Servicing the Hammond B-3 Type Pre-Amp

  
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Designed in the fifties, this pre-amp remained in production until 1975. And, it remains fully functional and serviceable today. This discussion is designed to help owners do some basic troubleshooting and, we`ll get into some depth of circuit operation as well as common problems usually found here. If the vibrato switches are set to vibrato off,
Servicing the Hammond B-3 Type Pre-Amp - schematic

the signal from the matching transformer goes into Point A , a solder terminal located on the preamp. If you had a problem with the non-vibrato sound, it would likely be in this area. Basically the signal is routed to the input grid (pin 1) of the 6AU6A tube for amplification. The voltages shown are usually the first thing to check. If the tube is good, then the two most likely sources of problems will be R6 (plate load resistor) or R7 (screen grid resistor). If you check the plate voltage for example and it reads low, then the plate load resistor R6 is probable going bad. (opening up) If however the voltage on the plate reads high, then the screen grid resistor R7 is going bad. (opening up). The vibrato input is nearly the same as the above but for a few differences. First, the 1. 7Vdc is actually created here on V2 ²s pin 7. This reference voltage is used on several other tubes. Second, the output goes into the vibrato delay line driver tube. This is one-half of V3, the 12BH7A output tube. Again the plate and screen voltages should be checked on V2 like above. Additionally, the voltages on V3 should be checked. Any loss of amplification here will cause the vibrato to either be weak or not work. V4 is a dual triode tube. The first stage V4a accepts signals from the non-vibrato preamp, the vibrato scanner, and the percussion circuit. These signals are again amplified to a sufficient level to drive the expression control....



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