Ts4d Motorola

[dieseljeep]

Originally Posted by timmy

Well still cannot find the source of the hum but is it at all possible to get a hum like this from hv, the 1b3 gets its heater voltage from the hv coil so from there I don't know. The Motorola Manuel states that there is a 22 ohm resistor between the crt and the audio amp heaters to keep audio hum down but that resistor is there.

The 6SQ7 is a high mu tube. Try a 6SR7 tube, that has a lower mu.
The 22 ohm resistor in parallel with the 6SQ7 heater has to be in there, as the 6SQ7 tube has a heater rated at 300ma, directly in series with the CRT, which has a 600ma heater.
I'd be tempted to rewire the socket for a 6J5, which is a half of a 12SN7, but for now the 6SR7 is a plug and play replacement.
I have to look at the schematics again and do some comparisons with the various versions.
I never ran into this version, only the ones using the 12SN7.