Cathode bias levels & tone

Discussion in 'Amp Tech Center' started by jsnwhite619, Dec 3, 2019 at 11:44 PM.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've played around enough with my fixed bias amps that I've figured out I generally like fixed bias Fender amps set in the 55%-60% range. I don't really shoot for that level, but playing with it long enough and tweaking it to what I like with my guitars, it seems to always be the area I end up setting it. But, that's easy enough to play with using a bias pot.

    I don't have wide selection of wirewound cathode resistors on hand, and would like to avoid a useless endeavor if possible. Using my 5e3 as an example -- does anyone here set those with cooler bias or prefer it instead of the "safe 100%" level? Mine is set up with a 330v PT and around 370v B+ using a 270R resistor. I've measured it before and I can't remember where it was at -- 100% +/- 5% maybe. So, it's not a situation where I'm running 420v in my Tweed Deluxe and need to change it. I'm just wondering how much effect it has?
     
  2. Phrygian77

    Phrygian77 Tele-Afflicted

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    I agree. I like JJs in a PR or DR biased a bit hotter though, at 65%.
     
  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I seem to like the 65-70% range for a "change is better" moment, have fun with the lower headroom for a while, then when I finally play with it again and lower it...."Ah, there's my Fender!" To me, even though the hotter bias is easier to overdrive, I still like the sound and feel of the cooler bias overdrive - in most cases. It's just louder than Hell's Bells when you get there... ;-)
     
  4. Phrygian77

    Phrygian77 Tele-Afflicted

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    I let a customer go with a PR built with new production Tung-Sols running at 70%. I'd been so used to JJs that with the bias resistors, I couldn't get it any lower. He still loves it though. I'm constantly checking on him... lol.

    The Vibro Champ I built with a 5881, I used two 3watt 470s in parallel with a 5 watt 500 ohm wirewound pot for bias. I wanted to run it all the way up to 23 watts, but I stopped at 21 because it just sounded good right there.
     
  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I was talking about it in another thread recently, but my 5f1/5f2 setup now gets dead on vintage levels with a 500R cathode resistor and a 275v tap. I think that worked out to around 75% on Rob's calculator, but allows for a GZ34 to go in and still be around 95-100%. Now that Hammond seems to have discontinued the 290CAX I was using for everything in that chassis cutout size, it looks like I'll be swapping to their 290AX PT. BUT, they've remade it to have a 275v & 325v tap, and a 100mA rating, plus 3A rectifier rating, and it's a good bit smaller than the CAX, so I'm satisfied. To have the full rectifier option and sacrifice 15mA and a lot more room for the export 240 wiring on the CAX, I can live with it. I've only used the AX for a Princeton Reverb, but it was amazing. I told the guy to be sure and let me know if he sells it. His response was "Talk to my kids."
     
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  6. peteb

    peteb Friend of Leo's

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    is 21 W versus 23 W the plate dissipation or the power out?


    I just want to add that if it is plate dissipation, then possibly you are getting more clean power out at lower plate dissipation, and maybe that is what you like about that setting. Not sure how that works, just a thought.


    a champ is cathode biased and single ended. what works on a champ might not be what works best on the cathode biased push pull tweed deluxe.
     
  7. Phrygian77

    Phrygian77 Tele-Afflicted

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    It was plate dissipation, Pete. And yes, you're correct in that in cathode biased class A/B push pull that can and should be lower, even though people seem to run those too hot also. I think a lower plate to cathode voltage compensates to a degree. You're knocking off a good bit of voltage just by being cathode biased.
     
  8. King Fan

    King Fan Friend of Leo's

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    Jason, you’re just asking what might be the effects of lowering bias on your 5e3 currently biased around 100%? Or was there some other question?

    If so, also, I wasn’t sure how were you proposing to cool the bias?

    Sorry, I got confused by the thread so far... I probably missed a turn somewhere.


