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    Dynaco PAS 3 - high voltage at 12AX7

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    anti-doom

    Posts : 3
    Join date : 2013-12-27

    Dynaco PAS 3 - high voltage at 12AX7

    Post by anti-doom on Fri Dec 27, 2013 2:53 pm

    My PAS 3 got tone control removed and power board upgraded. All measurements at 12X4 tube are within range per schematic. But I got high voltage at 12AX7 tubes. I got 280 V at pin 5 (should be +/- 11V), 280V at pin 6 (should be 200 V) and 280V at pin 9. However, the preamp is working fine, sound really good and the power transformer is not even warm. Please help me to troubleshoot the high voltage. Should I add 1 uF at output to block DC leaking will fix the issue? Thanks

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Dynaco PAS 3 - high voltage at 12AX7

    Post by sailor on Fri Dec 27, 2013 6:01 pm

    Pin 4 and 5 are the filament or heater. Test the voltage between these 2 pins it should be around 12 volts AC not DC as I remember the PAS 3 is AC heated.

    anti-doom

    Posts : 3
    Join date : 2013-12-27

    Re: Dynaco PAS 3 - high voltage at 12AX7

    Post by anti-doom on Fri Dec 27, 2013 6:42 pm

    Yes I got about 11.8V AC between pin 4 and pin 5 of the 12AX7 tube. But PAS schematic page 11 shows pin 5 to ground at 12AX7 should be around 11V DC. Measure pin 5 to ground of my PAS, I got 290V DC which I think too high. Thanks

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Dynaco PAS 3 - high voltage at 12AX7

    Post by sailor on Fri Dec 27, 2013 11:02 pm

    I just checked the schematic and I was wrong. It looks like it has rectifying diodes. So it is DC. I also noticed that there is a mistake in the schematic. It has a grid that is pin 7 labelled as pin 9. Check the voltage between pin 4 and 9 and between 5 and 9. That should get you the 5.5 volt reading you are looking for.

    anti-doom

    Posts : 3
    Join date : 2013-12-27

    Re: Dynaco PAS 3 - high voltage at 12AX7

    Post by anti-doom on Sun Dec 29, 2013 9:43 am

    Problem solved. I did not connect point #4 on PC-6 to ground. Now it works fine and all measurements are within specs. Thanks

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