Simple Hifi Home Stereo Amp Builderino

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Feb 8, 2019.

  1. Billnchristy

    Billnchristy Tele-Afflicted

    Sep 21, 2017
    Georgia
    I agree with an asterisk. I have used 3w SET amps on 87db efficiency speakers and 90db is adequate volume but the dynamics aren’t there because when the music needs 8w, you just can’t deliver.

    You could jam some folk music with a watt or two.
     
  2. kleydejong

    kleydejong Tele-Holic

    649
    Aug 30, 2010
    Orange City, IA
    Hmm, I did just go through my tube stash and have a few old Baldwin 12AU7's that might be perfect for this project. I like this idea.

    [​IMG]

    I have some 1N4007 diodes on hand for rectification.

    The spec sheet I found for my PT indicates 120ma on the B+. Probably not enough for 6L6GC's. JJ 6V6S?
     
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  3. kleydejong

    kleydejong Tele-Holic

    649
    Aug 30, 2010
    Orange City, IA
    My current setup is nearfield in a home office.

    Good idea, I'll see if I can rig up the inputs and outputs to test it out.
     
  4. jsnwhite619

    jsnwhite619 Friend of Leo's Ad Free Member

    Age:
    34
    Sep 10, 2013
    Georgia
    1/8" to 1/4" adapter is all it needs. Won't be stereo through an amp combo, but nothing fancy just to hear it.

    Sent from my XT1585 using Tapatalk
     
  5. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Near field, like you said, that would work fine with 6V6. There's nothing really lost because if you did double the power the volume is not appreciably greater, and speaker efficiency is where you can realize gains if you need it.
     
  6. David Barnett

    David Barnett Doctor of Teleocity

    Overnight sensations are likely less than 87dB/1W/1M. I can't find a sensitivity spec on Paul Carmody's site, but a pulled-from-my-ass guess would put them in the 84-85 range - they're 4.5 liter boxes.
     
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  7. kleydejong

    kleydejong Tele-Holic

    649
    Aug 30, 2010
    Orange City, IA
    Guess I should put a new speaker build on the project list! I think I'll give it a shot and go from there.
     
  8. Billnchristy

    Billnchristy Tele-Afflicted

    Sep 21, 2017
    Georgia
    I have built a couple pair of Jordan single driver full range speakers but they aren’t super sensitive. I’ve always wanted to do some fostex but they are so dang ugly!

    I had a friend that had fostex 12” full range driven by a 1 watt set amp, he loved it.
     
  9. shortcircuit

    shortcircuit Tele-Meister

    147
    Sep 9, 2017
    North Carolina
    I built one based on the Lacewood amp a while ago. I listen at a low to modest level in my office, and the amp is plenty loud. My schematic and pics:

    Hifi.jpg outside.jpg guts.jpg

    Instead of a choke, I used hefty caps to filter the power supply. The amp is dead quiet and sounds great. What I'd do differently though is instead of a 6SN7 for the driver, I'd use a 6SL7 for more gain, no cathode bypass caps for the driver, and global negative feedback to the cathode of the driver.
     
    Last edited: Feb 9, 2019
    kleydejong likes this.
  10. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Cool build Shortcircuit. I can't imagine your gain is too low. Or maybe you are saying you would trade more gain to be reduced by NFB and no bypass. Kleydejong, that would be one way to use your stash of 12AX7's.

    err... that would be 680 ohm cathode for center bias with 12AU7 at your voltages.... sorry.
     
  11. kleydejong

    kleydejong Tele-Holic

    649
    Aug 30, 2010
    Orange City, IA
    That's extremely helpful, thank you! Great looking build, very much what I'd like to make here.

    Regarding the power supply - just confirming some details so I understand them. Please correct any mistakes:

    Because you used diodes you can pretty much use as large of a capacitor as you want. You wouldn't necessarily want to run the first B+ node before the 500r/50w resistor because we're dealing with a hifi amp and the extra filtering is needed whereas a guitar amp you can kind of get away with it being a bit more noisy and unrefined. The larger the filter cap the more filtering that occurs.

    The 500r/50w resistor must be 50w because there may be upwards of 130ma of current going to the four plates at 330v, therefore 330*130 = 42.9 watts. In most power supplies I've built for guitar amps the heavy current load going to the output transformer / power tube plates exits before any dropping resistors - hence the 2w-ish dropping resistors.

    Is the value of the 500r important? Other than to pinpoint your B+ for the 6V6 plates? I may have an 8r 50w resistor in the parts bin as a backup for my resistive load box.
     
  12. kleydejong

    kleydejong Tele-Holic

    649
    Aug 30, 2010
    Orange City, IA
    I have a big multi pack of almost every resistor a guy could ever want at 1/4w - so I'm okay to tweak the plate / cathode resistors a bit to work well with whatever tube and voltage setup I can get.

    To determine that center bias are you drawing load lines? I really need to wrap my brain around that...
     
  13. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Load lines work well, but I use a program called TubeCad that I have found to be quick and very accurate. It's available from GlassWare Audio, but unfortunately it does not work on win 7 and newer OS.

    If you dropped in a 12AX7, 100K plate and 1500 cathode is center biased. With no cathode bypass and a little negative feedback like Shortcircuit suggested you would have better linearity and tamp down the gain a bit.
     
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  14. shortcircuit

    shortcircuit Tele-Meister

    147
    Sep 9, 2017
    North Carolina
    Yep.

    Exactly, because of the diodes, I can use large capacitors, and I want the extra filtering. The big caps and the resistor are cheaper than a big choke.:D

    I'm using the 500R resistor to drop the voltage from 333V to 290V. The voltage drop is 43V, so the current across the 500R resistor is 86ma. The dissipation (voltage drop x current) is 3.7W. As you can see, a 50W rated resistor is totally overkill. I used the 50W resistor because I wanted to bolt it to the chassis. It was only a couple of bucks. The outside of the chassis where the resistor is bolted does get kind of warm though.

    The reason I wanted 290V going to the output transformer is that it will drop to around 275V at the plates of the 6V6 with my OT's. The guy who designed the Lacewood amplifier did optimization tests on the 6V6 tubes and reported there is point of diminishing returns on power vs THD above 275V on the plates. https://www.cascadetubes.com/optimization-of-the-6v6-se-ul-amp/ Can I tell the difference with my crummy speakers? No. But with the bias set at just above 9W and the low plate voltage, the 6V6's will last forever.
     
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  15. shortcircuit

    shortcircuit Tele-Meister

    147
    Sep 9, 2017
    North Carolina
    BTW, in the picture, I have a set of NOS Sylvania tubes, but I am running Russian 6N8S (6SN7) and 6P6S (6V6). I didn't like the Russian tubes for guitar, but dang, they sound really good for hi-fi.
     
  16. bermuda

    bermuda TDPRI Member

    Age:
    53
    50
    May 15, 2017
    Florida
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