Valvecaster With Some Changes

Discussion in 'Project Depot' started by mozz, Dec 18, 2018.

  1. mozz

    mozz Well-Known Member Supporting Member

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    Been on a fuzz building kick lately. 3 rangemasters, fuzzface, Maestro fz-1, etc. Bought some more pcb's for fuzzfaces, more rangemasters, astrotone (sam ash fuzz), couple simple boosters (npn, pnp, fet, LBP, Electra).

    Came to the conclusion that it's easier to find tubes from the 20's than to find certain transistors from the 60's -70's. Will not try to think about using IC's such as lm308 and those bucket brigade things. Next hamfest i will be more equipped with lists of stuff to look for.

    Anyway, in my tube stash i have about 150 pieces of 6211, it is basically a 12au7 (low gain) that was used in computers way back in the 50's-60's.

    I tried to use these back 30 years ago in guitar amps and all they did was howl, terribly microphonic. The point is, the circuit i tried to use was meant for 12ax7, as are 99% of tube guitar amps out there. The whole family of 12A*7 have the same pinout but different gain, different everything.

    Fast forward so fast you are seeing UFO'S. Light bulb goes on. Long lost project comes to fruition.

    Got a deal on some (36 pcs) small transformers, 80% off list price, (sucker for large quantities of electronic stuff i will never fully use) Triad, dual 12 volt secondaries. Tube pedal time. For enclosures i think i may be able to use 1590c but this thread is about the breadboard guinea pig that is alive.

    1.jpg

    Building the circuit around the parts you have. The Valvecaster uses either 9volt or 12volt to power the entire tube. Thanks to Matsumin for the initial build and webpage dedicated to it. It's known as a starved plate design when running the tube voltage well below what would normally be used. Transformers are giving me about 20vdc for the plate and the other winding, once dropped a bit, gives me 12.6vac for the tube filament in series mode.

    2.jpg

    I'm not going to get into how modern "tube" pedals are claiming "starved plate" mode when they are still using a 12ax7, they just don't work with low voltage and sound like crap. They do work but not in true starved plate mode. 12AU7 and other tubes work much much better on low voltage. They are much cheaper in NOS form too. That's all i know, 2 zinc cents worth.
    BK Butler Chandler pedals used a 12ax7 so i am gonna have to study the schematics from them and see what he did there.

    Here is the flea market enclosure i built it in for now. It works, it jams, not quite a fuzz or distorion. No rangemaster Sabbath tones, but more general rock, Skynyrd, Billy Thorpe, Crazy Horse dirt.

    Most parts except the power supply are soldered right on the tube socket itself. Once i figure out how to put a small transformer and a tube socket on a 1590c, i will be much happier. Don't ask about the meter, it's not hooked up, i just wanted the slant project cabinet.



    3.jpg
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    Here is the distortion meter showing 37.5% dirt and the oscilloscope showing what that dirt looks like, it removes the original signal and shows you the distortion component. Weird waveform but as i said, not as much heavy fuzz with sub-octave low notes.

    Testing was all done at 1khz and 70mv.

    5.jpg
    6.jpg


    Next page will just be 3 pictures, the original schematic, my few changes i made to the schematic, and a final tube chart showing tubes i tried, about 30 different ones, distortion measured and gain measured.

    I didn't like the gain pot on the tube cathode. Every time you adjust the pot, you hear scratchy noises. That's because there is voltage there on the cathode (duh) and pots don't like voltage on them. Why it was made like this i don't know. A few builders have gone to a fixed value resistor, bypassed, and some have used a gain pot on the second triode. I picked 1k with a 22uf and it works fine.

    For now it has no gain pot, if you use the volume control on the guitar, it cleans up nice. I was surprised by this, as that is how a Rangemaster works. It was still loud yet a lot cleaner. I have read you can use these with certain amps and other amps it will sound like crap. You can go from Bob Segar "turn the page" to Metallica's butchered version just by cranking the volume on the guitar.

    Ok, now the data, your results may vary. Input 70mv @1khz, volume max, tone max. Remember, your guitar can put out 1 or 2 volts, that's 2000mv. Some tube amp testers will use 100mv, or test at 400hz, to me that's fairly hot, i have settled at 70mv and use 1khz, i have seen how many amps and pedals will react to that, then i crank it up and see how much headroom there is and how the tone stack or controls work.

    You may be shocked, shocked i say at 40% distortion. My old Supro was over 70% cranked way up, think "Good times bad times" lead break dirt.

