Behringer Vintage Tube Overdrive VT911 = B.K. Butler Tube Dr

A forum devoted to mod, tips and suggestions for upgrading and rehousing your VERY CHEAP commercial stompbox to near boutique excellence.
aot
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Post by aot »

Hwasser wrote:Wow, those resistors are really small! xD I bought the ones suggested here, they are little larger in than the original ones, but they fit. Having a hard time soldering them without them moving around (because of how light weight they are), how do u guys solve this without a third hand?
I've done quite a lot with SMD components, so all I can really say is that it's practise. A good pair of fine-nose stainless steel tweezers, a fine tipped temperature controlled soldering iron and some fine 60/40 solder also helps.
Hwasser wrote: Remove R8: am I going to connect it together with copper-wire here or am I supposed to leavy an open space where the resistor was?
just remove, if anything else was necessary then it would say so.
Hwasser wrote: put a 100k resistor between pin 7 of the 12ax7 to the top of R17: is pin 1 the one to the left when you see the tube from below?
Have a look at the 12ax7 datasheet: http://pdf1.alldatasheet.com/datasheet- ... 12AX7.html

Hwasser wrote: 10k after C12: exactly, what? between C12 and what? Am I supposed to drill a hole somewhere?
After meaning it is in series with C12. I used a 10k 1206 SMD resistor, and this was tricky, I had to rest the two components up against each other in an upside down 'V' shape. Or in other words:
Before:

[PAD] - [C12] - [PAD]
[PAD] - [C12] - [10k] - [PAD]

livo
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Post by livo »

aot wrote:Hi Niels,

These are the changes that I made:

Change R9 to 10k
Remove R8
Change R17 to 22k
put a 100k resistor between pin 7 of the 12ax7 to the top of R17
change R16 to 100k
change R18 to 22k
10k after C12
change R14 to 1M

I used 1206 surface mount resistors to make the modifications.

Good luck!

Aaron
This list contains only 8 swap-outs / insertions, while Bajaman's modded schematic shows 10 (11 if you count OPAMP swap). What about the 10k and 4k7 resistors near the OPAMP? Are these unnecessary if you don't swap the OPAMP from original CoolAudio Quad to the TL074?

aot
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Post by aot »

I didn't make that swap. Changing the op amp results in lower noise, and adding the two resistors gives the final op amp some gain rather than it being a unity gain buffer. On paper not a huge difference, but if you like how it sounds but want less noise then perhaps worth it.

To be honest I swapped out the components and then the pedal has sat in a box because it just sounds like a fuzz pedal. My plan is to eventually gut it and use the enclosure for a custom pedal...

livo
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Post by livo »

Thanks aot. So what your saying is that even after this mod, it still sounds like a fuzz.

I posted another explorative question about the BITMO 2BO mod here.
https://www.freestompboxes.org/viewtopic ... 3&start=40
It explains my impression of the pedal out of the box and it's problem plus what Behringer did and said about it.

According to the OP of that thread, the BITMO 2BO mod is apparently 2 resistors and 2 caps (added in, swapped out, other removals or connections???? I don't know. Even though repeatedly asked he failed to expand or clarify anything. Then the thread was hijacked to cover the VT999). Supposedly this mod makes the Gain / Drive pot more widely variable or controllable. So rather than having all the adjustment available compressed between 0 and 1, you get a more expanded adjustment or finer control. This would be a big improvement in my opinion.

Some other reading I have done simply says that swapping out the tube to a good one is all that it takes. I haven't tried this yet.

Then there is the mod that you've done, (minus the Opamp swap) but you say it is still just fuzzy.

I'm wondering if it is worth pursuing or not and just doing what your suggesting is ditching the guts of it and using the box shell.

livo
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Post by livo »

OP of other post has provided me with details of mod kit. I'll give it a try.

ksandbergfl
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Post by ksandbergfl »

Hi there... just got one of these from Musicians Friend for $19.

The included wall wart is 9v at only 300ma. Has anyone tried using a beefier power supply, like maybe a 12v at 500mA or even 1A? My intuition tells me that this could sound better (the tube would be hotter), but might reduce the life expectancy of the tube and/or melt something on the circuit board.

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FiveseveN
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Post by FiveseveN »

Your intuition is wrong. You can't force more current through the device, it will only use whatever it needs, to put it in simple terms.
And even if electronics worked that way, "hotter tube => better sound" is meaningless. You'd have to first define what makes a sound "good" and then show how a tube's temperature influences those properties.
Ignorance more frequently begets confidence than does knowledge. (Charles Darwin)

ksandbergfl
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Post by ksandbergfl »

I tried a 12v 1.5A power supply -- yes there is a big difference in sound quality! i wouldnt describe it as a "warmer" sound but the clarity/articulation is better, it sounds much more like a tube preamp and a little less like a fuzz.

i also replaced the 500k Drive pot with a 100k audio taper pot - it was harder than i anticipated but well worth the effort. now i have a full range of Drive to play with, the gain doesnt max out at 1 anymore... it rolls up evenly from 1 to about 8 or 9. I am very happy with this pedal now.

