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    KT 88 Bias voltage

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    Tube Nube

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    KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 11:36 am

    Just replacing a toasted KT 88, now. I've lost my manual.

    Can some kind member suggest what the bias voltage should be?

    Thanks in advance.

    corndog71

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    Re: KT 88 Bias voltage

    Post by corndog71 on Thu May 05, 2016 11:58 am

    0.40vdc should be fine.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 12:13 pm

    Thanks Corn',


    Bob Latino
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    Re: KT 88 Bias voltage

    Post by Bob Latino on Thu May 05, 2016 12:17 pm

    Tube Nube wrote:Just replacing a toasted KT 88, now. I've lost my manual.

    Can some kind member suggest what the bias voltage should be?

    Thanks in advance.

    In a VTA ST-120 the recommended bias voltage is .55 VDC per each output tube. This is a good compromise between long tube life and quality sound in the VTA ST-120. A .40 VDC bias setting per each output tube (IMHO) is just a little too low for a KT88 in the VTA ST-120.

    Bob

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 12:35 pm

    Tube Nube wrote:Just replacing a toasted KT 88, now. I've lost my manual.

    Can some kind member suggest what the bias voltage should be?

    Thanks in advance.

    Noob, the manual says .6/tube (1.2 for the pair) on the 125s.That's too high for KT-88s.  Holger ships his at .5 (1.0 for the pair) and those are KT-120s, which are pretty ballsy tubes. My experience is to start at about .45/tube (0.9 for the pair) and gradually inch 'em up to .50. Run them up to .6/tube but I bet you won't hear a difference, and the toobs will love the lower bias.
    I just read HRH Bob Latino's post whilst I was writing mine and he is the pro here. It is of course a subjective listening experience. My advice? Start low and work your way up until you've found bliss.


    Last edited by deepee99 on Thu May 05, 2016 12:52 pm; edited 2 times in total (Reason for editing : Commentary after reading Bob's post.)

    corndog71

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    Re: KT 88 Bias voltage

    Post by corndog71 on Thu May 05, 2016 12:56 pm

    Bob Latino wrote:
    Tube Nube wrote:Just replacing a toasted KT 88, now. I've lost my manual.

    Can some kind member suggest what the bias voltage should be?

    Thanks in advance.

    In a VTA ST-120 the recommended bias voltage is .55 VDC per each output tube. This is a good compromise between long tube life and quality sound in the VTA ST-120. A .40 VDC bias setting per each output tube (IMHO) is just a little too low for a KT88 in the VTA ST-120.

    Bob

    Bob,

    Do you know what the typical dissipation of a KT88 is in the ST120 with the bias set to 0.55vdc?

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 1:20 pm

    Thanks for all your biased input, heh heh.

    Just one thing--I dont imagine it makes a dofference, but I'm still running. Vta 70, not a 120.

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 1:23 pm

    You have been remitted to the pun jail. Good luck in there: Kentley, sKIzo and I skulk the place, and sKIzo has photo-shop. Be very, very careful.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 1:33 pm

    It was beyond my capacity to resist. Consequently I was induced to commit that pun. Hope it wasn't too much of a battery upon your senses. I could try harder to keep them to myself, but as you know, humour is no fun in a vacuum.

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 1:40 pm

    Tube Nube wrote:It was beyond my capacity to resist. Consequently I was induced to commit that pun. Hope it wasn't too much of a battery upon your senses. I could try harder to keep them to myself, but as you know, humour is no fun in a vacuum.
    You are hereby inducted into the Hall of Shame, if you can withstand resistance for it. Welcome Ohm, Henry.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 2:15 pm

    I am truly transformed by this award!

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 2:24 pm

    Tube Nube wrote:I am truly transformed by this award!
    Well just don't get wound up about it. We are, after all, bi-polar and quite biased to the Truth, although we are quite Jouley about such matters.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 2:29 pm

    Ohm I G_d, how long can this continue?

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 2:32 pm

    Riddle me this, Batman: Which one of the 3 Stooges is nominally better qualified to serve as a conductor?

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 2:51 pm

    That would be Mo.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Thu May 05, 2016 3:57 pm

    Heheh! THat was too easy for the likes of you!

    deepee99

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    Re: KT 88 Bias voltage

    Post by deepee99 on Thu May 05, 2016 4:07 pm

    Tube Nube wrote:Heheh! THat was too easy for the likes of you!
    Although your circuits can get Curly, there is the Larry factor. I just found this essay by Larry about the Wife Acceptance Factor (WAF), and if you don't fall off your chair turn the volume down a bit:
    https://en.wikipedia.org/wiki/Wife_acceptance_factor


    Last edited by deepee99 on Thu May 05, 2016 4:08 pm; edited 1 time in total (Reason for editing : grammar)

    audiobill

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    Re: KT 88 Bias voltage

    Post by audiobill on Thu May 05, 2016 4:17 pm

    Back to regularly scheduled discussions......

    arledgsc

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    Re: KT 88 Bias voltage

    Post by arledgsc on Thu May 05, 2016 8:55 pm

    I measured the ST120 plate voltage a week or so ago using a Weber WZ68 rectifier in the amp.  I got 490V under load with 119V line voltage.  Bob's cathode voltage recommendation for tube current is right in there for 70% plate dissipation of a 35W max. tube like the KT-88.  For my setup I actually calculated 0.53v measured across the 10 ohm cathode resistor yields 50mA of plate current.  About 5% of the current through the cathode resistor is screen current that is summed with plate current.  So 490V x 50mA = 24.5W or 70% of the maximum plate dissipation of 35W. 

