Fender Silverface combined cathode/grid bias...Yay or Nay?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Muttlyboy, Nov 30, 2019.

  1. Muttlyboy

    Muttlyboy Member

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    I'm working on a friend's AB568 Vibrolux Reverb.

    I'm going to change out the bias balance set up to the "blackface style" bias control.

    I'm wondering what the general consensus is about the semi cathode bias system.

    Right now the amp is set up with TAD 6L6 WGC tubes running at 33mA @417V which seems a bit cold but the amp sounds pretty good
     
  2. HotBluePlates

    HotBluePlates Member

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    It's fine.

    The "general consensus" will probably be to repeat Gerald Weber's suspicion (as printed in at least one of his books) that it's no-good. (EDIT: It was Ken Fischer that implied it was bad in the Trainwreck Pages; Weber gave a half-$&% explanation of it in a book, then dismissed it as something to remove for black facing an amp.) But then again, Weber implied that everything from the silverface era was bad, yet there are plenty of good sounding silverface amps.

    The benefit of cathode bias is it is safer because it is self-limiting. If something goes wrong (leaky coupling cap, tube shorted) and the tube tries to draw 300mA, the large voltage drop across the cathode resistor turns the tube off. If it cannot for some reason, the cathode resistor can burn open and all tube current drops to zero.

    The drawback of cathode bias in an amp making large output power far into Class AB and close to Class B operation is that it is self-limiting. Cathode bias gives a stable bias voltage when the average tube current is near-constant, as it is with Class A (current-reduction in one side is offset but current-increase in the other side). Venturing only a little into Class AB, the average tube current rises only a little, and cathode biasing still works pretty well. Far into Class AB, one tube turns off and the other tube tries to reach a peak of 3-5 times idle current; the average tube current rises as you push to higher power output, but the cathode resistor tries to counteract that.

    Meanwhile, fixed-bias doesn't care. It is a set voltage, whether your output tubes are idling happily or melting down.

    I think Fender experimented with adding a little cathode bias to give a little more compression and a slight governing of maximum power output. It's a "little safer" than pure fixed-bias, but not as constricting of power output as pure cathode bias.

    Why not just add a bias adjustment pot before the existing bias balance pot? Get the best of both worlds. That way, you can plug in any pair of 6L6s, adjust the balance pot to give equal idle current, and then tweak the bias adjustment pot to land on your target idle current.

    You will not get "every last clean watt" from an amp that has two 6L6s that are not matched for Gm and idle current (at multiple points of operation), but an awful lot of guys seem to not be able to crank up their 2x6L6 amps without the use of an attenuator...

    If the amp sounds good, it is good. Have your friend play it while you adjust up/down between some safe limits you establish, and see where he likes it. Your friend's 6L6s will probably last longer if they're biased cooler, and if it sounds good who's to argue?
     
    Last edited: Dec 1, 2019
  3. zenas

    zenas Member

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    Yeah this ! Why easier than the old convert the phase inverter to blackface style. You solder a trim pot on the balance pot where that resister is now, probably have to change that resister too.
    If I had one of those fixed/bias drip edges I'd do that, replace all the electrolytics and service it. Leaving the original circuit in tack but you get it working right. Then play it awhile. If you decide to blackface mod it you always can later on.
    I've just avoided drip edges because they're priced like they are blackfaces. Part of that is confusion is because the early ones were and the ones that weren't probably still got a blackface tube chart glued in the cab. On Super Reverbs they may have an AB763 chart as late as 1970. That's like three circuit changes later, AB763 charts even after the drip edge went away.
     
  4. Muttlyboy

    Muttlyboy Member

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    I like the trim pot idea. I've never done it before but it makes good sense.

    It took a while of experimenting with resistors to get the the tubes heated up from about 9.5 watts (22mA) to 13.7watts (about 33mA).

    I think that the negative bias voltage went from -43v to about -30v (I'm not sure if I remember right, but those numbers are pretty close)

    The stock resistor on the pot was a 15k, and now it has a couple parallel resistors which work out to 9k.

