Bypassing tone controls from Dyna Pas preamp

Discussion in 'Audio Hardware' started by fortherecord, Feb 6, 2011.

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  1. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    Just wondering if there was a simple way to bypass the tone contols in the pas preamp. I believe there is a feedback circuit involved, so wasn't sure where to start.
     
  2. misterdecibel

    misterdecibel Bulbous Also Tapered

    Start by downloading this:

    http://www.curcioaudio.com/pas_mnl.pdf

    Go to Page 11:

    First, de-solder all the wires that go from the PC-5 to and from the tone controls.

    Remove the four 0.0075uF capacitors from PC-5. Replace two of them, the one that go from Eyelet 6 to Eyelet 6 and the one that goes from Eyelet 13 to Eyelet 11, with wires. Replace the other two, the one that goes from Eyelet 6 to Eyelet 5 and the one that goes from Eyelet 13 to Eyelet 12, with a pair of film capacitors. These caps need to be 400V DC or better, 0.22uF or larger. I like to go bigger than 1.0uF here, if you can find something that fits on the board.

    That should do it. Essentially, you're removing the entire tone control assembly from the feedback loop, closing the loop, and providing new output caps.

    Oh, and be careful with that soldering iron, it's way too easy to damage the old Dynaco circuit boards. The green fiberglass ones are somewhat more durable, but the brown phenolic ones are quite fragile, the copper trace will lift right off the board if you're not very quick and very careful when applying heat.
     
  3. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    Thanks for the info.I do have some .22uf orange drops around, I'll have to check their voltage, otherwise, it seems pretty simple. I just installed,a new pc5 boars last night. What about removing the filter switch? I've already removed the loudness switch, which just meant unhooking it.
     
  4. Scott in DC

    Scott in DC Forum Resident

    Location:
    Washington, DC
    Tone Controls

    The best tone controls are no tone controls. I have always used minimalist equipment that dosn't have tone controls.

    I used to own a Quad 33 and later upgraded to the Quad 44 preamp. The had tone controls that you could switch out the the circuit. I liked that approach, you could use them if you wanted or needed but you had the option for non at all.

    Scott
     
  5. misterdecibel

    misterdecibel Bulbous Also Tapered

    I meant the one that goes from Eyelet 6 to Eyelet 4...
     
  6. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
    You should get a lot more clarity after removing the tone controls. Also, if it's like the SCA-35 the gain goes up quite a bit, so you will only be using the lower 1/3 of your volume control.

    Also, if it's like the SCA-35's interstage/coupling caps, using orange drops is fine for a while, but if you are adventurous and open minded, using some better caps will sound better - assuming the rest of the system is 'transparent' enough.

    I was quite pleased going from stock to orange drops, then to some $8 (at the time) caps, I think Hovland Musicaps and RelCap RTXs.

    I don't know if the PAS is similar to the SCA-35 in that the volume control had a tap that was used for a 'variable loudness' of sorts, and was taken 'out of circuit' at the halfway (IIRC) point. I replaced mine with a good Alps pot. One may use a different value (?) to allow use in the 'center' of travel, which some folks say sounds better. That may only have been true with old-style carbon pots though ...
     
  7. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
    After downloading the manual, the tone circuits in the PAS are more complex than the simplified ones used in the SCA-35, so what I said about gain may not be true ...? I bet it will sound better though!

    It does look to me that unless you disconnected the 'tap' lead on the volume control, the loudness part of the circuit would always be 'on'.
     
  8. misterdecibel

    misterdecibel Bulbous Also Tapered

    There'll be no change in gain, it's nothing like an SCA-35.
     
  9. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    The tone controls in an sca35 are very easy to bypass because all you have to do with that amp is just disconnect them. I did that previously with mine, and it did sound better.
     
  10. misterdecibel

    misterdecibel Bulbous Also Tapered

    This should be the end result:

    [​IMG]

    Pardon my quickie attempts at editing with Windows Paint...

    If you wish to also remove the filter, remove the 1500pF and 0.004uF caps, the 18K and 3.3M resistors, and place a straight wire where the 18K resistor was.
     
  11. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
    Not to usurp the thread, but the SCA-35 was my introduction to tube audio back in 1996. Had it for 7 years, did simple mods, it was excellent, amazing for a $100 amp (plus upgrade parts and time). It pushed my Yamaha A-700 integrated into (passive) subwoofer amp duty.

    I bet a PAS (and ST-70), modded/upgraded, sound pretty amazing as well.
     
