Greetings;
I have always assmed that if it's a Henry, it runs forever, and
to specification. That having been said...
I own a 3K Classic-X of the 8877 vintage.
Two nights ago, I got on and one of the locals commented about
my signal having an AC component on it. Several others confirmed
this. Being technically competent and certainly, not shy, I dug
into the unit right away.
A high voltage probe on the plate voltage (3800V) revealed that
with only 100 ma of current, I had 170VAC riding on top of the
DC. The next day, I brought home a Sencore L/C analyser and
verified that the three oil caps had not changed their value
significantly. At the same time, I checked the rectifiers for
uniformity of avalanche voltage and they were okay also. While
I had everything disconnected, I put the Sencore on the AC
filter choke. I was surprised to note that instead of 8 Henrys
inductance (per schematic)it was 6. The surprise was because
there were no signs of windings shorting out, like charring,
stink, seepage, etc.
One has only to do the math to understand the function of that
choke and two .1uf filter caps in parallel. They are in series
with the rectified AC as a trap for 120 Hz, prior to the 16 uf
filter cap.
By design, .2uf x 8H = 125 Hz. Close enough for 120 Hz.
Currently .2uf x 6H = 145 Hz. Too far off?
So, you power supply gurus. I'm not certain of the Q of that trap,
but is being 10% off frequency enough to compromise effectiveness?
Is it normal for high inductance devices to change their value by
25% over time? This amp is around 20 years old.
I have contemplated adding a third .1uf 7.5Kv cap in parallel with
the existing two, or replacing them all with a .3uf 7.5Kv cap.
.3uf x 6 Henrys = 119 Hz
Any input would be appreciated before I jump in.
Posted to Amps and Henry@contesting.com
73, John, NJ0M
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