VPM-1

[kong53]

I recently built the DIY VPM-1 kit …I get a signal and I’ve trouble shooted to the best of my abilities but I can’t get the pedal to change volume (in other words there is no sweep).
I tested all the points on the voltage chart and the Red 5 volts points register 4.15k and the Green 9volt points register at 8.80k. I’m not sure how to test the -9volt points. I get a clean signal just no volume taper.

Any suggestions ? Thanks for your time,

[brach]

I’m sorry about your VPM-1 troubles.
Are you saying you’re measuring 4.15 and 8.80 thousand volts?! Are you sure you are using your multimeter correctly?
I don’t want to insult you, but you may not have the experience to successfully build this type of project. Unfortunately, the VPM-1 isn’t a very beginner friendly kit (as explained in the assembly manual)….the reason I’m bringing it up is because it may be difficult to help you in this forum if you don’t have the correct tools or much experience using them. You may need to take this kit to an electronics technician to get some live face-to-face help.
I’m sorry about this.
-Brach

[kong53]

Thank you for your reply …not insulted I meant ohms I think …I would say I’ve built simpler mods successfully in the past but I definitely try to attempt projects I shouldn’t and this turned out to be one of them and it seems the the more probing and testing I do the more lost I’ve become.

I’m finding continuity at a couple of spots where there shouldn’t (at the LED and the S2 dip switch) be and no continuity on pin 2 of Q1 at this point I’m afraid I might have reached the point of no return.

[Scott]

I built and installed a VPM-1 kit and it is working fabulously. I now have it in the effects loop chain of my Fender Hot Rod Deluxe. If I set the volume pedal at it lowest volume before the sounds starts breaking up, I can turn up the preamp on the amp for a really nice sound at a quiet level. What I would really like to do is turn the gain setting on the pedal way down so that I can get the same effect and use the full range of the pedal. I have not been able to fine schematics for this pedal. Is anyone able to share information on what mode could be done to turn the output gain down another 10-20 dB?

[Scott]

I built and installed a VPM-1 kit and it is working fabulously. I now have it in the effects loop chain of my Fender Hot Rod Deluxe. If I set the volume pedal at it lowest volume before the sounds starts breaking up, I can turn up the preamp on the amp for a really nice sound at a quiet level. What I would really like to do is turn the gain setting on the pedal way down so that I can get the same effect and use the full range of the pedal. I have not been able to fine schematics for this pedal. Is anyone able to share information on what mode could be done to turn the output gain down another 10-20 dB?

[brach]

Scott,
I’m not sure I totally follow you, but if you want full range of motion with the treadle there is a trick you can do…
You can calibrate the pedal by pressing the first 2 or 3 points at the bottom of the treadle position (heal down) and then equally space out the other points between the bottom and the top of the treadle motion. Each point is 25% of the full range…the 5 points are 0%, 25%, 50%, 75%, 100%. So, if you put the first 2 points at the bottom of the range of motion and then space the other 3 points out equally between the heal down position and the toe down position, then the heal down position will be 25% of the volume, going up to 100% when in the toe down position. Hopefully, 25% of full volume (the heal down position) will set your amp to a level just before it breaks up. I hope that makes sense…and is helpful for doing what you want to do.
-Brach

[Scott]

I’ll try to explain my question about the gain knob again. (When I said the word “breakup”, I did not mean about amp distortion breakup. What I meant was that the output of the VPM-1 when the pedal is at the very bottom of it’s range starts to stutter).

What I want is to have the volume pedal at 100%, then use the gain know to turn down the output another 10-20 dB. It seems like the gain know boosts the signal, but does not cut it. I always use the gain knob at it’s lowest setting. I want it to go much lower. I’m guessing that swapping out a couple of resisters on the op amp would drop the overall output.

