Thursday, October 21, 2010

Tube Stomp Box

Tube Stomp Box for digital recording

A good friend of mine is an accomplished guitarist who has a recording setup for his band. Part of his rig is an amp simulator plug-in that he uses with digital input (AD converter) to simulate the tone of an amplifier. Recently he asked if a tube guitar pedal would add that tube 'roundness' to enhance the sound of his recordings. This sparked an idea..

I figured that any stomp box has to be small and portable, powered via wall-wart and have a bypass switch. Having lots of tube stuff and parts around I scrapped a prototype together on the bench. In the photos above and below you can see the first attempt at the circuit. To fit the requirement of wall-wart power meant that a high-voltage B+ supply had to be derived from a low voltage source. I settled on a 24VAC @ 1 amp supply sourced from Triad, which when run into a voltage quadrupler yields 160VDC at about 200 ma. The filament supply would have to come from the 24VAC, padded down with series resistance to get 12VAC. OK, so with the PSU figured out it was on to the tube gain stage.

My first attempt was to use the venerable 12AX7, which as you will see proved problematic. I used both halves of the tube to make two identical stages with a gain of about 35dB each. I put a volume control between stages figuring that the guitar volume control could be used a a gain and the stomp box control could be used for drive so you could get some overdrive. The gain of the whole circuit was way too high to input into a guitar amp (or amp simulator) so the plan was to knock it down with an output transformer.

Alright, so far so good.. looked clean on a scope, got plenty of gain and overload margin was acceptable. Then I put it in a box and hooked it to an amplifier- BUZZ! HUUUMM! Oscillation! Microphonics! High frequency rollof! I was surprised, everything looked fine before putting it in the box. Too much gain coupled with wiring capacitance and high impedance created all sorts of problems. I worked on adding more filtration to the power supply, DC filaments and shielded cable to no avail (high-gain, high impedance tubes such as the 12AX7 have issues like this).
I had to look for another solution.

Well, of course! Change the tube to a 12AU7, get rid of all that gain and all the associated problems go away too. And as fate would have it, the plate and cathode resistor values worked out perfectly for the given voltages so the 12AU7 could just drop right in. Less gain equals lower miller capacitance, meaning flatter frequency response and less sensitivity to induced hum, microphonics and oscillation. This tube also has lower output impedance making it easier to drive long(ish) unbalanced interconnects which are likely to be use with this device. So with less gain and lower output impedance the output transformer could be eliminated too.

So, done for now. I will ship this out to him to see what he thinks, probably end up making some changes according to his feedback and then who knows? Maybe another Elliott Studio Arts product?? We'll see..

 Nope! This project was a failure... My friend could not get rid of the buzz. Probably due to sharing the same supply for the filaments and the B+. He still has it so I have no way to experiment and figure it out~ Oh well, next time I'll do an isolated DC supply on the filaments..

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