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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. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    Hi all,

    Having recently come into ownership of a Silverface Deluxe Reverb, I've begun to consider plate voltage even more than previously. It seems pretty common to find plate voltages in these amps at about 450 or even higher when running 6V6 tubes at about 70% mpd, especially on high-voltage days when I've got 122 volts coming through the line or whatever. I realize that some folks in other parts of the country can have even higher voltage than that.

    So I started thinking about how to get plate voltage down, the first and cheapest way being a change in rectifiers from a stock 5U4GB in a Silverface DR to a 5R4GYB. I didn't like the sound of it, though, and when the 6V6's were biased at the same point (68%), I only gained about 7-8 volts with the 5R4. That made me not want to pursue other ways of bringing down the voltage, such as getting a variac or volt limiter like The Brown Box or whatever.

    But this got me reading a bit. A member here reminded me that Jim Kelley has been known to run 480 volts on 6V6GT's. So I found an article about Kelley and his amps. Here is an excerpt; "One spec that has often raised eyebrows is that Kelley ran the 6V6GTs at upward of 485 volts DC on the plates – hotter than even the notoriously hot-running blackface Deluxe Reverb, but he tells us that any concern regarding these numbers is unfounded. “Voltages aren’t the big issue with vacuum tubes, current is. They don’t handle high current very well, so you do have to watch the current very carefully if you’re running at the upper end of the voltage range.”" (https://www.vintageguitar.com/26793/jim-kelley-facs-reverb/)

    On the other hand, there is Gerald Weber's A Desktop Reference of Hip Vintage Guitar Amps, where he comments on 6V6's in a Deluxe Reverb: "Biasing is critical in a Deluxe Reverb, because if the tubes are biased cold, the plate voltage will increase beyond the limits of a 6V6. 30 mA per tube is what I recommend [my comment: a 12-watt tube at 30 mA of current and 425 plate volts is at 106% mpd!]. After biasing, I check the plate voltage and, if it is over 425 volts, I bias the output tubes for more current, until the plate voltage is no more than 425 volts" (p. 66).

    Discuss!
     

  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    There's two common specs listed for the 6V6 that give 12 or 14 watts max dissipation. Some very knowledgeable people just bias all 6V6's as 14 watt tubes.
     

  3. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    If I recall correctly, those different specs are "design center" and "design maximum," where the "design maximum" is 14 watts. If I recall what @Opaltone said correctly, at some point the spec was changed to where the "design maximum" was the primary spec instead of the "design center." Two conclusions might follow from this, both suggested at some point by @Opaltone (I do hope I'm remembering correctly!):
    1) Many claim that the RCA 6V6GTA's can handle more current because they're 14 watt tubes. But this is really just a case of the "14 watt" number coming from a change in the way that specs were listed as primary, as noted above. So they're not really any different than previous tubes whose specs were given/weighted differently.
    2) It might follow, too, that if, say, RCA 6V6GTA's can be treated as "14 watt" tubes, then, at least in theory, so can earlier models of 6V6GT's.

    But still: the comments of Kelley and Weber noted above cut in totally different directions, as far as I can tell. One is very concerned about plate current (Kelley), while the other is very concerned about plate voltage (Weber).
     
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  4. Opaltone

    Opaltone Tele-Holic

    518
    Aug 12, 2004
    Berkeley, CA
    You recalled correctly - at least what I'd previously mentioned to you. The 14W max dissipation can be applied all the way back through 6V6GT, 6V6G and 6V6*, as far as I understand.

    *watch the pin connection in metal tubes as the grounding may be tied to the metal shell, making it "live" and possibly transferring the ground through your body (i.e. a very serious electric shock hazard).

    Redplating results from surpassing the tube's current capacity, and it can kill your tubes AND possibly damage your amp. If the voltage is too high, however, what I'd be concerned about is high-voltage arcing, which would also lead to rapid failure. I don't know how Jim Kelley became confident he could push voltage so high, but I suspect he pushed voltage even higher, until he found some failure limits, then dropped back from there. I'd be curious to hear about that.

    - T
     
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  5. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Here's a question I don't know the answer to. If you guys, there's at least three of you, don't like the DR sound when the rectifier drops the voltage, would you like the sound when the voltage is dropped by other means?



    If you lower the voltage of the power supply, watch out that you don't lower the filament voltage too much, it should not go below 6 volts. What does it measure now?