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Post by FiveseveN »

That's fine as long as you realize it's the extra voltage (on both the heaters and the anodes) that's causing this, not the extra current capability. Using 12 V is even suggested on the old Tube Driver schematic at Muzique.
Ignorance more frequently begets confidence than does knowledge. (Charles Darwin)

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Post by alexirae »

Sorry for my ignorance but should I leave the pin9 of the 12AX7 tube disconnected? I can't see in the schematics where to connect it, thanks in advance!

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Post by FiveseveN »

Yes, leave it floating. Pin 9 is the heaters' center tap. This application uses the heaters in series, so you have to leave it disconnected.
Ignorance more frequently begets confidence than does knowledge. (Charles Darwin)

alexirae
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Post by alexirae »

Thanks for the quick response FiveseveN!, I was asking this also because I'm having a weird issue with the pedal I made using the veroboard version of tagboardeffects blog (Behringer VT911 w/ Bajaman mods), 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.

Any info about this is appreciated, thanks in advance!

ksandbergfl
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Post by ksandbergfl »

My guess is that this is caused by a bad tube. I bought several different tubes to try in my pedal, and they all work fine except for one.... I am getting very similar symptoms with 12AT7, if that makes a difference. 12AX7, 12AY7, and 12AU7 all work fine.

For what it's worth , I liked the sound of the 12AU7 the best, that's the one I left in the pedal.

alexirae wrote:
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.
\

alexirae
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Post by alexirae »

Thanks for the info ksandbergfl.

Definitely you were right, it was the tube! I bought a new one and now the pedal works fine, sound more or less like a fuzz though but I guess that's how the original pedal sounds :P

wildschwein
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Post by wildschwein »

livo wrote:Thanks aot. So what your saying is that even after this mod, it still sounds like a fuzz.

I posted another explorative question about the BITMO 2BO mod here.
https://www.freestompboxes.org/viewtopic ... 3&start=40
It explains my impression of the pedal out of the box and it's problem plus what Behringer did and said about it.

According to the OP of that thread, the BITMO 2BO mod is apparently 2 resistors and 2 caps (added in, swapped out, other removals or connections???? I don't know. Even though repeatedly asked he failed to expand or clarify anything. Then the thread was hijacked to cover the VT999). Supposedly this mod makes the Gain / Drive pot more widely variable or controllable. So rather than having all the adjustment available compressed between 0 and 1, you get a more expanded adjustment or finer control. This would be a big improvement in my opinion.

Some other reading I have done simply says that swapping out the tube to a good one is all that it takes. I haven't tried this yet.

Then there is the mod that you've done, (minus the Opamp swap) but you say it is still just fuzzy.

I'm wondering if it is worth pursuing or not and just doing what your suggesting is ditching the guts of it and using the box shell.
Hi,
Here's a copy and paste of what I sent to others about the Bitmo mod over the years.

"I'll tell you the values and where the components go here - if you want to post it later it's all yours.

Parts list:
100k resistor (was supplied with a tan carbon comp)
460 OHM resistor (blue - metal comp )
1200pF capacitor (metalized polyester type)
.0047uf capacitor (not sure of this one - it was aqua-coloured and axial type - was marked with the number 472.)

You have to unscrew the tube board to do the job - but that's all. Looking at the tube board from the solder side there are the 9 pins - 1-9 going clockwise from the gap at the bottom of the board. The .0047uf cap goes between pin 1 and pin 7 of the the tube on the tube board. The 460 OHM resistor is connected to pin 7 and also the negative lead of the electrolytic capacitor (on the other side of the board). You should see the electrolytic cap's 2 solder connections just slightly above pins 6 and 7.

On the drive knob you just connect the 1200pF cap and the 100k resistor in parallel and solder them to the two outer pins of the pot - the instructions advise turning the knob all the way down before soldering.

Remount the tube board and that's it. You should notice when you plug it in that it has only a fraction of the gain it used to. I found the lower gain 12AU7 was no longer enough to get adequate output from the pedal so I dropped in a Sovtek 12AX7A - which was the highest gain tube I had laying around. The original Chinese tube seemed to not sound quite as good to my ears."

