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    Removing PAS-3X Tone Control.

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    Tiziano73

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    Removing PAS-3X Tone Control.

    Post by Tiziano73 on Sun Feb 03, 2013 1:05 pm

    Hi to everybody.

    I did try to make Curcio Audio PAS-3X Tone Control removal but something seems to not be ok.
    Joe Curcio say to begin with the Left Channel and connect a jumper from terminal 4 to terminal 6 on the foil
    side of PC-5, then connect another wire jumper from terminal 6 to terminal 7 of PC-5. De-solder the wire connected to terminal 3. You will find that it is connected to
    the center lug of the Left Treble Control and once desoldered from PC-5 terminal 3, you may cut it at the connection to the Left Treble control.
    Next we will make the same changes to the Right Channel.

    Now if you follow this modifications on the PAS schematics:

    http://i10.servimg.com/u/f10/17/84/41/68/scherm13.jpg

    The red line is that Curcio say. As you can see the DC component go to the output with a 47K resistor. It's all right?!?!
    What do you think about?
    Regards.

    Tiziano

    tubes4hifi
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    Re: Removing PAS-3X Tone Control.

    Post by tubes4hifi on Tue Feb 05, 2013 10:16 pm

    you are correct, the cathode voltage is going thru the 47K to the output. It's probably got a couple volts on it.
    Here is the way I would do it - cut each of the wires on the tone controls, and remove them.
    Then jumper eyelet 6 to 7, and 13 to 14. You're done. And BTW, you don't need those 510K resistors across the output jacks!
    This schematic is also easier to read . . .


    PeterCapo

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    Re: Removing PAS-3X Tone Control.

    Post by PeterCapo on Wed Feb 06, 2013 2:51 pm

    Roy,

    Wouldn't it be desirable to retain the 1.0uf cap in line with the output to block the small amount of DC from getting to the output? I have been under the impression that this 1.0uf cap also makes it possible for the PAS to be used with amps having an input impedance as low as 100k vs. 250k without the 1.0uf cap.

    Although, if you jumper only from 6 to 7 (while also having cut the leads to the pots), it looks like it leaves one of the 0.0075uf caps in the feedback loop that would then block the DC, I believe. But, that 0.0075uf is one of the 50 year old paper jobs that I wonder might better be left out - in addition to it being 50 years old, how might it affect the feedback circuit? In any case, without the 1.0uf it still leaves the question of the change in compatibility with the amplifier’s Z-in…

    Peter

    tubes4hifi
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    Re: Removing PAS-3X Tone Control.

    Post by tubes4hifi on Wed Feb 06, 2013 4:50 pm

    you've still got C4 (and C12) blocking the DC from the output, and this value should be fine for most people, but it could be made larger if you need freq response below 20Hz. Of course on any 30-50 year old PAS3 I'd be replacing ALL of the capacitors!!
    C5 needs to remain in the circuit for loop feedback.

    PeterCapo

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    Re: Removing PAS-3X Tone Control.

    Post by PeterCapo on Wed Feb 06, 2013 5:19 pm

    Sorry, Roy, maybe I am missing something. But, in the original PAS-3X, I believe the X-mod bass pot shorts the wiper to both ends of the pot when it is centered (unlike your schematic), which would therefore short-out both 0.0075uf caps per the schematic at Tiziano's link (not the red lines drawn-in, but the original circuit showing the X-mod bass pot centered)? If this is the case, then C5 would not be able to be part of the feedback loop with the bass pot centered in a PAS-3X, would it?

    And wouldn't the small amount of DC be coming from the cathode of the first section of the 12AX7 via the 47K resistor, which C4 would not be able to block from reaching the output (again, assuming C5 is shorted out by the bass pot and the 1.0uf cap is not in the circuit)?

    Thanks, Roy.

    -Peter

    tubes4hifi
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    Re: Removing PAS-3X Tone Control.

    Post by tubes4hifi on Wed Feb 06, 2013 8:29 pm

    I realize that schematic is drawn the same as the one in the PAS-3X manual, but as far as I know
    (someone who actually has a PAS-3X correct me please) the circuit actually works as I've redrawn it.
    So C5 is in circuit as is part of the loop feedback.

    Tiziano73

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    Removing PAS-3X Tone Control.

    Post by Tiziano73 on Thu Feb 07, 2013 3:13 am

    Probably I think to understand what Peter want say. I modified tubes4hifi schematics with original Dynaco Pot control.
    With that PAS pot (the original ones) in center position, C5 and C12 are short circuit and C7 and C14 too is paralleled with the 1uF cap.

    http://i10.servimg.com/u/f10/17/84/41/68/pas-pc11.jpg

    So the output coupling cap for each channel is Cout= http://i10.servimg.com/u/f10/17/84/41/68/formul10.png

    Isn't it?