[brach]

Scott,
That is correct about the gain knob, it is not a volume knob, so it doesn’t reduce volume. The circuit itself, does have a slight drop in amplitude when the gain knob is at fully the counterclockwise position, but it’s only a few dBs. The opamp circuits in the pedal are non-inverting, so they cannot lower gain, the lowest they can go is unity. But I’m not sure that lowering the volume/gain level of the pedals is going to solve your problem. It sounds like you are asking for a wider dynamic range of the pedal, which lowering the gain setting will not give you.
You can test if this by adding another pedal (with a volume control) to the effects loop after the VPM-1, and with that pedal set to unity gain, adjust its volume control lower. This will at least give you an idea of weather lowering the VPM-1’s output gain will solve your problem. If that helps the problem you are experiencing, then you could modify the voltage divider circuit on the VPM-1’s output. I can tell you what resistor to change when and if that time comes.
I think what will work better for you is if you do what i was suggesting in the previous post about setting the treadle calibration differently. If you set it to where the volume on the VPM-1 is at 25% in the heal down position, then set the preamp on the amp for a nice sound at a quiet level. Then you’ll have the entire range of treadle movement to turn up the amp. You are already using the VPM-1 in this way, where you get a sound that you like in the heal down position, but if you set up the calibration this way you won’t hear the noise artifacts at the low end of the treadle position anymore.
-Brach

[Scott]

Brach. Thanks for the thorough information! It would be very helpful if you could let me know which resistors make up the divider.

I put another pedal after the VPM that allowed me to turn down the volume further, and I got what I want. It gave me the full range of the pedal, but also allowed me to use a greater range of the gain controls on the amp. Now I can run the clean channel on my amp with the knob set at something like 4-5 at a comfortable listening level, (and I can even push it higher if I want gentle saturation).

I learned a couple of things for this experiment:

On the positive side, it is easier to adjust the level on the amp when the knobs are at higher numbers. (Without the gain reduction, a premap setting of 1 is a comfortable level, 2 is loud, and three gets complaints from the family/neighbors. The volume curve seems to be flatter at higher numbers.)

My other goal was to get the “better sound quality” that happen when the tubes are run hotter. This experiment debunked that. First, I played it with no gain reduction and the Hot Rod preamp set at 2. Then I used a sound meter and the gain reduction pedal to get the same sound level with preamp set at 5. I couldn’t hear a difference. I think that all the stuff about “the tubes being happier” when the preamp is higher is just that our ears hear things better at moderately loud volumes. (flatter frequency response, and quiet details aren’t lost in the noise floor).

So, I would still like to modify the pedal to get a lower volume output, but now the reason is more pragmatic than achieving harmonic bliss at low levels.

(One more things I learned. when the Hot Rod preamp is pushed to high levels, some of the signal bleeds into the power amp even when the pedal effects loop is at zero. So, if you want dead silence when the pedal is heal down, you can only run the preamp at moderate levels).

[brach]

Scott,
Thanks for getting back to me. The resistors that make up the voltage divider are R8A (1k) and R9A (100k). They are located under the output jack, so you’ll have to remove that to access them. Look in the troubleshooting guide to see how to properly remove the jack.
If the pedal that you put after the volume pedal to reduce the volume level was set to unity gain, you might be able to measure the resistance of its volume knob to get an idea of what the voltage divider resistance values should be. Otherwise, you can make note of the signal level change by using an oscilloscope.
With tube amps, the preamp tubes don’t usually offer much tonal change between loud and soft volume levels…this is because they are able to handle a lot of head room giving a relatively clean tone. The real tone (increased odd harmonic distortion) comes from pushing the power tubes hard…but the volume level from the speakers usually gets really loud before they really start sounding good. This is why “power brakes” are used…to attenuate the already distorted tone from the loud power tubes. In your application, you were just attenuating the hot preamp signal, which has too much voltage headroom to break up nicely, while leaving the signal getting to the power tubes relatively low. Since from a tonal perspective, it doesn’t matter if the preamp is turned up or not, I suggest leaving it relatively quiet, so the signal hitting the volume pedal is lower. This way, it won’t bleed through as much when in heal down position.
I hope that makes sense.
-Brach

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