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    50-25-25-25 cap board for Mark III

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    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    50-25-25-25 cap board for Mark III

    Post by j beede on Wed Mar 02, 2011 9:57 pm

    I've built a nice high voltage 50-25-25-25µF under chassis cap board to replace the quad cap cans on my Mark IIIs. Should I put the 50µF segment on the GZ34 side of the choke or on the "away side" of the choke? FYI: I have already inserted 1n4007 in series with pins 4 and 6 on the rectifier sockets.
    ...j

    tubes4hifi
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    Re: 50-25-25-25 cap board for Mark III

    Post by tubes4hifi on Wed Mar 02, 2011 10:20 pm

    IMHO, I wouldn't connect the 50uF to the output of the rectifier. Most rectifier tubes have a maximum rating of being connect to 50uF, but why push it. You may lose 3 or maybe 5vdc by using one of the 25uF sections first.
    I assume you are then going thru a choke or resistor, into the 2nd cap section (and 3rd and 4th).
    Great using the diodes, that will also protect the tube rectifier.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Thu Mar 03, 2011 4:17 pm

    I thought most 5AR4/GZ34 were rated for 60µF on pin #8. I am currently using Sovtek 5AR4 and haven't been able to track down a datasheet for that tube. So would you put the 50µF on the "far side" of the choke away from the 5AR4 (where the Mark III has 20µF from the factory) or would you drop the whole board down to 25-25-25-25µF?

    Anyone one out there regularly using 50µF or more on pin #8 of their 5AR4?
    ...j

    Bob Latino
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    Re: 50-25-25-25 cap board for Mark III

    Post by Bob Latino on Thu Mar 03, 2011 5:00 pm

    J,

    If you are going to use that cap board I would do it 25, 50, 25, 25 with the 50 uF AFTER the choke. That said, If you use the 80, 40, 30, 20 uF 550 volt F&T quad cap you would have more capacitance and your Mark III would also look more original with a quad cap sticking up like on the original amp.

    Bob

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Thu Mar 03, 2011 9:53 pm

    Bob,
    The board I built is 4" x 2.5" 1.2" it will be hidden inside the chassis under the PT like the Stokes board. The board is mounted to the #8 screw that usually holds the cable clamp for the PT. I disconnected the quad cap and am leaving it in place to maintain the stock look. I know you are a proponent of the quad cap--it is my second least favorite Dynaco component after the primitive sliding power switch Smile I'll post a picture once I figure out the hosting stuff. Isn't the 5AR4 rated for 60µF? I gather you guys have learned that this is optimistic?
    ...j

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Fri Mar 04, 2011 12:46 am

    Can you see this? First attempt at posting a photo.
    ...j


    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Fri Mar 04, 2011 1:07 am

    ...and here it is "tacked" in place for a trial run. As suggested I put the 50µF cap on the side of the choke away from the 5AR4. Amp sounds quieter for sure after taking that ~48 year old quad cap out of the circuit. I will leave the disconnected quad cap on the chassis for nostalgic reasons only. Now I need to make a second copy...


    tubes4hifi
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    Re: 50-25-25-25 cap board for Mark III

    Post by tubes4hifi on Fri Mar 04, 2011 3:04 am

    nice job! I agree with Bob on the hookup, you'll still get all the power you want but reduce power-up stress on the tube. Yes, I checked tube data and they are RATED at 60uF but do you see how many posts there are about them going bad? Play it conservative and it will last years instead of weeks.
    Now all you need is our VTA MK3 upgrade, same exact circuit used in our M125 monoblocks.
    www.tubes4hifi.com/MK3.htm

    stewdan

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    Location : Houston Texas

    Re: 50-25-25-25 cap board for Mark III

    Post by stewdan on Tue Mar 08, 2011 11:10 am

    J,

    Where did you get the 25MF @ 450V Caps? Are they Snap-In Types or something else?

    Can you post a picture of the underside of the perf board?

    Stew

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Tue Mar 08, 2011 4:51 pm

    stewdan wrote:J,

    Where did you get the 25MF @ 450V Caps? Are they Snap-In Types or something else?

    Can you post a picture of the underside of the perf board?

    Stew

    Stewdan,
    The caps are 47µF, 450V with radial leads. Wired in series with balancing resistors they should be good for about 900 WVDC. Now I can try high B+ via bridge rectification and KT-88s without "boiling" the quad caps. I just bent the cap leads to wire them point-to-point on the back of the perf board. The boards are busy making music at the moment, but I'll try to remember to photograph the backside next time the amps are on the bench. The boards measure 4" x 2.5" x 1" and they fit perfectly between the input board and fuse holder in my Mark III amps. These caps are short and fat so there is plenty of clearance to the bottom cover. Mark III owners should just say "no" to 525V quad caps! I may even pull the quad cap "decoys" off my Mark IIIs so I can better see the 6550s glow!
    ...j

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Cap Board Resistors

    Post by stewdan on Tue Mar 29, 2011 3:17 pm

    J,

    What value resistors are you using? I am having trouble reading the colors in the pictures. They also change color when I print the picture, so I need to ask.

    Thanks for the picture of the underside of the perf board which I saved to my hard drive. I saw it originally in the Forum but then it disappeared. Stew

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: 50-25-25-25 cap board for Mark III

    Post by j beede on Tue Mar 29, 2011 11:38 pm

    stewdan wrote:J,

    What value resistors are you using? I am having trouble reading the colors in the pictures. They also change color when I print the picture, so I need to ask.

    Thanks for the picture of the underside of the perf board which I saved to my hard drive. I saw it originally in the Forum but then it disappeared. Stew


    I used 220K, 1/2W, 2%. I think Sheldon Stokes uses 330K on his board. I continue to enjoy my reworked Mark IIIs. I had never heard Mark IIIs before finishing this set.




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