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    The Dynaco PAM-1 (mono preamp)

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    Mark Korda

    Posts : 23
    Join date : 2009-12-29

    The Dynaco PAM-1 (mono preamp)

    Post by Mark Korda on Mon Dec 17, 2012 4:10 pm

    Hi,I might be out of my mind,but I have 2 Dyna PAM mono preamps.I also plan to use them with a pair of yet unbuilt Mark 3's,which have the octal tube socket to plug the PAM's umbilical cord to power the tube heaters.My question is:(Is it possible to remove the tone controls from the PAM's circuit?,and how would I do it.This might border on the immpossible because Frank VanAlstine,on another forum said the tone controls were part of the feedback circuit,which I know nothing about.I am going to try to include the on line manual in this post.It has a giant pictoral near the end which might help.Any new ideas would be appreciated....thanks...Mark Korda.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: The Dynaco PAM-1 (mono preamp)

    Post by PeterCapo on Mon Dec 17, 2012 5:13 pm

    I had a brief look at the PAM schematic, and, although drawn differently, the tone control section looks similar to, if not the same as, the PAS. "Removing" the tone controls from the PAS means shorting the ends of the bass pot together and electrically lifting the wiper of the treble pot. But, don’t just take my word for it. I suggest you trace through the two schematics (PAM and PAS-3X) comparing them, and do further research on-line about removing the tone controls from the PAS, as a point of reference. You might also consider increasing the PAM’s output coupling capacitor from 0.05uF to 0.22uf, as this change was also done in the early releases of the PAS.


    Last edited by PeterCapo on Mon Dec 17, 2012 10:09 pm; edited 1 time in total

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: The Dynaco PAM-1 (mono preamp)

    Post by Bob Latino on Mon Dec 17, 2012 6:37 pm

    Below is the PAM-1 pictorial to help you find your way around inside the preamp ...

    Bob



    Mark Korda

    Posts : 23
    Join date : 2009-12-29

    The Dyna PAM-1

    Post by Mark Korda on Mon Dec 17, 2012 6:46 pm

    PeterCapo wrote:I had a brief look at the PAM schematic, and, although drawn differently, the tone control section looks similar if not the same as the PAS. "Removing" the tone controls from the PAS means shorting the ends of the bass pot together and electrically lifting the wiper of the treble pot. But, don’t just take my word for it. I suggest you trace through the two schematics (PAM and PAS-3X) comparing them, and do further research on-line about removing the tone controls from the PAS, as a point of reference. You might also consider increasing the PAM’s output coupling capacitor from 0.05uF to 0.22uf, as this change was also done in the early releases of the PAS.
    Hi Peter,thanks for that info,I will study the PAS tone controls.When you say short the bass control,do you mean leaving the wires intact,including the wiper(2)and running a little wire from the bass pots 1 and 3?Also what do you mean when you said electrically lifting the wiper of the treble pot?Thanks for your help Peter...sincerely Mark Korda

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: The Dynaco PAM-1 (mono preamp)

    Post by PeterCapo on Tue Dec 18, 2012 2:20 am

    I'll try to correct and expand what I said before. In the schematic for a PAS having the X-mod tone control pots, the bass pot when "centered" shorts the entire pot with its double-pronged wiper. The X-mod treble pot has a break in the middle of its resistance element, and, when "centered," the pot is completely open. The pots in the PAM and non-X-mod PAS do not have these features.

    The idea is to adjust the wiring so that you emulate centered X-mod pots all the time, which should result in flat frequency response as the tone controls would then be effectively "out of circuit." To do this for the bass pot (X-mod type or not), simply short all three lugs together (per channel) without removing or changing any other attached wires. To emulate a centered X-mod treble pot, if you have an X-mod treble pot just disconnect the wire going to the wiper. If you have a non-X-mod treble pot, disconnect whatever is attached to the wiper, and, in my opinion at this time, also remove whatever is connected to either one of the end lugs (0.02 or 0.002 cap, respectively). Be sure to insulate any bare leads of wires or parts that were disconnected in this process.

    As an alternative, have a read here: http://www.curcioaudio.com/PAS%202-3-X%20Tone%20Control%20Removal.pdf If you identify the corresponding eyelets on the PAM PCB that are specified in the Curcio document for the PAS, then you might be able to add the jumpers in a similar manner as described in Joe Curcio’s instructions, although I would still also open one end of a non-X-mod treble pot.

