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Hammond preamp




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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Topic - Hammond preamp - Thermionic27609 13:40:09 04/21/15 (12)

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