Tube Filter

Enveloppe filter is maybe one of my favorite effect . I’m not a fan of flanger, chorus ect… these effects are too much invasive to the sound whereas an enveloppe filter adds expressivity and madness. I had few years ago a  tube zipper from electro-harmonix and the effect was really uber-cool , it could bring some amazing sound. A really must have ! Unfortunately I was stupid enough to trade it for a boss phaser . The boss was good but it got sold too….

Anyway, here’s a similar idea to create a filter with tube . The idea came to me by a post on the JC morrison and his creation the “tube EQ” sold under the EHX brand.

My project is a simpler one . A Twin T filter introduced in a feedback loop of a gain stage . Instead a creating a Notch (which a twin T filter does) it actually increases the frequencies in the peak and reducing the others above and below . It’s a wah wah effect . Is a topology found on maybe inductorless wah wah pedal (colorsound or the zvex wah probe which uses the same circuit).

The drawback of this simple circuit is that It didn’t give an effect as deep as an inductor based wah can offer. The transistor version has the same drawback. It’s more softer and work better with a small of distortion after.

As I have my tube tremolo going nowhere  I took the yesterday afternoon to try this circuits out. I also have few optocoupleur in my mess .


Once again , this kind of working isn’t safe . It must be done with care and attention. Even if I managed to not put High voltages in the breadboard things can get ugly if you’re messing around. The orange wood plate in here to protect the high voltage tube circuit.


I first powered up the circuit with a pot controlling the led (in the optocoupler) and it gave a nice “fixed wah “with a “nasal” sound .  Then , I put an envelloppe follower to create a enveloppe filter (the LEDs react with the pick attack) . I still have to make the circuit better , the LRD resistance get too much low for the effect. No big deal , just tweak the circuit until it gets right.

The effect isn’t agressive (without the cathode cap) or deep as you could except and the impedance stuff  must tame the thing a bit. However , put before a distortion it can adds some delecious distortion. Of course, you have to deal with all the hardware a tube has to have to work (high voltage , power transformer , ect…)

The cathode resistor has to be left unbypassed otherwise serious oscillations can occur. A switch can be installed to engage or not this cap. I think a potentiometer can be used as well .


This circuit can be implated with a pentode/triode (the ECF80/6BL8) as well (the ouput buffer is extra). But the gain has to be set proprely . Even if pentode can give large gain I’m not sure that larger gain will be better.

pentode wah

I think such circuit can be interesting with more experimentations , that I’ll do !  I’ll try to better up the enveloppe follower . But LFO or even a kind CV control like the ones used in Synth world could aslo do the trick !

to be continued….

here’s the JC morrison post :

I highly suggest to read the whole blog. Although it isn’t guitar related , but I must admit that good stuff isn’t , it exposes really really interesting circuits and electronic tube designs !


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