KT66 & Cathode Bias?

Discussion in 'Building the Classics' started by Gene Ballzz, Jan 6, 2022.

  1. yladrd61

    yladrd61 Well-Known Member

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  2. Gene Ballzz

    Gene Ballzz Well-Known Member

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    I had not noticed before that that particular calculator could be used for figuring out cathode bias component values.
    Thanks For That!
    Gene
     
  3. Pete Farrington

    Pete Farrington Well-Known Member

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    I think it is assumed, otherwise there will be cathode degeneration, which will greatly increase the signal voltage at the grids required for full output.
    Interestingly, with a shared cathode resistor, the bypass cap has little effect until the signal level becomes high, moving operation into the class B section of the loadline.
    At lower signal levels, with both push pull sides conducting for the full cycle, the signals at the control grids will also appear at the cathodes and so will largely cancel each other out. So other than at large signal peaks, shared cathodes are mostly self bypassing. Hence it may be noticed that at lower power outputs, removing the bypass cap has little effect.
     
  4. yladrd61

    yladrd61 Well-Known Member

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  5. Spanngitter

    Spanngitter Active Member

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    Bear in mind that when you want to run KT66 in Cathode Bias each tube should have it's own Rk/Ck combination as per KT66 Data Sheet.
    Marshall is doing that in their Astoria Series (anyone has schematic for the Classic for me?), two nice 10W Welwyn Enamel Resistors accompanied by 2 uncomfortably close capacitors..
     
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  6. Pete Farrington

    Pete Farrington Well-Known Member

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  7. Gene Ballzz

    Gene Ballzz Well-Known Member

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    It sure would be nice to know the values of those Rk/Ck pairs. I'd also like to know the plate voltage (or OT center tap voltage) on an Astoria? That would likely point me in the right direction, at least for ballpark values! Astoria is the only amp I know of that "natively" uses the KT66 in cathode bias configuration.
    Thanks,
    Gene
     
  8. 2L man

    2L man Active Member

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    This loadline calculator is very fast tool to try different setups. For cathode bias one tube resistor is simply:Grid Voltage divided by Current in Amperes. For cathode bias actual operative B+1 is a sum of plate and grid voltage. Screen voltage has significant effect to grid voltage. When Headroom is inserted there appears Distortion estimates.
    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
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  9. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @2L man & All Endeavoring & Kind Enough To Help Me,

    To understand the kind of help I'm asking for, a good start is to read post #19 of this thread.

    Then you need to understand that while I love learning, I have some health conditions that are certain to take me out in the next 3-ish years. I also love physically working with my hands and tools at the workbench, as well as singing and playing guitar in a performance environment. I estimate that I have only another 18 to 24 months that I'll be able to continue performing. There is not really enough time in this life cycle for me to gather, study and develop the knowledge and expertise to completely design an amp, from the ground up, but am fully capable of performing a nice assebmly, once the electronic details have been narrowed down. I simply want to end up playing through an amp that is not currently being made, with features and aspects that 50+ years of performing has shown me will likely work well for me. I've already built several somewhat standard amps, with a few small mods and modified/repaired many other amps.

    Load lines and all the technical jargon that surrounds the use and understanding of what they really mean are Greek to me and there is simply not enough time to dig in and study it all. I am however a good solder monkey, with good fabrication skills and a pretty good guitarist/singer/performer!

    So what I'm asking you great folks (who already know and understand much of this stuff that I don't know) for here, is to help me determine some of the technical details of how to accomplish my goal! If you need descriptions of why I want to do what I'm proposing, I'll gladly share my assumptions that are based on my obsevations and experience. I'm certainly not averse to being told that my assumptions are false and need to modify my direction and plan.

    I hope this makes some sense?
    Gene
     
  10. _Steve

    _Steve Well-Known Member

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    Ive just gone through this process with an EL84 amp. In the end I had to gator-clip in a few different R values to get the ~97% dissipation as recommended by Rob Robinette.

    I'd recommend you do the same, just start with some ballpark R value and take the measurements as per Robinette's site. Keep gatoring until you get it.

    For the bypass cap I just went for whatever 'big' 50V cap I had on hand which was 100uF which I think is way higher than necessary. You could probably go way lower, but you'll need to do the math to make sure its sufficiently large so that none of the frequencies are being attenuated.
     
