The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    Automatic Disconnect for ST 70


    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Automatic Disconnect for ST 70

    Post by TN Allen on Sun Feb 10, 2013 12:06 pm

    I recently upgraded a Dynaco ST 70 tube amp. The old tubes seemed to survive the inconsistent AC power we have in this part of Maine, but newer rectifier tubes are evidently more delicate. The first did not survive the first few flickers in the power on a windy day a few weeks ago.

    I'm wondering if there is a quick disconnect I can use that would shut the power off immediately when there is a sudden short interruption in the AC power. I've looked at surge protectors, but don't see that these would trip if the power is just briefly interrupted. I suppose I can buy more tubes, or only use the amp on days that seem unlikely to have power interruptions, or move to another part of the state, but a way to shut the amp off automatically seems a better choice if such a device exists.

    Thank you, tna

    Posts : 195
    Join date : 2008-11-30

    Latching Relay

    Post by DynakitParts on Sun Feb 10, 2013 12:29 pm

    What may work would be a 120 vac latching relay. When the power is interrupted
    the contacts will open. You will need a manual momentary switch (push button) to
    re-energize the relay coil once the power is restored. Do a google search for this
    simple circuit.


    Posts : 489
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Automatic Disconnect for ST 70

    Post by arledgsc on Sun Feb 10, 2013 2:02 pm

    The VTA relay board would be a good option for you VTA Relay Board. Or at the very least in your environment the rectifier tube socket diode mod described here (yellow sheet mod). Diode Mod

    The relay board opens upon power failure and times out for 17 sec. upon each power reapplication. The yellow sheet diode mod adds solid state diodes on the rectifier tube HV AC inputs. During a power failure the diodes prevent reverse arcing through the rectifier tube. I use a variation of the yellow sheet mod using instead one diode on the rectifier output. Either of these will only help your situation.

    Posts : 959
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Automatic Disconnect for ST 70

    Post by peterh on Sun Feb 10, 2013 3:50 pm

    I use SSR to turn on poweramps, these i feed via 12V from my preamp ( takes a few mA )
    But in your case i'd go with a variant :
    Install an ssr, then two toggle buttons ( really cheap ones will do)
    Additinally you will need a 4.5V flashlight battery ( or 9V ), a 100ohm resistor and
    maybe a 1n4007 diode.

    The first toggle will connect the battery to the ssr, making it close. As it closes
    i will use any suitable power ( rectified filament ?) through a resistor to keep
    the ssr in ON state. The second toggle will shortcirquit the ssr control input, thus shutting the amp down.

    To clarify : you amp will have one "on" button and one "off" button. The battery
    will serve as "bootstrap" and will last very long. If the amp is in "on state" and
    power fluctuates it will torn off.

    In my last implementation of ssr ( in a vta70) a have the ssr in paralell with the
    ordinary cirquit breaker, thus either "on" from preamp or "on" with the breaker
    will turn it in ( for those cases where i use the amp without my usual preamp).

    Ed Cheek

    Posts : 6
    Join date : 2013-02-10

    Re: Automatic Disconnect for ST 70

    Post by Ed Cheek on Sun Feb 10, 2013 5:50 pm

    TN Allen,

    I am an electrical engineer. I can tell you what we do in industry to protect against power flickers. Try a uninterrupted power supply, or UPS. It plugs into your outlet and has a battery backup and inverter. If your power flickers the UPS automatically kicks in and keeps power going. When power comes back on the UPS goes back into charge mode to charge the battery. They work great and are not very expensive. You can even get them at Sam's Club.

    The system would not let me post a link. Go to and search for uninterrupted power supply and check it out.

    The ones Amazon have are used mainly with computers and home entertainment equipment. I hope this helps.

    Ed Cheek

    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Thank you

    Post by TN Allen on Sun Feb 10, 2013 6:29 pm

    Thank you for the information, all of the ideas seem good. I'll look into them in detail tomorrow. Perhaps something I can plug the amp, preamp, turntable, tuner and CD player into would be a good idea, although a circuit inside the amp also makes a lot of sense to avoid trouble in a situation where I might not have the external device.

    Posts : 496
    Join date : 2008-12-05

    Re: Automatic Disconnect for ST 70

    Post by PeterCapo on Sun Feb 10, 2013 6:54 pm

    For power interruptions, ground fault safety and convenience, I find this works well:

    If the power goes out, the GFCI opens, which cuts the power to everything plugged into the strip. Just press a button to reset and you're back in business. But, it would probably be a good idea to let a few minutes pass - it's not good to rapidly power cycle tube amps.

    Posts : 305
    Join date : 2010-02-03
    Location : Costa Rica

    Re: Automatic Disconnect for ST 70

    Post by baddog1946 on Mon Feb 11, 2013 10:23 am

    There are several manufacturers that GFI outlet modules and power strips with remote controls out that allow reset and on-off functionality. The GFI system has been bulletproof for me in Costa Rica where we have a nightmare power grid with voltage spikes and quick on off incidents almost daily. I am even considering building one of these into into the chassis of one of my custom amps.

    Posts : 26
    Join date : 2011-05-07

    Remote Control Power Strip

    Post by bcinnamon on Mon Feb 11, 2013 1:14 pm

    Get this:

    It works great. Momentary outage requires a manual restart so you can't short-cycle your equipment. Also, you can plug in all your audio devices to this power strip and control power remotely - eliminating standby power usage.

    Every VTA customer should get either Bob's Time Delay Relay built into their system - or one of these power strips.

    My TV and entertainment toys were drawing about 60 watts even when they were not on. Using this power strip I can basically "unplug" everything without crawling behind my components. BTW, 80 watts 24x7 here in California, where electricity is $0.30/kwh, is $210/yr!

    TN Allen

    Posts : 124
    Join date : 2013-01-01

    Re: Automatic Disconnect for ST 70

    Post by TN Allen on Mon Feb 11, 2013 1:37 pm

    BTW, 80 watts 24x7 here in California, where electricity is $0.30/kwh, is $210/yr![/quote]

    I've thought we pay too much at about $.15/KWH(most of that is "delivery"), but $.30/KWH is outrageous!

    I've been looking at the various possibilities, and plugged the amp, preamp & etc. into a Tripp Lite Isobar I've had for 20 years. I'm thinking though of the internal circuit as well, seems good insurance as 2 5AR4's cost about what the VTA kit costs.

    Thank you for all the suggestions. tna


    Posts : 179
    Join date : 2011-04-04

    Re: Automatic Disconnect for ST 70

    Post by Tom on Thu Feb 14, 2013 11:58 am

    Kind of on the same topic, here at KSC, we're demoing these powerstrips with a "remote" on/off that you can hang on the wall like a lightswitch. Easy off for all those "vampire" power drains.

    Smile Tom

    Sponsored content

    Re: Automatic Disconnect for ST 70

    Post by Sponsored content

      Current date/time is Tue Jun 19, 2018 10:16 am