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In Reply to: RE: Hammond preamp posted by Triode_Kingdom on April 22, 2015 at 08:16:10
Unfortunately, a lot of organ techs aren't really tube amp techs. They know how the hook the equipment up, but they don't really understand the circuits or how they work. Plus, I would imagine that in the pre-internet days, finding Type 56 and Type 57 tubes was a headache. The 1930s transformers fail more often than later ones, and the solid-state preamps don't require an outboard power supply, making the organ more flexible in terms of using one with, say, a Leslie 147 that doesn't supply B+ via a standard hookup.I'm one of the few organ techs I know who rebuilds tube preamps. It's tedious work having to replace the point-to-point wired components mounted under the chassis on turret boards.
IMO, Hammond really got their preamp design right in 1950 with the B2/C2 series. They added to it with the 3-series, but that basic circuit was used from 1950 through 1975.
Edits: 04/22/15Follow Ups:
Yes, I understand the difficulties of maintaining old tube gear first hand. It sounds as though you've got a pretty good handle on this. I'm not sure exactly why you want to make these modifications, but it's not too difficult to do. My only advice is to test thoroughly after each change. Given your experience with Hammonds/tubes and the rarity of this model, I doubt you'll find much specific help here. LTSpice could be a great aid in determining beforehand what to expect in terms of changes to voltages and gain.
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Buy Chinese. Bury freedom.
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.
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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