    Another thing is that the amp has the older AA964 *schematic (*tube chart) which calls for a GZ34, but the amp actually matches the AB568 which calls for a 5U4.

    I'm thinking that this particular amp has the earlier power transformer because even though the amp is presently biased a bit on the cold side (13.7 watts) the plate voltage is 417v while the schematic shows 440v.

    Maybe I should tell my friend not to run the amp too much until we swap in a gz34

    * I wrote schematic when I ment to say tube chart
     
    Last edited: Dec 1, 2019
  5. Vanyu

    Vanyu Member

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    Now when you say the amp has the AA964 schematic, are you saying that it has an AA964 tube chart, or that the amp has been converted to AA964 specs?

    Don’t read much into Fender’s posted voltages, they’re usually not that right. 417v is a reasonable voltage for a 5U4 rectified Vibrolux.

    Running a 5U4 is fine, swap in a GZ34 if you feel like you don’t have enough headroom with a 5U4. Truth be told, a 5U4 will be easier on things than a GZ34, no need to restrict use because it has a 5U4.
     
  6. HotBluePlates

    HotBluePlates Member

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    You mean "tube chart"?

    The blackface schematic shows -42v of bias, while the AB568 silverface schematic shows -38v to -41v. I'm actually more concerned that your 6L6s need -30v with 400+ volts on the screen to pass 33mA. Either your 6L6s need replacing, or you need to investigate the parts attached to the 6L6 cathodes.

    The graph below is taken from G.E.'s 6L6GC data sheet, and shows plate current for various G1 voltages when G2 is at 400v (probably mighty close to what you have now on the VR's screens). The red lines show that "book value" of plate current for -30v is 90mA(!) 33mA through 150Ω gives ~5vdc, so your effective bias is -35v, but following the vertical red line to a point halfway between the -30v and -40v gridlines still lands on 70mA. The data sheet curves show that you need -50v bias to get 30mA of plate current.

    This amp has the 5µF white Mallory cap still in place between the 6L6 cathodes? If yes, you need to replace it (those white Mallory electrolytic caps are notorious for failing), and you may want to disconnect it for now while you bias to be sure it doesn't influence your readings.

    [​IMG]
     
  7. Muttlyboy

    Muttlyboy Member

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    Yes...I meant to say Tube chart, not "schematic"

    I don't recall seeing a 5uf cap between the Cathodes. I'll look into that.

    Thanks for the heads up
     
  8. Sweetfinger

    Sweetfinger Silver Supporting Member

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    It's non-polarized, something you don't see in a lot of Fenders.
     
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  9. Steppin' Wolfe

    Steppin' Wolfe Member

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    Muttlyboy, Fender used the charts until they were gone.....tube charts may have noth8ng to do with the amp one has in front of them. Although tube charts usually do refer to the model of the amp in which they are placed, sometimes that is not so. The oddest tube chart/amp combinations I have owned were a near mint ‘57 5E3 Deluxe that had a 5F6A Bassman chart and a SF Dual Showman Reverb that had an AB165 Bassman tube chart. The circuit tells the story.
     
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  10. HotBluePlates

    HotBluePlates Member

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    Looking at photos of 1968 Vibrolux Reverb chassis online, even amps that appear untouched inside did not appear to have the white bypass cap connecting the cathodes. So I guess it's possible it was on the schematic but not installed.
     
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  11. Muttlyboy

    Muttlyboy Member

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    So, I take it that the 5uf non-polarized cap is not actually required to be in there?

    If I were to put one in, would two 10uf regular caps in series connected negative to negative work?

    I never knew about that bias chart that you posted. The tubes are new and I got them from a trusted TGP vendor.

    I got them as a matched quad and put two in one of my amps. (Cathode Biased and the recommended tubes from the Builder) I did not check voltage or current or nuthin...Amp sounds sweet (early Christmas present to myself...Carr Rambler at a nice price from the Emporium)
     
  12. Sweetfinger

    Sweetfinger Silver Supporting Member

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    It was on my combo bias Bassman. You have to look for photos showing from the front side looking down. The cap is small and can be hard to spot from a direct overhead chassis shot.
     