    Tiziano



    PeterCapo

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    Re: Removing PAS-3X Tone Control.

    Post by PeterCapo on Thu Feb 07, 2013 12:55 pm

    I stand corrected, Tiziano. When the X-mod bass pot in a PAS-3X is centered, C5 appears to be shorted, but C7 is in parallel with the 1.0uf cap. The key, I think, is that the pitchfork wiper in the x-mod bass pot shorts the three terminals of the pot when centered. If you envision the action of the pitchfork wiper, you can see that C5 is not only shorted when the pot is centered, but C5 is also shorted for one half of the pot’s rotation.

    Your formula for C-out total might be heading in the right direction, but I think it might depend on the position of the bass and treble pot wipers at any given moment(?)

    But, if you are talking about taking a minimalist approach to bypassing the tone controls, then, IMHO, I have always thought it would make sense to emulate the condition of the circuit when the two X-mod pots are centered – but without the parts. In particular, I suggest that the output from C4 (0.2uf) would go only to:

    • the parallel combination of the 47K resistor and the 33pf capacitor

    • the 1.0uf cap (to block DC from V1 pin 3) which then goes to the 62K resistor and then the 510K resistor if you keep it


    I believe the method Roy describes will indeed bypass the tone controls, but, in my mind, it leaves a question of the necessity of C5 (considering in the original design C5 could be shorted-out much of the time). I think there might also be a question of the effect of C5 on the feedback parameters with the rest of the tone control circuit removed - I believe C5 was originally intended to only work as part of the bass control circuit.

    With regard to the advantages of the 1.0uf cap, the PAS is known to be sensitive to loading on its output. I base my opinion on the content of the original Dynaco manuals. The original manuals state that the PAS-2X/3X is compatible without modification with amplifiers having a Z-in as low as 100K, where the non-X-mod PAS only works best with amplifiers having a Z-in of 250K with modification and 470K without modification. The 1.0uf cap is one of the features that distinguish an X-mod from a non-X-mod PAS.

    Here’s an interesting thread that has some relation to the discussion: http://www.audioasylum.com/forums/dynaco/messages/1/14568.html

    Of course, I say this in all deference to Roy or anyone else who disagrees. I could be wrong about all of this, but this is how I have understood it, FWIW. If anyone would care to explain it differently in detail, I’d be only happy to listen.


    Last edited by PeterCapo on Thu Feb 07, 2013 2:41 pm; edited 2 times in total

    tubes4hifi
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    Re: Removing PAS-3X Tone Control.

    Post by tubes4hifi on Thu Feb 07, 2013 1:25 pm

    those plots are pretty ugly, with the original circuits the PAS3 MUST see a 100K load.
    I guess that's why some people MUST have tone controls, to try to fix a bad circuit and incompatible amplifier load.

    PeterCapo

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    Re: Removing PAS-3X Tone Control.

    Post by PeterCapo on Thu Feb 07, 2013 2:07 pm

    Pretty ugly indeed - with solid state amplifier input impedances that didn't exist when the PAS was designed.

    With some research and elbow grease, it can still be turned into a good preamp again, even with the original circuit. I rebuilt one to a minimalist original circuit, no tone controls, etc., but with your power supply board. It exceeded my expectations.

    It is interesting to note that the manuals for some of the Dynaco transistor amps like the Stereo 120 and Stereo 150 have instructions for modifying the PAS to work with them. In the case of the Stereo 150 having a Z-in of 35K, the manual states that the X-modded PAS can be made compatible but not the non-X-modded PAS. That 1.0uf cap seems to make a difference. As I recall, GP49 often customizes PAS outputs to be compatible with different amps.


    Last edited by PeterCapo on Sat Feb 09, 2013 12:41 am; edited 1 time in total

    Tiziano73

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    Removing PAS-3X Tone Control.

    Post by Tiziano73 on Fri Feb 08, 2013 4:24 am

    Whith all yours considerations I believe that the fastest and less invasive way ro retain original Hafler project without the tone controls (minimalist approach) is:

    1) disconnect the center lug and Short circuit the two external lugs of Bass Control then connect one of them to terminal 6.
    2) disconnect the center lug of Treble Control.
    3) Make the same modification to the other channel.

    In this way the Ctot that the 47K resistor can see between it self and the amplifier output is C7+1uF and there is no direct connection for DC component with the output stage. I want remember that stock MKIII and ST-70 are direct coupled to the input (there isn't a capacitor on tube imput grid).

    http://i10.servimg.com/u/f10/17/84/41/68/pas-pc12.jpg

    Tiziano

    peterh

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    Re: Removing PAS-3X Tone Control.

