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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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Bob Latino
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    A Question About ST-70 Can Capacitor Upgrades

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    A Question About ST-70 Can Capacitor Upgrades Empty A Question About ST-70 Can Capacitor Upgrades

    Post by quadaptor Sun Nov 01, 2009 12:58 pm

    I noticed that one of the current Dynaco kit manufacturers has a new (at least new to me) electrolytic can capictor for ST-70s that has higher ratings. The capacitor is advertised as being for thir MK III amp kit, but will also work with ST-70s. I asked the manufacturer to compare the capacitor with their other can capicitor upgrade for the ST-70 amp and got the following response:

    The only thing common to these two capacitors is the MFD values and physical dimensions.

    Our # 284320-2 is a traditional can capacitor design made for us here in the US by CE-Distribution and sold through Antique Electronics and others. These traditional style capacitors are made on the original Mallory equipment.

    Our # 284320-6 is basically an aluminum can with (4) discrete modern F&T low ESR electrolytic capacitors fitted within the can. Three sections are rated for 550 vdc continuos 600 vdc surge and the remaining 30 MFD section rated for 525 vdc continuos 575 vdc surge.

    We selected this German made capacitor for our Dynakit Mark III amplifier.

    I have not personally compared these two capacitors to each other in either the ST-70 stock unit or Mottram's VTA upgrade. Most who purchase the # 284320-6 use this in the MK III amplifier. I doubt you will hear any measurable difference.

    I would suggest you pose this same question to Bob Latino at the Dynaco Tube Forum. I believe he has tried both of these can capacitors in his VTA upgraded ST-70.     [/i]

    Does anybody have experience with this capacitor in both stock and VTA boarded ST-70 amps?
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    Post by Bob Latino Sun Nov 01, 2009 1:23 pm

    I have used both the CE Distribution quad cap capacitor and the F&T quad cap capacitors. They are both well made and seem to work well in the ST-70 or Mark III. The CE is made in Arizona, USA on older Mallory equipment as you mentioned and is rated at 525 volts on all four sections. The F&T is made in Germany and rated at 550 volts for the first three sections and 525 volts for the the 4th section. The F&T has a slight advantage in voltage handling capacity but costs about $15 more than the CE capacitor. IMHO either one is acceptable in both the ST-70 or Mark III. The ST-70 at the first section of the quad cap has a nominal DC B+ voltage of about 420-430 volts. The Mark III is about 475 volts. On turn on the (surge) voltage will be slightly higher the these voltages just mentioned. Both are at least 50 volts above the normal B+ voltage rating steady state handling capacity for EITHER amp. If you are on a budget then get the CE cap. If you feel you need the extra 25 volts of handling capacity then get the F&T made cap from Dynakitparts.

    Bob
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    A Question About ST-70 Can Capacitor Upgrades Empty Thanks

    Post by quadaptor Sun Nov 01, 2009 1:45 pm

    Thank you very much!
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    A Question About ST-70 Can Capacitor Upgrades Empty st 70 can capacitor

    Post by kost Wed Nov 04, 2009 9:18 am

    hi

    Do you think that 500v rated capacitors are safe for replacing the st 70 can capacitor?
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    Post by Bob Latino Wed Nov 04, 2009 11:49 am

    kost wrote:hi

    Do you think that 500v rated capacitors are safe for replacing the st 70 can capacitor?

    Kost,

    As I said above, the operating high voltage on an ST-70 is maybe 420-430 volts DC but it may surge above that value when you turn the amp on. Dynaco rated their original quad cap at 525 volts but you *probably* could get by with a 500 volt rated capacitor on the quad cap. IMHO you should stick with a 525 volt or higher rated. The absolute BEST capacitor for a replacement quad cap for a Dynaco ST-70, Mark II, Mark III or Mark IV is the 550 volt rated quad cap from Dynakitparts. Link below ...

    Dynakitparts 550 volt rated quad cap

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    Post by dyna70 Fri Apr 09, 2010 8:46 am

    Hi folks.

    Probably a 'dumb question' but.... just received an Authenticap direct from 'Ask Jan First' in Germany. Looks well made, and the voltage ratings are all 600V surge, apart from the 30uF section which is 575V surge.

    Silly question is: the mounting tabs on most can caps usually require the tabs to be twisted a quarter turn around to fix the cap in place. However on the Authenticap I can see no signs of a pre-scored line on the tabs for twisting. I assume then this means the tabs should merely be bent over (away from the center of the cap) to secure them? I did e-mail Jan and he said they should be twistlock, but looking at the tabs I'm not so sure, and don't want to risk breaking one off! The last Quad cap I had had slight score lines to 'ease' the twisting of the tab at the correct point.

