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    Resistors for Quad Cap and Bias for Dyanco ST-70

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    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Mon Jul 29, 2013 12:41 pm

    Hello Everyone:

    I will be replacing the quad cap for the ST70. I will be using the 80/40/30/20 uF @ 525vDC from CE. While I am replacing this cap, I would also like to replace the two filter resistors. The original layout refers to a 22,000 and 6,800 ohm resistors. So my questions are:

    (1) Should I keep the same values?
    (2) What type should I use (metal oxide or carbon film)?
    (3) What is the recommended wattage rating for each resistor?

    The other item I will be replacing are the two 50uF bias caps. They will be replaced with two 100uF @ 100v electrolytic caps. The layout refers to two 10,000 ohm resistors. So my questions are the same as above (1, 2, and 3).

    By the way, I was able to bias the amp properly and it appears to be holding. Overall the amp sounds good, although my speakers are old. I am simply doing the above as preventive maintenance.

    Thank you

    Bally

    Posts : 25
    Join date : 2010-05-18

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bally on Mon Jul 29, 2013 2:13 pm

    BNR

    If you are using the VTA board by Roy, then use the 22000uF resistor.

    If you have the original board then use the 6800uF.

    The resistors should be metal, carbon would not last too long with the heat generated by the amp.

    Wattage rating should be 1 watt.

    Don't forget if you are using the 22000uF resistor, then the 80/20uF sections must be shorted.

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Mon Jul 29, 2013 2:36 pm

    Thank you Bally.

    By the way, the board is original but the quad cap use both the 22K and 6.8K ohm resistors. I am not clear as to the choice of either the 22K or 6.8K resistor.

    Just to be clear, I wasn't considering carbon composition resistors but possibly the use of carbon film resistors.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bob Latino on Mon Jul 29, 2013 3:01 pm

    To Bally - The VTA driver board uses a 2,200 not a 22,000 ohm resistor on the quad cap. The original Dynaco board does use a 22,000 ohm resistor.

    To BNR_1 - If you are replacing the original 22,000 and 6,800 ohm resistors on a new quad cap, any 2 watt metal film, carbon film or metal oxide resistor will be fine.

    Bob

    Bally

    Posts : 25
    Join date : 2010-05-18

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bally on Mon Jul 29, 2013 3:58 pm

    Thank you Bob. Correction is noted.

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Mon Jul 29, 2013 4:09 pm

    Thank you Bob for clarifying. I did not realize with the updated VTA board their is only a single resistor (2,200 ohm) on the quad cap.

    As to the bias resistors are 10K 1W carbon film (5%) acceptable?

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bob Latino on Mon Jul 29, 2013 6:16 pm

    Hi,

    Yes - A 10 ohm 1 watt carbon film or metal film resistor will work OK in there as a bias resistor ..

    Bob

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Mon Jul 29, 2013 6:35 pm

    Thank you Bob. I assume you meant 10K ohm.

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bob Latino on Mon Jul 29, 2013 6:43 pm

    If you mean the bias potentiometers on a stock amp - yes - 10K not 10 ohm.

    Bob

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Tue Jul 30, 2013 9:01 am

    I am refering to the two 10K resistors that are connected across lugs 1&2 and 3&4 on the 7 lug bias strip....not the pots. Yes, I am refering to the stock ST70.

    Bob Latino
    Admin

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    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by Bob Latino on Tue Jul 30, 2013 9:23 am

    Yes - those two 10K resistors should be 1 watt. Those are bias power supply resistors. The BIAS resistors on a stock Dynaco ST-70 are two 15.6 ohm resistors that go from pins 1 and 8 on both tubes of the pair (on each channel) to chassis ground.

    Bob

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by BNR_1 on Tue Jul 30, 2013 2:05 pm

    Very good....thank you Bob

    GP49

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    Re: Resistors for Quad Cap and Bias for Dyanco ST-70

    Post by GP49 on Wed Jul 31, 2013 2:16 pm

    Bob Latino wrote:Yes - those two 10K resistors should be 1 watt. Those are bias power supply resistors. The BIAS resistors on a stock Dynaco ST-70 are two 15.6 ohm resistors that go from pins 1 and 8 on both tubes of the pair (on each channel) to chassis ground.
    Bob

    Strictly speaking, the 15.6Ω resistors are CATHODE resistors and must be referred to as such in other amplifiers; but in Dynacos (after the early Mark II) they serve a second purpose, to measure bias (mighty clever!), so they get called "bias resistors."  Yes, Bob knows the difference!

    Isn't technical nomenclature FUN?

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