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## [AMPS] Power Supply Question

 To: [AMPS] Power Supply Question jhill@visi.com (jhill@visi.com) Wed, 12 Jul 2000 14:26:14 -0500 (CDT)
 ```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 -- FAQ on WWW: http://www.contesting.com/FAQ/amps Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com ```
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