CHAMP ELECTRONICS -" THE VALVE AMP
HOSPITAL"
NOTTINGHAM, ENGLAND
CORRECTION OF FAULT ON A CORNELL PLEXI
45/50 AMPLIFIER HEAD
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.
This is just so
beautiful.
The chromed
chassis
& transformer end-shrouds.
Note the
large nickel-plated washers used on all the transformers’
mounting bolts. Great
idea…adds strength & rigidity to the fixings.
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!”
100% Perfect
Product!!