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    Tung Sol KT120 heater current values and concerns



    Posts : 93
    Join date : 2014-08-29

    Tung Sol KT120 heater current values and concerns

    Post by eickmewg on Thu Nov 01, 2018 11:50 am

    New Sensor lists the Gold Lion KT88 reissue heater current as 1.6 A. New Sensor lists the Tung Sol KT 120 heater current as from 1.7 to 1.95 A. Why the range? Is this normal production variation or something else? It would seem to me they should have the current requirements nailed by now, whatever the actual value.

    I currently have an octal driver board in my VTA ST120 with 3 6SN7's that each draw 0.6 A at 6.3 V for a total of 1.8 A. Four KT120's at 1.7 A@ would be 6.8 A which with the 6SN7's would total 8.6 A, well within the total of 10 A for the 2 5 A 6.3 V windings. However, at 1.95 A for the KT 120's it would total 9.6 A, very close to the transformer rating. Should this be a cause for concern? I would like to try the KT120's instead of the GL KT88's. I am using a Copper Cap WZ68 rectifier so I am not using the 5 V PT windings, which should reduce the stress somewhat. So, can I safely combine the octal drivers and the KT 120's?

    TIA for any information!

    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Tung Sol KT120 heater current values and concerns

    Post by Peter W. on Thu Nov 01, 2018 1:32 pm


    I am sure Mr. McShane will be far more versed in tube variations than I. But, as a filament load, the amperage drawn is a function of the voltage delivered. They are not like motors that draw at their rated amperage to failure.

    0.25A is a 15% variation, and about the same as the +/- variation-from-tariff you will see at the extremes at a typical Utility Wallplate. Here in PECO territory, the tariff is for a nominal 120/240 VAC single phase service in to a residential customer. The tariff states that variation may be +/- 5%, but not more than an 8% spread overall. So, if your tube is pulling 1.7 A at 114 V, then it would be pulling 1.87 A at 126 V.

    I am sure that New Sensor sends tubes all over the world, and to many areas not nearly as tightly regulated as the US. And, they send tubes for many different types of equipment that may or may not have taps matching the local voltage as closely as they might. So, input voltages may vary such that their tubes may be pulling more amps in some situations than in others. And a single rating would not serve unless additional parameters were defined. BUT!!

    Read that rating more closely. If it is the same as the Tung-Sol sheet, that variation is given as applicable at the nominal filament voltage. Meaning that they see this as the 'normal' factory variance. Which, given that this is a relatively new post-blight tube running at a very high power, the manufacturers still have not gotten it down pat.

    Keep in mind that if the output of every present-day audio tube manufacturer were counted, what they produce in a year would likely not match just the rejects from Sylvania or RCA in a week, if even that. That is why you need to deal with the McShanes of the world and not unknown sources. QC does not happen at the factory, and not much even at the distribution level - but at the retailer.


    Posts : 93
    Join date : 2014-08-29

    Re: Tung Sol KT120 heater current values and concerns

    Post by eickmewg on Thu Nov 01, 2018 1:48 pm

    Peter, thanks for your concise explanation. I hadn't considered the voltage aspect of this equation. I am running this amp through a bucking transformer so the amp is "seeing" about 115 V, which should put me near the bottom of the current draw range. This should make the power transformer happier.

    I just found it strange that all the other tube data sheets I look at for a variety of power tubes seem to give a single heater current value at 6.3 V.

    In fact, I did get my KT 120's from Jim McShane but I hadn't at the time considered if the extra current requirements for my three 6SN7 drivers might be a problem. Jim was great to work with and I've come to realize he is a better tube source than some unknown seller on ebay.

    I did install the KT 120's and my VTA ST120 is sounding great. I just don't want to compromise the health of my power transformer in the long run.

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    Re: Tung Sol KT120 heater current values and concerns

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