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I'm going to be rebuilding a 1930s Hammond Organ preamp, and I was hoping to get a second set of eyes on the original design. Hammond played around with a lot of different biasing schemes for the input pentodes in their early preamps (using one 57 pentode and one 56 triode). The first two versions used standard cathode biasing for the pentodes with electrolytic bypasses, but then they switched over to using voltage dividers to set the 57 (6J7 in this case) cathode voltage. One short-lived version used dividers to set both cathode and screen voltage, but that idea was abandoned. One of the design imperatives guiding these choices seems to have been that Hammond was intent on avoiding the use of electrolytic capacitors at all costs. The 4uF filters are oil-blocks, not electrolytics.
In this Concert Model E preamp, there are two input channels, and the preamp draws ~9mA via a 20k wirewound/380R divider just to set the cathode voltage on the 6J7s. Since B+ comes from the power amp through a 10k resistor in the power amp, this shunt regulation current lowers the B+ voltage to what I'd estimate to be 185-190V. Then, both 6J7s share a single 500k plate resistor, so looking at the schematic, it seems to me that plate current at idle must be very low.
The only volume control is upstream from the inputs, so 100% of preamp noise is heard via the power amp.
In terms of rebuilding, it strikes me that in 2015, one could fix the cathode voltages with a string of diodes, avoid the need for the 20k/380R voltage divider, and boost the B+ for more clean headroom. (Other Hammond two-tube preamps use the same output stage, but draw less current, resulting in higher B+ in the 235-260V range.)
So, does the preamp pictured here seem like an intelligent design or one that was based on idiosyncratic design parameters that we don't need to be bound by in 2015? We have silicon diodes, metal film resistors for lower noise, better quality electrolytic caps, etc...
I have broad experience with most Hammond preamps, but only about 750 of these Concert Model E organs were ever made, the last being made in 1942, so there's not a lot of collective experience out there about these preamps and how well they work.
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I absolutely would not make changes to the circuit topology or component values in that organ. Hammond went to great lengths to create exactly the sound they wanted, and it has been my experience that there is typically a great deal of interaction between the various circuits. A cap that appears to limit highs unnecessarily in one stage may be compensating for a response peak or key click generated several stages back. It's all too easy to make what appear to be reasonably appropriate changes, only to find that a specific characteristic of the tone has been degraded, or a new problem appears. Also, the Model E was specifically designed and "tuned" to compete with church pipe organs. I doubt that anything that changes the tone will actually be beneficial.
I wouldn't hesitate to replace all the filter/bypass/coupling caps with modern equivalents of the same value, and to ohm out all the resistors to see which ones are out of tolerance. At that age, nearly all the smaller parts will need replacement.
Love these tonewheel organs, my own is a M-102 spinet. Hoping to pick up a Leslie for the living room this weekend. :)
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Buy Chinese. Bury freedom.
I'm a professional Hammond/Leslie tech, and I've been working on Hammonds and Leslies since 1989. I have most of the preamps memorized, but this is the first Model E preamp I've seen due to the low production numbers.
One reason I'm asking about this is that the owner wants to see if it's possible to interface a Trek II spring reverb unit with this preamp, and it might help to maximize clean headroom from the input stages of this preamp. (I'm in conversations with Trek II about circuit considerations.) The Trek II unit is designed for Hammond tube preamps from 1950 to 1975. Hammond was obviously trying to regulate the cathode voltages of the 6J7s with a voltage divider, but they were locked into sourcing B+ from the power amps, making B+ variable depending on circuit draw. If I fixed the 6J7 cathode voltage with a string of diodes instead of the voltage divider, B+ voltage would rise to ~250V.
Yes, Hammond did make many careful design decisions, but this preamp is from around 1938 when they were still making lots of running changes. For example, they went through five different preamp designs between 1935 and 1939 in their standard line and three different designs in the Concert E organs. The M100 series came out in 1961, so your M102 is quite a different animal (including built-in spring reverb).
In organs I encounter in the field, the majority of these prewar organs have had these early preamps replaced with the Trek II SSP-1 solid state preamp, but this doesn't work with the Concert E, one of the very few Hammonds with two independent volume pedals.
"the majority of these prewar organs have had these early preamps replaced with the Trek II SSP-1 solid state preamp"
That's a shame. Funny, we just had a thread here about butchery. This puts me on the other side of the fence...
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Buy Chinese. Bury freedom.
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: 04/22/15
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.
There is some genuine ingenuity in that design. The only thing I might mess with is the bypass on the 2.7K resistor.
The other design choices you mention, though strange at first glance, are very intentional.
Would you care to elaborate on the circuit's ingenuity?
In the first version of this family of preamps, the output triode's cathode is relatively fully bypassed, but in the second version, they went to a 1uF bypass and stayed with that through all the remaining versions. I'm not entirely sure why they did this for an organ with tones down to 33Hz, but the organ did have a mechanical tremulant that might have created some LF pulsing. I also imagine that the 12" speakers used in Hammond tone cabinets of the era might not have handled the low pedal tones that well. But since anyone directly involved is no longer around, all we can do is infer. We do know that Hammond was aware of ways of fooling listeners into thinking they were hearing LF pedal tones that Hammond's tone cabinets couldn't quite reproduce.
If you consider that the designer was really limited by the capacitors they wanted to use, a lot of the oddities in this circuit are actually intended to keep it quiet, despite the power supply limitations.
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