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    Triode questions

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    audiobill

    Posts : 271
    Join date : 2014-03-13
    Location : Philadelphia

    Triode questions

    Post by audiobill on Fri Mar 28, 2014 2:41 pm

    Hi all-

    If I run my m-125's in triode mode, three questions:

    -With reduced power output, can I plan on longer tube life?

    -Should bias, which drops about 4ma when switching from pentode mode, be restored to 1.2 v?

    -Does it become more feasible to use a GZ34 rectifier tube in lieu of the GZ68 Weber Copper cap?

    Thanks for all thoughts!!

    Bill

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Triode questions

    Post by sKiZo on Fri Mar 28, 2014 3:56 pm

    Good question - maybe help clear up some of MY misconceptions.

    I've always gone with the theory that both modes present the same load to the amp, and any volume change is due to the triode mode's relative inefficiency compared to pentode. So ... why triode? Because some find it more sonically appealing than pentode for certain genres. Jazz seems to be the hot ticket item for that. I don't much use it myself as I prefer the punch of the pentode. Maybe it's a volume thing - I tend to push it to what the aforementioned "some" might find extreme.

    That said, the GZ34 has always been a bit underpowered for these amps, working right at the limit of their design. If you do decide to go with bottles, I highly recommend the GZ33 or GZ37. I got a deal on a NOS GZ37 a while back and it's my #1 choice now, driving a quad of KT120's. Amp just sounds a lot more fluid now and doesn't seem to be straining at all like it did with the GZ34. It also sounds somewhat less clinical than the copper cap.

    Probably worth mentioning - I got an ST120 ... same thing, only different ...

    *Also, I don't believe you should be seeing any drop in the bias when switching modes, at least nothing that should be a concern. You should be able to just flip the switch on the fly under load without having to make any other changes. Maybe an out of spec resistor in the PT network?

    corndog71

    Posts : 444
    Join date : 2013-03-19
    Location : It can get windy here

    Re: Triode questions

    Post by corndog71 on Fri Mar 28, 2014 5:01 pm

    I just reread Skizo's build thread and Bob said not to worry about the slight drop in bias voltage when switching to triode mode.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Triode questions

    Post by Bob Latino on Fri Mar 28, 2014 6:39 pm

    audiobill wrote:Hi all-

    If I run my m-125's in triode mode, three questions:

    -With reduced power output, can I plan on longer tube life?

    -Should bias, which drops about 4ma when switching from pentode mode, be restored to 1.2 v?

    -Does it become more feasible to use a GZ34 rectifier tube in lieu of the GZ68 Weber Copper cap?

    Thanks for all thoughts!!

    Bill

    Hi Bill,

    1. I have never explored the question that you bring up about longer tube life by using the triode mode BUT > IMHO you probably won't get any longer tube life by playing the amp in triode mode ..

    2. The bias will drop slightly when you switch from pentode ultralinear mode to triode mode. It's about 4% or so on all the VTA amps. On an ST-70 you would go from .400 VDC to maybe .385 VDC when you make the switch. On a VTA ST-120 you will go from .550 VDC to maybe .530 or something like that. On the VTA M-125 the (4 output tube mode bias) will drop from 1.20 VDC to about 1.16 VDC. It is not necessary to raise the bias back up to the recommended pentode ultralinear voltage .. NOTE - the bias recommendations for the 3 VTA amps are just that > RECOMMENDATIONS ... They were chosen to give a combination of good sound and good tube life .. The higher you crank up the bias, the shorter is your tube life.

    3. The GZ34 tube rectifier will work fine in all the VTA amps as long as ..
       A. You don't push the amp really hard a lot of the time
       B. Your room is of modest size
       C. Your speakers are of reasonable efficiency .. maybe 89 - 90 dB or higher
       D. The load that your speakers present to the amp is not overly complex and the speakers are easy to drive

    OTOH > If you like to "crank it up", you have a LARGE room, your speakers are in the 84 to 88 dB efficient range, you have electrostatic speakers or some other type of speaker that presents a complex load, you should probably use the Weber WZ68 solid state rectifier in your amp. The Weber WZ68 will not sag the B+ voltage anywhere near as much as a tube rectifier and will be able to supply current to the power supply much better at higher volume levels.

    The third option is a Mullard GZ33 or GZ37... These two bad boys are TUBE rectifiers that can flow more current than a GZ34. The bad news is that they are not made any more AND they are $100 or so .. (expensive). If you can ever get a hold of one - use it .. they have a slow warm up and the they will not sag the high voltage as much as a GZ34/5AR4.

    Bob

    audiobill

    Posts : 271
    Join date : 2014-03-13
    Location : Philadelphia

    Re: Triode questions

    Post by audiobill on Fri Mar 28, 2014 6:47 pm

    Great, thanks all.

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