Tube parametric

[lowell]

hello all,
want to build a tube parametric guitar pedal. i can't find a whole lot as far as schematics go online. does anyone know if the "simple parametric" schematic on geo would work w/ tubes in place of the opamps within the gyrators?

[cbarrow7625]

The short answer is "no, it won't work". The gyrators are non-inverting op-amps. A single tube stage is inverting. Tubes also have a very high output impedance that won't work well with this design.

You could try to make a non-inverting tube stage with a low output impedance by cascading two tubes plus a cathode follwer. I think you'd be creating a mess though and it still wouldn't work all that well.

Many 'Pro' tube parametric EQ's are actually completely passive with tubes surrounding the circuit for gain manipulation. Here's a good example:

http://www.manleylabs.com/promain99.html


To do an "active" tube EQ you would need generous doeses of local negative feedback. If you are doing that how "tube-like" would you expect the results to sound?

Hopefully somebody else has better news for you.

[lowell]

hmm... ok bummer. but maybe a tube input and output stage w/ the geo parametric in b/t using the opamps?

[cbarrow7625]

Sure, why not. That should work fine.

[lowell]

thanks for your input!

[R.G.]

It's also worth noting that just rubbing tubes on some circuit does not necessarily make it sound any better, just like using germanium transistors doesn't automatically make a pedal sound better. Unless you know what you're doing with the tubes (and the germanium) it doesn't help with anything except truth in advertising.

[lowell]

I would think a tube input and output should add some warmth to the signal. Especially if the tube circuitry is maximized for that...?

[Shea]

I would think a tube input and output should add some warmth to the signal. Especially if the tube circuitry is maximized for that...?

Is this for a rack-mounted effect or a pedal? If it's a pedal, it would be such a hassle to stuff a power supply for a tube in there, not to mention the tube itself, that it'd be much more efficient in terms of time, effort, and money to build a solid-state parametric EQ that is totally clean, and rely on the tubes in your amp or in another effect to do the things that tubes do.

But OTOH, David Gilmour's pedal board has a tube buffer before and after every single pedal, so maybe there's something to the idea.

Shea

[lowell]

Shea,
hmmm that's very interesting I didn't know that... very cool. Gilmour has some killer tone. My idea w/ this pedal is to use 1 6021 mini dual-triode as the input and output buffers. I found a cute little toroidal transformer and hammond chassis w/ air vents that should do the trick. I really don't think the power supply will be very complex or hard to wire. Gonna add a mini fan to the equation too. Probably doesn't need it but why not. I ABSOLUTELY hear what you're saying as far as time and money go, but since I'm making this by hand and I want it to be original I'm willing to invest in it. There are plenty of "affordable" pedals out there that do the trick... but that's not what I'm going for. I might build a ss and tube version and compare them... we'll see.