    EDITED: I thought you wrote 'that made me want to drop the voltage by other means' when you wrote 'That made me NOT want to drop.........' which means my first question doesn't make any sense, so never mind.
     

  6. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Try #2 at making a valid comment:

    you didn't like the result when the rectifier tube change dropped the plate voltage by 7-8 volts, and that makes you not want to try dropping the voltage by other means.


    I would think that a change of eight volts on the plates would be hard to hear the difference of. Is the difference that you are hearing the small change in the plate voltage or does the rectifier change change the sound, tone, feel for reasons beyond just the small voltage drop?
     

  7. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    Thanks, peteb. I don't know how else to account for what I heard. Maybe there is something else going on with the rectifier and the resultant sound other than the voltage drop. But it definitely changed the sound and response of the amp. In a few words, I would say that the 5R4GYB made the SF DR: thicker, less sparkly, woofy in the bass, break up earlier, and a bit hairer, with less definition between the notes. I give an account here of what I did (post #111): http://www.tdpri.com/threads/the-pearl-of-great-price-nad-1976-deluxe-reverb.731668/page-6

    And in light of @Opaltone 's post above: For the purpose of discussion, let's--for the moment--treat all 6V6GT's as 14-watt tubes. If we re-work Gerald Weber's suggestion of 425 plate volts with 30 mA of current, we get 90% mpd, instead of 106% mpd if it were with a 12-watt rated tube.
     
    Last edited: May 26, 2017

  8. Opaltone

    Opaltone Tele-Holic

    518
    Aug 12, 2004
    Berkeley, CA
    There are definite sound and feel differences when dropping B+ - especially feel! And the current running through the rectifier is in dynamic flux, so there may also be a difference in the way the rectifier changes the power with its unique delivery signature. I can perceive (hear/feel) differences between rectifier types and different manufacturers of the same type of rectifier. What theprofessor has described is right in line with my own experience.

    At least it doesn't calculate out into the known/predicted meltdown zone.

    - T
     
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  9. slider313

    slider313 Tele-Holic

    Age:
    60
    915
    Jan 6, 2011
    NC
    Okay here's your post with the numbers you're getting:

    "Then I switched out the JJ's with my matched pair of Tung-Sol black glass 6V6GT's. Have I ever said how much I love those tubes? I think so. Anyway, they sounded best to me right at about 70%. (Note to self: at 437 plate volts, my "Motorola"-labeled one was at about 19.2 mA, and my "Delco"-labeled one was at about 19.0 mA."

    I wouldn't be too concerned with the 437v plate at all. The amp does sounds good, right? Tung Sol made a very well built 6V6GT and an even better 6V6GTA.

    Now try Gerry Weber's theory. Crank the bias until the plate voltage comes in @ 425v and record your current draw in ma. What's the plate dissipation? How does the amp sound? How long do you want those Tung Sol's to last? Do you know when you play the amp, and push signal through the power tubes, the current goes UP and voltage goes down?
     

  10. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    Thanks, slider313. Yes, that was the first day I had those 6V6 GT's in with the 5U4GB. I think the wall voltage was right at 120 v AC that day. The day I recorded the numbers on the 5U4 versus the 5R4 rectifiers, the voltage at the wall was 122 AC. The plate voltage was higher--about 452.

    I'll have to try out Weber's theory sometime and see where those Tung-Sol 6V6's come out with plate current in mA. The thing is, I really like the sound where they are, before things get "flooded" or overly warm and loses some definition. I want the tubes to last a long time, and so Weber's theory doesn't sound like the right one for me!

    In line with @Opaltone 's posts above, if we rate the Tung-Sol black glass tubes as 14W tubes, let's do the math at 455 plate volts: 14/455 = .030769 x 5.9 = .181538 (= about 18.2 mA). So if they're really 14W tubes, mine are currently biased at about 59% (I already had the 18 mA plate current number, which is why I inserted the multiplier as 5.9 above, or 59%).
     

  11. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    It's well known that changing plate voltage affects the sound. But, 7-8 volts is less than 2 percent of 450, so the difference should be barely perceptible at most, it should not wreck your tone. What you describe is a browning of the sound but 440 volts on a 6v6 isn't brown territory. What about amps that put 400 volts on a 6v6? Are they all lousy amps? Or, does the magic of the DR appear when the plate voltage gets up to 450?




    Just some thoughts



    Isn't part of this thread about recognition by long time DR players that that's the way the DR is, it works that way, and it sounds really good that way. There is nothing to change.


