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    Coupling cap options in Dynaco tube amps

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    Bob Latino
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    Coupling cap options in Dynaco tube amps

    Post by Bob Latino on Thu Mar 28, 2013 6:18 pm





    As an original Dynaco tube amp gets older it is a good idea to change out the main coupling caps and possibly the interstage cap as well. In an original ST-70 you have a .05 uF interstage cap and two .10 uF main coupling caps per channel - so 6 caps altogether. After many years these caps can start to leak a little and degrade the sound. You can put in some other film caps or upgrade the caps. The four main coupling caps are important because all four are directly in the signal path and are responsible for feeding each of the four output tubes with the audio signal.

    The top photo shows a Russian Teflon cap. These may work well in some applications but in general Teflon caps are just too large to fit into most Dynaco tube amps. Yes - I am sure you could squeeze them in somehow but IMHO they are impractical because of their size.

    The next two caps are Russian PIO caps. The upper one is a K40-Y and the next one down, the green cap, is a K42-Y. Both of these are small enough to fit in either a stock driver board or a VTA driver board. The K40-Y is an earlier series and a little larger while the K42 is a little smaller. Some favor the K40-Y but after extensive listening to TWO Dynaco ST-35's - one with the K40-Y's and one with the K42-Y's, I have to say there is really little to choose between them. Both give a very large and deep soundstage with a very smooth and detailed sound. The K42's are physically smaller and can fit into areas that the K40's would have trouble.

    The Auricap is a reasonably priced (about $15 a cap) small film capacitor that is very small and will fit almost anywhere. These sound good but IMHO not as good as either of the two Russian PIO caps. They don't seem to have a "full" sound as the Russian caps. They maybe sound just a little thin at least in the applications I have used them in.

    Have you changed your coupling caps in your original ST-70 or any ST-70 variant or any Dynaco tube amp ? Any comments or conclusions?

    Bob



    gener8r

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    Re: Coupling cap options in Dynaco tube amps

    Post by gener8r on Fri Mar 29, 2013 9:44 am

    On Latino Kits I too have used the Russians (recieved from you) and they are quite good. Price to performance is hard to beat.

    My favorite combo to date on my own scratch builds with the VTA board is a Cardas golden ratio solid tinned in the 0.10uf interstage and a Mundorff Silver in Oil at 0.33uf in the coupling positions. Besides being a better fit they come accross, to my ears anyway, as better sounding too, and worth the additional expense. But I'm using them in combo with a Tomiko Tranny and KT90s set at a bias of about 0.525 volts, so your results may vary. Wink

    As with most good tube equipment, the coupling/output capacitor is the one thing that can make the greatest difference. I find they make more of a difference in a preamp than an amp, but they do make a difference in the ST70.

    simonfck

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    Re: Coupling cap options in Dynaco tube amps

    Post by simonfck on Fri Mar 29, 2013 2:34 pm

    will the caps 1.0" D x 1.4" L fit VTA driver board ?

    poconoman

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    Re: Coupling cap options in Dynaco tube amps

    Post by poconoman on Fri Mar 29, 2013 7:40 pm

    So, on an original ST70 board, 2X.22uf 500v and 4X.1uf 630v green PIO?

    Bob Latino
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    Re: Coupling cap options in Dynaco tube amps

    Post by Bob Latino on Fri Mar 29, 2013 7:48 pm

    simonfck wrote:will the caps 1.0" D x 1.4" L fit VTA driver board ?

    On the VTA driver board you have a 1.62+ inch or about 1 5/8 inch spacing between the two outermost solder points. A 1.40 inch cap should fit in there OK and still leave room to bend the leads down to fit the hole spacing. The one inch diameter is fairly thick for a coupling cap but you should be able to install these caps in C5 - C8 OK ..

    Bob

    simonfck

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    Re: Coupling cap options in Dynaco tube amps

    Post by simonfck on Fri Mar 29, 2013 11:12 pm

    Bob Latino wrote:
    simonfck wrote:will the caps 1.0" D x 1.4" L fit VTA driver board ?

    On the VTA driver board you have a 1.62+ inch or about 1 5/8 inch spacing between the two outermost solder points. A 1.40 inch cap should fit in there OK and still leave room to bend the leads down to fit the hole spacing. The one inch diameter is fairly thick for a coupling cap but you should be able to install these caps in C5 - C8 OK ..

