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    Project not going well

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    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Project not going well

    Post by Tom_Johnson on Wed May 09, 2018 4:39 pm

    I am building a power amplifier with 6CZ5 and 6GH8 tubes. I am using a Knight KA-55, Stereo 40 amplifier that probably never worked because it was wired wrong. I purchased two St-70 boards from ebay. They came from tubes.rs and look real good. My output transformers are 6.6k and were checked by injecting 6.36 volts into the 8 ohm taps. Nothing smoked and I observed 185v and 184v between the plate windings and 2.18v from each center tap.  My problem is simple I am sure but not for me. My voltage on the left channel is way off and after the 820k resistor I get only 18v. On the right channel I get 48v. should be around 40v and I will take care of that after I figure the left channel problem out. I went ahead and built the second board and switched them out to no avail, it did the same thing. I am going back over the boards tonight. There is a short somewhere.
    I am not allowed to post a link for the schematics used to build the PC3 board and the amplifier. Maybe an admin can give me permission to post the link. It is an RCA high fidelity design for 7199 and 6973 tubes. If any of you have any thoughts, ideas or comments please share them.
    Thanks, Tom
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    Peter W.

    Posts : 709
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Project not going well

    Post by Peter W. on Thu May 10, 2018 8:15 am

    Two suggestions, generic rather than specific.

    a) Put everything aside for a minimum of 24 hours - 48 is better. What happens is that you become too close to the problem such that you will gloss over something otherwise obvious. If you have a friend and/or "Elmer" that can go over the boards and what you have - they will be far more likely to spot the error than you are.

    b) When you step back into it, start at the beginning and go step-by-step. Look for physical issues such as solder blobs, cold solders and similar. Also look for misplaced components, mis-read resistor values and similar simple errors.


    TAKE YOUR TIME and remember: This is a hobby and so should be fun!

    Dale Stevens

    Posts : 89
    Join date : 2014-07-06
    Age : 69
    Location : Loris, SC

    Re: Project not going well

    Post by Dale Stevens on Thu May 10, 2018 2:50 pm

    Welcome, Tom. Stick with the folks on here and we will get you fixed. Several goto guys on the forum which
    Peter is right on top of the list. Dale

    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Re: Project not going well

    Post by Tom_Johnson on Thu May 10, 2018 4:43 pm

    Thanks for the encouragement guys. I did step away from this project for a few days, still thought about it though. Am am an amateur, I understand electricity, the dangers involved, know how to read a schematic and I know how to solder. Just enough to be dangerous, ha ha.  I checked the resistors and capacitors and solder points, all were good. I had the 6v power light hooked to the right side, disconnected it and that wasn't it. I might switch sides L to R and see if the problem follows. All supply voltages are spot on or a couple volts below specs.

    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Re: Project not going well

    Post by Tom_Johnson on Thu May 10, 2018 5:35 pm

    Here are the schematics I am working with. Thanks







    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Re: Project not going well

    Post by Tom_Johnson on Fri May 11, 2018 12:02 pm

    Here are a few pictures of what I am working with. I am going to enjoy my birthday weekend, my wife is taking me away for a Surprise Birthday Party. Secrets are hard to keep or easy to see. Thanks






    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Re: Project not going well

    Post by Tom_Johnson on Fri May 11, 2018 12:42 pm

    This is the schematic I am basing the the amp on.



    Tom_Johnson

    Posts : 6
    Join date : 2018-05-08
    Age : 61
    Location : Michigan

    Re: Project not going well

    Post by Tom_Johnson on Mon May 14, 2018 10:55 pm

    Well Heck, I may have figured it out. Kind of late to try it though. There is supposed to be a jumper wire between Tube Pins 6 and 9 on the side I am having problems with. I thought they were test points. Left side in the picture. Right side on my amp because the board fit better that way. I have attached a picture of the driver board.
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    Peter W.

    Posts : 709
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Project not going well

    Post by Peter W. on Thu May 17, 2018 9:11 am

    We all knew you could do it. Just took a bit of space!

    Congratulations!

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