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In Reply to: RE: Hammond preamp posted by Triode_Kingdom on April 22, 2015 at 10:37:25
The interest in modifications is this:
The owner, my customer, wants to use this preamp with the Trek II RV-1 aftermarket spring reverb unit (probably the best sounding spring reverb I've ever heard), which was designed for use with the AO10 and AO28 preamps in Hammond 2 and 3 series organs. It intercepts the signal to the output triode (half a 6SN7 or 12BH7, very similar to this older preamp) and adds reverb. Its input impedance is slightly lower than the 1M grid resistor here, but it's above 500k. If raising B+ would increase signal swing from the input stage, this might make the preamp and spring reverb more compatible. Trek II has never sold one for this application, and knowledge of these preamps is scant, even among Hammond/Leslie techs. You just don't see them that often. I've tried the Hammond/Leslie lists, and no one bit. Unfortunately, a lot of the older guys who used to like to talk about this stuff have drifted away from the groups.
Complicating matters is the fact that the organ and owner are in another state. It's being sent to me because I work on a lot of Hammond preamps, but I've never worked on this model. So I'm trying to go into this knowing as much as I can... If it weren't for the owner's interest in the reverb unit, it would be a straightforward rebuild job, and I wouldn't need to ask any questions.
Follow Ups:
I understand, and I'll say it again - you could spend days or weeks analyzing the circuit and making the changes empirically. Or... you can download LTspice (if you haven't already) and simulate the circuit. Nothing is faster than this when you're trying to set up DC values and maximum headroom and response. I won't even build a relatively simple circuit now without analyzing it this way first, just because it saves so much time later. Seriously, when you put this into SPICE, it's like having a whole team of experts giving you the best advice on the planet. :)
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Buy Chinese. Bury freedom.
I've actually never used SPICE, being a Mac user, but I see it's now available for OSX. I guess it's a learning curve I need to climb.
Believe me, it's not nearly as difficult as it first appears. There are a number of tutorials on YouTube, and tube models are available free from several online sources. Have fun!
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Buy Chinese. Bury freedom.
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