Hope this helps, I sold the pedal recently so no longer have it as a reference."

wosinek
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Post by wosinek »

Adding the 1200pF cap and the 100k resistor to the Drive pot made the distortion more
manageable... before everything was too hot before you even got to 1 on the dial
and too sensitive to adjust. I didn't do the the other mods yet but will try another tube
to soften up some of the fuzziness maybe.
for $20 from Musicians Friend closeout this pedal is worth it just for the parts.

ksandbergfl
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Joined: 16 May 2015, 14:52

Post by ksandbergfl »

I made a demo video of my VT911 hack. I simply replace the 500k Drive pot with a 50k pot.

Search Youtube for "Behringer VT911 Drive mod" or use the URL below:

https://youtu.be/f9xywtv6Oqg

I did not find the out-of-the-box pedal useful if the Drive got above "2". So I measured the Drive pot, when set to "2", and it measured 37k. So I figured, if I used a 50k pot instead... then my Drive at "10" would be where it used to be at "2", and I would get all the settings in between. :hmmm:

Based on the schematics, the 500k pot controls the gain of an op-amp stage, which feeds the tube stages. My theory is that 500k provides TOO MUCH op-amp gain into the tube section. I did not scope it, but it might even be possible that the op-amp stage is distorting too, thus feeding the tube stage a dirty/noisy signal to begin with.

Anyway, I hope the video might help someone, or give someone some additional ideas. I am very happy with my Vt911 now. I have done this mod on two VT911's now, with similar results.

Rock on!
--- Keith

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Post by DrNomis »

ksandbergfl wrote:I made a demo video of my VT911 hack. I simply replace the 500k Drive pot with a 50k pot.

Search Youtube for "Behringer VT911 Drive mod" or use the URL below:

https://youtu.be/f9xywtv6Oqg

I did not find the out-of-the-box pedal useful if the Drive got above "2". So I measured the Drive pot, when set to "2", and it measured 37k. So I figured, if I used a 50k pot instead... then my Drive at "10" would be where it used to be at "2", and I would get all the settings in between. :hmmm:

Based on the schematics, the 500k pot controls the gain of an op-amp stage, which feeds the tube stages. My theory is that 500k provides TOO MUCH op-amp gain into the tube section. I did not scope it, but it might even be possible that the op-amp stage is distorting too, thus feeding the tube stage a dirty/noisy signal to begin with.

Anyway, I hope the video might help someone, or give someone some additional ideas. I am very happy with my Vt911 now. I have done this mod on two VT911's now, with similar results.

Rock on!
--- Keith

I would have to agree with you, with a 500k gain pot the gain of the op-amp stage gets so high that the op-amp itself gets driven into hard-clipping which is one of the reasons why the VT911 sounds bad, incidentally, the VT911 circuit is the same as the BK Butler 3-Knob tube driver.


As a sidenote, I remember doing some signal-tracing of the BK Butler 4-Knob Tube Driver using a scope and signal-generator, and found that the op-amp gain-stage in the 4-Knob Tube Driver circuit had a tendency to sharp-clip the signal on the top peak at high Drive settings, when this happens the 4-Knob Tube Driver circuit tends to sound a bit hoarse rather than smooth..... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

synapzedog
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Post by synapzedog »

Behringer_VT911_Mod_Bass_or_Guitar.pdf
Schematic with notes
(303.48 KiB) Downloaded 72 times
I bought one of these to play around with and use it with my Bass, but the same mod will surely work for guitar as well.

You will absolutely have to modify this pedal as it is totally useless out of the box. If you are not a tinkerer- buy something different. Here is the modifications I did and it is now a useable pedal.

1. (absolute must-do) Replace VR3 drive pot with a good ~20-25k pot (not sure if it was a linear/log/audio pot it was a spare). The one I used I mounted to the box and ran wires to the board. If this step is all you do you will improve this pedal by light years. I
2. (if using with a Bass, may not be required but I did it just to make sure the low end was there) add 0.1uF in parallel with C5, C11 & C12
3. (optional) with the stock 12AX7 the pedal is a little crispy for my taste, but you might like it that way. I prefer a lower gain tube like a 12AU7/ECC82.



For me 'by ear' the distortion starts around 3 on the drive and is nice an linear. If you have one of these lying around you can bring life to it!!!!
Attachments
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Post by stojanos »

I bough one of VT911 on ebay. It is broken. Maybe I have killed it plugin 9VC DC with wrong + and - sides. With a proper power supply, led is on but It has very loud noise that you can barrely hear otherwise very nice worm overdrive distortion. This device is not moded. Everything looks original.

I've checked all capacitors and they are OK - even replaced the main one close to power supply (100uF) together with 1407 diode but loud noise is still the same. Before I attempt more expensive tube swap or more complicated op amp swap can somebody help me to trace the problem? I am not sure if i am getting proper voltage on the tube. It does not look like glowing at all.

Any help is welcome.

Cheers,
Srdjan

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