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RE: 6080 tubes for MA1 MKIII

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Ralph

In Reply to: RE: 6080 tubes for MA1 MKIII posted by Lew on January 26, 2008 at 20:16:16

OK- our experience is that the tube fails fairly quickly. We designed the amps for the Russian/Chinese 6N13 which is an equivalent. The specs are slightly different; with the American tubes the amps do make more power. We believe the failures in the American tubes are due to the grid structure warping a little, which causes the tube to arc. The specs do say 'not recommended for fixed bias service' and that is exactly what we are doing. OTOH and FWIW, there is no such comment anywhere in the Russian or Chinese spec sheets and they hold up fine.

Follow Ups:

RE: 6080 tubes for MA1 MKIII - pathologymd 14:48:15 01/28/08 (7)
Hi Ralph,

I am curious to know if the output stage change from plate to cathode resistors would make the American tubes possible now? I know since the change, I have not had one Russian tube go south in over three years. Of course it may not have anything to do with it, I am just curious.

Thanks,

William
- RE: 6080 tubes for MA1 MKIII - Ralph 15:22:35 01/28/08 (6)
  We think that that change and the associated move to 5 ohms rather than one ohm, does mean that the American tubes will hold up better.
  
  The evidence suggests that the current limiting resistors in the grid circuit may need to be a little higher value. This may limit output power very slightly, but it could also prevent the grids from warping, thus reducing arcing.
  - Question - Lew 06:25:13 01/29/08 (5)
    How are the "current-limiting resistors in the grid circuit" implemented? Are you referring to grid-stoppers? Thanks.
    - RE: Question - Ralph 10:39:25 01/29/08 (4)
      Yes. In out amps the 6AS7 runs a grid current window that is about 30V wide; about -15V with respect to the cathode (that seems to be where grid current begins) and about 15V positive. So the resistors help prevent current hogging.
      
      The 6C33 has a tighter window which is almost like a pentode- driving the gird into current means the tube is nearly saturated.
      - RE: Question - Lew 12:20:10 01/29/08 (3)
        
        In Reply to: RE: Question posted by Ralph on January 29, 2008 at 10:39:25
        
        So you are saying that the cathode resistors affect grid current by ameliorating the problem of current hogging. This has nothing to do with the grid-stop resistors.
        
        RE: Question - Ralph 13:04:33 01/29/08 (2)
        
        In Reply to: RE: Question posted by Lew on January 29, 2008 at 12:20:10
        
        Sorry- maybe I misunderstood. The cathode resistors were increased to 5 ohms, which really helped reduce current hogging in the cathode circuit, but the 'grid stop' resistors are also there to prevent current hogging in the grid circuit.
        
        RE: Question - Lew 08:13:00 01/30/08 (1)
        
        In Reply to: RE: Question posted by Ralph on January 29, 2008 at 13:04:33
        
        I thought that the raison d'etre of a grid-stop resistor is to reduce or eliminate ringing or hf oscillation when driving high transconductance tubes (such as the 6C33C). The grid-stop resistor in combination with the Miller capacitance seen at the grid forms a low pass first-order filter.
        
        RE: Right - Ralph 10:16:12 01/30/08 (0)
        
        In Reply to: RE: Question posted by Lew on January 30, 2008 at 08:13:00
        
        We never had much issue with oscillation, but in this case we're basically killing two birds with one stone...

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