Vintage PA Grampian type UA ENSA

[maninashed]

I've recently unearthed (no pun!) this venerable PA system. It's a Grampian type UA no2737 badged 'ENSA'. The valve line up is like nothing I've ever seen before, EF39, CL33,CL33, and 2 rectifiers type CY31. It's powered through a mains dropper so must be an AC/DC live chassis set! It's no wonder some performers were electrocuted! On this set the mains dropper is past redemption. The set is complete, and in quite good cosmetic condition. The pair of speakers clip on and cover the amp forming a carry case so it's been protected while being stored.

The set also includes a period Grampian microphone which sits in a felt pocket in the rear of the case along with the wiring. Can anyone throw any light on this amp and if possible does anyone have a circuit diagram? I really need to know the value(s) of the mains dropper so I can source/make up a replacement.

I've included a few photos for your perusal.
Bill

[GrimJosef]

Is it one like this https://www.vintage-radio.net/forum/...ad.php?t=76064 ?

Cheers,

GJ

[maninashed]

Hi GJ, yes it looks excatly the same, although I'm not sure yet if the schematic is the same. This particular amp has two rectifiers and no barretter.

Bill

[mark pirate]

It looks like the speaker is a replacement, it looks the same as the speaker fitted to my mid fifties Ekco TC208 console TV.

Other than the dropper, it looks to be in good shape. Nice to have its original mic present and correct, should sound good when restored with those CL33's in push-pull.

Mark

[maninashed]

Hi Mark, yes I think you are correct about the speaker because the other one is a Celestion of the same type/size. It's in remarkable condition and should restore nicely, but I need that dropped before I start work on it. Anyone any ideas of the possible value of the dropper?

[OscarFoxtrot]

Looks like this on on ebay

which has a circuit diagram on the listing

[julie_m]

Should be a simple Ohm's Law calculation based on the ratings of the valve heaters. They're all 200 mA, add up the voltages, subtract from 230 to get what appears across the dropper and then divide by .2 (or multiply by 5, whichever is quicker). I get 116 V for the heaters leaving 114 V for the dropper; NPV is 560 ohms and it'll be dissipating over 22 watts.

[Jonster]

Nice! Hope you get it going. The microphone is a Grampian DP2, I had one a few years ago.

[Herald1360]

Presumably in its original state the microphone would have been input via a microphone transformer which would have provided the requisite isolation (at least by the standards of the day).

[maninashed]

Thanks OscarFoxtrot,

I've managed to contact the vendor and he's kindly emailed me a copy of the cct diagram! I've attached it to this post, I'm sure you will find it interesting.

Chris you are right, there is a microphone transformer, but one side is connected to chassis via a 5N Hunts cap, and we all know how good they are. It looks like the dropper is 640R, do any of you chaps have one of those lurking at the bottom of your spares box? The circuit is simple enough and providing the wound components are ok this should be a straightforward rebuild.

Bill

[techiesteve]

Originally Posted by maninashed

It looks like the dropper is 640R, do any of you chaps have one of those lurking at the bottom of your spares box?

The dropper is shown as 640R, but that is its total value, for 250 VAC as marked on the top tap. It would be lower for our present UK mains.

[Herald1360]

I would definitely replace that 0.005uf capacitor with a 4n7 Y type, though possibly the suggestion to remove the link altogether has some merit.

If the circuit makes sense, though, the mic transformer primary does go to ground, rather than the mains connected HT- (presumably chassis, though it may well be floating with the chassis actually grounded).

[maninashed]

Yes Chris, the 5N cap will be replaced with an X rated one of similar value. The chassis can be grounded (there is an earth connection on the outside of the cabinet connected to chassis), the HT- is floating. I've included a photo of the amp under chassis which looks in pretty good fettle considering this is 60 years old.

Bill

[julie_m]

It should be a Y-rated cap between mains and earth. (remember, X is a cross, so it sits across the mains). I'd also apply a Megger to the input transformer - primary connections, shorted, to one probe; secondary connections, shorted, to the other probe - before touching that mic.

If in any doubt whatsoever, use an isolating transformer. 100 VA ought to be adequate (46 VA for the heaters and dropper resistor, and the amplifier itself doesn't look like it will need more than about 30). If you can get one with a 115 V tap on the secondary, you won't even need the dropper.

[maninashed]

Thanks AJS, you correct of course, it should be a Y rated cap, I think the red wine had taken its toll when I posted that!
I do have a Megger, and an isolating transformer and will do as you advise. I've no intention of using this amp on a regular basis, its more of an interesting period relic, but it will be one that works !
Bill

[Neil Purling]

Do you recall what kind of microphone the Grampian DP2 is?
I am curious about the sort of input signal level would be required to give full output.
As this amplifier is not working we don't have the ability to test it.

[maninashed]

If you would like more pictures of the Mic, I will certainly do that for you, as far as specs on the mic, I'm afraid I can't help, but I ll bet someone else on here will have it squirrelled away somewhere! It's not working yet, but as soon as I can source a 620/640 ohm dropper I'll soon have it up and running.
Bill

[Jonster]

Its a dynamic (moving coil) mic and I think the one I had was 4k impedance.

[Neil Purling]

Just wondering about the sort of signal the pre-amp gets:100mV or greater?
Did Grampian ever issue specifications for their amplifiers?

[OscarFoxtrot]

I'm suspect, like Rolls-Royce cars, that the power was merely described as 'ample'

Unlike Coomber, who used a rather ingenious combination of PMPO and badminton courts.