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    Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

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    Bob Latino
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    Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by Bob Latino on Sat Dec 05, 2009 9:51 am

    Roy Mottram of Tubes4HiFi was kind enough to let me post his board layouts for the PC-5 line stage and PC-6 phono board as found on the PAS-2 and PAS-3 preamps. Dynaco never listed the parts on these two boards in their assembly manual. When you got your PAS-2 or PAS-3 kit the boards were already preassembled.

    Bob

    tubes4hifi
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    resistors on PAS3 PCBs

    Post by tubes4hifi on Wed Dec 15, 2010 3:23 pm

    I've got a pair of PCBs in front of me with the schematics. The listing for the 100K 1w is incorrect,
    on the schematic of the PC-6 phono board the 150K is marked as 1w. The actual boards I have in front of me
    has a 1/2 watt resistor on it, it's 40 years old, doesn't even look like it ever got warm.
    Using the voltage chart from the PAS3 manual, that resistor has 95v across 150K equals .63ma and
    that is .06 watts. The one resistor with the most power across it is the 100K plate resistor on PC-5 line amp.
    It has 130v across 100K for 1.3ma and that is .169 watts, about 1/3 of 1/2 watt rating.
    On the actual boards I have in front of me that 100K is a 1w resistor even though the schematic doesn't
    spec that. It is an original part. None of the cathode resistors use more than .06w - See updated board layouts below ...

    Roy @ www.tubes4hifi.com


    turbodan

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by turbodan on Thu Nov 01, 2012 10:00 pm

    Can anyone provide some insight as to why in the PC5 diagram provided by tubes4hifi the 7500pF caps are substituted with 6800pF replacements? It seems like this would have a negative effect on treble response....

    tubes4hifi
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by tubes4hifi on Sat Nov 03, 2012 7:44 pm

    in reply to the last two posts, the .002 cap connected to eyelet 4 is part of the treble tone circuit.
    as for replacing the 7500pf with 6800pf in the bass section, two reasons I do this, one, try finding a 7500pf cap . . .
    2nd, both the treble controls and the bass controls have WAY too much gain in a stock PAS, so subbing a 6800pf for a 7500pf
    is a fairly minor change and might only give you 12db (that's 4X !!) of boost/cut instead of 15db.

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Fri Apr 05, 2013 12:14 am

    Just wondering what wattage the resistors are suppose to be and what voltage the caps are suppose to be? thanks

    tubes4hifi
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by tubes4hifi on Fri Apr 05, 2013 2:13 pm

    look at the info at the top of this page - all resistors are half watt except one is marked one watt.
    Most capacitors should be rated at 400vdc (you can always check the schematics to be sure of anything).
    Here is a copy of the original PAS schematic that I've labeled so you can tell what is what,
    then two much better re-drawings of the two PCB schematics.
    In an original PAS preamp, PC6 eyelet 4 and eyelet 11 have two separate connections to chassis ground,
    when I service these preamps I cut both ground wires out and run a single ground connection to eyelet 10,
    and then connect eyelet 4 and eyelt 11 together with a wire, as shown on the bottom schematic.






    Last edited by tubes4hifi on Sat Apr 06, 2013 7:27 pm; edited 2 times in total

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Fri Apr 05, 2013 2:54 pm

    Thanks, I find the original diagrams confusing, I didn't see where they labeled the voltage rating of the board capacitors, I've been trying to figure out what the voltage should be at the last stage of the filter to the boards.


    Last edited by thinker1920 on Fri Apr 05, 2013 2:55 pm; edited 1 time in total (Reason for editing : typo)

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Sat Jul 06, 2013 1:07 pm

    Does any one know what the current draw is for each board? I had it written down and I can't seem to find it. I was trying to figure out the power dissipation for the power filter resistors, I'm guessing that 2 watt resistors would be Ok but that's just a shot in the dark.


                  Thanks Smile

    tubes4hifi
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by tubes4hifi on Sat Jul 06, 2013 4:35 pm

    see all my notes above, including a voltage chart from the PAS3 manual. None of the caps have over 200v on them.
    You can calculate the current for everything in the circuit using ohms law. The PC5 uses a total of 2.5ma. The PC6 uses a total of 3.1ma.

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Tue Aug 13, 2013 9:07 pm

    After some listening, the PAS 2 seems to add a fair amount of bass and attenuate the highs, any way to fix that?

    tubes4hifi
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by tubes4hifi on Wed Aug 14, 2013 12:32 am

    sorry to say, but the 50+ year old design isn't the best. Easy solution, turn down the bass and turn up the treble.
    Better solution, replace all the boards and power supply with new designs. ( www.tubes4hifi.com/PAS3-CAT.htm )
    Best solution, sell it and buy a new preamp.

    Bob Latino
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by Bob Latino on Wed Aug 14, 2013 8:02 am

    I agree with Roy .. A lot of people want a vintage system with a Dynaco amp and Dynaco preamp BUT > The original Dynaco PAS-3 was only a mediocre preamp. Read what I said a couple of years ago on another post. Link below ..

    Using an original Dynaco PAS-3 preamp in a modern music system

    Bob

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Sun Aug 25, 2013 4:25 pm

    I think I found the problem Very Happy I don't have the tone controls connected Embarassed Is there something I could connect to bypass them? Like put a resistor so it would be as if the tone controls were set to the middle? Thanks

    tubes4hifi
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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by tubes4hifi on Wed Aug 28, 2013 11:40 am

    there are at least 3 or 4 threads on this forum alone dealing with tone control removal, and probably dozens on the internet (use Google to search)
    here is the local list for this forum:
    http://dynacotubeaudio.forumotion.com/search?mode=searchbox&search_keywords=PAS+tone&show_results=topics

    thinker1920

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    Re: Board layouts for the PC-5 line stage and PC-6 phono board on the PAS preamps - photos ...

    Post by thinker1920 on Sat Aug 31, 2013 9:21 pm

    I thought about it for a while and decided to stick the tone controls back in. I followed the original instructions but, the controls don't really work Razz the treble only makes a difference if you turn it all the way up, and the bass control functions like a volume control, you turn it up everything gets louder, you turn it all the way down and all but the very highest treble disappears and that even is reduced in volume. Is that close to what the original was like? or is something really out of whack?

      Current date/time is Fri Oct 31, 2014 9:29 pm