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Old 24th March 2008, 08:02 PM   #1
jenks is offline jenks  United Kingdom
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Default replacing op-amps with thermionic valves

hello!

I'm in the middle of designing a preamplifier circuit on multisim to go in an stomp box for bass guitar. I have two OPA627AM op-amps for the peak/notch part of my circuit but I was hoping to replace them with thermionic valves. I have two 12AX7 valves that I've noticed have a pair of triodes inside. Can I use one of these valves to replace both of my op-amps?

If so tell me how! and, if not, do I need a different type of vlave to do the job? I was initially gonna keep the valves for a 'drive section of the pedal but I haven't figured out how to do that yet. Any advice on perhaps using the valves as an intentional overdrive at a stage after the preamp section (and keeping the op-amps in the preamp stage as they're of rather high quality anyway) would be appreciated too!

Thanks! :-)

Jenks.
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Old 24th March 2008, 08:47 PM   #2
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Default Re: replacing op-amps with thermionic valves

Quote:
Originally posted by jenks
I'm in the middle of designing a preamplifier circuit on multisim to go in an stomp box for bass guitar. I have two OPA627AM op-amps for the peak/notch part of my circuit but I was hoping to replace them with thermionic valves. I have two 12AX7 valves that I've noticed have a pair of triodes inside. Can I use one of these valves to replace both of my op-amps?
Sure, it's possible, but why would you want to?

U1 is just a simple unity gain buffer. A cathode follower could replace it easily enough. As for U2, you treat the grid as the (-) inverting input, and the cathode as the (+) non-inverting input. All these op-amp based filters were originated back in the 1940s with VT implementations in mind. Of course, the 12AX7 won't have the open loop gain of the op-amp, and so will probably require component tweaking in order to get similar performance.

That schemo looks perfectly serviceable. If it's VT-type distortion you're after, an over driven VT stage after that will do it.
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Old 25th March 2008, 02:45 PM   #3
jenks is offline jenks  United Kingdom
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Default bass preamp & OD

Hi Miles!

Thanks for your reply!

Yes, I've had a little read and it definitely makes a lot more sense to include my 12AX7's in a seperate overdrive stage, after the tone shaping controls.

I've been looking at a few circuits this morning (which I've attached) I'm intending on using this pedal as my overdriven sound, as i have the MXR blowtorch for all-out distortion/fuzz so I was thinking to try using the 'soft-clipping' circuit. I'd love to try the MXR one too but I've no idea how it manipulates the signal so I'm tempted to leave it alone until I'm a bit more experienced on this topic!

Any advice on how to best utilise the valves I have for achieving a rich & warm tube overdrive sound in my pedal will be greatly appreciated!

thanks again.

Jenks.
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