Radio Craftsmen mono amps

Discussion in 'Tube Audio' started by Normo, Aug 17, 2013.

  1. jazbo8

    jazbo8 Well-Known Member

    Messages:
    823
    What's the bias setting on the 500A? Perhaps it still has a decent amount of class A output, out of the total class AB output
     
  2. jon_s

    jon_s AK Subscriber Subscriber

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    1,207
    Location:
    Florida
    It's very hot! 60mA is about the coldest mine will bias, which is right around maximum plate dissipation for a KT66 (~25W). They have no usable AB output, perhaps at most a few Watts for 1950s marketing purposes.

    Edit- And, the OPTs are 8.6k, where for AB you might expect half that.
     
    Last edited: Aug 11, 2015
  3. jazbo8

    jazbo8 Well-Known Member

    Messages:
    823
    That's pretty much class A, so much for my speculation...:no:
     
  4. tcdriver

    tcdriver AK Subscriber Subscriber

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  5. trobbins

    trobbins Active Member

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    tcdriver likes this.
  6. tcdriver

    tcdriver AK Subscriber Subscriber

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  7. bobla

    bobla New Member

    Messages:
    1
    Hello C500A owners, I have some good news about replacement power transformers for this model. The Stancor PC-8414, listed in Sams as the replacement for the 500A power transformer (with a requirement that new mounting holes need to be made), has become very difficult to find. After communicating with Hammond/Dahl transformers, they have decided to release a new power transformer model, #284X, which is an almost exact replacement for the 500A power transformer; even the mounting holes appear to fit perfectly to the originals! And, if the high voltage measures slightly too high, Hammond has included a 125V primary tap in addition to the 115V tap. The new transformer model also has TWO 6.3V 3A heater windings, besides the 5V 3A winding for the 5U4. You can either parallel the 6.3V windings, or, if you still would desire to have the HV somewhere in between what is possible with the 115 or 125V primary taps, you can use one of the 6.3V windings in an out-of-phase series connection with either of the primary taps (only one tap at a time, please), to achieve the desired HV. The only thing missing from the original transformer is a center tap for the 6.3V winding(s). You can pretty much duplicate this by adding two 100 Ohm 1/2 Watt resistors, connected in series across one of the 6.3V heater windings, and the mid point of these two series resistors connected over to the .25 mFd heater center tap bypass capacitor. If you're creative, you can accomplish this using only the resistor leads, from the right most output tube heater pins 2 and 7 over to the terminal strip tie point that used to connect to the old transformer's heater winding center tap. I understand that this transformer will be available in about 4 weeks from Digi-Key under their part number 284X-ND at a cost of $112.50 each. We have to keep these great amplifiers running!
     
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  8. tcdriver

    tcdriver AK Subscriber Subscriber

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    Wow, that is great news indeed. Thank you for letting us know. :thumbsup:
     
  9. sheltie dave

    sheltie dave Addicted Member

    Messages:
    6,517
    Location:
    St. Louis
    I hooked my RC500a up last night with the Fairchild 245, and got gobsmacked yet again. Ran it through a Klipsch ksb1.1, then an Altec Carmel. Played an early Motown collage, and the sound is so breathtakingly clear and right I had tears in my eyes. Wow!

    My test bench listening is not a posed photo, so please ignore the getting things outta the way clutter.

    [​IMG]
     
    Last edited: Apr 21, 2017 at 8:20 AM
    radioactive and soundmotor like this.
  10. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

    Messages:
    6,002
    Location:
    Hudson Valley, NY
    Sounds like a rare combination of rather special components there. Making me want to put my 500A amps into play this weekend.
     
  11. soundmotor

    soundmotor super modified Subscriber

    Are those cabinets porcelain over steel?
     
  12. sheltie dave

    sheltie dave Addicted Member

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    Yes, they are the original Crane cabinets from 1957, Soundmotor.
     
  13. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
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    Location:
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    Now all you need in the Kitchen is a GE Americana Double Oven Range.
    12_geovens_lg.jpg

    I paid $50.00 for a '74 model in yellow "gold". Replaced the upper oven gasket, timer/clock, and the upper oven element after two years. Only time I use the lower oven is Thanksgiving and Christmas and when I clean the upper oven. Pull off the sides, back and bottom panels, stuff in the lower oven and set to clean. Only have to clean the top of the upper oven. Even has a built in 2 spd exhaust fan with a HUGE motor. Damn thing is practically indestructable. Other than 2 nicks in the porcelain on the front edge that are less than 1/4" diam it's pristine.

    Too bad they didn't make them for gas.

    Larry
     
    Last edited: Apr 21, 2017 at 1:35 PM
  14. soundmotor

    soundmotor super modified Subscriber

    Fantastic condition from the pic too. It's amazing they did not get ripped out during a remodeling binge along the way. Call me officially envious.
     

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