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    Wiring dual bias in a "stock" Mk.IV



    Posts : 15
    Join date : 2009-04-08
    Location : New Jersey

    Wiring dual bias in a "stock" Mk.IV

    Post by Listens2tubes on Sat Apr 25, 2009 2:54 am

    Hi, I am very pleased with my modded original amps. PC-4 has been treated to modern parts and the amps sound great. For a while I've wanted to wire individual tube bias pots for the EL34's. It looks as simple as seperating the output tubes and adding another 13.5 ohm resistor and bias pot to V2. Without worrying about mounting the extra pot whatelse would I need to do? Shocked

    Bob Latino

    Posts : 2460
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Wiring dual bias in a "stock" Mk.IV

    Post by Bob Latino on Sat Apr 25, 2009 10:13 am

    I suppose you could wire in another bias pot, cut the line between pins 1,8 that runs between the two output tubes and add another bias resistor to the tube that doesn't have one now - BUT - I don't recommend doing so. You would have to drill another 3/8" hole in the chassis to accommodate the extra bias pot but as soon as you drill into the existing Dynaco chassis you will devalue your amps. Many times I have seen Dynaco amps that have been "improved" by drilling the chassis and tube cages for various reasons and then watch that amp sell for a lot less than it should on Ebay or Audiogon. Of course maybe you could mount the other bias pot inside the chassis somehow but then you would have to take off the bottom cover everytime you wanted to adjust the bias on that one tube.

    If you really want separate bias controls for each output tube then a better idea would be to purchase a pair of VTA driver boards for your Mark IV's which already have separate bias controls for each output tube built onto the board. By using the VTA's bias system you can now pull the original bias pots out of the amp and maybe add a triode/pentode switch in the 3/8" hole where the original bias pot use to be.



    Posts : 15
    Join date : 2009-04-08
    Location : New Jersey

    Re: Wiring dual bias in a "stock" Mk.IV

    Post by Listens2tubes on Sat Apr 25, 2009 1:46 pm

    Hi Bob,
    I did meantion not to worry about the extra pot, I will remove the power octal and mount a plate to it with the pot and 2 pin jacks for V2 & V3 which I'll label. If I can't fit the jacks I will have to drill 2 holes along side the power socket/V3 biaspot hole thus devaluing, even more, the already hole enlarged(WBT nextgen RCA's and Acme fuse holder)chassis. Very Happy
    I say let my children worry about selling these baby's at the estate sale in 50 or 60 years. lol!


    Posts : 760
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Wiring dual bias in a "stock" Mk.IV

    Post by GP49 on Thu May 14, 2009 2:48 am

    It is not difficult to construct a complete new bias adjust resistor string with two resistors and a miniature pot, on a tag strip. The pot can be soldered directly to the tag strip; no need to drill a hole, since bias adjustment isn't done often and the mounting of the pot need not be ultra-sturdy. The tag strip can be mounted on the top of the chassis using one of the screws holding the circuit board. The screw also serves as its ground and the new bias string is wired up to the 100K resistor in the bias circuit of one tube; the original one feeds the other tube and can be left intact. You have to separate the joint connection between the two 100K resistors where the original bias connection is made, easily done by lifting one end of a 100K resistor to tie in your new bias adjust string anyway. At the output tube sockets, separate the cathode resistors, using two of exact value; if you can't find a matching one to the one already in the amp, get two new matching ones instead. Value is not critical, anything around 10Ω is fine. If you don't use the Biaset octal socket for anything else, connect the two cathode resistors to different pins on it so you can measure the bias current on both tubes without having to probe under the chassis when setting bias.

    I have done this on one of my Mark II and will do the other one next time it comes out for servicing. If I had thought to take pictures when I did it, I'd try and post them here!

    With this dual bias adjust, I set one tube for the desired idle current at its Biaset point (50mA into an 11Ω resistor is 0.55 volts) and then connect my DVM across BOTH the Biaset points, then adjust the other tube until the meter reads ZERO volts.

    Solder Slinger

    Posts : 2
    Join date : 2009-05-24

    Individual Bias adjustments for each tube on a stock ST-70

    Post by Solder Slinger on Sun May 24, 2009 7:31 pm

    I'm interested in the circuit mods needed to bias the individual tubes. I want to mod a stock ST-70 with a setup similar to what is used on the DIYtube board, where there is a "balance" type control used to adjust the bias between the tubes and a bias voltage level control used to set the overall bias current. The mods should pretty much the same for a Mk IV and an ST-70.

    Since the ST-70 chassis was already drilled for additional pots,I'm not worried about the decreased value. I like the ease of zeroing out the difference between the two tubes, then setting the overall bias current.

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    Re: Wiring dual bias in a "stock" Mk.IV

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