Tubed Expression Pedal

[mashaffer]

I have been looking for an inexpensive first tube project. Idea one was a tube subwoofer crossover but I recently got a Yamaha keyboard which has provision for a sustain pedal but not for an expression (volume) pedal. I have a pedal from an old electronic organ so I have the machanicals (including several pots connected to it) and originally thought of making a stereo op amp variable gain circuit to place between the keyboard output and the mixer/amp.

Well that would be inexpensive and rather easy to build, but then I thought, "why not tubes"? I could probably come up with a reasonable power supply using standard 120v-12v transformers to drive a 12A series tube. If we needed a higher B+ I suppose the second transformer could be a 6v secondary (giving 240V for the plates).

What do you think of the idea? Would you use a standard CC circuit with volume control at the grid or some sort of variable feedback system to vary the gain?

mike

 

[ThSpeakerDude88]

if you wanted to use two 12v transformers back to back I suppose you could, although it would probably be cheaper to do this :http://www.tubesandmore.com/scripts...evclients/cemirror/ELEVATOR.FXP?item=P-T262E6


its cheap ( usually $20) and puts out 130vac on 120v lines, with an additional 6.3v heater. I am currently using one to power a 6aq5/6au6 guitar combo.

If you needed higher voltages, you could use a full-wave voltage doubler to get around 270-300v depending on load, which would not be much with a 12xx7 tube.

Hope this helps!

 

[alexmoose]

I could probably come up with a reasonable power supply using standard 120v-12v transformers to drive a 12A series tube. If we needed a higher B+ I suppose the second transformer could be a 6v secondary (giving 240V for the plates).
mike

if you're going to do that, either make sure that the current rating on the 6v is really high, or that you instead use two transformers in series. which is indeed the safer way to go

 

[mashaffer]

Getting Proper output imp

I was looking at the 12au7 since I don't really need substantial gain and wanted low Ra. In my searching I also came across the 5687 which has even lower Ra. I might try that if I can find some at comparable prices. The question is how well will an Ra of 3k-7k drive a high impedence mixer input in CC mode? Should I just use CF? I really didn't want to mess with transformer output on this one. It seems like the singal levels involved should allow me to go CF without using a bipolar (as in +/- not as in transistor) PS. Does that seem correct to you?

I appreciate your patience with the greenhorn.

mike

 

[mashaffer]

Never Mind

Total brain spasm on my part. Completely flaked out on CF bias process.

mike

 

[mashaffer]

Playing with Oregano

I just installed Oregano so I thought I would play arund drawing up what I have in mind. First run looks like this. I have not done any real design work on it yet just plugged in an Rk that would give me about 1ma of cathode current. Does this seem like a reasonable direction to go?

mike

 

[Merlinb]

Perfectly reasonable. Though a 12AU7 or 12AT7 would work very well at a fairly low HT voltage, unlike the 12AX- just thought I'd mention it.
Plus, you'd then get quite low output impedance even without a cathode follower- again unlike the 12AX7.

 

[mashaffer]

Thanks for the input Merlinb. The diagram shows an AX because that is the mdel that Oregano has installed I had actually considered using an AU because of the lower output impedence. I was not sure however if the higher mu of the AX would make for a better CF. At first blush it looked to me like an AU in CC would still have a Zout in the thousands of ohms. I guess that would be alright but I was concerned that I might run into an amp with a Zin closer to 10K than 100K. Maybe that isn't too likely.

What do you think of the idea of using a CC with the pedal in a feedback loop to adjust gain that way? I am kind of reluctant to use variable feedback because I am worried about failure modes and I am not sure that the tone quality would not shift as feedback was varied.

mike