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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


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    ST70 pop on turn off

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    sailor


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    Post by sailor Fri Mar 23, 2012 5:18 pm

    I have a ST70 that works just fine but it pops on turn off. Is this normal? Is the switch going bad and needs to be replaced or should it be shunted with say a .01/400volt cap?
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    Post by Bob Latino Fri Mar 23, 2012 6:23 pm

    Hi Sailor,

    Both of your solutions will probably work. Either replace the switch OR run about a .02 uF cap @ 200 volts or higher across the two terminals. Personally - I would replace the switch. Radio Shack has a suitable replacement SPST slide switch that will fit right in there. The only problem with the Radio Shack switch is that it is not threaded like the original switch. An easy solution is to just use a 4-40 nut over the end of the two screws that hold the switch on.

    Bob
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    Post by sailor Fri Mar 23, 2012 8:58 pm

    Thanks Bob, you make this the best audio board on the net.
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    Post by sailor Fri Mar 30, 2012 3:59 pm

    I replaced the the switch and added a shunt cap for good measure the pop is now completely gone.
    While I was soldering on the bottom I decided it was time to replace the original bios caps.
    When I turned the amp. on I was greeted with far more detailed sound and it seemed to have better Bass.
    The problem is once you change 2 things out you don't know which made the difference.
    I would assume that either the switch was so bad that it was limiting the amount of power to the transformer or more likely, the 2 caps were very noisy and changing them out cleaned up the sound. I had no idea that the bios caps could make that much of a difference?
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    Post by Tube Nube Fri Mar 30, 2012 4:05 pm

    Can you tell us a bit more about what you replace, and what you put in as a substitute?
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    Post by sailor Fri Mar 30, 2012 5:01 pm

    Hi Tube Nube, A shunt cap is a low value cap around .01 to .02mfd 400volts or higher, across a switch to help stop a pop when you turn electronics on and off. Most amps and preamps already have this cap. The ST70 does not. I may not have needed it with the new switch replacement but I was already soldering so I put it in.
    As far as the 2 electrolytic caps in the bios supply, The original ones are either green or gold and I think 50mfd/75 volts. I changed them out with new 100mfd/100volt. I was holding bios just fine before they were changed out. But once removed there poor condition became apparent.
    I think all would agree: If you get an old ST70 at the very least you should replace the quad cap and the 2 bios caps as they are very likely to fail and do damage to your tubes and choke. I did not change the bios caps because I thought they were in better shape than the were. I won't make that mistake again.
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    Post by Tube Nube Fri Mar 30, 2012 6:04 pm

    "Hey there Sailor", (Couldn't resist saying that, and having done so, it feels a little too "Mae West")

    Seriously, thanks for that explanation. I wondered of I should be considering replacing caps like you did, but I'd guess not since my ST70 is one of Bob's, and is only a couple of years old.

    As for the shunt cap . . . that might be a good idea!

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    Post by tubes4hifi Fri Mar 30, 2012 11:49 pm

    TubeNube,
    it's just like the cap on your preamp power switch, I use 0.01uF 1000v rated capacitors
    ST70 pop on turn off Bc-85
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    Post by sailor Sat Mar 31, 2012 12:15 pm

    A picture is worth a thousand words.
    Nube your caps are just fine. Electrolytic caps have a shelf life of about 20 to 25 years. The caps in the old Dynaco equipment are closer to 50 years which makes them a ticking time bomb. They are going to fail just a matter of when.
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    Post by fedoragent Fri Dec 20, 2013 5:42 pm

    tubes4hifi wrote:TubeNube,
    it's just like the cap on your preamp power switch, I use 0.01uF 1000v rated capacitors
    ST70 pop on turn off Bc-85