  12. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    The sca35 and the st35 are great little amps, perfect for a pair of Quad esl speakers, wish I hadn't sold mine.
     
  13. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    Those caps and resistors were all on the pc5 board, right? I removed those parts as well as the tone controls, but now all hear through the pas is a loud hum, like that of a broken ground, on all selector settings.
     
  14. Rick58

    Rick58 Forum Resident

    Location:
    Dublin, CA, USA
    did you solder a wire where the 18k resistor was? (IIUC) You want it to connect the 'output' side of the first 12AX7 (thru the .02uf cap) to one end of the 1.2M ohm resistor (which is also connected to the input of the other triode in the 12AX7).

    IOW, the two 'halves' of the 12AX7 are coupled thru the .02uf cap. The input to the 'second half' of the 12AX7 has a 'load' resistor of 1.2M ohm. The .02 cap and 1.2M resistor form a filter as well, but it's too late in the evening for me to remember how to calculate what it is! errrr ... I think it's a 6.6Hz (DC blocking) filter.
     
  15. misterdecibel

    misterdecibel Bulbous Also Tapered

    Here's another quickie Windows Paint edit of the Dyna schematic, with the filter removed:

    [​IMG]

    If you've got a loud hum and no signal in both channels, it's probably something fairly simple, like an open circuit somewhere in the circuit. Also check to make sure that the ground wire from PC5 to the chassis ground wasn't removed along with the extraneous wires to the tone controls and filter.

    Maybe you could take a quick picture of the top and bottom of PC5?
     
  16. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    Thanks, I'll try and post some pictures later.
     
  17. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    Here are the photos of the PC5 board and volume/balance controls.
     

    Attached Files:

  18. misterdecibel

    misterdecibel Bulbous Also Tapered

    This would be so much easier if I had a digital camera and could show you mine...

    Anyway, doing a quick comparison between your photo and the working PAS in my lap, the main difference that I see is that you have left a few components that need to be removed, and removed a couple that need to be replaced. Looks like your jumper wires and the replacement output caps are in the right places.

    [​IMG]

    I've circled in red two caps and two resistors that can go away. The caps that I drew in green are the 33pF caps across the feedback resistors, I'm not sure if the circuit is stable without these.
     
  19. misterdecibel

    misterdecibel Bulbous Also Tapered

    And of course you'll need tubes in the tube sockets... :)
     
  20. indy mike

    indy mike Forum Pest


    The 2X/3x versions of the PAS preamp had the tone controls out at a particular point of the pot. Greg Dunn's excellent Dynaco site has these reviews of the 3X that mentions the centered pot/tone controls out of the circuit feature: http://home.indy.net/~gregdunn/dynaco/components/PAS3X/srpas3.jpg http://home.indy.net/~gregdunn/dynaco/components/PAS3X/hfpas3.jpg
     
  21. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    The only thing with that is, the signal still has to go through the extra wire and caps even with the tone controls at center on the 3x circuit.
     
  22. indy mike

    indy mike Forum Pest

    Not perfect by any means, but Dyna did recognize the problems the tone controls presented and made an inexpensive fix available - $10 upgraded the earlier PAS 2 or 3. Getting them out of the circuit altogether is the best solution.
     
  23. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    [​IMG]

    I've circled in red two caps and two resistors that can go away. The caps that I drew in green are the 33pF caps across the feedback resistors, I'm not sure if the circuit is stable without these.[/QUOTE]

    Thanks, I followed that and made the changes. I now hear a signal, but volume is way too high and volume and balance controls seem to be out of circuit somehow as they have no effect. Hum in all selection setting cept the one being used (fm mpx).
     
  24. fortherecord

    fortherecord Forum Resident Thread Starter

    Location:
    Upstate, NY
    I wonder if the brown 105k 400v caps I added when I bypassed the tone controls were high enough?
     
  25. misterdecibel

    misterdecibel Bulbous Also Tapered

    If the FM MPX input is the only one that has a signal source attached, then hum in the other ones would be normal if they're open circuit and the volume is all the way up. So it looks like the PC5 is probably behaving mostly normally now and you just have some kind of simple wiring error to troubleshoot around the volume/balance/stereo-mono part of the circuit.

    I'm gonna guess that when the scratch filter and the loudness control were removed from the circuit, somehow the ground for the volume and balance was lifted.

    Okay, looking at your picture, where you've run the short black ground wire from the vol/bal ground to PC5, I'd remove that wire and replace it with a long black wire going from the vol/bal common ground back to the chassis ground point at the power supply.
     
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