CHAMP ELECTRONICS -" THE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

CORRECTION OF FAULT ON A CORNELL PLEXI 45/50 AMPLIFIER HEAD

 


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Looks Fantastic!





THE QUIRKY TALE OF A BRAND NEW CORNELL PLEXI 45/50


During the summer of 2007, I received a phone call from a gentleman named John, who lives only about 20 miles away from my home town of Nottingham…here in the UK. John explained to me that he had, that very day, bought a brand new Cornell Plexi 45/50 head, and that he had just done a round-200 mile trip to take possession of it! He further explained that on picking it up from somewhere in the south of England, the salesman had told him that it had been on test for the whole of the previous day and that all was good. Fair enough.

So......on arriving home with his new-found baby, John eagerly and excitedly plugs her in for a good play. Apparently he played one chord and she went really very, very quite!!

John’s first phone call was to the place where he had just bought/collected it. They in-turn put him directly in contact with Dennis Cornell. Being disappointed but helpful, Dennis suggested to John that if he could find someone local to take a look at it, he would reimburse any costs. This would save John having to take it all the way back to the dealer. So this is now where I came in! John searched the web, found me, and made the obvious phone call. It was no problem for me to drop what I was already doing, get John to come straight over, and then find out what was wrong with the amp. This we proceeded to do.

On first putting it on the test gear it was indeed working but extremely quiet; about 2 watts! Checking all HT, bias and general voltages, everything seemed fine. I next injected the signal generator straight into the phase splitter, and the output stage burst into perfect life. OK, this eliminated a lot of possibilities but she still wasn’t working from the front-end.

Now, I diverse a little here......especially for the technically minded. The Cornell products are absolutely the neatest, nicest, hand-built amps I have ever witnessed! They are superb! (Check out some of the comments I have put on the photos.) They are an absolute masterpiece. The main component board is about 2mm thick and very solid. There is a solid copper earth (ground) buss running the full length of the board for all the ground returns. This is the correct and only way to do the grounds on any amplifier!

So then, what was wrong? Well, knowing the power supply, output stage and splitter were all OK, it obviously had to be something up at the front-end. Checking voltages on the first ECC83 showed everything was fine there. However, on the first half of the second ECC83, I wasn’t happy with what I was seeing. Turning the amp off to do some continuity tests revealed the problem. The bottom end of the components on the cathode of the first ECC83 were certainly on ground. In-fact, I could see a thin black wire coming from these components to the buss ground rail.

Looking at the same thing on the second ECC83, there was no black wire! So where were these components getting their ground from? My meter confirmed that there was indeed no connection here to the ground rail. I next removed the few nuts that hold the component board to the pillars and lifted it up as much as the fly-leads allowed. Then with a small torch and mirror I examined the underneath to find out what was going on there. The component board is simply that and not a PCB, but on close scrutiny I could see just one piece of circuit board track, running from the said main buss rail to a turret which is right next to the ground end of the components from the cathode of the second ECC83. Looking back on the top, this turret carries the ground of a piece of screened cable and….the said cathode grounds of this ECC83. The problem was that the riveting into the board of this turret wasn’t that firm. Consequently, as this is riveted through this one piece of circuit board track, the connection was iffy, hence an intermittent/bad ground! I simply soldered a small black wire from the ground end of the cathode components to the big buss bar and she all burst into life!! Whoopee!
 

I still to this day cannot understand why a fantastic product, designed and made like this, relied on an unsoldered, riveted turret to supply the grounds to the second tube stage, when it would have been so simple (and certainly more reliable) to solder a ground wire to the main buss like the rest of the tubes’ grounds!

Though I never got to speak to Dennis direct, I was informed that he was
not happy (rightly so) with one of his amps “going down”. I believe this problem has now been corrected.

Cheers for looking, John.

 

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This is just so beautiful.
 


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The chromed chassis & transformer end-shrouds.




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Note the large nickel-plated washers used on all the transformers’ mounting bolts. Great idea…adds strength & rigidity to the fixings.

 

  

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Absolutely superb! Note the individually twisted pairs of wires feeding the heaters of each tube, this ensures equal currant carrying throughout. Also, the large ground buss rail. When finished & tested, the component board is sprayed with a clear lacquer to keep out the damp & condensation. This whole amp is truly “well though-out!”


 


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100% Perfect Product!!




 

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