    When I used briefly straight diode rectification the plate voltage jumped up to 520V.  In this case I had to crank down bias to 0.5V to yield 47mA of plate current for the same 70% of max. dissipation operating point.  A lower plate voltage, as with tube rectifiers, may require more current for the same power at idle.  So knowing the your plate voltage will let you adjust bias more accurately for your setup.

    Weber has a nice bias calculator.  Mid-page just enter parameters...
    http://www.tedweber.com/webervst/tubes1/calcbias.htm

    GP49

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    Re: KT 88 Bias voltage

    Post by GP49 on Thu May 05, 2016 11:02 pm

    Tube Nube wrote:It was beyond my capacity to resist..

    In all seriousness...in all this thread, the role of the cathode RESISTOR has been ignored.  There was no mention in the first post of what amplifier was being discussed so we really don't know WHAT it is; it differs among different amplifiers.  Some amplifiers use NO cathode resistor...and no matter what, their so-called "bias voltage" (a misnomer, the actual bias voltage is the NEGATIVE voltage applied to the control grid) is zero.

    The value of the cathode resistor(s) must be known to provide the desired measurement of the voltage at the cathode.  What one is actually adjusting is the cathode CURRENT at idle, which by Ohm's Law results in a voltage at the top of the cathode resistor.

    In an amplifier with no cathode resistor, an provision to plug in an ammeter is usually provided, for direct measurement of idle current.

    corndog71

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    Re: KT 88 Bias voltage

    Post by corndog71 on Thu May 05, 2016 11:16 pm

    arledgsc wrote:I measured the ST120 plate voltage a week or so ago using a Weber WZ68 rectifier in the amp.  I got 490V under load with 119V line voltage.  Bob's cathode voltage recommendation for tube current is right in there for 70% plate dissipation of a 35W max. tube like the KT-88.  For my setup I actually calculated 0.53v measured across the 10 ohm cathode resistor yields 50mA of plate current.  About 5% of the current through the cathode resistor is screen current that is summed with plate current.  So 490V x 50mA = 24.5W or 70% of the maximum plate dissipation of 35W. 

    So under these conditions you could go as low as 0.36vdc for bias which puts you at 50% of max dissipation.  According to Roger Modjeski, a Sylvania engineer he spoke to recommended 50% of max dissipation for longer tube life.  He said you could potentially get up to 10,000 hours at that level.  Modjeski claims this performance is not uncommon with tubes in his amplifiers even with higher plate voltages.  Roger doesn't always go by the books because he found many of the "typical use" specs were arbitrary and better performance could be found with further experimentation.  I've been running my new issue Tung Sol 6550s at 0.40vdc for well over a year and they sound great.  I did lose one tube on power up at one point but the others are still holding up well.  I got those tubes from Upscale Audio.  At some point I also want to pick up some power tubes from Roger and see if his perform any better.

    Tube Nube

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    Re: KT 88 Bias voltage

    Post by Tube Nube on Fri May 06, 2016 1:14 am

    GP49 wrote:
    Tube Nube wrote:It was beyond my capacity to resist..

    In all seriousness...in all this thread, the role of the cathode RESISTOR has been ignored.  There was no mention in the first post of what amplifier was being discussed so we really don't know WHAT it is; it differs among different amplifiers.  Some amplifiers use NO cathode resistor...and no matter what, their so-called "bias voltage" (a misnomer, the actual bias voltage is the NEGATIVE voltage applied to the control grid) is zero.

    The value of the cathode resistor(s) must be known to provide the desired measurement of the voltage at the cathode.  What one is actually adjusting is the cathode CURRENT at idle, which by Ohm's Law results in a voltage at the top of the cathode resistor.

    In an amplifier with no cathode resistor, an provision to plug in an ammeter is usually provided, for direct measurement of idle current.


    I mentioned in a subsequent post that it's the VTA ST-70. I'm guessing that it's the same as the 120, though that's an assumption.

    corndog71

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    Re: KT 88 Bias voltage

    Post by corndog71 on Fri May 06, 2016 10:28 am

    Tube Nube wrote:
    GP49 wrote:
    Tube Nube wrote:It was beyond my capacity to resist..

    In all seriousness...in all this thread, the role of the cathode RESISTOR has been ignored.  There was no mention in the first post of what amplifier was being discussed so we really don't know WHAT it is; it differs among different amplifiers.  Some amplifiers use NO cathode resistor...and no matter what, their so-called "bias voltage" (a misnomer, the actual bias voltage is the NEGATIVE voltage applied to the control grid) is zero.

    The value of the cathode resistor(s) must be known to provide the desired measurement of the voltage at the cathode.  What one is actually adjusting is the cathode CURRENT at idle, which by Ohm's Law results in a voltage at the top of the cathode resistor.

    In an amplifier with no cathode resistor, an provision to plug in an ammeter is usually provided, for direct measurement of idle current.


    I mentioned in a subsequent post that it's the VTA ST-70. I'm guessing that it's the same as the 120, though that's an assumption.

    Yeah, the ST70 uses EL34s and I think lower voltages. You can still do the same measurements listed above to determine what your ideal operating points would be. 70% of the max dissipation is fine but will shorten the life of modern day tubes compared to tubes made back in the 50s and 60s. If you're not into rolling power tubes then I would recommend the lower bias point for longer tube life.

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