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Lux 300B SET hum after replacing PT

Posted on May 2, 2013 at 14:35:31

I've replaced the PT because the original was dead.
New one is made on specs >

0-230V +scr

400-0-400V 0.1A
400-0-400V 0.1A
3.15-0-3.15V 3A
0-5V 4A
0-5V 4A
0-5V 3A
0-5V 3A

I replaced the transfomer and neassured voltages, but al are little high now, but the main problem is that there's a hum on both channels. Amp is working but hum is to loud.

Anyone an idee what to do?

regards Dirk


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RE: Lux 300B SET hum after replacing PT, posted on May 2, 2013 at 17:16:34

DC readings are:
- 300B filaments 7,75V and 4,7V
- 1K resistors 97,2V and 95,6V
- 300B's pin 2 > ground 496V and 495V
- 300B's pin 2 > 4 398V and 397V
- 300B's pin 3 > ground 9,6V both
- EF86's pin 6 > ground 278 and 266
- 5687 pin1/pin9 > ground 284V and 280V

Little to high and amp is humming


RE: Lux 300B SET hum after replacing PT, posted on May 4, 2013 at 08:04:12


did you figure out what is wrong ?
do you have have schematics or know how to get them for the FCUPS power supply?



RE: Lux 300B SET hum after replacing PT, posted on May 5, 2013 at 00:06:14

Posts: 4175
Location: Somewhere nice on planet dirt
Joined: September 25, 1999

I'd say the voltages are way too high. Especially the 300B Filament at nearly 8V.

The hum comes probably from you either having disturbed the grounding or from the magnetic field of the new transformer being much stronger than the original one and being picked up. Did the orientation of the transformer change?

Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?


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