    I've only barely tried a DR, but I play 6v6s at 400v with no complaints
     

  12. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    I don't mean to suggest that 400v territory is bad for all amps. I even noticed a great deal of difference between my DRRI, which ran at about 415 with a 5V4 rectifier and this SF DR, which runs at about 445-450 with the spec rectifier, a 5U4. I changed the DRRI rectifier from a GZ34 because I liked the sound much better with a 5V4. Again, only a change of about 8 volts or so. But I heard a subtle difference. The difference between the 8 volts on the SF DR--between a 5U4 and a 5R4--was much more clear to me than the one I noticed between the GZ34 and the 5V4 with the DRRI. I was very, very happy with the DRRI at 415, and I might have been even happier with it closer to 400.

    The "browning" effect I'm describing was clear to me, even with an 8-volt drop. I had the tele in the middle position the whole time--both with the 5U4 and 6V6's biased at 68% and with the 5R4 and the same 6V6's biased at 68%. The tone was fatter and woofier with the lower voltage. Perhaps it is not so much what is happening with the 6V6's as what it is doing to the preamp. Is it essentially shifting more gain to the preamp stage?

    And, anyway: I'm still very, very interested in opinions on what seem to be opposing positions on the plate voltage and plate current on vintage 6V6 tubes by Kelley and Weber. It sounds to me like they are saying the opposite things.
     

  13. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    Might be the different current handling abilities of the rectifiers is causing the changes in tone ?
    I mean "sag". I suppose if a guy had a voltmeter set up to monitor things while playing, you could find out.
    I'm way too lazy to do that though.
     
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  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    BobbyZ makes a good point. When you lower the static, idle voltage with a lower output rectifier tube you also add dynamic sag so that could be part of what theprofessor is hearing.
     
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  15. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    That makes sense to me. I have normally thought of "sag" only in terms of slowed response and attack. But if it is indeed the "sag" that's contributing to what I'm hearing, then it seems like "sag" affects more than response time of voltage supply.
     
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  16. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    I'm with Keeley, volts is not a problem, in televisions they used to go over 1000V on the 6V6 with no detrimental problems. Current is what burns the tube up. The screens are the most likely to go. Keeley was fond of the Sylvania 6V6GTA and there big plates, and seem to recall reading where he commented that he finds the modern JJ's a good replacement for his amps.
     
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  17. theprofessor

    theprofessor Tele-Afflicted

    Aug 8, 2016
    Chattanooga, TN
    Thanks, Bendyha! That's just the kind of opinion I was looking for. I think you're right on both counts: the Sylvanias and the JJ's. Cheers!
     

  18. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Just for interest, here is a load line similar to that of a silverface Deluxe Reverb under schematic conditions. 415V plate, 6600Ω primary, but here the screens are at 400 instead of the schematic 410V. Drawn in is a load-line for 70% disipation. The steeper loadline representing the "B" part of the biasing where the second tube is in "Cut-off", and the flatter line the "A" part. As you can see, the tube is over the 14W line most of the time, so 70% is hotter than one might think. The cut-off is just above the knee, with a slightly higher screen voltage, maybe though the knee, so what with the 470Ω screen grid resistors, the screens should be safe.
    silverface deluxe reverb plot loadline 415V.png
     
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  19. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    I should have drawn the average disipation line in on the chart above, but I have gone and scrubbed the drawing now. If one takes the point where the load-line cuts the -0V (205mA, 77V aprox.) and enters a new point above the 77V at half the max 205mA = 102.5ma, and draw a line from ther to the Q point (415V, 23.6mA.) this line takes into account the fact that the tube is in a cut-off, cooling state nearly half the time. Then one sees that it does not stay above the 14W line for too long.
    I also see that I seem to have drawn the Q point in at 26.3mA instead of 23.6mA......not that it makes much difference to the drawing..
     
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  20. MatchlessMan

    MatchlessMan Tele-Meister

    416
    Oct 22, 2011
    Wiltshire UK
    I have a 5E3 clone that I built from a kit (Modulus). Wall voltage in my part of the UK is well above 240V AC whereas the mains transformer is spec'd for 220V with the primaries joined in series. As a result I found that, to get the dissipation down to reasonable levels, I had to up the power tube bias resistor from 270R to 470R! I have fitted a 270R resistor in the HT line, which has dropped the plate voltage sufficiently that I can use a 330R bias resistor. (I believe I am not the first to do this mod!)

    I don't know how well that would work in a DR though.
     

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