PV10BL Modifications
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~ The fix for whistling tubes ~

Discussions on the topic @ Vinyl Asylum:
                    http://www.audioasylum.com/scripts/t.pl?f=tubes&m=120675 
                    http://db.audioasylum.com/cgi/m.mpl?forum=tubes&n=178906&highlight=Conrad+Johnson+PV10A+w/phono&session=


Anumber1 @ the VinylAsylum helped me with this. He said:
"...we changed the 47.5 ohm resistor on the grid of  V4 (R29, right channel) leading to the Volume Control pot to 1k ohm. You would need to do the same for the left channel (V5 IIRC).
The Whistling/Microphonics are not really bad tubes but a parasitic oscillation that occurs due to the zero feedback design of the circuit. If you have tubes that will not work now they may be viable after the resistor swap. IIRC the job took me less than 20 mins to accomplish so it 'aint "Rocket Surgery". "

I took his advice and it only took 20 minutes. Dead quiet now.

For reference here is the entire board and the Schematic for the PV10B. It applies to the PV10BL minus the phono stage.

 Before w/ the 47k resistors


After w/ 1k resistors

2.21.2007


~ New output coupling caps ~




Original caps were 3.9uF 600V AXION caps w/ cj SN 150nJ 600V caps in parallel. cj soldered the small caps to the bottom of the board. They were not readily seen so I was unaware that they were there for running in parallel with the AXONs.



Replacement caps are Jentsen Cross-Caps 3.9uF 400V in parallel with Russian military #K40Y-9 .22uF 600v paper-in-oil. Thanks again, Al! You da man.




It's a tight fit. no room under the board for these fat Russian caps. The Jentsen is tight to the board and the Russians are damped to the Jentsens.After just 24 hours the sound is special. Every break/fix should entail an upgrade.


10.24.2007