  13. Muttlyboy

    Muttlyboy Member

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    Thinking about it, I took voltage from the plates to the chassis instead of plate to cathode.
    I forget what the voltage drop was across the 150 ohm resistors (about 5 volts) so probably not enough to clear up the issue of the bias voltage
     
  14. HotBluePlates

    HotBluePlates Member

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    That actually would place you even closer to my example at 400v on the plates (really 400v plate-to-cathode). ;)

    You don't have anything near 90mA per tube, so you might want to ask why.
     
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  15. Vanyu

    Vanyu Member

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    I’m actually not too shocked that you need -30v to bias to 33mA, that seems about right to me.

    For strictly fixed bias, yeah that’d be a problem, but I’ve used split biasing in the past and I found in my own amps that negative voltage requirements lessen significantly when any cathode bias is mixed in. My stock bias resistor was 47K and I’m fairly certain I bypassed it with a 4.7K to get my bias in range, though I don’t remember what my negative voltage was. I had individual unbypassed 100ohm resistors in my amp. Fender used 150ohm resistors, so this effect will be more extreme than mine.

    If your amp doesn’t have a bypass capacitor (which it should, was shared in stock config), then I’d add individual 470uF (to simulate fixed bias behavior) 100v caps over each resistor, though you could use smaller values (like 50-100uF) to keep some of that mojo that comes with cathode biasing. If your amp has the stock cap, then I’d just leave it as it is.
     
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  16. Muttlyboy

    Muttlyboy Member

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    I just thought of something...

    I measured the negative voltage before the 1.5k grid resistor at pin one...

    That means that the actual bias voltage is even lower
     
  17. HotBluePlates

    HotBluePlates Member

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    0.033A * 150Ω = 4.95v of additional bias.
    -30v on grid + 4.95v on cathode = -34.95v total bias.
    Graph in this post shows 70mA plate current for -35v bias and 400v plate & screen.

    33mA is alarmingly weak (under 50%) for a 6L6 tube in these conditions.

    I've got a tube tester that allows setting exact plate, screen and grid voltages, and will measure idle plate & screen current, as well as transconductance. Normally, I use the 250v plate & screen condition from the 6L6GC data sheet to test 6L6-type tubes (middle of page 2, "Class A Amplifier, Pentode Connection"). The condition shows the tube should idle at 72mA. The "average old production 6L6GC" among the 28 I've tested have an idle plate current of 61-80mA, with weak tubes idling down near 40mA and very strong tubes idling 80-99mA.

    This tells me "good 6L6 tubes" should be within a reasonable tolerance of data sheet values. @Muttlyboy's tubes don't seem to be within that reasonable tolerance.

    (I've also encountered old production tubes that will test "off the charts good" on a tube tester in this same setup. They might pass double the current the data sheet says for the electrode voltages applied. These tubes are not good, but are defective in a way that causes excessive current. Adherence to the data sheet works both ways.)​

    Unless your tubes are gassy (or the coupling caps are leaking d.c., which would skew the result in the other direction), then there is no direct current through the 1.5kΩ grid stopper, and thus no voltage drop across them. D.C. volts should be exactly the same on both side of the 1.5kΩ resistor, as well as on either side of the 100kΩ bias feed resistors (I assume you haven't swapped in 220kΩ resistors yet).
     
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  18. Muttlyboy

    Muttlyboy Member

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    No modifications other than swapping the resistor to lower the negative bias voltage
     
  19. Muttlyboy

    Muttlyboy Member

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    Vibrolux update...

    I changed out the bias balance set up to regular bias , but I kept the 100k bias feed and 47k plate resistors for the PI.

    I also jumped out the 150 ohm cathode resistors and clipped out that 5uf cap from between the Cathodes.

    So now the tubes seem to bias out closer to the chart above that HBP posted.
    406v plate
    38mA
    -41.3 negative bias voltage
     
  20. HotBluePlates

    HotBluePlates Member

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    Darn!! I was curious to get your feedback on how the mixed-bias setup sounded.
     
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