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Opinions on plate voltage and plate current on vintage 6V6GT tubes

Discussion in 'Glowing Bottle Tube Amp Forum' started by theprofessor, May 26, 2017.

  1. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    If I was wanting to drop the voltage supply on this amp, I would probably try a zener on the transformers center-tap, and while I was at it, I would take advantage of it to back-bias the amp from this new node, giving a much smoother negative voltage than the stock half-wave rectified negative supply, this would still work with the vibrato. Aikens has a very good article covering the options, where one can choose to have a bias supply that sags with the playing dynamics, or of a stable constant voltage bias supply.
     
    Last edited: May 28, 2017
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  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks so much, bendyha, for all your input. Unfortunately, much of it is over my head. I don't have enough of an understanding of how amps work (I should say, very little understanding of how amps work) in order to make sense of this. Perhaps as I learn more, however, I can re-visit these data. Thanks again!
     

  3. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Then the rectifier tube has its characteristic voltage drop at idle, and it has its characteristic sag voltage under load, both should be some what predictable.

    I have measured sag. I think a 5y3 sagged as much as 100 V under extreme load.


    In the DR the 5u4 may sag 50 v, from 450 to 400, while a 5y3 could sag 100 v, from 440 to 340. And the difference you are hearing is the 340 vs the 400 under load, not the 440 versus the 450 at idle.


    You've got to pummel the amp to get sag like that, probably very little sag when volume is half way up.
     
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  4. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    @peteb : where do you fall on the spectrum of emphasis on how to treat your vintage 6V6 tubes when biasing them?
    • worry mostly about excessive plate current, with the possibility that plate voltages might get very high (Kelley)
    • worry mostly about excessive plate voltage, specifically that it not exceed 425 or 430v, if at all possible (Weber). See also Mike Kropotkin of KCA NOS Tubes at the very bottom of this page of his website, where he states, "Be careful when going to a higher rated rectifier in 6V6 amps. You should always avoid having more than 430 volts across a 6V6 (plate voltage minus cathode voltage)."
    I realize that one can be concerned with both, but in the opening post I juxtaposed two quotations by two amp experts which seem to emphasize opposite things as being the most important. What's your opinion?
     

  5. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    As to the 12W 14W issue, this registration for designation 6V6GT from Nov. 1958 shows the changes of Hytrons 1939 originally released 6V6GT, and its alterations over the previous registration modification from 1953.
    Reading the July 1956 Sylvania regestration of the 6V6GTA shows that it specifies that all ratings and parameters are identical to the 6V6GT, with the exception of the filament heat-up time.

    upload_2017-5-28_19-53-55.png
     
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  6. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    I think Kelley was an engineer, and Weber was a former meat cutter turned technician, then guru.

    You often see violations of voltage OR current in guitar amps, but it is excess power that makes the plates glow and burn up.

    Weber's plan to increase the current to make the voltage sag to what he feels is a safe level may work on a particular amp, but bad advice in general. What if the power transformer has an overrated current spec? The voltage will hold up much better while the current climbs.
     
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  7. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you, Bendyha! What a great piece of data!
     

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you, FenderLover. That's what I thought, too: it is excess current that makes tubes go south--and fast. So I find it all the more interesting that Weber (and perhaps also Kropotkin, as stated in post #24 above) would emphasize the plate voltage number. How far should one turn up the plate current to get the plate voltage down to those numbers? That depends on the amp, I suppose. But in my case, it would be quite a lot. It seems my amp would soon go "nucular," as a former U.S. president pronounced it.
     

  9. danlad

    danlad Tele-Meister

    185
    Mar 24, 2015
    Here and there
    The thing for me is you design your output stage then hope to hell your power supply can feed it. Output stage at idle I can understand. Output stage driven hard into class B or beyond, well then it becomes something I didn't design. Even if it takes a new thread, any thoughts & insights, go on!
     

  10. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    I prefer low current, 50% bias, for very unscientific reasons. I've never had a 6v6 amp that wanted to put much over 400v on the plates, so I really can't compare what I've done to a DR. My main reason for low current is to take it easy on the PT and the tubes.


    Weber, who I generally listen to, seems to have a low number, 425, in mind, maybe it is out of date. He must have expererienced over voltaged 6v6s, as he strongly recommends staying away from a SS rectifier on a DR.


    I also think that yours is safe to play as is, because that's the way they are and have been for a long time.
     
