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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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    dual rectifiers for ST120 & ST70 amps

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    dual rectifiers for ST120 & ST70 amps Empty dual rectifiers for ST120 & ST70 amps

    Post by tubes4hifi Thu Jul 13, 2023 7:14 pm

    highly recommended . . . the primary problem with the ST120 amp is the output tubes draw too much current for the rectifier tube.
    These often last only a few months at best, and if they blow, they usually also short out the quad cap.
    Now available, a simple mod to use dual rectifiers, which gives you twice the current capability, and so the rectifier tubes should last years.
    This does require removing the quad cap, but if you blew out the rectifier and the quad cap, makes sense to use this.
    $25 bare board with two sockets, just add four capacitors (also available for $50 the set, about the same as a new quad cap).
    buy at www.tubes4hifi.com/DR.htm
    dual rectifiers for ST120 & ST70 amps Dr610
    dual rectifiers for ST120 & ST70 amps Dr710

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    Post by mantha3 Thu Jul 27, 2023 9:26 am

    The digital bias meeters on the front. Is there a kit / thread on how to do this? Thinking about updating my 13 year old ST120 with the digital bias and then also the dual rectifiers.
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    Post by kFish Thu Jul 27, 2023 10:00 am

    mantha3 wrote:The digital bias meeters on the front.  Is there a kit / thread on how to do this?  Thinking about updating my 13 year old ST120 with the digital bias and then also the dual rectifiers.  


    Same question here. Any kit, or even parts-list someone could share?

    Ken
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    Post by kFish Thu Jul 27, 2023 10:20 am

    [quote="kFish"]
    mantha3 wrote:The digital bias meeters on the front.  Is there a kit / thread on how to do this?  Thinking about updating my 13 year old ST120 with the digital bias and then also the dual rectifiers.  

    A quick scan of the sale site shows this option. I guess I need to look at tubes4hifi.com more often

    dual rectifiers for ST120 & ST70 amps Screen10


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    Post by tubes4hifi Thu Jul 27, 2023 11:22 am

    yes, there is a thread for this, and you're reading it . . .
    and yes, there is a kit available for the parts at www.tubes4hifi.com/ST70.htm
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    Post by 1bigpig Thu Jul 27, 2023 1:37 pm

    Just to be clear on the bias meter kit, is the "$35 for two pairs KIT" a meter for both the left and right or do I need to buy 2 kits for an ST-120?
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    Post by tubes4hifi Thu Jul 27, 2023 2:10 pm

    one kit is for TWO PAIRS of meters (just like it says, that's four meters, two for each side), so one kit needed for ST70 or ST120 or pair of MK3 amps
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    Post by jasn54 Thu Jul 27, 2023 5:56 pm



    Last edited by jasn54 on Thu Jul 27, 2023 6:01 pm; edited 3 times in total (Reason for editing : clarification)

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    Post by Madhyamika Fri Jul 28, 2023 5:24 pm

    The dual rectifier mod makes a lot of sense - I’ll probably order one for my ST120. I also have some M125s - will this mod work for them as well?
    Thanks!
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    Post by tubes4hifi Fri Jul 28, 2023 6:26 pm

    the mod itself will work with M125s, but this board won't fit an M125
    anybody else want this mod for their M125s?
    If we can get maybe 6 people to say yes, we can go for it
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    Post by Madhyamika Fri Jul 28, 2023 8:14 pm

    Cool possibility - thanks, Roy! For what it’s worth, I have 3 M125s so I’d be up for buying 3 mod kits (I use the M125s as left, right, and center channels in a 5-channel surround setup)
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    Post by Briansilcox Sun Aug 06, 2023 12:29 pm

    Just started the dual rectifier kit, reviewing the instruction sheet and photos on tubes4hifi site, I see that having the optional triode/pentode switches seems to offer an unplanned level of complexity.  Any recommendations for this scenario? I have no problem removing the switches and wiring, as I keep the switches in pentode mode. I could use some guidance in what wiring to remove or reroute with the switches removed.

    Thank you!


    Last edited by Briansilcox on Sun Aug 06, 2023 1:47 pm; edited 1 time in total (Reason for editing : Add info)

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    dual rectifiers for ST120 & ST70 amps Empty Parallel rectifiers

    Post by tubeguy8888 Wed Aug 09, 2023 11:21 pm

    I first tried dual rectifiers about 12 years ago.
    I had 2 rectangles literally burn up and meltdown internally.
    There are a couple of things to keep on mind.
    2.  The rectifiers must be wired in parallel ( easily done)
    3.  You double the current that can go to the power tubes AND
    4.  You cut the impedance by 50%  , so the response will be sharper.
    5   Dont use Russian/ Chinese tubes if you score some vintage Telefunken. RCA, Sylvania. Or other
        Quallity tube.  I switched to 2 x 274B.rectifiers from Emission Labs.  Or old Military spec 274B ( usually made by Sylvania)
    6  The 274b is a great substitute; it can handle 550 volts easily, and sounds great. It is virtually indestructible.  I have a pair of EML 274b for over 12 years,  first in a ST-70 clone, the in a VTA 120. With KT-88 outputs. The EML are expensive. But well worth it. Mine still tests as new.
    7. The 274B is indirectly heated.  I uze the mesh plate  versions.  They take a minute or 2 to warmup.
       So you dont hammer the cold power tubes with instant B+ of 550 V , so they. Protect power tubes.
    8. Just be sure your power transformer can handle the additional current. Required by 2 rectifier in parallel.  I used equipment from Roy Mottram and it was no problem.  You can use. Twin GZ-34,  274B, and 5v4.   Watch other rectifiers. ( such as gz37 or 5GB  types..  they require too much heater currrent  to use more than 1z
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    dual rectifiers for ST120 & ST70 amps Empty updated dual rectifier mod instructions

