Unknown Guitar Amp
 

Folks, Help &/or advice sought, please, on renovating a huge 1960's guitar amp. Its bespoke/hand built. The chassis & large component layout is excellent, although the soldering/wiring is a bit grotty in places. It feeds two huge Leak Sandwich600 speaker units. Voltage rectification is via semi-conductor diode bridges, and the amplifier valves are - 12AT7, 2x 6057's, 6J5G, 6SN7GT & two KT88's.
I've trawled through the Heathkit archive, but no luck. I dont know if Leak produced amplifier designs to drive their speakers. I'm not really an audio/hi-fi guy, but had promised my son that I'd have a go at renovation.
Hopefully, I'll attach some pictures for your perusal.

Regards, David

ps.Have only been able to attach a pic of the underside so far, shall try & reduce other picture sizes & post

Re: Unknown Guitar Amp
 

David,
I'd take bets that it's not Leak. It looks home-brewed. There's a peculiar mix of components, some much later than others. There look to be Plessey electrolytics, carbon/metal film resistors and carbon composition resistors. However, I don't see why it shouldn't work, but a bit of reverse engineering is called for.
Be careful with the speakers as they're worth serious money if they're in decent nick.
Alan

Re: Unknown Guitar Amp
 

This looks like a home built project, with the builder having access to a folding press, guillotine and hole punches, along with the components which look like RS, and the chocbloc connectors at a guess.

Geof

Re: Unknown Guitar Amp
 

Hi, Yes, definitely looks like homebrew kit. I'd give it a squirt with the can of "Bradex Easy Start" on the shelf, that might fix it!
Les

Re: Unknown Guitar Amp
 

Come on Geof, it's only aluminium. A stanley knife will cut it, a tank cutter will make the holes and a couple of pieces of angle and a vice will make the bends. At least, that's how I used to do it, untill work invested in a set of RS hole punches. That made it a lot easier. There were also firms who would make you a chassis, although the fact that the corners are not welded or riveted makes me think it's home made.
Alan

Re: Unknown Guitar Amp
 

Hello again Folks, Thanks for comments so far. The mains transformer, the big choke & the o/p t/former are all Forrests. The KT88's are GEC's. So someone had a lot of dosh to play with when it was built.
Have hopefully managed to reduce my picture sizes for further attachments.

Regards, David

Re: Unknown Guitar Amp
 

A fairly typical home-brew counstruction.

I would guess that it is a "standard" main amplifier, and a "standard" pre-amp both built on one chassis.
The 3 x ECC83's being the latter, with the octal valves the main amplifier.

As for the main amp design, and without even trying to follow wires etc from the photograph, there is a fair chance that it was, or was based upon, the GEC 100 watt as described in "An approach to Audio Frenquency Amplifier design", by GEC, which can be found on the internet, by searching for "A gec approach".

Re: Unknown Guitar Amp
 

I guess I was spoilt when I was an apprentice instrument maker in a large factory with access to all the toys. But 16 SWG, which is what it looks like, would take some cutting with a Stanley knife. Yes I noticed the just folded corners. This looks like someone's project kept under the worktop and brought out when no-one as looking, so I am told it went on :o

Geof

Re: Unknown Guitar Amp
 

Alan, This is just the info I'm needing. Shall follow up the GEC connection. I'd thought that the builder may have obtained a circuit diagram of a Fender or similar professional amp & had endeavoured to build a cheaper copy.
Obviously I could trace through all the wiring & eventually draw up a circuit diagram. Just wish the chap had stuck to one method of component mounting instead of a miss-match of vero board, tag boards & blocks. And had adopted a less random colouring & sizing of wires. The main chassis work is fine & the plywood cabinet is very well constructed joinery-wise.
After a careful brush & a good hoover, the amp has been moved from my woodworking shed to my wee electronic workshop, which is relatively dust free, for in-depth testing. The speaker cabinets just need re-varnishing, and I'll rub down & re-spray the amp cabinet.

Regards, David

Re: Unknown Guitar Amp
 

Geof, You ar so right. Its my suspicion also that the builder had access to a commercial workshop. And was possibly a 3rd or 4th year electronics apprentice with a keen 1960's interest in pop music & guitars. He might well have aquired the major components from work(with his boss's approval, Im sure), & carried out the under-chassis wiring at home with whatever wires & components could be scrounged.
Shall continue the hunt for a circuit diagram.

Regards, David

Re: Unknown Guitar Amp
 

Geof,
I agree, and the first time I saw it done I was fairly amazed myself. However, provided it's not hard Dural, a few passes with a Stanley knife really will cut it. It's even easier if it's a straight cut not a corner, because after a couple of passes you can bend it off and file the edge. The real danger is sliceing a bit off the end of the fingers holding the steel rule down :'(
David: Ah, the foreigner under the bench, it takes me back :)
BTW, there are too many Alans in this Forum. I shall have to invent a silly name.
Alan

Re: Unknown Guitar Amp
 

Much easier with a ten foot guillotine :) Even if it did take five minutes to get up to speed. The Stanley knife method does work but can cause a nasty injury with all the sharp edges :( This amp would have taken a long time to put together, but some of the wiring looks a bit iffy, and the mains connection should have some protection, the tag strip isn't the best way to bring in the mains, this looks very poor, as does the top wiring to the transformer.

Geof

Re: Unknown Guitar Amp
 

Geof,
I hadn't looked at the picture of the top - ooh, err, that's not the mains wire I would have chosen.
However, looking on the bright side, it looks like a good set of the most expensive components, so sorting it out properly shouldn't be expensive.
Alan

Re: Unknown Guitar Amp
 

Folks, Once again - thanks for response. Wonder if anyone has a copy of the aforementioned GEC Amplifier Design Book and could let me have a copy of the GEC 100 circuit diagram, &/or copies of the standard & pre-amp cct diagrams. Rather than me purchasing the whole book off Amazon or similar. Please.

Regards, David

Re: Unknown Guitar Amp
 

If I can find it, I've got a photcopy of the "Amplifier Circuits for the KT88" GEC application note which has several circuits up to a 400W parallel push pull monster. 6SN7s figure, ISTR. I copied the note from the Tech Library at Racal in the 70's when I came across a couple of lovely H/B 30W UL KT88 amps complete with Quad 22 preamp and some large, knackered, Heathkit speakers which were given to the local operatic society where I crewed. As luck would have it, after a chance meeting with the former stage manager I found they were still there some 25 years later, and yes, I COULD have them :)

I'll have a look tonight.....

Chris

Re: Unknown Guitar Amp
 

I have that 1957 GEC Audio Frequency Amplifier Design book but the whole thing is available free in pdf form here:-

http://www.pmillett.com/technical_books_online.htm

(It's the sixth one down) :)

Regards, Mick.