    Post by peterh on Fri Feb 08, 2013 5:28 am

    Tiziano73 wrote:Whith all yours considerations I believe that the fastest and less invasive way ro retain original Hafler project without the tone controls (minimalist approach) is:

    1) disconnect the center lug and Short circuit the two external lugs of Bass Control then connect one of them to terminal 6.
    2) disconnect the center lug of Treble Control.
    3) Make the same modification to the other channel.

    In this way the Ctot that the 47K resistor can see between it self and the amplifier output is C7+1uF and there is no direct connection for DC component with the output stage. I want remember that stock MKIII and ST-70 are direct coupled to the input (there isn't a capacitor on tube imput grid).

    http://i10.servimg.com/u/f10/17/84/41/68/pas-pc12.jpg

    Tiziano
    Thanks for the re-drawn cirquit, much clearer now.

    I wonder however, would'nt this be equivalent with
    a pas3x with tone ccontrols centered ? Or do i misread
    the schematic ?


    Tiziano73

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    Removing PAS-3X Tone Control.

    Post by Tiziano73 on Fri Feb 08, 2013 9:52 am

    I believe that is the way to exlude tone control by the cirtcuit.
    PAS manual say that in flat position tone control are trasparent.

    In this way the joint in circuit is not the crawling contact of the pot but a solder connection.

    Isn't it?

    Tiziano

    GP49

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    Re: Removing PAS-3X Tone Control.

    Post by GP49 on Fri Feb 08, 2013 11:31 am

    tubes4hifi wrote:those plots are pretty ugly, with the original circuits the PAS3 MUST see a 100K load.
    I guess that's why some people MUST have tone controls, to try to fix a bad circuit and incompatible amplifier load.


    No, a lot of people must have tone controls because not all recordings are perfect. A lot of them sound better with tonal corrections.

    I do not listen only to audiophile records.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Removing PAS-3X Tone Control.

    Post by PeterCapo on Fri Feb 08, 2013 5:05 pm

    Tiziano73 wrote:Whith all yours considerations I believe that the fastest and less invasive way ro retain original Hafler project without the tone controls (minimalist approach) is:

    1) disconnect the center lug and Short circuit the two external lugs of Bass Control then connect one of them to terminal 6.
    2) disconnect the center lug of Treble Control.
    3) Make the same modification to the other channel.

    In this way the Ctot that the 47K resistor can see between it self and the amplifier output is C7+1uF and there is no direct connection for DC component with the output stage. I want remember that stock MKIII and ST-70 are direct coupled to the input (there isn't a capacitor on tube imput grid).

    http://i10.servimg.com/u/f10/17/84/41/68/pas-pc12.jpg

    Tiziano

    Here's what I think the end result should be, regardless of how it is done, whether connecting a jumper here or there, or disconnecting this or that. The 0.0075uf cap in parallel with the 1.0uf is negligible. I don't think you need either of the 0.0075uf caps



    Last edited by PeterCapo on Sat Feb 09, 2013 12:52 am; edited 1 time in total

    Pillo69

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    Re: Removing PAS-3X Tone Control.

    Post by Pillo69 on Fri Feb 08, 2013 7:22 pm

    GP49 wrote:
    tubes4hifi wrote:those plots are pretty ugly, with the original circuits the PAS3 MUST see a 100K load.
    I guess that's why some people MUST have tone controls, to try to fix a bad circuit and incompatible amplifier load.


    No, a lot of people must have tone controls because not all recordings are perfect. A lot of them sound better with tonal corrections.

    I do not listen only to audiophile records.

    +10

    Tiziano73

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    Removing PAS-3X Tone Control.

    Post by Tiziano73 on Sat Feb 09, 2013 5:20 am

    Peter, i believe your schematic is the correct solution to the post:

    Removing PAS-3X Tone Control.

    It has been a pleasure.

    Tiziano


    tonebells

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    Re: Removing PAS-3X Tone Control.

    Post by tonebells on Tue Jul 23, 2013 8:39 pm

    tubes4hifi wrote:you are correct, the cathode voltage is going thru the 47K to the output.  It's probably got a couple volts on it.
    Here is the way I would do it - cut each of the wires on the tone controls, and remove them.
    Then jumper eyelet 6 to 7, and 13 to 14.   You're done.  And BTW, you don't need those 510K resistors across the output jacks!
    This schematic is also easier to read . . .


    I did the bypass as above and had to much bass. I add the jumpers from 4 to 6 and 11 to 13 and it flattened right out.

    tonebells

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    Re: Removing PAS-3X Tone Control.

    Post by tonebells on Wed Jul 24, 2013 9:12 pm

    Here is what I ended up with.
    PAS-PC5sch
    I put the 510K resistors back as well.

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