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    Strange coincidence, possible connection?

    LeGrace
    LeGrace

    Posts : 281
    Join date : 2016-08-07
    Location : Ontario, Canada

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    Post by LeGrace on Thu Feb 28, 2019 1:32 pm

    Just got back from a couple days of skiing. Boy was it cold, -22c at the summit, -30c with windchill!

    So today I swapped the leads over on my bucker transformer. Now with 122v in seeing a cozy 115v out to the M125's. Hoping this increases operating margins sufficiently to be able to safely use Triode mode. I also replaced one tube pair where the red plating was primarily noticeable, but not the other pair. Because I think I should be able to get much better then 2000 hrs as long as they haven't been baked. GZ37 soldiers on, amazingly robust tube.

    Will monitor the situation closely.
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    Jim McShane

    Posts : 213
    Join date : 2011-10-19
    Location : South Suburban Chicago

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    Post by Jim McShane on Thu Feb 28, 2019 2:15 pm

    Peter W. wrote:...incoherent mumblings....

    Once more - a power pentode is NOT a triode. Operating it such that it mimics a triode (will never be more than an emasculated tetrode in actual service) will do all sorts of things - not necessarily fatal, but certainly contrary to the proper function and actual design of the tube. Want another analogy - going off-label with a drug. Might be benefits and there will definitely be differences - and possible consequences.

    If that's your opinion you are certainly entitled to it. However to make the blanket statement that triode connected pentodes or tetrodes makes it a poor tube is simply not the case.

    Triode connected pentodes have been in use fior a long time and in many (most) cases they deliver excellent performance. There are even cases where pentodes have been triode connected inside the tube when it was made to create a "new" tube. The most widely known example of that is probably what Sylvania/Philips did with the 6AV5GA. By triode connecting it internally it performed just like a 6B4G - these were sold to the US military as JAN 6B4G tubes. Triode connected EF86s make a very nice tube as well.

    I have personally run KT-90 and KT-88 tubes in triode on many occasions and I have been VERY happy with the result. My own amp is running KT90s in triode and it is really a terrific amp according to people who have heard it.

    So if you don't like triode connected pentodes that's one thing. To say they are not able to function properly is a bit of a reach - don't you think?
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    Jim McShane

    Posts : 213
    Join date : 2011-10-19
    Location : South Suburban Chicago

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    Post by Jim McShane on Thu Feb 28, 2019 2:20 pm

    gktamps wrote:VTA M125 "emasculation" occurs through connecting the suppressor grid to the plate through a 100R resistor as a grid stopper to eliminate oscillation, just as you indicated earlier.

    Actually no - the plate is connected to the screen grid not the suppressor grid. In most applications connecting the plate to the suppressor will give you lots of sparks and smoke since often the suppressor is internally tied to the tube cathode.
    Peter W.
    Peter W.

    Posts : 1268
    Join date : 2016-08-07
    Location : Melrose Park, PA

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    Post by Peter W. on Thu Feb 28, 2019 2:51 pm

    Jim McShane wrote:

    So if you don't like triode connected pentodes that's one thing. To say they are not able to function properly is a bit of a reach - don't you think?

    And a few of the Cadillac 4/6/8 engines ran very, very well. I do not dispute that it is certainly possible to run a pentode as if it were a triode. De gustibus et coloribus non est disputandum.

    I do, however, strongly question why one would choose to do so. I think in this case, specifically, a series of coincidences put the OP's speakers at risk from parasitic oscillation. And I think that one of the contributing (and necessary) factors was the fact that the amp was in 'triode mode' at the time.
    gktamps
    gktamps

    Posts : 19
    Join date : 2017-07-30
    Location : Santa Rosa, CA

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    Post by gktamps on Fri Mar 01, 2019 2:04 am

    Oops, thanks for the correction, Jim. I intended to state screen grid but misstated. The schematic shows it clearly.
    LeGrace
    LeGrace

    Posts : 281
    Join date : 2016-08-07
    Location : Ontario, Canada

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    Post by LeGrace on Sat Mar 02, 2019 5:15 pm

    Zero issues so far, even after 10 hrs run time. Wonder if dropping the supply voltage did the trick?

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