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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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    One Hot A470

    Sled108
    Sled108
    Sled108


    Posts : 40
    Join date : 2010-02-03
    • Post n°1

    One Hot A470 Empty One Hot A470

    Post by Sled108 Sat Apr 10, 2010 12:12 am

    Please...Break it to me Gently...I have 1 Hot A470 on my ST70..The Other is not so Hot...Both channels sound Good, but I know that this is not a Good Thing...When I say Hot, I mean 3 Seconds on the Palm Hot. The other side is like Nothing. The Volume doesn't change it...Should I start looking for a "New" Old A470, or would I be Better Off a Pair of New Ones?...Either Chicagolands, or maybe Edcors? What would be the best way to go to in a Hard Working ST70?

    OK, After writing this, it has cooled down...But I also turned the Volume down a Lot.
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

    One Hot A470 Empty Re: One Hot A470

    Post by Bob Latino Sat Apr 10, 2010 7:53 am

    Sled108 wrote:Please...Break it to me Gently...I have 1 Hot A470 on my ST70..The Other is not so Hot...Both channels sound Good, but I know that this is not a Good Thing...When I say Hot, I mean 3 Seconds on the Palm Hot. The other side is like Nothing. The Volume doesn't change it...Should I start looking for a "New" Old A470, or would I be Better Off a Pair of New Ones?...Either Chicagolands, or maybe Edcors? What would be the best way to go to in a Hard Working ST70?

    OK, After writing this, it has cooled down...But I also turned the Volume down a Lot.

    Let me guess? It's the LEFT output transformer that gets hotter than the RIGHT output transformer? There is always an inequity in the heat on those two output transformers because the LEFT output transformer absorbs heat from the nearby rectifier tube. If you run the amp hard this will make the inequity in temperature of the two output transformers even more noticeable.

    That said, if that output transformer gets as hot as the power transformer there *could* be an issue with one of the five primary windings - all of which are carrying 420 to 440 VDC ? Personally, if the amp sounds OK I wouldn't worry about it but if it starts to blow fuses or you get some flashing inside the two output tubes on that channel you should probably replace the transformer.

    Bob
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    Sled108
    Sled108
    Sled108


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    • Post n°3

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Sat Apr 10, 2010 9:38 am

    Thanks Bob...You're right, it is the Left One. The thing is, the Amp is Diode Rectified. I'll have to keep a Watchful Eye on those Tubes....

    Here's ST70 in question...

    https://i.servimg.com/u/f68/15/08/77/85/st70_212.jpg
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    Bob Latino
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    • Post n°4

    One Hot A470 Empty Re: One Hot A470

    Post by Bob Latino Sat Apr 10, 2010 2:03 pm

    One Hot A470 St70_212

    Whose driver board is that on your amp (photo above)? It looks like two 6U10 compactron tubes ... I don't think I ever saw that driver board before ? I am not at all familiar with that driver circuit ? Do you have a photo of the inside wiring ?

    Bob
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    Sled108
    Sled108
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    • Post n°5

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    Post by Sled108 Sat Apr 10, 2010 8:02 pm

    I have been asking the same question about This since I got It....Over 25 yrs. ago, and yes, they are 6U10's....All I know is that it was Modified by a Place called Melos...Here's the Insides..Yet to be Identified...

    One Hot A470 St70_i10
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    xlr8
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    xlr8


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    • Post n°6

    One Hot A470 Empty Re: One Hot A470

    Post by xlr8 Sun Apr 11, 2010 9:46 pm

    Hmmmm.... Looking at the bottom of your amp, I'd say you could hardly call it an st-70 anymore. Looks more like "Melos" built their own amp and just used a few good st-70 parts. Laughing
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    Sled108
    Sled108
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    • Post n°7

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Mon Apr 12, 2010 5:38 pm

    How would it Work using these Edcors? They are only about 1/4" larger all around, so they would fit. But they are 4.2K Ohm, not 4.3K Ohm...Can they still be made to Work, and has Anyone heard These?

    http://www.edcorusa.com/Products/ShowProduct.aspx?ID=485
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    Bob Latino
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    • Post n°8

    One Hot A470 Empty Re: One Hot A470

    Post by Bob Latino Mon Apr 12, 2010 6:25 pm

    Sled,

    1. That model Edcor output transformer will not fit the stock Dynaco ST-70 mount holes. You will wind up having to drill 2 new holes for each transformer. The transformer will fit from front to back because there is enough play in the mounting slot. They won't fit from side to side.

