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    Powering up after years a ST-70 & PAS-3

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    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Powering up after years a ST-70 & PAS-3

    Post by Aljaheejus on Tue Aug 23, 2011 12:17 am

    I have a power control unit with which I can bring up the AC voltage slowly. I have been told to monitor the B+ voltage near the "power filter" as I bring up the AC. I don't know what location is best to monitor the B+. Can someone please advise?

    I have been told to start at 30 VAC, wait for B+ to come up, then proceed increasing the VAC by 10 volts each time. Do I wait until the B+ stabilizes each time before proceeding?

    Are there certain levels of B+ to look for at each stage, or just wait for it to come up and stabilize?

    Thanks!

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Powering up after years a ST-70 & PAS-3

    Post by tubes4hifi on Tue Aug 23, 2011 4:33 pm

    On an ST70 you can monitor the B+ voltage on pin 8 of V1 (the rectifier tube).
    If you are using a variac, I usually start at about 50-60% of full voltage (about 60-70vac).
    It will take at least 30 seconds to even read a voltage here with this low AC input. If nothing blows (let's hope not!)
    then slowly bring the voltage up about 10% or 10vac (on the variac) about once a minute, so after 5-6 minutes you should be at full voltage.
    With 115vac input you should read about +420vdc at this point.

    On a PAS3 you can monitor the B+ on pin 7 of the 12X4 rectifier tube, probably better where it goes to the quad cap and meets a 10K resistor.
    Same exact process as above, once you get up to full input 115vac you should have around 405vdc at this point.

    If you don't have the manuals for these, both are available here on my website: http://www.tubes4hifi.com/downloads.htm

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: Powering up after years a ST-70 & PAS-3

    Post by Aljaheejus on Tue Aug 23, 2011 8:59 pm

    Thanks for the great information. I will clip lead to those points and to the chassis/ground and take readings as I power up with my variac. I have 120VAC here so my VDC may be a bit higher than the VDC you quote for 115vac, I suppose.

    The gentleman who sold my first ST-70 to me gave me the advice about powering up and included a special tube to use in place of the 5AR4 rectifier tube. It is a 5U46B, and he suggested I use that with the variac to further protect the amp. That's what I will try.

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Powering up after years a ST-70 & PAS-3

    Post by GP49 on Tue Aug 23, 2011 11:42 pm

    That's 5U4GB (a "G" as in "GP49"). The 5U4 series of tubes do not have a delay-start as do the 5AR4/GZ34 that is the standard rectifier in the Stereo 70, so they begin to feed DC as soon as the filament is warm enough to emit electrons. I think your friend was afraid that the 5AR4 would hold off starting up until all of a sudden it would "turn on" and possibly overstress long-dormant capacitors downstream, instead of being under control by your use of the variable AC source.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: Powering up after years a ST-70 & PAS-3

    Post by Aljaheejus on Wed Aug 24, 2011 12:43 am

    Thank you, and yes, it is a "G." From the etching on the glass it is clear, but earlier I was reading handwriting. It did feed voltage immediately, as I have about 30 VAC feeding as I type. The B+ is around 15 vdc and climbing. I will tune in again after either full voltage or some catastrophe ends the power-up. Thanks for the explanation of the difference in these tubes.

    Full power-up has been a success and B+ was measured at 420 vdc. Thanks to all for the help.


    Last edited by Aljaheejus on Wed Aug 24, 2011 1:09 am; edited 1 time in total (Reason for editing : Update on progress.)

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Powering up after years a ST-70 & PAS-3

    Post by tubes4hifi on Wed Aug 24, 2011 5:47 pm

    420 vdc is perfect and my preference is the 5AR4 over the 5U4GB due to slower turn-on.
    See other posts on this forum about adding two 1N4007 or UF4007 in series with the rectifier tube to protect it.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: Powering up after years a ST-70 & PAS-3

    Post by Aljaheejus on Wed Aug 24, 2011 7:07 pm

    Thanks. After using the 5U4GB tube to power up successfully with the variac, I replaced with the 5AR4. Listening now!

    I am familiar with the use of the diode to replace the selenium rectifier, but not the use of "two". Is that correct? In reviewing the posts, I only see the need for "one." Can you explain, or point me to the specific post describing the need for 2. Thank you again!


    jjones3318

    Posts : 57
    Join date : 2011-04-05
    Location : Boulder, CO

    Re: Powering up after years a ST-70 & PAS-3

    Post by jjones3318 on Thu Aug 25, 2011 12:12 am


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