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In Reply to: Re: Re plate or cathode resistors posted by hitsware on December 27, 2005 at 09:35:23:
The instructions for the amplifier kits years ago had a copy of a data sheet from Tung-Sol for the 6AS7. It does not mention a fusible link either. There is a diagram of the Tung-Sol tube and it is shaped like the Sovtek or Svetlana. But I bought, in ignorance, RCA 6AS7 and it is smaller, just an inverted U-shape without the heat dissipating structure on top. Those tubes arc pretty quickly, as I learned. I looked at them and at the 6SN7's and their links look no stronger or weaker than the ones we're talking about. But the two RCA tubes I tried as emergency replacements, for all their arcing, still have their links intact.The original update installed 1 ohm resistors at the plates and 150 -180 ohm resistors at the grids.
Follow Ups:
The 150- to 180-ohm resistors at the grids are "grid-stoppers" and have nothing to do with the subject of this thread. Grid-stoppers prevent hf oscillation caused by an interaction related to the Miller capacitance of the tube itself. Typically grid-stoppers are needed where the tube has a very high Gm. It's those one-ohm resistors, which in a circlotron could be installed either at the plate or cathode, that are there to ameliorate imbalances between the tube sections that are in parallel with each other.
I think for current sharing the 1 Ohms need to be at the cathode. As you said before, the high output impedance of the plates should make a 1 Ohm resistance almost moot. (I'm relating to SS cicuitry (all I know anything about)) so may be missing something......mike
Thanks for the information. I'm very ignorant and try to keep my comments to just what I have experienced with my amplifiers. The rest of the time I lurk and learn. If resistors at either plate or cathode can have the same effect, can you say why they are now put at cathode rather than plate?
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