The 6AX5 that I used with no Hum issues was RCA branded.
As far as I can tell there were only 2 companies that made the 6AX5 and the rest of the companies that sold them either got them from RCA or GE. The construction differences between the two are easy to see if you look around ebay or google.
As I said previously, I switched back to using the 6X5, and also have used a Bendix 5852 and a Sylvania 1274 and could not detect any sonic differences. Most of them were in the $5 neighborhood, but I think I paid $13 for the Bendix.
When I researched the failure issues with the 6X5 tubes, the way I interpreted the issue was this:
Early versions of the 6X5 (pre World War 2) were made with ST glass envelopes and used by Zenith in their large console radios that had a large number of tubes - some models having somewhere around eleven. The current required to drive those circuits resulted in some tube elements distorting from heat causing the filaments to short out. Apparently the larger ST envelope also provided less support to the internal tube elements than the 6X5GT design.
Where the circuits had a center tap to ground on the power transformer secondary winding, the shorted rectifier filament wound take out the secondary winding.
The SP14 doesn't have a grounded secondary winding by design, and is operated conservatively with regard to the Tube data published for the 6X5 back in the 1940's and 50's when most of the 6X5GT's with the smaller glass envelope were manufactured.
So particularly in the SP14 application, I think the probability of 6X5 failure occurring is low, and a failure causing secondary damage to the power transformer is even less.
I also need to make a correction to my previous explanation of the SP14 circuit.
The regulated filament voltage to the 6SN7's is converted to DC using solid state diodes before being floated on the B+.
I had said it was 6.3V AC previously.
As far as I can tell there were only 2 companies that made the 6AX5 and the rest of the companies that sold them either got them from RCA or GE. The construction differences between the two are easy to see if you look around ebay or google.
As I said previously, I switched back to using the 6X5, and also have used a Bendix 5852 and a Sylvania 1274 and could not detect any sonic differences. Most of them were in the $5 neighborhood, but I think I paid $13 for the Bendix.
When I researched the failure issues with the 6X5 tubes, the way I interpreted the issue was this:
Early versions of the 6X5 (pre World War 2) were made with ST glass envelopes and used by Zenith in their large console radios that had a large number of tubes - some models having somewhere around eleven. The current required to drive those circuits resulted in some tube elements distorting from heat causing the filaments to short out. Apparently the larger ST envelope also provided less support to the internal tube elements than the 6X5GT design.
Where the circuits had a center tap to ground on the power transformer secondary winding, the shorted rectifier filament wound take out the secondary winding.
The SP14 doesn't have a grounded secondary winding by design, and is operated conservatively with regard to the Tube data published for the 6X5 back in the 1940's and 50's when most of the 6X5GT's with the smaller glass envelope were manufactured.
So particularly in the SP14 application, I think the probability of 6X5 failure occurring is low, and a failure causing secondary damage to the power transformer is even less.
I also need to make a correction to my previous explanation of the SP14 circuit.
The regulated filament voltage to the 6SN7's is converted to DC using solid state diodes before being floated on the B+.
I had said it was 6.3V AC previously.
