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steefdebruijn

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Posts posted by steefdebruijn

  14. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted

    Why don´t you buy a good matched quartet and tests? You have several websites with good prices for JJ EL34


    I thought I did that already but seems not the case. I deliberately avoided vague eBay sellers and went for a seemingly reputable seller (not through eBay) in Germany. And maybe the tubes *are* okay and something else is wrong with my build. That is why I seek for alternative measurements on the tube side as well as on the amp board side. Will ponder about buying extra tubes though. Any suggestions for sellers in Europe?


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  18. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted · Edited by steefdebruijn
    added 30+40k option to get to 300vDC

    Question: Is it a good or a bad idea to take the 7812 off board to a bottom-of-case-mounted 532-529802B25G (at 3.7c/w) for example? (assuming the 7812 produces the most heat of the 7812/7912 pair). The 7912 stays onboard on the 588-EA-T220-64E in this setup.

    Question: should I overshoot or undershoot the 300vDC? If I calculate correctly I get 292vDC with 18k drop resistor (too low according to Jose 's findings) and 310vDC with 16k drop resistor. Or should I parallel the current 21k resistor with about 100k to get 17k36 drop resulting in about 298vDC? [edit] combining a 40k+30k gives me 17k14 resulting in 300vDC so I think I should go for that option.

    Update: I consulted with a local tube seller for testing/matching my tubes to create a box of 12 good-matched tubes.

  20. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted

    Thanks Jose, I learn a lot by reading a lot but it takes time to sink in ?

    DC offset one channel -23vDC (shifted to -40 after a while) and +1vDC, other channel -52vDC (also drifting to -35) and -13 vDC, this results in balances of 23-40vDC for both channels.

    I read here that too low 300v line can cause things to oscillate. So maybe I should lower the drop resistor to about 18K first before measuring again?

    I did not measure my tubes myself but did buy matched stuff from banzai-music in Germany. So I really want to rule out improper matched tubes ?

    About the LV I did see that mw used a 12v secondary supply for his recent build so I think I should go that route. A bigger heatsink on the LV will not fit on the board.

    Yes I use the LV for the 12Ax7 tubes (x as in X and U ? )

    Thanks for the help

    Steef

  21. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted

    Yep,  each channel with ground. 
    Sorry I got confused (HV vs LV) I used a large headsink on my Square maybe you can used this  (mouser reference 588-EV-T220-64E ) on LV  
     


    I used 588-EA-T220-64E. At 8c/w vs your 10c/w but should I worry about the 60 deg celcius? I guess I should use a lower sec winding voltage transformer then. Maybe 12-13vac or so?

    To be sure what I am supposed to measure on outputs. As far as I understand the outputs are balanced outputs. So a “+” and a “-“ pin per channel - no gnd output. I measured dc between these pins. If I measure pins to gnd I get 2 measures per channel.


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  22. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted

    I answer based on the little knowledge I have.
    1. Yes 
    2. You need lower resistor a bit 
    3. Yes
    4. offset is measured with each channel with respect to ground. I'm not sure what you're doing (I think you're measuring the balance)
    5. I use a large heatsink. maybe it would be convenient for you to think about that when you do your chasis. 
    First check the offset, if it is low and the Bias is correct you should not have problems connecting your stax.
     


    Thanks for the quick response. I guess I should lower the resistor to about 17k5 then if I calculate correctly.

    I measured the DC on the output pins per channel. Should I measure each pin against gnd?

    The LV heatsink stands tall on the PCB so can’t bolt on chassis.

    Steef


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  23. Megatron Electrostatic Headphone Amplifier

    in Do It Yourself

    Posted

    Hi all,

    After a l.o.n.g time first start of my Megatron, fully bare version for now. Scary as hell but no cracks, fumes, funny smells etc. Pfew. I did check PSUs with variac first. Did measure filament AC on trafos first and DC out on the PSUs unloaded. Did them loaded with some load resistors a long time ago also.

    Amp board full power on first launch with all tubes in (after triple checking the wire mess, that is ? )

    Some operational parameters

    • Balanced & matched 12AU7EH and 12AX7EH tubes
    • Matched octet 6CA7EH tubes
    • Bias +570VDC (on testpoint)
    • HV +-457VDC after delay of about a minute after filament power on
    • 300V line is about 265-270VDC
    • all 4 leds are glowing
    • all filaments too ? 
    • LV +-12VDC
    • Shorted inputs gives a steady 19VDC on one output and a jumping 20-25VDC on the other output
    • HV regulator heatsink L-bar not warm at all (about 15 minutes uptime)
    • Bias regulator heatsink also not warm at all
    • LV regulator heatsink about 60 degrees Celsius (seems a bit hot to me)
    • AC input on LV section 2x15VAC

    Questions

    • Is bias voltage OK (a bit lower than 580)
    • Is 300V line OK (or should I lower the 21K resistor value to get closer to 300VDC)
    • Is +-457VDC OK (is a bit higher than 450)
    • are the output DC offsets OK (especially the jumping one)
    • should I do something about the LV section (maybe a larger heatsink although I have a high 60mm version, or get me a lower secondary transformer)?
    • should I check some other parameters...
    • ... or should I just plug in a Stax and listen ?

    Thanks so far,

    Steef

    IMG_0266.jpg

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