    As I mentioned before, succeeding at bypassing PAM tone controls using the techniques for the PAS assumes that the PAM's tone control circuit really is the same as the non-X-mod PAS.

    While you're at it, in addition to changing the 0.05uF capacitor to 0.22uf, consider also adding the 1.0uF cap in series with the output like the X-mod PAS had. It will block any small DC voltage from getting to the output. I think it might also make the PAM somewhat more flexible interfacing with amps having lower input impedance than, say, a Stereo 70.


    Last edited by PeterCapo on Wed Feb 06, 2013 9:37 pm; edited 1 time in total

    Mark Korda

    Posts : 23
    Join date : 2009-12-29

    The Dyna PAM-1(mono preamp)

    Post by Mark Korda on Tue Dec 18, 2012 7:36 pm

    PeterCapo wrote:I'll try to correct and expand what I said before. In the schematic for a PAS having the X-mod tone control pots, the bass pot when "centered" shorts the entire pot with its double-pronged wiper. The X-mod treble pot has a break in the middle of its resistance element, and, when "centered," the pot is completely open. The pots in the PAM and non-X-mod PAS do not have these features.

    The idea is to adjust the wiring so that you emulate centered X-mod pots all the time, which should result in flat frequency response with the tone controls effectively "out of circuit." To do this for the bass pot (X-mod type or not), simply short all three lugs together (per channel) without removing or changing any other attached wires. To emulate a centered X-mod treble pot, if you have an X-mod treble pot just disconnect the wire going to the wiper. If you have a non-X-mod treble pot, disconnect whatever is attached to the wiper, and, in my opinion at this time, also remove whatever is connected to either one of the end lugs (0.02 or 0.002 cap, respectively). Be sure to insulate any bare leads of wires or parts that were disconnected in this process.

    As an alternative, have a read here: http://www.curcioaudio.com/PAS%202-3-X%20Tone%20Control%20Removal.pdf If you identify the corresponding eyelets on the PAM PCB that are specified in the Curcio document for the PAS, then you might be able to add the jumpers in a similar manner as described in Joe Curcio’s instructions, although I would still also open one end of a non-X-mod treble pot.

    As I mentioned before, succeeding at bypassing PAM tone controls using the techniques for the PAS assumes that the PAM's tone control circuit really is the same as the non-X-mod PAS.

    While you're at it, in addition to changing the 0.05uF capacitor to 0.22uf, consider also adding the 1.0uF cap in series with the output like the X-mod PAS had. It will block any small DC voltage from getting to the output. I think it might also make the PAM somewhat more flexible interfacing with amps having lower input impedance than, say, a Stereo 70.
    Thanks Pete,and Bob for the diagram.So Peter,with what you said about the treble control,basically I would cut every thing on all 3 tabs and back to the eyelets or strip where they originated from?,letting the treble control alone with nothing connected?By the way,I love that picture of your ST-35.I rebuilt one too and am using it presently with a passive transformer preamp I made in a PAM chassis.If I can get your e-mail I'd like to show you some pics.I can't get thru to Photobucket with out going thru a lost pass word process.Thanks again....Mark.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: The Dynaco PAM-1 (mono preamp)

    Post by PeterCapo on Tue Dec 18, 2012 9:15 pm

    I would not make any irreversible or not-easily-reversible changes. If you disconnect anything, desolder it vs. cutting so the end of the lead will remain intact, and do so very carefully so that you can put it back if this doesn’t work.

    Again, please keep in mind that I may be wrong. Success also depends on the tone control circuit in the PAM being the same as in the non-X-mod PAS. I am leaving it to you to make the comparison between the schematics. Also, this procedure relies on the PAM being wired properly to begin with, which is the way shown in the original schematic.

    For the treble pot, I'd disconnect the 1K resistor lead from the wiper (middle lug, #2). Since the resistor will be left hanging by its other end, do something to ensure both ends of this resistor cannot short to anything. I suggest not completely removing and discarding the resistor at this point.

    Then, I would disconnect the lead from just one of the other two treble pot lugs (#1 or #3) – doesn’t matter which one. There’s a 0.02uF capacitor on one and a 0.002uF cap on the other. Like with the resistor, disconnect just the lead from the lug, and then make sure the capacitor’s leads don’t short to anything.

    Yes, the Stereo 35 is a little gem. Mine is the Dynakitparts reissue.

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