
greg.

[Herald1360]

From the original post, kit or not isn't the issue, only whose kit is it?

A kit for that particular Mullard design would have been a no brainer to some enterprise or other at the time.

[Diabolical Artificer]

Those grey RS caps reform well. I've found they're pretty good quality. Might save a few £'s and time.

Andy.

[vidjoman]

Originally Posted by greg_simons

http://www.r-type.org/articles/art-003f.htm
the above web address seems to illustrate a remarkably similar layout.
greg.

I think that is it. Look at the component layout drawing and it leaves out the heater wiring (for clarity) and on the OP photo the heater wires have been added after the main construction. Normally the twisted heater wires would be next to the chassis to minimise hum.

[Restoration73]

The UK input plug/socket is called a "Screenector" and I have a spare plug.

[Sideband]

Originally Posted by pmmunro

I would agree that the use of Radiospares grey capacitors would suggest a home build project rather than a kit.

The, rather thin, leads broke off the solid dielectric version of these capacitors very readily, but what was/is their service record apart from that? They may not need to be changed.

PMM

From experience, those capacitors are not much better than the Hunts 'Moldseal' (the brown resin type) and will more than likely be unserviceable. I believe they are made by Dubillier and they also go leaky with age. I recently helped clear out a workshop and found tins full of new (old stock) grey RS caps just like those in the amplifier and they all ended up in the bin.

[Nuvistor]

I always thought those grey RS capacitors were paper dielectric so I would not expect them to be any more reliable than any other 50 year old paper caps. Certainly need testing but I would just replace with modern types.
Frank

[greg_simons]

Hi frank,
many thanks for your input, those capacitors are going!, they're probably not going to be much good and in any case those ecl86's are precious and not to be stressed in any way, ironically enough if it had been the standard 5-10 configuration with el84's wouldn't have been so bad as I have plenty of stock.
greg.

[Ed_Dinning]

Hi Gents, not at home at present, but there was a Mullard app note on a stereo amp using ECL 86 in one of the more obscure publications.

Ed

[Valvepower]

Hi,

If it helps this amplifier was discussed in the thread below…

https://www.vintage-radio.net/forum/...ad.php?t=99399

Terry

[greg_simons]

Hi ed and terry,
That's really good info, especially for lowering the input gain a little and altering the feedback slightly, restoration 73 has kindly offered me another 'screenector'plug so all I need now is some time to get stuck in.
as a much much younger person I used to look with longing at this stuff in the practical electronics magazine and now after fifty years I have one on my bench to do.
greg.

[trobbins]

Greg, there is a subtlety with the input stage EF86 pentode to triode conversion I think. Pin 8 can be seen connected to anode or cathode. In practise, the connection to cathode may allow better shielding around the valve terminals for the input pin 9. This was a situation described in the 6AU6 app note:
http://dalmura.com.au/projects/6AU6pAWV.pdf
Ciao, Tim

[greg_simons]

Cheers tim, I will study that info in more detail.
greg.