  11. _Steve

    _Steve Well-Known Member

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    Heres what the Mullard book says about split cathodes. Maybe Leo would probably disagree:

    Separate cathode-biasing resistors are used in the output stage to limit the out-of-balance direct current in the primary winding of the output transformer. The use of other balancing arrangements has not been thought necessary although it is likely that some improvement in performance, particularly at low frequencies, would result from the use of d.c. balanc-ing. It is necessary in this type of output stage for the cathodes to be bypassed to earth even if a shared cathode resistor is used. Consequently, a low-fre-quency time constant in the cathode circuit cannot be
    eliminated when automatic biasing is used
     
  12. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Thanks for that! :D That is pretty much my intention and also why I've been asking about the ASTORIA to get me into said ballpark, as the ASTORIA seems to be one of the few amps that actually uses KT66s in cathode bias configuration. The fact that Marshall chose to use two separate cathode resistors (and apparently separate caps) lend a bit of credence to what the tube data sheets lead me to surmise! I'm thinking that if I somewhat "mimic" the cathode bias arrangement of the ASTORIA, I'll be somewhere in the ballpark. I just some values from someone who has one these amps on hand! Again, looking for cathode bias resistors and caps values and plate/OT CT voltage. Those tid bits of info should get me on my way!

    Thanks Again,
    Gene
     
  13. neikeel

    neikeel Well-Known Member

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  14. yladrd61

    yladrd61 Well-Known Member

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    I have it switchable from fixed bias to cathode bias on my 5E8A copy with (2) 7581A (2) 5U4GB rectifiers, and 5F8A copy with (4) 5881's and (1) GZ34 it has been a while since I have been inside either amp so I do not remember what the voltages and resistor values I ended up with.
     
  15. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Thank you sir! Been there, done that, and that very pic is what confirmed for me that there are two cathode bias resistors, although I was a dumba$$ to not notice the cap values! Sure would be handy if the resistors had the values facing up to see them? In case you hadn't noticed, I'm a member over there also! I've even got a question there asking for the resistor and B+ values!

    On a side note, those amps are examples some of the nicest use of combined PCB and turret construction that Marshall has done! And FWIW, the trim pot that looks like a bias adjust is, IIRC, kinda like a hum adjust pot that has been known to fail and many simply replace it with a 100Ω or 100k resistor. There's a lengthy discussion over there about it!

    Thanks Again,
    Gene
     
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  16. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Grrrrr! I've scoured the whole darned innernest looking at ASTORIA gut shots and the very few that actually show the resistor values oriented to see them, were such low resolution as to not be able to read them when zoomed in! Somebody around here must own one that they could look at?
    Still Lookin'
    Gene
     
  17. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Maybe @anitoli might shed some light here?
    I'd So Appreciate It,
    Gene
     
  18. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Gene look at this Genalex data sheet way down at the bottom for push pull class AB1. At 450v the Rk ( cathode resistor value) is listed at 500 ohm with a separate resistor on each tube. At 250v anode listed at 200 ohm. I think this would be a good place to start.

    https://www.thetubestore.com/lib/thetubestore/genalex-kt66.pdf
     
  19. Pete Farrington

    Pete Farrington Well-Known Member

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    Note that the g2 voltage there is down at 300V.
    With it up near 450V, anode current would be much higher, probably about double in fixed bias, though self bias would act to counteract quite such an increase.
    With regard to Rk value, triode or UL operating idle conditions may be seen to be more relevant to typical valve guitar power amp / supply circuits.
     
    Last edited: Jan 16, 2022 at 6:57 AM
  20. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Thanks to all for sharing so much great info! The reason I bothered @anitoli is that I thought he actually owned an Astoria. Even though I'm close to the "ballpark" it would still be nice to know the power tube/OT-CT voltage and the actual values of the cathode resistors in the Astoria, if anyone actually gets their hands on one? This would nor only get me into the "ballark" but actually have me up to bat, at home plate!

    By the time I order my kit from Modulus, lead time, shipping, etc, I likely have at least a couple/few months before I actually start building. That should give me plenty of time to complete my research!

    Thanks Again,
    Gene
     
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