Monday, 8 April 2013

Behringer VT911 w/ Bajaman mods

Bajaman modded version of the humble 9V tube circuit. Tube section and output buffer have seen a quite nice remodelling job :) You could run this with 12V too, but not higher as the tube heater filaments are not meant to handle higher DC voltages. Try out other tubes like 12AU7 in there too... This one should also fit in 1590B with careful measuring...


33 comments:

  1. Just received a non working VT911, I think that as usual, the problem is the tube.

    All smd mini pcb. Will check the tube, and it that's not the problem, will replace the original pcb with this one.

    BR

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  2. Killer! This could be my next project...

    Thanks Miro!

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  3. more and more tube pedal!!!

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  5. Tag this one - i've just completed the build with a 12ax7 as described - very lush, controllable overdrive. the layout is perfect. my only change was the tone pot, i used a 25k lin to get a bit more sweep in the tone. Verify!

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  6. Miro,

    the "components" layout doesn't match the "cuts and links" layout. You have to move the top left link and cut one column over each. Everything else looks fine to me.

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    1. Damn.. I'll fix that once i'm back home. Thanks for heads up!
      +m

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    2. Fixed. Sorry that it took this long.. I had an issue with my main computer's system drive...
      +m

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  7. So, I put a 1N4005 in place of the 4001 and I took the top 220u and moved it over two columns (i.e., on the other side of the link there). Are these things okay? I think they are but sense tells me to ask.

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    1. I also put in three out of five bipolar 10uF electros. That's okay too, right?

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  8. Can you tell me what the caps need to be rated at? how much Voltage? Is 16V ok?

    Gunni.

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  9. Would it make sense to supply it with 12V? At least the heaters... Sorry for the n00b question, I'm tottally green when it comes to tubes.

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  11. Another one: as I see you put the polarity protection diode D1 in series with the 9V input. I've seen other schematics where is it put in paralel, that is between GND and 9V, so that it shorts the power if it is connected with reversed polarity. The "series" solution obviously has the disatvantage to lower the 9V with the diode's opening voltage drop, while the "paralel" solution shorts the power, so probably it is worse for your power source. Which one do you recommend and why? Thanks!

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    1. You could use 12V with no issues. Original unit uses 9V.

      Unless schematic clearly states 1N400x in series with supply, we use 1N5817 instead for lower drop. There is a very good reason why we don't use protection diodes from ground to supply. With diode in series, wrong polarity will just result in circuit not working. While parallel diode protection will explode with wrong polarity, taking a chunk of the board with it in worst cases. To me, that's just bad engineering. That's why we always either omit the protection, or use a diode in series.

      1N5817 has maximum of 0.45V drop, typical 0.42V when passing 1A of current - a current that we'll never see in a pedal. With common pedal currents, the drop will stay closer to 0.1V.
      www.irf.com/product-info/datasheets/data/1n5817.pdf
      +m

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  12. Thanks, mirosol. Do you think this would sound good driven by another OD in front of it? I'm thinking on something like a TS (or rather the Maxon OD808), a Klon or a Boss Blues Driver (one of my favourites)? I'm planning to build a double pedal, one side more like a milder OD, while the other one would rather be a kind of distortion. And I'm thinking to make it similar to the Deucetone Rat, where the two sides are independent and can be cascaded. I'd be interested what you would suggest for the OD side with this one or any other distortion + OD setups that could be built into one box. Thanks!

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    1. I see no reason why that wouldn't work. Haven't built this myself, so i'm not sure about the output level this is capable. However, all the above will sound different to this one so any of them should be a good choice.
      +m

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    2. Thanks. Seems, I'll have to build all of them to compare :D Plus I've just found the Boss OD1... So many things to build and so little time. But I guess this is all of us are struggling with :)

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  13. Finally the parts arived fromTayda and after some struggling (one wrong link, forgot to solder one of the ICs legs, etc) I got it working. I'm very happy with the sound, and it is quite sensitive for what tube you use - at the end one of my TAD tubes turned out to sound the best. I only have one issue: it clicks quite loud when engaging the stomp switch. I see that there is already a pull down resistor of 1M from SW4 to ground (or at least I think it is that), could something else be done? Compared to my other builds the click here is quite loud and disturbing, so I'd really like to get rid of it. Would maybe converting it to true bypass help? Thanks!

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    1. There are no pulldowns on input, nor output. 1M as input pulldown and 220K as output pulldown should fix it. You could possible solder those resistors straight to the jacks. Even though some people tend to despise the method, it's used in almost every point-to-point amplifier around.
      +m

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    2. Thanks miro, I'll give it a try.

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    3. Finally I had the time to try it and it worked perfectly, no clicks anymore. Thanks!

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  14. Is there anyway to wire this to use 2 12ax7 tubes?

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    1. 12AX7 is pin to pin equivalent to 12AT7 and 12AU7. You can use any of those tubes with this circuit.
      +m

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  15. what I mean is can this be wired up to cascade 2 valve tubes one into the next?

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  17. VT9 doesn't appear in the layout, should I leave that pin disconnected?

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    1. Correct. It's part of the heater connection. But in this design, only 4 & 5 get used. Not 9.

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  18. Thanks for the response smarkalet, I was asking this also because I'm having a weird issue with the pedal, I checked that the soldering is fine, the links and cuts are in the right place, resistors, capacitor values, etc.

    What is happening is that I connect the power supply to the pedal (12V, 1.5A) and the first 2 seconds the sound is muted then the sound from the guitar comes up nicely, but after 3-5 seconds the overdrive start to sound like a diying battery/fuzz. Does this issue sounds familiar to anyone here?, is there a way to debug what's going on?, I can provide voltages measures if necessary, what I need to know is where in the circuit to take the measurements.

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  19. I Just finished. But I have a problem. Gain pot is like a volume (volume pot work) and tone pot is like a gain!? What I do wrong? Thanks!

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