The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Driver Board Cap Question

    Share

    harpy

    Posts : 94
    Join date : 2012-02-04

    Driver Board Cap Question

    Post by harpy on Sun Jun 24, 2012 12:25 am

    Say on a stock Mk III does the .1uf need to be the same type of cap as the .25uf coupling caps?

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Driver Board Cap Question

    Post by Bob Latino on Sun Jun 24, 2012 7:49 am

    No - You can use most any kind of cap for that .10 uF cap. Dynaco used 600 volt rated caps for the two .25 uF main coupling caps but I believe that .10 uF cap was 400 volt rated. So just make sure that the .10 uF cap you use in that position is rated 400 volts or higher.

    Bob

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Driver Board Cap Question

    Post by PeterCapo on Sun Jun 24, 2012 1:31 pm

    Yeah, that 0.1uF cap was rated for 400VDC. Another consideration is the proximity of this cap to the 6AN8 tube on the circuit board. I've never measured the temperature right next to this tube, but I wouldn't want to grab it while it is operating. Just about any capacitor would probably still work even though it might be getting baked, but why stress your parts any more than necessary?

    A radial leaded capacitor here would stand up higher and closer to the tube, so I suggest using an axial-leaded capacitor of 400VDC rating. The 600VDC parts tend to be bigger and would sit a little higher. I might avoid polystyrene and almost certainly avoid a wax-filled capacitor here. Generally speaking, a 400VDC axial polypropylene or Teflon cap would probably fare best in terms of avoiding and tolerating the heat from the 6AN8.


    Last edited by PeterCapo on Sat Nov 10, 2012 6:30 pm; edited 1 time in total

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Driver Board Cap Question

    Post by harpy on Sun Jun 24, 2012 11:33 pm

    Good info as always, Thanks. Here is a mock up. I would cut the leads and mount wire leads (Dynakit hookup wire) at the center, but I am a little worried about them being so close to the KT-88s. Would they get any feedback, or too much heat from the tubes? The Russian Teflons are just nutty big and forget about putting one in front.







    Some different angles.




    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Driver Board Cap Question

    Post by PeterCapo on Mon Jun 25, 2012 10:25 am

    Well, I don't see any reason not to give them a try especially since you already have them. Since the two big ones are Teflon and military spec, I suppose they would be among the best at tolerating any heat. I don't think there would be any issues with arcing, unless uninsulated leads get too close to grounded surfaces including the capacitor's body if it is in contact with the chassis anywhere.

    I don't know if you need to cut the solder lugs (if that is what you meant). Just solder the wire to the lug and then insulate it with a couple of layers of heat shrink tubing. If you don't have a heat gun, there should be some scaled-down models available for shorter $. Or, maybe just some plain Teflon tubing might be better here.

    Before soldering, I would take some scotchbrite or steel wool and clean the solder lugs so the solder will flow and bond well – clean the lugs with isopropyl after resurfacing them to remove any scotchbrite or steel wool particles.

    It looks like you already have a piece of paper under the body. Maybe also use a strip of electrical tape along the bottom-facing side of the capacitor body plus some paper or whatever, like you already have. I'd be pretty thorough about this.

    Similarly, I'd consider finding a way to secure the big Teflons in place so they can't move around. Their metal bodies could possibly short the leads of other components or the eyelets. I'd take similar precautions (the tubing) so the solder lugs can't short to anything like the tube bases, chassis, etc...

    The K40Y-9 looks fine in the 0.1uF position. Should sound nice when it's done! Good luck!

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Driver Board Cap Question

    Post by harpy on Mon Jun 25, 2012 8:46 pm

    I'll give it a try. I was going to mount them with 3M 4941 VHB Acrylic foam tape. It will support 90 lbs. per square in, is good till 300 degrees f, and is UL listed. The tape is .45mm think and I may stack two pieces to give it a little more height off the board. What's on there now is the backing paper of the tape. They still need to be trimmed the length of the cap.
    I was thinking of cutting down leads about of a 1/4" and drilling a new mounting hole to run the wire. Once soldered, shrink tubed. I was going to give up on them all together because they didn't fit on the stock board, but the Dynakit board packs the front resistors tighter. They are kind of ugly, but I am out of real-estate underneath.
    Someone said on another Forum they sounded better cutting off the metal case off. It looked like a marshmallow. Anyway..... Thanks for the help.

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Driver Board Cap Question

    Post by hawaii.ken on Mon Jun 25, 2012 10:54 pm

    Can they be mounted on the bottom of the board?

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Driver Board Cap Question

    Post by harpy on Tue Jun 26, 2012 9:08 pm

    Yes, but it is still a tight squeeze as they have to be fairly close to the board to avoid contact with the bottom cover. They are also very heavy and I wouldn't trust the industrial tape as the sole mounting method. This gives you a rough idea of them underneath the board.



    To be honest it's probably not worth the effort above or below. I am still waiting on two that are somewhere between Ohio and Moscow. If they haven't showed by the time I get the second amp built, I may just use K-40's all the way around and be done with it. I had seen pictures and knew the dimensions, but didn't quite grasp their shear size. I think the saying is: "Well it looked good on paper."



    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Driver Board Cap Question

    Post by hawaii.ken on Tue Jun 26, 2012 9:39 pm

    You could try a layer of clear heat-shrink around the body of those caps and attaching them to the board with stranded wire. Then use your foam tape on the bottom cover to add some support (sticky side to the bottom cover and don't remove the paper from the side facing the caps).

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Driver Board Cap Question

    Post by harpy on Wed Jun 27, 2012 3:40 pm

    Thanks Ken that's a good idea. I hadn't though of shrink tubing the the cap. I picked up some 1.5" clear shrink tubing and will explore that option further. Very Happy

    Sponsored content

    Re: Driver Board Cap Question

    Post by Sponsored content Today at 7:28 pm


      Current date/time is Sun Dec 04, 2016 7:28 pm