Parkemo amplifier

[djrm]

   

I got this old amp out of storage today, its not been very well looked after over the years since we stopped using it in about 1975. 

It is one of a pair of nearly identical I own, they were bought as army surplus back around 1968 I think. I remember moditying them to have a wider frequency response as they were limited with very small interstage coupling capacitors and other filters. These drove my Dad's lowther speaker and another speaker of some unknown kind. I had one and my sister the other for a while, they are reunited now. 

I've been listening to shortwave radio on it today on the old Lowther, next I'm going to play an old record I bought in a charity shop this morning - The King and I.

I'll get the other out and test that too now. A web search has found them to be very highly regarded in hi-fi circles. Perhaps the schematic will turn up if I look a bit more. I tried to show the glow of the valves in the photo, I'll try a bit harder next time.
David.

[ppppenguin]

Presumably you mean Parmeko, who were well known for their high quality transformers. I didn't know they made amplifiers too. The company survived until 2014: https://www.thegazette.co.uk/notice/2109991

I don't know the circuit but if there are any capacitors coupling to the output valves these need to be checked and probably changed. Even a small leakage here can damage the output valves and transformer.

[djrm]

Sound advice there Jeffry, of coarse, it is a Parmeko design.
The photos here are of the other amplifier, it looks newer than the other and has glass base output valves.
I tried this one and it was quite distorted, I have changed the coupling capacitors and now its sounding sweet again.
This amp has a seperate bias supply of about -33V, one grip was at -15V due to the leakage.
The three capacitors are visible in this photo taken before the change.
There are some schematics on the interweb of a similar design but I cant find the same one.
           

[dawson5]

Great tag board in the first picture. All looks good quality stuff.

[Ed_Dinning]

I see there are some high value resistors 2.2 Meg and some load resistors 47K there. it would be as well to check these as they often go out of spec

Ed

[djrm]

Hi Ed,
I've now checked all the resistors and was surprised to find all but one within an acceptable margin, the 2M2s were about 2M5. The faulty 100k was more then 200K so I replaced it. I found a schematic for something similar here: A.P. 32112 My amps are type A.P. 32020.

This particular resistor is the anode load of the second preamplifier.
The first pre-aplifier is not used, it has a microphone transformer input feeding into the second preamplifier which also has an input from the front panel jack socket.

Last night I played some bass heavy music through it into the lowther horn speaker, much better than my other speakers. The workshop was rocking.

Picture shows new components fitted.
   

[Diabolical Artificer]

They look nautical, something like a ships PA/comms and as others have said, well made. Those ceramic standoff's are pretty standard on the Neptune tfmr Parmeko made, well tasty. The brown C core tfmr looks like a Partridge. In pic two is the first valve a relay? Some sort of soft start?

Nice amp's, but components worth more on their own probably, the GEC KT66/88's are worth a few bob by themselves as you probably know.

Andy.

[djrm]

I seem to have a vauge reccolection that the amps were sold as ex Admiralty and I agree they look like they could be ships equipment.
Not quite so robust as the B40 receiver but not far off. They are quite heavy.

The odd looking valve is all tat is left of a CV1863 rectifier valve which got smashed by accident, Ive replaced it with semiconductor diodes and a limiting resistor soldered into the valve base.
I need to buy another glass rectifier but have you seen the price lateley! I have seen some similar amplifiers sold by online auction going for hundreds of pounds.

[Murphyv310]

Do I see a 6F23 on the third picture? Odd for a TV valve of the 60's to be there.

[djrm]

Hi Trevor, Well spotted.
The original valve was probably an EF86 and used as a mic amplifier. I must have robbed it for something else in my youth.
It isnt being used now anyway. I think I put this in just to look complete, shame I lost the screening can in the process.
David.