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    TDR time delay resistor selection

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    eickmewg

    Posts : 77
    Join date : 2014-08-29

    TDR time delay resistor selection

    Post by eickmewg on Sat Jan 28, 2017 8:44 am

    I have been using a Weber WZ68 in my VTA ST120 and I finally decided to get a TDR kit. The amount of time delay can be modified with a change in R3 resistor value. Once installed with the supplied 470K resistor, I measure a delay of about 12 seconds before the green LED lights (supposedly it should have a 17 second delay.) So, for those who are using the TDR, what do you feel should be the "best" time delay for amp and tube health?

    I was thinking that with the LED's on the top of the TDR board which then faces the bottom cover, you wouldn't be able see the color change. Actually you can easily see the change through one of the chassis holes near the left OPT in a darkened room.

    Finally, even with the TDR installed, would the amp still benefit from a current in-rush thermistor, such as a NTC CL80?
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    arledgsc

    Posts : 466
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: TDR time delay resistor selection

    Post by arledgsc on Sat Jan 28, 2017 11:41 am

    In the ST120 I use a solid state rectifier (WZ1 - no B+ dropping resistor) and set my TDR to about 36 seconds delay with a 1M ohm resistor. After reading datasheets for old 6SN7s they specify about 10 - 17 sec. minimum for warm up of the cathodes.

    I've had zero power-up issues with this length of delay. I'm in the camp of a little extra warm-up time is a good thing.

    eickmewg

    Posts : 77
    Join date : 2014-08-29

    Re: TDR time delay resistor selection

    Post by eickmewg on Sat Jan 28, 2017 1:43 pm

    So I put a stopwatch on the TDR activation with the 470k resistor and it came out at 19 seconds; I need to speed up my "one Mississippi." That seems more than reasonable. arledgsc, is the B+ for your ST120 still appropriate with the WZ1? I think the WZ68 drops about 14 volts. Is your B+ too high?
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    arledgsc

    Posts : 466
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: TDR time delay resistor selection

    Post by arledgsc on Sat Jan 28, 2017 2:48 pm

    eickmewg wrote:arledgsc, is the B+ for your ST120 still appropriate with the WZ1?  I think the WZ68 drops about 14 volts.  Is your B+ too high?

    With the SS WZ1 rectifier B+ was a little high so I wired the power tranny +5V rectifier heater winding as a bucker to the AC input voltage.  I used the AC neutral to keep AC voltage in the secondary near ground potential.  Drops high voltage about 5% or in my case or from about 525V to just under 500V when I measured it under load at 119V ac input.  

    I did this about a year ago and have had no issues.  The amp runs a little cooler as well.

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