    Bob


    Thanks Bob,


    sKiZo

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    Re: Coupling cap options in Dynaco tube amps

    Post by sKiZo on Wed Apr 03, 2013 5:14 pm

    Hi Bob! Is it here yet??

    (Just checked, and according to OOps!, the amp is in Maumee OH and should get here tomorrow ... little early to head out and tap my foot by the mailbox ...

    One concern I've seen with the PIO caps is the metal shell shorting out or arcing to nearby components. I know careful planning will help, but do you think an extra layer of insulation would be a good thing?

    I got some of this stuff and was thinking of painting it on the caps before installing them ...



    Experiment a bit first to see how smooth a coat I can get so it looks purty ... and I get my choice of colors! Maybe disguise them as bumblebees. ;-}

    Bob Latino
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    Re: Coupling cap options in Dynaco tube amps

    Post by Bob Latino on Wed Apr 03, 2013 5:29 pm

    The outer metal case on the K40-Y caps is conductive - BUT - if set up as the VTA board manual suggests, there is no real danger of the outer case touching the driver board. The VTA board manual tells you to cover the leads with insulated tubing (supplied) and then bend the leads as shown in the photo below. If you do this, then the two leads will drop right into the two outer eyelet holes for the Russian caps. You solder them in like this and they are held about 3/16 of an inch above the board. They will not touch the driver board if set up like this. Last week I took off the bottom cover of my own ST-120 to check some voltages using the Weber WZ68 vs a GZ34 tube rectifier. The Russian caps in my own amp are still there in the same position about 3/16 of an inch above the board. The Weber WZ68, by the way, gives about a 12 to 15 volt increase in the B+ voltage which necessitates a slight bias adjustment on the 4 output tubes.

    If the outer metal case still bothers you, then wrap some black electrical tape around the outer metal case.

    Bob




    Last edited by Bob Latino on Wed Apr 03, 2013 5:32 pm; edited 1 time in total

    poconoman

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    Re: Coupling cap options in Dynaco tube amps

    Post by poconoman on Wed Apr 03, 2013 5:31 pm

    I'd wrap some electrical tape on the PIOs.

    hawaii.ken

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    Re: Coupling cap options in Dynaco tube amps

    Post by hawaii.ken on Wed Apr 03, 2013 6:07 pm

    poconoman wrote:I'd wrap some electrical tape on the PIOs.
    How about some clear heat-shrink instead.

    sKiZo

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    Re: Coupling cap options in Dynaco tube amps

    Post by sKiZo on Wed Apr 03, 2013 6:23 pm

    I kinda like the idea of painting them up like bumblebees ... that'd confuse some people ...



    So, like ... tell us again why you put those freakishly huge resistors on your VTA board?

    Probably worth mentioning ... I saw a post somewhere or other about making sure the leads don't stick out too far on the top of the board as they can short on the chassis ... shouldn't be an issue if you bend them as per the instructions.

    Wonder if heat would be an issue? Add a layer of ... whatever ... and you wouldn't get the same heat transfer to air? Maybe that's why they build em bare metal in the first place?

    poconoman

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    Re: Coupling cap options in Dynaco tube amps

    Post by poconoman on Wed Apr 03, 2013 6:54 pm

    hawaii.ken wrote:
    poconoman wrote:I'd wrap some electrical tape on the PIOs.
    How about some clear heat-shrink instead.

    I don't see why not.

    tubes4hifi
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    Re: Coupling cap options in Dynaco tube amps

    Post by tubes4hifi on Wed Apr 03, 2013 9:22 pm

    recently I've been using black heat shrink, but I should switch to clear ! Smile Or maybe a custom paint job!!
    Electrical tape in various colors! red, green, yellow, blue, hey, it's really a 100w resistor!! lol!

    sKiZo

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    Re: Coupling cap options in Dynaco tube amps

    Post by sKiZo on Wed Apr 03, 2013 9:51 pm

    Why stay 2D when you can go holographic?