    This photo is a little bit out of focus.  Are there certain pins that this cap needs to be on?  I'm going to use a .01 Orange Drop that is rated for 630V.  This should do the job I hope.
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    Post by tubes4hifi Fri Dec 20, 2013 10:52 pm

    there are two wires to the power switch, one lead of the cap goes to one each of the wires
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    Post by hoangtu_09 Mon Dec 26, 2022 12:58 am

    sailor wrote:I replaced the the switch and added a shunt cap for good measure the pop is now completely gone.
    While I was soldering on the bottom I decided it was time to replace the original bios caps.
    When I turned the amp. on I was greeted with far more detailed sound and it seemed to have better Bass.
    The problem is once you change 2 things out you don't know which made the difference.
    I would assume that either the switch was so bad that it was limiting the amount of power to the transformer or more likely, the 2 caps were very noisy and changing them out cleaned up the sound. I had no idea that the bios caps could make that much of a difference?

    Do you have pictures what you replaced?
    Thank you
    lorne
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    Post by lorne Mon Dec 26, 2022 5:39 pm

    Laughing I'm really glad to see this thread. I've been using a Melos GK 1+1 tube preamp I bought 35 years ago. Eventually it will be teamed up with my ST-70. It has been popping and driving crazy. I should have caught on to the possibility that it was the power switch. Suspect So, I'll change out the switch and put in a cap.

    Q: What is a good type of cap to use? What about a ceramic disk type? I have some in my bins.
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    Post by Bob Latino Mon Dec 26, 2022 6:44 pm

    lorne wrote: Laughing I'm really glad to see this thread. I've been using a Melos GK 1+1 tube preamp I bought 35 years ago. Eventually it will be teamed up with my ST-70. It has been popping and driving crazy. I should have caught on to the possibility that it was the power switch.  Suspect    So, I'll change out the switch and put in a cap.

    Q: What is a good type of cap to use? What about a ceramic disk type? I have some in my bins.

    It doesn't really make any difference what cap you use > A film cap or a ceramic disc cap will work fine. Years ago I used a .02 uF @ 200 volt film cap on my own original Dynaco ST-70 and it worked fine to stop the turn off pop.

    Bob
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    Post by lorne Tue Dec 27, 2022 5:39 am

    Thanks Bob. I must have a lot of choices in my parts bins.

    After some research, I am having some doubts as to whether it really is the power switch, or something else.

    When I shut down the prepamp, I switch to 'mute'. I make sure the volume is turned right down, because the 'pop' is attenuated. Then I flip the power switch to 'off'. POP!

    I have also tried switching off at the consent that supplies the VAC to the preamp, albeit with the preamp's power switch at 'on'. POP!

    I'll sort out the preamp's power switch with a capacitor and see if that works. If not, I'll have to trace for another cause. I won't trouble everyone on this forum further, cuz it's not Dynaco related. But, I will add a short note here when I find the solution.
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    Post by peterh Tue Dec 27, 2022 5:45 am

    lorne wrote:Thanks Bob. I must have a lot of choices in my parts bins.

    After some research, I am having some doubts as to whether it really is the power switch, or something else.

    When I shut down the prepamp, I switch to 'mute'. I make sure the volume is turned right down, because the 'pop' is attenuated. Then I flip the power switch to 'off'. POP!

    I have also tried switching off at the consent that supplies the VAC to the preamp, albeit with the preamp's power switch at 'on'. POP!

    I'll sort out the preamp's power switch with a capacitor and see if that works. If not, I'll have to trace for another cause. I won't trouble everyone on this forum further, cuz it's not Dynaco related. But, I will add a short note here when I find the solution.

    Why don't you turn off the power amp first and wait some 10s before turning off the pre ?

    quad44, Dave_in_Va, 10-E-C and lorne like this post

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    Post by lorne Tue Dec 27, 2022 6:04 pm

    peterh wrote:[Why don't you turn off the power amp first and wait some 10s before turning off the pre ?

    Now that's a good suggestion. As yet, I have not waited after turning off the two Trio / Kenwood mono amps.

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