Grampian 506

[Newgirl2013]

Hi Everyone
I have just received a Grampian 506 Tuner Amplifier in need of a complete restoration
The valves were packed seperately
Please could anyone tell me which valve goes where
Also it needs a main smoothing capacitor
Please cound anyone help me there
Thanks very much Sharon

[julie_m]

If you post some pictures of the chassis and a list of the valve numbers, you will stand a better chance of getting an answer. This is because you will have opened up the field of potential answerers from "people who currently own or have used or owned a Grampian 506 and still remember everything about it" to "anyone who is not necessarily intimately familiar with the Grampian 506 circuit in particular, but still has enough general knowledge to be sure of a reasonable idea how to proceed if they had the set in front of them".

I would have thought whoever is doing the complete restoration for you would have been able to work out for themselves which valve goes where, though ..... After all, you're paying them to know how to do the job!

[Station X]

Pics in this thread:-

https://www.vintage-radio.net/forum/...d.php?t=158981

[ms660]

Take a look at this photo:

https://www.vintage-radio.net/forum/...1&d=1277330178

The two valves to the left:

The uppermost should be a UCH42

The lower one should be a UAF42.

The row of valves to the right:

The uppermost should be a UY41

The next one down should also be a UY41

The next one down should be a UL41

The next one down should also be a UL41

The bottom one should be a UAF42

So far as I can make out:

https://www.vintage-radio.net/forum/...ad.php?t=55578

EDIT: Post crossed.

Lawrence.

[julie_m]

Oooh, Lawrence is on the case now! Well, this one should soon end up sorted, then!

You're even both in Cornwall -- though I'm not going to make the usual city-dweller's mistake of forgetting Cornwall is actually a huge county, with lots of space between each little village where everybody knows their neighbours and the next .....

[ms660]

I don't live near Hayle although I do know the area well as I was once a West of the Hayle River resident for lots of years. Amongst other things the area was famous of smelting, the foundary, the explosives works (the dunes/towans and dynamite quay) and a certain Peter De Savery.

End of OT bit.

Lawrence.

[ms660]

Looking at the under chassis photo in the link I posted I can't see any phase splitting going on so it looks like a single ended output stage, the two UL84's in parallel?

Lawrence.

[Newgirl2013]

Thanks Laurence
Got it all valved up
I just need to find a double electrolytic Smoothing Capacitor to solder in
Does anyone know what value this should be and where I might find one
Sharon

[Newgirl2013]

Hi Julie
Well thanks to Laurence I have got it all valved up
Doing the reastoration job myself
I was in the radio repair trade years ago, long since retired but I like to keep my hand in
The last time I repaired a Grampian 506 was in 1982
Replaced the biassing components and it nearly deafened me
Hope I get the same good quality out of this one
Sharon

[Nuvistor]

There a selection of smoothing/reservoir caps here, I don’t know what value the 506 uses.
https://www.cricklewoodelectronics.c...-and-Tags.html

[Newgirl2013]

Hi Frank
Thanks for thje info
I used to have a valve amp and the main Smoothing capacitor was 50+50 350Volt
I will look on that
Thanks
Sharon

[Radio Wrangler]

Look up the rectifier valve data. It should specify a max reservoir capacitance which is a good value to go for. For the smoother, the same or a bit larger is fine.

David

[julie_m]

Originally Posted by Newgirl2013

Hi Julie
Well thanks to Laurence I have got it all valved up
Doing the reastoration job myself
I was in the radio repair trade years ago, long since retired but I like to keep my hand in

Ah, that makes sense.

The UY41 rectifier is rated for a reservoir capacitor of up to 50µF and your set has two of these, so you can get away with a reservoir up to 100µF. Double capacitors are available in values up to 32+32µF. Larger double capacitors may be available, as they were used in TV sets. You can certainly still get single capacitors of 47µF, 450V; four of these in two parallel pairs would be suitable. You can make the smoothing (second) capacitor even bigger than the reservoir by a factor up to about 5 times, if needs be. There probably will be room to mount single capacitors below the chassis, with the original multi-can left in place but electrically disconnected just to preserve appearances.

