Elevated heater potential is commonly advised when stacked tubes, such as a mu-follower, is used.
But let’s say I have 320V at B+ and will use a 6DJ8 in a mu-follower topology. 100V at the bottom tube anode, 120V over its anode load and 100V over the topp tube. That will put respective cathodes a couple of volts above gnd respective 220V.
Elevating the heater to 100V from gnd will put it -120V below the topp tube which is at the limit for a 6DJ8 (U+k-f).
But from what I understand the limit is harder for the bottom tube cathode to heater (U-k/+f), stating only +60V.
Have I misunderstand the specs or is the limitation of heater elevation for 6DJ8 actually 60V, setting a limit for the top tube catode at 180V (60+120).?
But let’s say I have 320V at B+ and will use a 6DJ8 in a mu-follower topology. 100V at the bottom tube anode, 120V over its anode load and 100V over the topp tube. That will put respective cathodes a couple of volts above gnd respective 220V.
Elevating the heater to 100V from gnd will put it -120V below the topp tube which is at the limit for a 6DJ8 (U+k-f).
But from what I understand the limit is harder for the bottom tube cathode to heater (U-k/+f), stating only +60V.
Have I misunderstand the specs or is the limitation of heater elevation for 6DJ8 actually 60V, setting a limit for the top tube catode at 180V (60+120).?
The stated limit is the stated limit; 60 +120. Whether you want to violate that limit is up to you. It's not clear how manufacturers actually determined the limit, or whether they considered it a 'hard' safety limit or a 'soft' performance limit. Philips quoted it as 50V & 150V, RCA quoted 200V for both sections, etc. Was there really a difference in the tubes or just in the manufacturers' definitions?
The heater-cathode potential can cause leakage over time. Certainly conduction if the heater is negative w.r.t. to the cathode. This occurs over time with better tubes, no estimate can be given.
You are best to use another winding or transformer to power the heaters "on top".
You are best to use another winding or transformer to power the heaters "on top".