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Thread: Can I use different matched sets of tubes together?

  1. #1
    TDS
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    Can I use different matched sets of tubes together?

    Would it be okay to use two sparately matched quads of output tubes in an amp, where an octet of matched tubes would normally be used?

    The amp has DC bias adjustment with a balancing adjustment between th two banks. It also has AC balancing adjustment at the PI to balance the in-phase and out-of-phase banks.

    I would think that it should be possible to achieve a balance of cathode currents between each bank and offset the likely difference in AC gain as well by using the two adjustments available.

    I have a matched quad available and it would be a saving if I only need to buy another matched quad. I would use one quad set in one bank and the other quad set in the other bank.
    Last edited by tboy; 09-22-2007 at 09:05 PM.

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    Supporting Member loudthud's Avatar
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    Your question is somewhat difficult to answer without knowing the characteristics of the tubes involved. If you can achieve AC and DC balance with a quad on each side, this would be preferable. However, if AC balance cannot be achieved, you should try splitting the quads with two on each side to see if AC balance can be achieved that way. There may be a slight imbalance between tubes on one side, but it should not be so great as to cause a problem.

    How will AC balance determined? Some people apply a sine wave and adjust for a DC null on the output, others insist on a distortion analyzer.

  3. #3
    TDS
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    Incidently, the circuit is my turret board version of a Marshall VBA400 Bass amp and using JJ KT88's

    Each bank has a 1 ohm resistor between each group of cathodes and ground. The DC balance would be measured with no signal, by adjusting the the bias and balance to get each bank biased at the correct level and adjusted for the minimum voltage differential between the two groups of cathodes. In other words, equal voltage drop across the two 1 ohm resistors.

    The AC balance is adjusted with a trimpot in series with a resistor on the plate of the in-phase triode of the PI. Balance would be measured with a sine wave signal applied and the trimpot adjusted for maximum voltage diffential across the same cathode test points.

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