    Post by tubes4hifi Thu Aug 10, 2023 2:19 pm


    instructions . . . . http://tubes4hifi.com/DR.htm
    here's some added instructions to go with photos already posted, to clear things up . . .

    dual rectifiers for ST120 & ST70 amps Drmod_11
    dual rectifiers for ST120 & ST70 amps Drmod_10

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    Post by Briansilcox Thu Aug 10, 2023 4:19 pm

    I completed the dual rectifier kit wiring /installation last night.  When I first set out with documentation, I realized the optional triode pentode switching I had built in originally had to go.  While it looks like a big job, it really wasn’t difficult.  After reconfiguring the wiring to the output tubes, I had a few questions regarding the optional SCM module, and differences between the VTA70 and 120.  Roy very quickly responded, and I completed the wiring of the chokes, etc to installation of the board.  The above post clarifies all that.

    I powered it up this morning, no issues, and was slightly surprised that no bias adjustments were necessary.  I did install a new pair of J&J GZ34s just purchased.  I am not really qualified to offer subjective review, but to my ear, I detected a notable improvement in bass response (welcomed).  In the year I have run my VTA120, the only issues I have had are one blown rectifier and two fried bias resistors, all easily detected and resolved.  I do run the Circuit Specialists variac at 117v, per recommendation of Bob Latino, as my line voltage is about 121v.

    I know that Roy is stepping aside to enjoy retirement, I just want to take a second to say thanks…I built my first Dynaco (ST120) 50 yrs ago age 18, and have seriously enjoyed the kits Roy and Bob have developed for us, and will very likely be building more.dual rectifiers for ST120 & ST70 amps 20230812

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    Post by cgtayloriii Fri Aug 18, 2023 4:58 pm

    I have purchased the dual rectifier kit and hope to install it soon. I also have the triode/pentode switches and want to retain them. My plan is to increase the length of the spacers on the DR board so that I have room to adjust the triode/pentode switches under the board with a finger or pencil. All wiring from below will come up through the holes left by the old rectifier tube and quad cap.

    I don't see a problem, but am I overlooking something?

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    Post by tubes4hifi Fri Aug 18, 2023 5:29 pm

    that should be fine, you can probably use 1" spacers
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    Post by penguinpages Sun Sep 03, 2023 1:48 pm

    cgtayloriii wrote:I have purchased the dual rectifier kit and hope to install it soon.  I also have the triode/pentode switches and want to retain them.  My plan is to increase the length of the spacers on the DR board so that I have room to adjust the triode/pentode switches under the board with a finger or pencil.  All wiring from below will come up through the holes left by the old rectifier tube and quad cap.

    I don't see a problem, but am I overlooking something?


    How did the concept of raising the DR PCB board up (and maybe cutting down height of toggle switches a bit, to allow retaining of triode/pentode switches?

    Can you post how you also wired or attached terminal post inside?

    This would mean all wires routed through the holds from old rectifier port and Quad cap port, but what I am trying to work out before I start pulling things apart is how it effects steps for the upgrade. Steps seem to imply to just tear out all the Rectifier/Quad cap, and triode/pentode switches... then start.. But what is diagram if triode/pentode switches is left.

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    Post by cgtayloriii Sun Sep 03, 2023 3:42 pm

    Haven't had time to make the modification, yet. Will post when I do.
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    dual rectifiers for ST120 & ST70 amps Empty Repositioning triode/pentode switches

    Post by Ianedwin Thu Sep 07, 2023 10:28 pm

    Hello

    Just sharing a couple of photos of my dual-rectifier conversion. I moved the triode/pentode switches further forward to either side of the PCB. See the images taken from above and from the front. Straightforward to do and means you don't need to elevate the DR board. All good so far! Hope this helps when you are planning your own conversion.dual rectifiers for ST120 & ST70 amps Img_5810
    dual rectifiers for ST120 & ST70 amps Img_5811

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    Post by penguinpages Fri Sep 08, 2023 7:43 am


    That looks like solid workmanship.

    I was trying to figure out how to move the switches to the front of the chassis but it was quickly getting ugly. This gives me idea to also make it work.

    How do you connect your Digital Multimeter for BIAS setting? I am currently working out means to hack the two port bias port upgrade to have holes in front , next to the bias meter ports..

    I will post when / if I figure it out... My guess is this is why you have face plate bump outs...so you still have bias contact ports in front.



    Would you mind posting picture if you have it of inside? I am still trying to work out wire diagram based on instructions... likely my being a bit dense and that I am not an EE.
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    Post by Ianedwin Fri Sep 08, 2023 8:35 am

    Will post more photos and some info on how I wired it. Based in Australia so forgive the time zone delay!

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