    2. That model only has one a single 8 ohm output tap. You need the 16 ohm tap for the feedback line unless you change the feedback resistor value for use off the 8 ohm tap.

    3. Since it is larger, it would look funny paired with a stock A-470 output transformer if you are just replacing ONE output transformer.

    IMHO you should spend the extra money and get a quality replacement A-470 from Dynakitparts. They look identical to a stock A-470 and have secondaries for 4, 8 and 16 ohms.

    Bob
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    Sled108
    Sled108
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    • Post n°9

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Mon Apr 12, 2010 7:01 pm

    Hey Bob....As XLR8 said, mine is hardly a Dynaco anymore...It has both the 16 & 4 Ohm Leads cut. And, I would be changing Both of the OT's..The holes would need 1/8" of Rat-tailing ea....What I wonder is, with my Setup how these at 60 Watts will Perform against the 35 Watt Dynakits? Maybe run a bit Cooler?
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    Bob Latino
    Bob Latino
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    • Post n°10

    One Hot A470 Empty Re: One Hot A470

    Post by Bob Latino Mon Apr 12, 2010 7:24 pm

    Sled,

    In the photo of your amp's wiring the 16 ohm lead is not connected to the binding post but that wire has been extended back to the feedback eyelet on your driver board. Again, you can just use the 8 ohm tap for feedback but you will have to change the feedback resistor value on your driver board (any maybe the small picofarad capacitor that parallels this resistor) to keep the same amount of feedback.

    Since these transformers have a larger core, they would undoubtedly run cooler than the stock Dynaco A-470's. They also have a different set of wire colors than original Dynaco transformers.

    Bob


    Last edited by Bob Latino on Fri Oct 22, 2010 11:25 pm; edited 1 time in total
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    Sled108
    Sled108
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    • Post n°11

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Mon Apr 12, 2010 7:42 pm

    It is a bit hard to tell from that Pic, but you Nailed it...Both Leads are cut, and the Feedback is coming from the 8 Ohm Tap to those Resistors and Bundle of Mial 3900J Caps..
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    Sled108
    Sled108
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    • Post n°12

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Thu Apr 15, 2010 12:47 am

    Before I Spring for a New Pair of OTs...Is there any chance that maybe I have a Bad Power Resistor or Cap in Here? Not believing my Ringing Ears, I hooked up my Dual Dancing Needle mA Meters..And it seems that they are Well Balanced up to 12:00 on the Knob...After that, the 'Hot' side runs Lower than the 'Cool' side..On this I hope that I can believe my Lying Eyes...Each KT77 has its own 10K Trimpot set at .5v .47mA, so they are Good...Could there be Something else going Soft besides the Tranny?
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    Sled108
    Sled108
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    • Post n°13

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Tue Oct 12, 2010 6:03 pm

    Cooler weather has arrived, and I've finally swapped out the A470's for a pair of Edcor CXPP60-8-4.2K's. I put the Blue Edcor Endbells on the back and painted the fronts black, after a little work with the Rat-Tail file they installed just fine. You'd have a hard time telling that they're not the Dynaco's. I'm bringing them up slowly and watching the Bias, but so far they really Sound Good...I guess my ST70 is not really much of a Dynaco anymore...Next up will be the PA-060, that will pretty much make it Final...
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    Sled108
    Sled108
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    • Post n°14

    One Hot A470 Empty Re: One Hot A470

    Post by Sled108 Fri Oct 22, 2010 11:21 pm

    Well, it is Cooler now, around 68 Deg. in my Listening Room. I'm running this now at 2 O'Clock thru a Marantz 7T and the OT's are so Cool that it's hard to tell if they're Warm...For that matter, the PT is also so Cool that I can barely warm my Fingers on it...These Edcors sound Great!
    And I have those KT77's at .5A & .55v...


    One Hot A470 50281407


    Last edited by Sled108 on Thu Mar 31, 2011 12:13 am; edited 2 times in total (Reason for editing : Added the Cooler...)
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    j4570
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    • Post n°15

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    Post by j4570 Wed Oct 27, 2010 9:00 pm

    Now that's a truly custom amp.

    Heavily Modified.
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