A simpler solution for amps with cathode biased output tubes, is to hook the heater winding CT up to the cathode(s) of the output tubes - that will elevate the heater winding to the cathode voltage of the output tube - which should at least quieten the potentially noisier pre-amp hum from the heaters. And yes - you should keep the heater AC pairs twisted together.critter01 wrote:Alan,do you think that something like this would eliminate that 50 hertz hum?
" On a SE amp wire heaters as a twisted pair, no CT, no 100ohms, no ground reference at all. If you do miss a ground reference you will fry your PT!
From 1st filter cap run a 2W 220K to one side of the heaters. From the same side of the heaters run a 0.5W 27K to ground. You should now have 35-40vdc biasing the heaters. Tweak 27K to make sure that you don't have less than 35v or more than 40v (SF voltage champs might need nearer 22K?)."cheers
But hum can come from several sources. Apart from heater hum and poor lead dress, you can get hum from SE amps just from a poorly filtered power supply. In fact, in SE amps you will be more likely to get more hum than in PP amps because in a PP OT, any residual ripple hum riding on the signal gets cancelled out in each side of the OT primary (whereas you don't have that ability with SE OTs). So a really good solution for SE amps, is to use a choke in the filter. A choke acts to oppose changes in current, so it takes the ripple right out of the power supply. For a SE 1 x 6V6 amp, you only need a 40-50mA choke to filter the whole amp. So build it like a normal 5F2A, but put the choke in series with the power supply rail between the rectifier and the 6V6 plate supply node, and put another 500V 16-22uF cap to ground on the rectifier side of this choke. (So you have PT- rectifier- cap- choke- OTcap-10kresistor -screencap- 22kresistor- pre-ampcap)
