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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    ST 70 Choke

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    Don_U

    Posts : 2
    Join date : 2020-12-20

    ST 70 Choke Empty ST 70 Choke

    Post by Don_U on Sun Dec 20, 2020 11:10 am

    Hi Guys:
    I'm new here. Was an application engineer for years. I rebuilt a ST-70 from the chassis up with a 6gh8a driver board and improved the PS with PP bypass and better coupling caps. This amp uses the C-24X triad used by Mr. Latino. While that some what addresses max current and is physically  compatible it still seems like you are pounding on it. It also does not have the same Henry value as the orginial. I'm installing a 6sn7 Mullard type driver circuit in the new ST70 I'm building. Without the consideration for component dimension, would a choke with larger electrical capacity i.e. 1.5 H @ 300ma/ 400 WVDC 40ohm DC resistance be better suited for this? Hammond makes this spec a 158TA, it can be applied with an accommodation for adding a small of height to the chassis.

    It would seem that a larger amperage capacity of 300ma in max current and an1.5 Henry value closer to the original of 1.75H would be more of an improvement. The chassis mod is incidental to me.

    Thanks up front for your input.
    D-
    WLT
    WLT

    Posts : 145
    Join date : 2013-07-13
    Location : Rochester NY

    ST 70 Choke Empty Re: ST 70 Choke

    Post by WLT on Sun Dec 20, 2020 11:25 am

    It seems better in some respects but what about the 400 VDC max limit? The ST70 has a higher B+ so it looks like it is really not suited for this application. Then you might ask what would be the failure mode. Doubtful it will fail turn to turn. If the winding insulation is not rated high enough it may fail to ground. Not good.

    It may be worth contacting the vendor for further clarifications on the 400 VDC limit.
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    ST 70 Choke Empty Re: ST 70 Choke

    Post by Bob Latino on Sun Dec 20, 2020 12:40 pm

    Don_U wrote:Hi Guys:
    I'm new here. Was an application engineer for years. I rebuilt a ST-70 from the chassis up with a 6gh8a driver board and improved the PS with PP bypass and better coupling caps. This amp uses the C-24X triad used by Mr. Latino. While that some what addresses max current and is physically  compatible it still seems like you are pounding on it. It also does not have the same Henry value as the orginial. I'm installing a 6sn7 Mullard type driver circuit in the new ST70 I'm building. Without the consideration for component dimension, would a choke with larger electrical capacity i.e. 1.5 H @ 300ma/ 400 WVDC 40ohm DC resistance be better suited for this? Hammond makes this spec a 158TA, it can be applied with an accommodation for adding a small of height to the chassis.

    It would seem that a larger amperage capacity of 300ma in max current and an1.5 Henry value closer to the original of 1.75H would be more of an improvement. The chassis mod is incidental to me.

    Thanks up front for your input.
    D-

    The Hammond 158TA choke will not fit into a Dynaco ST-70 chassis or a VTA ST-70 chassis. If you want to use a custom chassis with a height of about 2 1/2 inches, you can use the Hammond 158TA choke. The original Dynaco C354 choke was not well made and does not deal with amp heat very well. Many original Dynaco ST-70 bottom covers will have a waxy material deposited directly under where the choke was located. The Dynakitparts C354 choke is much better made than the original Dynaco choke and is a good replacement. The Triad C-24X choke is very similar to the Dynakitparts choke but will handle slightly more current (200 milliamps vs. 240 milliamps). In any ST-70 type amp either the Dynakitparts choke or the Triad choke will work just fine. Either will also handle the extra current required by 6SN7 driver tubes.

    Bob

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    Don_U

    Posts : 2
    Join date : 2020-12-20

    ST 70 Choke Empty Re: ST 70 Choke

    Post by Don_U on Sun Dec 20, 2020 1:39 pm

    So, I'm trying to understand. If my tubes require ~200ma by themselves how much current is left for the 6sn7 drivers? Also, it does not matter then that the value of the C24-X is only 1 Henry compared to 1.75 Henry value of the Dynakit? How does something that seems underrated for the application keep up in the duty cycle? How much current is required from the power transformer does it take to supply these 7 tubes? I'm assuming design tolerances that take into account 20ma max current from the 6sn7 drivers. Making that value nominal it doesn't leave much headroom for PS current transients. Or is this choke not that critical to current transients in the PS rail?
    -D

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    mchavez

    Posts : 1
    Join date : 2020-02-12
    Age : 64
    Location : Seattle

    ST 70 Choke Empty replacements for ST-70 C-354 choke

    Post by mchavez on Thu Jan 07, 2021 11:58 pm

    The Triad is 1.0 H at 240ma and a resistance of 50 Ohms and a much wider tolerance....but the size is perfect.  and the lower H value is ok?

    I just looked at a hammond 156R at 1.5H, 200ma, 56 ohms, 400V the height is 1.69" which clears the bottom with the slimmest of margins. the mounting is the same spacing. would that work? link: https://www.hammfg.com/part/156R?referer=947
    MechEngVic
    MechEngVic

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    Join date : 2019-01-16

    ST 70 Choke Empty Re: ST 70 Choke

    Post by MechEngVic on Fri Jan 08, 2021 1:02 am

    Don_U wrote:So, I'm trying to understand. If my tubes require ~200ma by themselves how much current is left for the 6sn7 drivers? Also, it does not matter then that the value of the C24-X is only 1 Henry compared to 1.75 Henry value of the Dynakit? How does something that seems underrated for the application keep up in the duty cycle? How much current is required from the power transformer does it take to supply these 7 tubes? I'm assuming design tolerances that take into account 20ma max current from the 6sn7 drivers. Making that value nominal it doesn't leave much headroom for PS current transients. Or is this choke not that critical to current transients in the PS rail?  
    -D  

    The choke (filter) is used in the power supply to block higher frequency AC, which smooths out ripple. It offers the next circuits lower DC resistance than a resistor. A smaller value choke offers even less DC resistance, which provides more voltage for use by the outputs and results in higher power output. So you don't have to worry too much about a choke's current (as long as it's in the neighborhood) because AC is its main provision. Significant increases or decreases in choke value should be accompanied with corresponding reductions or increases in filter capacitance.

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