[Chas]

The earlier "S" (balloon) bulb 226 could be "damaged" if it was dropped and did not break. The elements, because of being secured only at the press could move side to side thus the relationship of the filament to the rest of the tube structure can cause an out of balance condition and exacerbate the likelihood of hum. Meaning, the use of 226 could still have a hum issues despite balancing. Therefor, to minimize the chance of hum the use of the 26 "ST" envelope that has support devices fastened to the tube inner structure that wedge against the shoulder of the glass envelope keeping the elements aligned. The changeover to ST bulbs also included changes to the oxide filament coatings to improve emission..

Note that a new power transformer will not likely develop the same B+ under load that the original did, the respective resistors connected to the center of the hum balance resistors are for bias. Insufficient or excessive bias will be a problem. The tube will have excessive plate current thus further dropping the B+ or the tube will be near cutoff causing a clipping distortion. Those values of resistors are not called out in the Riders printing of the schematic. If that resistor is still intact in an OEM SPU it should be measured to get a starting value for the bias...

Note too, to little bias in the RF stages will lead to excessive gain and oscillation of the RF stages... The RF stages are not shielded for high gain. In fact alignment of a normal receiver must be performed on a wooden bench or some 4" from a metal bench to prevent interaction with the RF coils...

GL

Chas