Don't overlook the other supplies that bias the tube{s}. Thoroughly check
them out also. Problem might be more minor.
Aloha & 73,
Ed Pagaduan
----- Original Message -----
From: <jhill@visi.com>
To: <amps@contesting.com>
Cc: <henry@contesting.com>
Sent: Wednesday, July 12, 2000 9:26 AM
Subject: [AMPS] Power Supply Question
>
> 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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