    Last edited: May 28, 2017
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  11. Jim Kelley

    Jim Kelley TDPRI Member

    14
    Jan 11, 2014
    Rural Orange County
    A few things to consider here:
    *Try to always run your amp at the line voltage for which it was designed.
    *Not all 6V6GT's have the same specifications, and databooks from the 50's and 60's don't necessarily give accurate maximum values for tubes manufactured in later years.
    *Current measured at the cathode is the sum of plate current and screen current.
    *The screen grid is the most delicate current carrying electrode within the structure.
    *Tubes with contamination, poor vacuum, or a leaky control grid are likely to suffer catastrophic failure far more often with high plate voltage than at low plate voltage.
    *For the gospel according to RCA, please turn to pages 129 & 130 in your RC-29 receiving tube manuals for additional perspective on absolute voltage limits for small vacuum tubes. (3A2 and 3A3) :)
     

  12. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Dear Sir,
    Thank you very much for your input. I certainly hope I have not mis-represented any of your views or couched them in any misleading context. I am a beginner in my understanding of the ins and outs of tube amps, and I greatly appreciate your expertise. Thank you for taking the time to comment.

    If I may:
    - Given the first point you posted, is it advisable, in your opinion, to use voltage limiters like a variac or Brown Box or Amp Preserver to keep the line voltage within vintage spec?
    - It seems to me that the remainder of your comments assume the foundation of a high-performing, well-built vacuum tube. So in that respect, everything rests on which tube and on having one with optimal performance.

    Cheers!
     
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  13. Jim Kelley

    Jim Kelley TDPRI Member

    14
    Jan 11, 2014
    Rural Orange County
    I really didn't intend to take issue with anything anyone said - only to add data points to the discussion.

    Yes, there should be some means of adjusting the voltage - either up or down, depending on the situation. Just to clarify, these devices are usually a type of variable transformer.

    With regard to using optimal
    components, yes. I have found over the years that the cause of tube failure is too often mistakenly attributed to the amplifier.
     
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  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you very much. I did not perceive your comments to imply your taking umbrage with anything anyone said. But out of respect, I wanted to offer that comment, just to be sure.
     

  15. alnicopu

    alnicopu Friend of Leo's

    Oct 3, 2009
    georgia
    Don't worry about it. It's a 30+ year old amp and still working. If you do want to worry about it, stick a 5v4 in there. My sf Princeton reverb was sitting at 440vdc with the 5u4gb that came with it. The 5v4 dropped it down to almost exactly what the original schematic said, under 410vdc.
     
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  16. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    What anyone says is you need to watch the volatage and the current because their both going to be high. I think what G. Weber is saying is don't go too high on the voltage, it's over spec and will fail if pushed, how do you know when it's too high voltage? So he sets a line in the sand, and raises the current, and follows the regular rules about current. of it red plates, it's too high. That's the pint. The current is not good, but at least you can monitor it.
     
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  17. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    *For the gospel according to RCA, please turn to pages 129 & 130 in your RC-29 receiving tube manuals for additional perspective on absolute voltage limits for small vacuum tubes. (3A2 and 3A3)

    Now that is high voltage! Okay, it is an octal diode with the plate cap on top, but it is still a smallish tube. upload_2017-5-30_10-6-53.png
     
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  18. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

  19. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    All diodes drop a certain amount of voltage, think of them as being like a gate that only lets voltage through in one direction, but you need a couple of volts energy to push the gate open.
    Mostly rectifier diodes need about 1V to let the positive wave through, but shut-down when the negative wave is trying to get through.
    Zener diodes tend to be used the otherway around, and rather than being trimmed for efficiency of letting as much through as possible, they are made using different mixtures of minerals, etc., to have a higher voltage voltage needed to get them to open up. Usually available in stages from about 3.3V to 10V in 1/2V steps, then up to 40V in 1V steps then 5V steps......up to 200V or more. Because of the current passing through them, and the energy they use-up themselves, gets turned into dissipated heat, one can buy them in wattage rattings from 0.5W, 1.3W 5W......25W...(there are some massive industrial ones that are available) The voltage getting through, once the set voltage is reached, or for our purposes "Dropped", is no longer affected by the Zener thus it causes no sag.

    Depending on how they are used, zeners can be used to drop a set voltage, as it might get utilized in the above case, or to keep a voltage at a constant level, dumping any voltage above thier set limit to ground, and affectivly eliminate any tendecy to sag that a suppy might suffer from under changing load conditions.
     
    Last edited: May 30, 2017
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  20. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    thanks Bendyha
     

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