Mystery power supply , preamp and power amp!

[brymac]

This was brought into Chailey Repair Cafe (which I run) today and looks like a beautifully made bit of kit. Very similar to what you would find in an early stereo Dynatron.

See photos attached - the blown rectifier on the power supply has had its markings rubbed off - any guesses please? (Impressively wired underside of chassis also shown)

And - what will the two missing valves on the power amp be? My guess is EF86s...

The only identifier on the whole thing is a plate which reads Universal Electronics Products London W1 (see photo)

Many thanks!

Bryan

[GrimJosef]

The broken valve certainly looks like a short-mesh EF86. Since the rectifier is a Mullard valve (there's enough of the logo left to tell) it should have a small type/factory/date code on it somewhere. The type part of the code will tell you what it was and the rest will tell you when and where it was made. The details are here https://frank.pocnet.net/other/Phili...CodeListAB.pdf. If it's flat(ish) topped it could be a GZ34.

Cheers,

GJ

[Tim]

A look at the rectifier valve socket wiring will tell you if it is a directly or indirectly heated rectifier.
The heater voltage measured and pin out(ascertained form the base wiring) will help narrow it down further

[brymac]

Many thanks for the helpful replies! Any more advice welcome...

Bryan

[trobbins]

More photos of power amp chassis (top and underside), and top of power supply chassis ?

[snowman_al]

Originally Posted by brymac

... the blown rectifier on the power supply has had its markings rubbed off - any guesses please? (Impressively wired underside of chassis also shown)

And - what will the two missing valves on the power amp be? My guess is EF86s...

The rectifier is a GZ30 (aka 5Z4GT) and the broken valve is indeed an EF86.
Yes, very nicely made. Can you tell us what other valves are used?

[brymac]

Here is some more information and photos as requested - but who would have made this

Valves:

Power supply - ?GZ30

Preamp - 4 X Ef86
1 X ECC83

Power amp - 4 X El34 and 2 ?EF86

I'm working on this with two of my volunteers from the Repair Cafe and we think this design features in a book of Mullard designs for amps - and that it's based on the 5-10 with a Mullard pre-amp.

The caps are pure Mullard - mustards and those little blue electrolytics you always find in Philips kit.

Many thanks for all the advice - we'll go and find a GZ30 and a couple of EF86s and fire it up!!

Bryan

[Sideband]

Originally Posted by brymac

and fire it up!!

Bryan

I hope that is not an inappropriate use of metaphors!

If it has EL34's it might be more like the 5-20 with pre-amps.

[govjohn]

If its a 5-20 it will be 1, EF86 &1, ECC83. It should be easy enough to tell which is which by traceing back from the O/P valve grids. Anyway they look like a nice pair of amps.
John.

[GrimJosef]

If there are 4 x EL34s then the amp is a stereo pair of 5-20s, not 5-10s (the 5-10s used EL84s). If that's the only rectifier then it will be a GZ34, not a GZ30. The GZ30 can only supply 125mA. Even a GZ34 will be working very hard to supply 4 x EL34s.

EDIT: looking at the tagboard I see the EL34 cathode resistors are 500ohms, a little more than the standard 470ohms spec'd for the 5-20. This will lower the EL34 quiescent current a little, giving the GZ34 a slightly easier life.

Cheers,

GJ

[brymac]

Thanks - one of us (not me!) spotted that it should be a GZ34, not a 30!

Bryan

[GrimJosef]

Do you know when it was last run ? I should add that if you just 'fire it up' then there is significant risk that you'll destroy one or more of the high-voltage electrolytic capacitors. Over a long period of disuse the insulating surface layer which forms the dielectric in these re-dissolves into the electrolyte. It needs to be reformed for the capacitor to work. Your electrolytics might be late enough that there is also an insoluble anodised layer on the foil, in which case their chances of surviving a hard switch-on are improved. But if it goes bang, possibly destroying your replacement rectifier too, then you'll know why. You can find out how to avoid this risk by searching for 'capacitor reforming' here.

Cheers,

GJ

[snowman_al]

Originally Posted by GrimJosef

If there are 4 x EL34s then the amp is a stereo pair of 5-20s, not 5-10s (the 5-10s used EL84s). If that's the only rectifier then it will be a GZ34, not a GZ30. The GZ30 can only supply 125mA. Even a GZ34 will be working very hard to supply 4 x EL34s.

EDIT: looking at the tagboard I see the EL34 cathode resistors are 500ohms, a little more than the standard 470ohms spec'd for the 5-20. This will lower the EL34 quiescent current a little, giving the GZ34 a slightly easier life.

This is correct. The GZ30 is not up to the job in this amplifier. Which could explain its demise.
You should replace it with a GZ34.

[brymac]

No - it's demise was caused by the whole lot being chucked into a cupboard with stuff put on top of it.

GJ - fear not - this will be very gently warmed up through my variac - not fired up....honest!!

Bryan

[GrimJosef]

Even a variac may not do the job. At low variac volts all the valve heaters will be too cold for the cathodes to emit. Nothing will happen. So you'll wind the variac up and up and up and eventually current will start to flow. But the mains transformer will now be supplying quite a lot of HT. The caps might be OK or they might not.

Specialised capacitor reformers have been developed to do just this job and if there was any other safe way then they wouldn't be needed. They're not difficult to fettle up. A couple of solid-state diodes and a suitable resistor tacked onto the rectifier socket pins can do the job with all the valves removed (just check that none of the other caps in the circuit will be over-volted). It would be a shame to do damage rather than doing this right.

Cheers,

GJ