On Jan 29, 2005, at 7:09 PM, Robert F.((Bob)) Jones wrote:
----- Original Message -----
From: "R. Measures" <r@somis.org>
To: "W0OXJ" <rfjones@ris.net>
Cc: <amps@contesting.com>
Sent: Saturday, January 29, 2005 6:33 PM
Subject: Re: [Amps] TL-922 Troubles
On Jan 29, 2005, at 4:33 PM, W0OXJ wrote:
I have been running a balanced pair of 3-500ZG's for 3 years.Recently
the
amp made a loud pop and began arcing while at idle.I turned it back
on
after
a period of time and noticed a loud hum and the filament transformaer
was
getting warm.
A filament got pushed sideways and shorted against the grid cage. Are
you using the stock vhf suppressors? Did you install a glitch R?
The amp is 100 percent stock,Richard.What is a glitch R?
Robert -- A glitch-R is a resistor in series with the HV+ that safely
limits peak discharge current during an anomaly. A glitch resistor
must be able to withstand a direct short to ground (the" big
screwdriver test") without damage of any kind or it will fail to do
it's job. A glitch-R is definitely not supposed to act like a fuse.
As to the value, I like to limit discharge current to 100A-peak in
1500w-out heater-type tube amplifiers and 200A-pk in 1500W
filament-type tube amplifiers. In 3-500Z amplifiers, a glitch R helps
to prevent filament-helix bending and fil/grid shorts.
One tube has fresh paint on it and the other had faded paint which
seemed
strange.The tube with fresh paint does not look used at all.The tube
with
faded paint had a red plate after 45 seconds of excitation under low
drive
and a loaded final.I swapped out tubes with a spare Eimac 3-500 and
the tube
with fresh paint proved to be causing the heavy draw on the filament
transformer.I also notice no idle current where I used to have 150
ma.
The amp seems normal,it loads but I didn't try to load it fully.The
relay
seems to operate OK,the 3-500ZG plate stills glows faintly red.
In the past I have noticed one tube would turn red after a period of
keydown
but was not concerned as the amp has delivered 800 watts out for
three
years
.
Now I plan on replacing the defective 3-500ZG but would like to know
what
could be causing the imbalance? and is this the reason for the failed
tube.Will a new tube be matched enough to give good service with the
existing tube?Why no idle current?
Both tubes check out with an ohm meter,no shorts between the three
elements.
The tube with the bent filament only shorts when the filament is hot.
Operating a stock 922 with a grid-fil shorted tube will destroy T2.
There is an article on the 922 on my Web site.
I'll take a look at your website and certainly consider the
suppressors.
I can't do any voltage checks until I replace the 3-500ZG and will
follow
the instructions in the service manual.They mention a bad zener,bad
relay
and a bad tube as causes for this condition.
The bad tube is the result of an intermittent oscillation c. 120MHz.
Good luck, Bob.
As always thanks Richard.
Any advice or experiences would sure help as I would like to insure a
few
more years of reliable operation.
Many thanks and 73,
Bob
W0OXJ
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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