    I made some inset panels out of the stuff for the saddlebags of my bike years ago ... really lights up.



    hawaii.ken

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    Re: Coupling cap options in Dynaco tube amps

    Post by hawaii.ken on Thu Apr 04, 2013 1:40 am

    lol Laughing

    Bigron865

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    Re: Coupling cap options in Dynaco tube amps

    Post by Bigron865 on Sat Apr 06, 2013 9:17 am

    Just a couple questions for all you experienced guru's. lol! Ok... Got new EH6CA7's new trio of EH12AU7's and wl be adding the upgraded quad cap and the K40y PIO caps next week. From what I've read, these tubes need time to burn in and so do the caps. How/what will I perceive when they are broken in? Will it be a BAM! Kind of awakening or something very gradual? What have your experiences been with burn in on these tubes and caps?

    Thanks, Ron

    Bob Latino
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    Re: Coupling cap options in Dynaco tube amps

    Post by Bob Latino on Sat Apr 06, 2013 9:37 am

    Hi Ron,

    In general coupling and interstage caps burn in slowly over a period of maybe 100 to 200 hours. This time period can vary widely depending on the cap.

    Re: The Russian PIO main coupling caps ... I find that the larger the voltage rating the SLOWER the cap takes to burn in. On the VTA amps you can use 400, 500, 630 or 1000 volt rated main coupling caps. The 1000 volt rated caps are physically larger and really a bit TOO large for the area so I don't use them any more and stick to the lower rated voltage caps. The 1000 volt rated caps use to (seem to) take forever to break in while the 400 volt rated caps broke in sooner. When the caps break in something happens inside the cap on the molecular level after the passage of an audio signal for many hours and the sound seems to smoothen out and become more "effortless" and more "natural".

    I am going to get in contact with Jim McShane and see if he will comment on tube break in in this post.

    Bob


    Last edited by Bob Latino on Sat Apr 06, 2013 9:58 am; edited 1 time in total

    Bigron865

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    Re: Coupling cap options in Dynaco tube amps

    Post by Bigron865 on Sat Apr 06, 2013 9:56 am

    Thanks Bob. You are so helpful. I'm really enjoying this forum and the amp.

    Jim McShane

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    Re: Coupling cap options in Dynaco tube amps

    Post by Jim McShane on Sat Apr 06, 2013 5:02 pm

    Tube break in is a controversial subject. There is no question that something happens, and there are theories as to what it is. But there is not much concensus beyond that.

    A few of the theories make some sense to me - first off, even a brand new current production tube (let alone a 30-40-50 year old tube) undergoes some changes on the surface of the cathode with use. Contaminants boil off and expose fresh cathode surface which improves the ability of the cathode to emit electrons, and also provides the getter an opportunity to capture loose contaminants liberated from the cathode and other internal parts during those first hours of operation. Those processes certainly will affect tube operation.

    A second theory I find plausible is the idea of internal parts "bedding in", that is to say they are establishing their final alignment, positioning, etc., as the tube goes through heating/cooling cycles, is exposed to small vibrations, etc. Again, to me that makes sense.

    There are others, but I think the two I mentioned seem to be the soundest (no pun intended) and achieve a modicum of acceptance.

    Most people say the break in type processes I've described so far take somewhere between 10 and 100 hours to occur. Why the wide range?

    I believe that much - if not most - of tube "break-in" is simply the listener becoming adjusted to the differences in tonal balance between the new tube and the tube it replaced. It's quite common for tubes to sound quite different, and it takes a period of time for your brain to accommodate the revised presentation.

    I've heard audiophiles attending live music performances say things like "the top end is too soft", "the bass is weak", and so on. Yes, the acoustics of the venue have some effect - but most often the comment is made while mentally comparing the live sound to the home system sound they have become accustomed to.

    This phenomenon could account for the wide range of "break-in" time reported by different listeners. Of course, what the tubes are used in will affect the process as well; some gear babies them, some does not. And that could make a difference as well.

    That's my $0.02 worth anyway!

    sKiZo

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    Re: Coupling cap options in Dynaco tube amps

    Post by sKiZo on Sat Apr 06, 2013 5:19 pm

    Me at a live performance ... "It's not loud enough!"

    As far as your 2¢ worth goes, I think we got a bargain. Also, you didn't mention the word "psychoacoustics" once ... and for that, we thank you. bom



    Bigron865

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    Re: Coupling cap options in Dynaco tube amps

    Post by Bigron865 on Sat Apr 06, 2013 9:17 pm

    Thanks Jim. Makes sense. It seems that one the upgrade bug bites, we or I tend to keep trying to make it better and expecting big changes to achieve the perfect sound nirvana (which may never be attained) but it sure is fun trying!

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