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In Reply to: Re: DIY posted by hitsware on December 23, 2005 at 07:45:00:
Thank you for pointing out the errors in my thinking. Tubes can fail in either mode, as an open or closed circuit, but that does not usually happen except that the 6AS7 has an internal fuse which can blow and give you an "open", in which case one would measure no current across its plate (or cathode) resistor. With regard to my statement about what you would see if you measure V across one of the plate or cathode Rs in the case where the tube is functional, Thievenen's (sp?) Law would suggest you are correct.The reason for the introduction of the plate or cathode resistors in the Atma-sphere circlotron was indeed to equalize the current draw for each tube under dynamic conditions. One might argue that the very low value of the R used (don't remember but ca one ohm) would not be sufficient to have a significant effect on this phenomenon, given that each tube has a plate resistance of (don't know for a 6AS7, but greater than 100 ohms). I have no opinion because I have never installed this option into my Atma-sphere MA240s which use 6C33C output tubes.
Follow Ups:
Why isn't the 'fusible link' mentioned or shown in the poop sheet?
Or is it simply a weak spot in the current path that 'acts' as a fuse? ....... mike
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.
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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