Jim
70.90.144.126
I had been making some changes to my amitys, trying to get rid of a small buzz I had in one amp. I had originally set them up using the 6.6 volt windings from the mains transformer (lundahl) and dropping resistors to get 5 volts for the 300B filaments. This resulted in some longer runs across the chassis, and I was pretty sure this was the source of my buzz. I decided to place a pair of hammond 5 volt 2 amp transformers close to the tube sockets. I still had to use dropping resistors to get the filament voltage correct, two .1 ohm 2 watt per tube. I powered the unit up without b+ to measure filament voltages, and was surprised that I had hum, and if I pulled one tube, I had a lot of hum. I immediately blamed this on the location of the hammond transformers, as they are fairly close to the OT, but at right angles. I put a piece of steel between the filament trans and the OT, no change in hum, I pulled the 300b, dead silent, put 300b back in, hum was back, about 3mv across speaker terminal. I pulled the 300b, and placed a dummy load (4 ohm non inductive test load) across the filament socket, dead silent, no reading at speaker terminal. My conclusion at this point is the hum is being directly induced in the 300bs. This might account for the start-up hum that many report when they first power up dht amps. With b+ applied, the hum drops, one amp measures 0.2mv (meter reads .4, but it reads .2 with the unit switched off), the other amp is .8mv. I did get rid of the buzz! This might be interesting for those who have built amps and are trying to get rid of that last little bit of hum - problem may reside in the tubes themselves. Additional info - I use a separate winding for each 300b with its own hum pot for ac balance, and I use an additional pot between the two tubes for dc balance. The pots are 50 ohm units with 22 ohm resistors from the wipers to each leg. The 300bs are EH gold grids.
Jim
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