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Thread: 12AX7 Voltage Problem(s)

  1. #1
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    12AX7 Voltage Problem(s)

    Hi,

    I'm building a 12ax7-based preamp/overdrive (based on the McTube schematics from http://www.dogstar.dantimax.dk/tubestuf/mctube.htm, which I had to modify to accomodate different power supply). I encountered the following problems:

    1. I included 12V voltage regulator to power the filaments (connected in series) and they do not glow. They do if I remove the regulator (although the voltage is a bit too high then). The regulator gives stabilised 11.8V output, which IMHO should be OK (the tube should accept 6.3V with 10% tolerance). Why is it happening?

    2. The voltage on one of the plates drops to amost half of what it should be. I supply around 135V (no load) and it sags to about 35V (should be around 60V considering the resistor value). I cannot find any fault in the power supply. What else could cause excessive voltage drop on the tube plate?

    Any help would be appreciated

    Maciek

  2. #2
    Old Timer tedmich's Avatar
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    the heaters might still be heating...are they hot? The regulator needs to move a good amount of current, and heatsinked or it will do a thermal shutdown; was the voltage running through pin 4/5 to ground when hooked up? The voltage sag is too much as the Real McTube B+ = ~140VDC see here:
    http://www.diyguitarist.com/Schemati...McTubeText.doc
    and here
    http://amps.zugster.net/articles/tube-pedals



    Fred Nachbaur was a very nice man and helped me through this, my first tube build, he will be missed.

    his web site is going away, the Google cashed page has a lot of info for a while longer:
    http://209.85.173.132/search?q=cache...&ct=clnk&gl=us

  3. #3
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    Thanks a lot for reply.

    I didn't know that Fred passed away, checked on the net and it is so indeed. Very sad, I never contacted him personally but from his articles he appeared to be a very nice bloke.

    I managed to get the McTube working (or "sort-of" working). I think the tubes (I used Electro-Harmonix ones) seem to be very fussy when it comes to filament voltage. Trouble is that a 12V regulator will never give you 12V, let alone 12.6V (or 2x6.3). Mine produced 11.5, hence each filament got 5.75. Doesn't sound like much of a difference but it seems to be. The regulator could deliver 1A, so it wasn't a problem, got warm, but not hot.

    So, I got the sound out, still hovever got these issues:

    1. Don't quite like the sound (a bit harsh, more solid-state like). Also I haven't got enough gain. At low settings there is hardly any distortion, then in goes into a really harsh crunch. The whole thing works better if I plug an normal (solid state) preamp before it. Then it gets nice, long and screamy sustain, so it is encouraging.

    2. The box shuts down the sound quite randomly (not gig quality just yet :-)). It will do it definitely when I plug/unplug things while it is on (?). It then needs some time to sober up and is fine again. Also after a while it seems to shut down by itself for no reason (no sound, while evertyhing appears to be working). I had some problems keeping negative voltage on the grid of the frist triode. Would that cause the tube to shut down if the grid went positive?

    To fix the sound issue I plan to modify the cirtuit by adding a few more steps (2 or 3 tubes). In that case I could run first triode with high plate voltage to get a good signal boost, attach tonal controls to it and use the remaining triodes as clipping sections with varying plate voltages (decreasing). Then I can switch them on or off depending on how much distortion I want. And the first "booster" triode should help me with the gain problem.

    Does that concept make sense? (pardon my ignorance, I'm quite new to tubes).

    Cheers

    Maciek

  4. #4
    Old Timer tedmich's Avatar
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    Trouble shooting the McTube isn't something I'm good at, some of the real amp techs around here could probably help you out (Enzo for instance). The Mctube uses an extreme "starved plate" design and small changes in voltage, like are expected with a new power supply design, can push it out of the "sounds good" area pretty quickly. In my experience building an exact copy, making it work and THEN changing it up is the best practice, especially when starting out.

    Adding gain stages to the McTube is logical but that (along with more standard high voltages) makes it into a conventional preamp; it would be best to go with some of the many excellent proven designs for this available online.

    The AX84 tube guitar amp cooperative has some tried tested and true designs that they will help you make, if you like more gain the Lead II preamp sound killer with just 1-12ax7 and 2-12au7s:
    http://www.ax84.com/static/corepream...tom_LeadII.mp3

    (the DIY KT88 power amp helps too )

    and your Aussie AC can make the required voltages just with a bridge and filter caps (~340VDC) but for safety use an isolation transformer like Fred did.

    Also see the 18watt.com site if you like Marshall tone

    Also AC heaters are AOK 90% of the time so don't sweat a regulator (12VAC should give a beautiful 6.4VDC though, see the National Semi LM338 design pages)

  5. #5
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    Thanks for the advice once again.

    I my supply I used toroidal transformers (in an effort to get less hum, which I think worked - no hum at all), but they produce different load voltages under near no-load conditions, so I think you right about the whole circuit being thrown out of balance by that.

    I actually plan to build an "exact" copy. It is in quotes because I need to do some gymnastics to get the same voltages from 240VAC mains, so it'll not be exact copy, but as close as possible. No trick with 2 plugpacks would work for me unfortunately.

    For this purpose I assembled a few bread board tube sockets, so I can experiment a little (you can buy them for $15-$20, while the parts cost $3). Not sure how a breadboard would take 340VDC, but it should be fine with 120VDC. Also I am planning to build a few variable voltage sources so I can adjust things like plate voltages and simply hear how the sound changes hoping to find the sweet spot that way. Might try it with a few different designs.

    I haven't seen the DIY KT88 project before, thanks for the tip, I'll have a look.

  6. #6
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    You should give correct B+ to 12ax7 to make it sound right. It would be around 300 volt for B+ and around 128volt on plate.
    You could see in many schematics on preamps that B+ around 300volt. That's what make 12ax7 to singing.
    And in my experience build some stompox with 12ax7, marshall design preamps is the best. If you just want boost or bluesy may think fender preamps.
    Why not try both and decided what you like

  7. #7
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    Instead of a regulator to power the filament, why don't you try plugging in ohms law and use a power resistor instead? 5 or 10 watts ought to be enough. The tube will never sound quite good enough unless the filament is getting the voltage it needs.

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