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Next up a PSE Flea powered amp - the Mighty 71-A

Discussion in 'Tube Audio' started by analog addict, Feb 19, 2018.

  1. analog addict

    analog addict Glory or Death!

    Messages:
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    After the usual long hiatus, I'm embarking on the next proof of concept, a parallel single ended 71-A amp featuring a stout power supply with an oil filled cap and two chokes, a voltage regulated plate supply for the dual 6SN7 front end, independent center tapped filament supply for each output channel, and three, count 'em three 71-A output tubes per channel, combining for a mighty 2 to 2.5 WPC.

    Why? you ask. Well, first off, because. Secondly, to prove something I've discovered with my last build, a PSE 45 featuring two 45s per channel. IMO, the 3.5 watts generated by the PSE amp has more oomph and bottom end than a similar single 2A3 per channel. Thirdly, because the rumor is that as you go down the old 4 pin triode list, the lower the output of the tube, the sweeter the sound. So far, I believe this is true, so this experiment to see if the trend continues. Anyway, here goes...

    A mock up of the layout....

    [​IMG]71 A mock up by Analog Addict, on Flickr

    The mostly cut chassis, deburred.

    [​IMG]71-A build by Analog Addict, on Flickr

    The corner welds filed down to provide a seamless surface for painting.

    [​IMG]71-A build by Analog Addict, on Flickr


    I had to cut the power switch and power light holes myself, which I hate doing, since I can't drill a hole to save my life. Here's what I came up with...

    [​IMG]71-A build by Analog Addict, on Flickr

    These bulb holders are cool, because as I understand it, they were originally developed for WWII planes, so that the light can be closed off to help avoid detection. Stealth lighting to go with the missile switch.

    The painted chassis, inside and outside, featuring a hammered black all in one paint.

    [​IMG]71-A build by Analog Addict, on Flickr

    [​IMG]71-A build by Analog Addict, on Flickr

    And some of the components painted a gloss black...

    [​IMG]71-A build by Analog Addict, on Flickr

    [​IMG]71-A build by Analog Addict, on Flickr

    Next up, populating the chassis....
     
    asilker, jeb98, Ziradog and 7 others like this.

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Very interesting! And a very purdy chassis too.......

    Dave
     
  3. analog addict

    analog addict Glory or Death!

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    Thanks Dave, High praise coming from you. I'll post the power supply schematic when I lay my hands on it, but since I've used some of Dennis Had's circuit ideas, he's asked me not to post the actual schematic. Perhaps I can post a redacted version....
     
  4. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    We likes it, preciousss. Very nice indeed.
    I just finished a PSE EL84 with Edcor transformers. I was somewhat surprised that the paralleled output tube circuit sounds so excellent. Some believe that the paralleling of output tubes would create a blurring of the sound since the outputs are split then combined again. I found that this was not true at all. In fact the sound is the same if not better than the single output tube just twice as powerful.
    What kind of OPT will you be using? Happy soldering.
     
    Last edited: Feb 19, 2018
  5. analog addict

    analog addict Glory or Death!

    Messages:
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    I have two pair of Transendars that I've been hanging onto. This build will use one of them...
     
  6. nerdorama

    nerdorama AK member Subscriber

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    Where did you source your chassis box? I'll enjoy watching your project.
    Thanks,
    John
     

     

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  7. analog addict

    analog addict Glory or Death!

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    That came out of Dennis Had's shop. Afraid I don't have the answer for you
     
    s-petersen likes this.
  8. Joe Roberts

    Joe Roberts Agent Provocateur Subscriber

    Messages:
    1,174
    Location:
    DC
    Excellent plan! 71A is an outstanding tube. If you like 45s, you might like this tube even more.

    Try some globe 71As for extra psychedelic sound!
     
  9. x3workshop

    x3workshop AK Subscriber Subscriber

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    Watching with great interest!
     
  10. Mike Stehr

    Mike Stehr Poverty Audio

    Messages:
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    Location:
    East Wa.
    I had a pair of LaScalas that I cobbled together years ago. I know everyone has their preference, but the only tube amp circuit that I preferred with the LaScala was a SRPP 6SL7/71a circuit I pilfered off the internet and made a bread-board. The filaments were DC heated with old Xentek regulated power supplies, with James universal OPTs.

    My AC heated 6EA7/bridged Allied IT 2A3 SET amp had a good sound with the LaScalas, but there was a organic tonality about the 71A circuit that just lashed up better with those speakers.
    SEP 6BQ5 amplifiers sounded good, but in kind of a "meh" way...

    At 800 milliwatts, the circuit would only get so loud, and ran out steam pretty quick, but the sound/tonality was amazing. 71a breadbored.jpg
     
    drtool likes this.
  11. analog addict

    analog addict Glory or Death!

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    Again, that's the point of the parallel outputs. All the 71-A goodness with the power of a 45...:rockon:

    I've used a SRPP front end in the first iteration of my 2A3 amp. I decided it really was lacking something I couldn't quite put my finger on. Took it out when I rebuilt it and went with parallel 7308s, i.e. one tube with both sides in parallel for each channel. It was a proof of concept for this amp, which will feature a parallel/parallel design. One 6SN7 in parallel for the front end, and three 71-As in parallel for the amp section.

    Heh, I'm all for full blown psychedelic sound. Unfortunately, I don't have three pairs of the globe 71-As yet. I think currently I have two pair. But I'm still looking.....
     

     

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  12. Mike Stehr

    Mike Stehr Poverty Audio

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    Grunt/drive, from what I recall...
     
  13. kcline

    kcline Tube Audio Collector Subscriber

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    very nice will be watching. Globe 71-A are not that hard to find, just good ones are $$,duds have worth if globe shape. at lease you do not need 71 not A those are very hard to find good
     
  14. analog addict

    analog addict Glory or Death!

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    AlTinkster92, Leland47 and x3workshop like this.
  15. epifanatic

    epifanatic acoustically suspended

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    :lurk:
     
    Paully likes this.
  16. Ziradog

    Ziradog AK Subscriber Subscriber

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    First time in the history of hi-fi that this emoji :rockon: and 2.5 watts were in the same sentence.
     
    drtool and Leland47 like this.

     

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  17. analog addict

    analog addict Glory or Death!

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    more progress. I gotta get over to Dennis' shop on the other side of town to pick up the OPTs. His set up man was painting them for me...

    [​IMG]Untitled by Analog Addict, on Flickr
     
  18. analog addict

    analog addict Glory or Death!

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    Ziradog likes this.
  19. IPADave

    IPADave Which one's Pink? Subscriber

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    Lookin' good! :beerchug:

    -Dave
     
  20. drtool

    drtool It might get loud In Houston Subscriber

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