    10% just starts to add some hair, 40% you can't play 7th chords and some minors and other stuff, they sound crappy.

    This circuit was designed for a 12AU7, the tube and it's variants does shine here. 5963, 6211, and 12AT7 was sounding good too. You can see the (relative) gains up and over 1000mv, that will nicely drive your amp into overdrive. You can only coax so much gain out of a tube, the distortion you can vary somewhat by going into non linear regions of the curves. But since this is starved plate design, all bets are off.

    12ax7,5751 meh, either very clean or they had the distortion but the gain was gone, all the way up you lost signal compared to true bypass. Remember, this circuit was not designed to use a 12AX7 , so the low voltage shines with lower gain tubes and they actually blow away the 12ax7 in terms of output.
     
    Last edited: Dec 18, 2018
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  2. mozz

    mozz Well-Known Member Supporting Member

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    MatsuminValveCaster.jpg MozzsumiValveCaster.jpg
    Values in the chart are not real life values, you may use humbuckers with higher output and see certain tubes come to life, or they may overload terribly. It just shows a comparison at 70mv at 1khz. This circuit really fits in with a 6211 (12au7) gainwise and i am really happy with it so far.

    You can see the 12au7, they have 10% and a little bit more distortion. That just gives it a little hair and bite, this circuit is very good using that tube. I have read using a boost before this really gets it going. There is also a 2 tube(4 triode) version for those who want more gain.

    I have also read of people using the original circuit, using 18v and was a much improved sound. I think by me using a 20vdc for B+ is what gives me some extra gain if needed and overdrive when using the guitar volume control.

    VALVECASTERTUBETRIAL-1.jpg
     
    Last edited: Dec 18, 2018
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  3. nomadh

    nomadh Well-Known Member

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    I just want to be your lab monkey while you explain things.
     
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  4. jtcnj

    jtcnj Well-Known Member

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    nice thread.
    Really would like an O-scope.
    the crackle is ok, see Zevex Super Hard On!
    dc volts across pot add some crackles, maybe some cackles as well.

    6e69770f94839f853c296b490445107a.jpg
     
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  5. mozz

    mozz Well-Known Member Supporting Member

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    Have a extra scope here, might be a B&K, 60 mhz? How far are you from PA border?
     
  6. idiotsdelight

    idiotsdelight Well-Known Member

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    Seriously. This is the stuff I'm talking about. Please do more of these threads.
     
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  7. Manodano

    Manodano Well-Known Member Supporting Member

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    That's a great VU meter enclosure. I think if you could figure out how to make the needle wiggle, that would be really cool. Especially if the meter lights up.

    I'm digging into my parts box and planning to build one of these in the next few weeks. I plan to use a "turret" tube socket. I'm pretty sure I can build the whole circuit on the the socket, and get this put together in no time flat. I've got a pretty hefty and clean 12v DC supply laying around (old Yamaha keyboard a/c adapter originally) and the enclosure, pots, resistors and jacks in my stash. I'll have to dig a little to see if I have the few needed caps.

    I have to believe that the the circuit with the 50K pot on pin 3 sounds a lot different than the 1k cathode resistor and 22 uF bypass cap, so I plan to try both.
     
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  8. golem

    golem Well-Known Member

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  9. mozz

    mozz Well-Known Member Supporting Member

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    You can build it right on the tube socket, next one will be soldered before the socket is riveted to the chassis. Actually think you can get by with 10k-25k, i didn't like it because how very scratchy it was, way to touchy to adjust. You can put a 500k vol pot where the 470k resistor is, if you want a gain control.


    Few tips on the 12v supply. 12a*7 family with series filaments draws 150ma at 12.6vac. If your supply is a lot more than that, your voltage will be high, i had to use 2 small dropping resistors to get the voltage down, it was over 14v and tubes were pretty bright, = short life.

    You also need a center tap on the 12v for a filament ground reference, if none available, you need to put 2 resistors from filaments to a ground point. This will even out the draw at 6.3vac from each leg to ground. Otherwise you get induced hum. Don't use pin 9 as a center tap to ground. I think i used 270 ohm. That is about 23ma each, so 46ma plus 150ma from tube = 200ma. My secondary's are rated at 250ma so i wanted to be under max. Most of these valvecaster drawings show using the rectified voltage for both filament and plate, tried that, didn't work as quiet.

    My transformers have 2 separate secondaries, so one was filaments, other was used with a bridge rectifier for the plate voltage.
     

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