    Thanks,

    - John


    Last edited by dyna70 on Fri Apr 09, 2010 8:52 am; edited 1 time in total
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    Post by Bob Latino Fri Apr 09, 2010 8:51 am

    Hi,

    Yes - You are correct - there is no pre-scored line on the tabs of those caps. You just bend the tabs the 1/4 turn and the cap will hold on securely.

    Bob
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    Post by dyna70 Fri Apr 09, 2010 8:55 am

    Hi Bob - thanks for the speedy reply Smile

    So they are meant to be twisted then? OK Cool. Looking at it though I guess they could be bent over instead - just seems like a more secure way of doing it to me. I guess either way works... and probably spending way too much 'audiophool' brain power thinking about this Wink

    Cheers,

    - John

    P.S. Any suggestions for polishing the cap? Brasso? Wire wool?
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    Post by Bob Latino Fri Apr 09, 2010 12:08 pm

    To polish the cap I would use a metal polish like "Brasso". The metal polish won't leave any scratches. Probably very fine (0000) steel wool won't scratch either? I have always used metal polish on these can caps ...

    Bob
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    Post by DC92 Thu May 28, 2015 1:13 pm

    So must I replace the Quad cap with the same 30/20/20/20 ratings or can you go higher without any modifications? Thanks and apologize in advance for my ignorance.
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    Post by Bob Latino Thu May 28, 2015 1:18 pm

    DC92 wrote:So must I replace the Quad cap with the same 30/20/20/20 ratings or can you go higher without any modifications? Thanks and apologize in advance for my ignorance.

    No - You can use the 80, 40, 30, 20 uF CE quad cap in place of Dynaco's 30, 20, 20, 20 cap. When you install the new (upgraded) cap just make sure that the 80 section is facing the BACK of the amp. If you do this then all the other sections will be in the correct place ..

    Bob
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    Post by DC92 Thu May 28, 2015 5:10 pm

    Thanks, Bob! I just talked to the kind folks at Dynakit, and he's sending me a 80, 40, 30, 20 uF CE quad cap and a schematic. I should be able to replace it myself. My biggest concern is the stability of the machine itself, since it's so heavily tilted towards the back. Just having it upside down seems like the most difficult part of the operation.
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    Post by DC92 Wed Sep 02, 2015 4:31 pm

    Hi Bob,

    Just wanted to send a note of thanks. I bought the Quad Cap you suggested, and I just installed it. The buzz is gone, and I can bias the tubes and it sounds like gold.

    This is an awesome forum, and Bob, you're awesome for helping out us novices!



    Bob Latino wrote:
    DC92 wrote:So must I replace the Quad cap with the same 30/20/20/20 ratings or can you go higher without any modifications? Thanks and apologize in advance for my ignorance.

    No - You can use the 80, 40, 30, 20 uF CE quad cap in place of Dynaco's 30, 20, 20, 20 cap. When you install the new (upgraded) cap just make sure that the 80 section is facing the BACK of the amp. If you do this then all the other sections will be in the correct place ..

    Bob
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    Post by Bob Latino Wed Sep 02, 2015 4:46 pm

    Electrolytic capacitors do undergo chemical changes as the years go by .. They don't last forever .. The real Achilles heal of the original Dynaco ST-70 was the quad cap. That quad cap was the amp's entire high voltage DC storage system. The FIRST two things that you do to an original ST-70 if you are restoring the amp, is to replace the quad cap and also replace the selenium rectifier with a diode. Selenium is a bad poison and if the stuff ever leaks out of that rectifier, you have a hazardous waste situation. I would wear rubber gloves removing the selenium rectifier from the amp on the off chance that some did leak out.

    Read below from a 2012 post here on the forum ..

    Selenium rectifier post - May 2012

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    Post by howlin' hoosier Tue Nov 08, 2022 11:01 am

    Bob Latino wrote:
    DC92 wrote:So must I replace the Quad cap with the same 30/20/20/20 ratings or can you go higher without any modifications? Thanks and apologize in advance for my ignorance.

    No - You can use the 80, 40, 30, 20 uF CE quad cap in place of Dynaco's 30, 20, 20, 20 cap. When you install the new (upgraded) cap just make sure that the 80 section is facing the BACK of the amp. If you do this then all the other sections will be in the correct place ..

    Bob

    Is it safe to assume that the back of the amp is the end with the speaker terminals/RCA jack/power cord?
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    Post by Bob Latino Tue Nov 08, 2022 11:09 am

    Yes - the back of the amp is where the speaker posts, power cord and fuse post are located.

    Bob
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