CHAMP ELECTRONICS -" THE VALVE AMP
CORRECTION OF FAULT ON A CORNELL PLEXI
45/50 AMPLIFIER HEAD
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
John explained to me that he had, that very day, bought a brand
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
on test for the whole of the previous day and that all was
good. Fair enough.
arriving home with his new-found baby, John eagerly and excitedly plugs her in for a
good play. Apparently he played one chord
went really very, very quite!!
phone call was to the place where he had just
They in-turn put him directly in contact with Dennis Cornell. Being disappointed but helpful, Dennis
suggested to John that if he could
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
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
about 2 watts! Checking all HT, bias and general
seemed fine. I next injected the signal generator straight
phase splitter, and the output stage burst into perfect life. OK,
a lot of possibilities but she still wasn’t
working from the front-end.
Now, I diverse a little
here......especially for the technically
Cornell products are absolutely the neatest, nicest,
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
the ground returns. This is the correct and only way to do the
grounds on any
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
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!
were these components getting their ground from? My meter confirmed that there was indeed
no connection here to the ground rail.
removed the few nuts that hold the component board to the pillars and
up as much as the fly-leads allowed. Then with a small torch
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
see just one piece of circuit board track, running from the said
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
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
hence an intermittent/bad ground! I simply soldered a small black
the ground end of the cathode components to the big buss bar and she all burst into life!!
I still to this day
cannot understand why a fantastic product, designed
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
rest of the tubes’ grounds!
Though I never got to speak to Dennis direct, I was informed that he
happy (rightly so) with one of his amps “going
down”. I believe this problem has now been corrected.
Cheers for looking, John.
This is just so
& transformer end-shrouds.
large nickel-plated washers used on all the transformers’
mounting bolts. Great
idea…adds strength & rigidity to the fixings.
superb! Note the individually twisted pairs of wires feeding the
each tube, this ensures equal currant carrying throughout. Also, the
ground buss rail. When finished & tested, the component board
with a clear lacquer to keep out the damp & condensation. This
whole amp is
truly “well though-out!”