The capacitors connected to the first grids of the UL41s must be replaced. Any modern polyester or polypropylene film capacitors of the correct capacitance and same or higher voltage (the maximum they are likely to encounter in practice is about 350V) can be used. If these capacitors are faulty, then the output valves can pass excessive current and the loudspeaker transformer (which is Stercora equi ludibrio, as they might have used to have said in Ancient Rome ) can be ruined. The only exception is if they are Mullard Mustard capacitors; which are literally the first modern polyfilm capacitors anyway.

Note also that the common negative connection to the power supply capacitors should be as short and thick as possible, and not have any branches off to anywhere else in the amplifier circuit; and finally, the use of "U" prefix valves suggests this is a live chassis appliance, so be very careful. If your house wiring includes an RCD, this will protect you from electric shocks -- just tell anyone else who may be present and using a computer to save their work (now, there's a story for another time.)

Grampian were Public Address specialists, so pretty much anything they made should be capable of plenty of volume! PA amplifiers intended for reproducing mainly speech often included (explicit or implicit; the design of the transformer between an amplifier and its loudspeaker has a strong influence on its working frequency range, and hence the working frequency range of the whole system) bandpass filtering restricting them to the frequencies present in the human voice; but this having a radio receiver may well be intended to have a better bandwidth, and so be capable of reproducing music more faithfully. (The total amount of steel used in the core determines the lowest frequency that can pass through it; and the thinness of the laminations determines the highest frequency that can pass through it, thinner being better.)

[Nuvistor]

Anyone have circuit for the 506?

[ms660]

In the photo's so far, the neutral of the mains lead appears to be connected to the chassis but I can't see any isolation between the chassis and the metal case.

That aside, so far as I can make out there are two series heater circuits one for the scale bulbs, UCH42 and UAF42 and another one for the UL41's, UY41's and the other UAF42....Auto transformer with two taps...?

Definitely paralleled UL41's so far as I can see, common 75 ohm cathode resistor, common 680k grid return resistor, individual 2.2k grid stoppers, looking at the wiring and the output transformer suggests the output transformer might be of the hum cancelling type, the 1st AF (on radio) uses grid current bias, the grid resistor is 22meg, the anode is fed from a decoupled supply (100k and a big wax TCC at least) the anode load is 220k, the screen feed is via a 3meg.

The surge limiters for the rectifiers are 180 ohm fed from the mains via the on/off switch, there's a switched mains outlet socket on the rear of the chassis.

Lawrence.

[Techman]

Originally Posted by ms660

In the photo's so far, the neutral of the mains lead appears to be connected to the chassis but I can't see any isolation between the chassis and the metal case.

No need to be shocked at that, Lawrence, it's Grampian, what do you expect

I've had experience with their stuff before and it's usually over engineered in the wrong places with scant regard for any electrical safety...but I love 'em

I'd love to go back in time and meet the designers of some of their stuff and talk to them about some of their methods. It could of course be just one person who had some very strange ideas - we'll never know!

If it's got Grampian written on the front, then be prepared to be electrocuted

[Newgirl2013]

Hi Techman
Yes very unusual
I haven't known any radio equipment containing a mains transformer having neutral connected to chassis
AC DC with a mains dropper, yes, but not a mains transformer
Just wondering if there's any way I could isolate the chassis
The other Grampians I have are a 641 Tuner amp and a 642 Amp all isolated
Wish I could get hold of a circuit diagram for the 506

[Nuvistor]

Is it an fully isolated transformer or an auto transformer? Other makes have used that system, the Pye R33 for one with a tap for the valve heaters.

[Newgirl2013]

Hi Frank
I've been told it's an Auto Transformer
Shaz

[Herald1360]

Then you're stuck with neutral to the chassis / negative HT rail. As a standalone item (not connected to any other, earthed, gear) for your own use it should be fine, though a dedicated RCD plug top wouldn't come amiss if your house supply isn't already thus protected.