    Sent from my iPad using Tapatalk
     
  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yes, in short. I rechecked mine to be sure and it is 370v B+ and 93% on Rob's calculator with a 270R cathode resistor. It's just one of those things that with so many posts about 5e3 clone voltage being too high, there's lots of talk about increasing the resistor to get out of triple-digit bias readings. But, there isn't much about "I cooled my 5e3 bias down to the 70-80% range and it was ...." Now, admittedly, there may be a very good reason that you don't see people raving about lowering the bias on their 5e3. But, I was just curious due to the lack of info I found, and because I don't really want to do the work myself to experiment... ;)
     
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  10. King Fan

    King Fan Friend of Leo's

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    I'm the opposite of an expert, and I don't know what the sonic effects of further lowering bias would be.

    Personally, if I were running at 370 B+ and 93% dissipation, I wouldn't worry about other posts about 5e3 clone voltages being "too high." Seems like most of those have dang high B+ and dissipation over 100%.

    I'll stand by for informed responses. :)
     
  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, I'm not worried about it at all. Just one of those things where, I seem to like a cooler bias on fixed bias Fenders, so I wonder what it does with cathode bias. I set everything to 70% when I started because that's what you read a lot of online. After I started experimenting with it, turns out I like it cooler.

    BTW, I played with tubes and took measurements on my 5e3 this afternoon. I'll post numbers when I get a chance. Nothing that really matters, but it's data. And we love numbers around here! ;-)

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  12. peteb

    peteb Friend of Leo's

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    What tone or sound are looking for?
     
  13. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, I didn't do anything to the bias resistor this afternoon, but I swapped some tubes around and took some measurements for my curiosity, and have some data on here for others in the same situation. When I used the bigger tubes, I played it just a few minutes to hear it, but less than 10 minutes run time for each setup. The shop was around 60 degrees, laser thermometer had the 5 watt around 140 degrees at the top, but I didn't measure it during normal operation, so grain of salt there. All bias calculations done on Rob's site.

    Amp specs:
    Hammond 290BX Deluxe Rev. PT; 330-0-330, 138mA, 3A on the 5v & 6.3v taps
    Hammond 1750E Tweed Deluxe OT; 15 watt, 8k, 8ohm
    270R, 5 watt bias resistor, 25uf Sprague bypass

    My stock tubes, all JJ's 5y3 & 6v6's measure:
    • 370v B+
    • 341v plate to cathode
    • 21.5v @ bias resistor
    • 37.7mA per tube
    • 12.9 watts dissipation
    • 92%
    GZ34 with 6v6's. I'd tried this before and played it, but with the bias so hot, I just took the measurements today and that was it. If memory is correct, it was extremely clean & punchy, kinda Blackface, but I didn't do much because it was SO high.
    • 433v B+
    • 399v plate to cathode
    • 25.8v @ bias resistor
    • 45mA per tube
    • 18 watts dissipation
    • 129%
    5881's and GZ34
    Old Groove Tube/Sovtek 5881's that came out of a used Hot Rod Deluxe I used to have. Cleaner than the 6v6's and a sound I really liked. Noticeably louder than stock.
    • 427v B+
    • 390v plate to cathode
    • 29.8v @ bias resistor
    • 52mA per tube
    • 20.3 watts dissipation
    • 78%
    5881's with 5y3 Not nearly as loud and hard hitting as the GZ34 (obviously) but much cleaner than stock and still a good tone. Kinda like a fixed bias 6v6 sound.
    • 360v B+
    • 328v plate to cathode
    • 24.7v @ bias resistor
    • 43mA per tube
    • 14.2 watts dissipation
    • 55%
    I've been comparing the 1482 & 1483 Silvertone schematics - two amps that I recapped for a guy in the spring. Dang near the same circuit for each except 6V6 & 6L6. Both 5y3 rectifiers, both 350v-360v B+ voltage. The OT on the 1483 Silvertone was pretty small - no bigger than a Deluxe Reverb I would think, if that. I know I don't want a full 25-30 watt amp that gets into Proluxe territory. If my numbers are right, 2x 6L6/5881 & 2 preamp tubes would still be less stress than a standard Deluxe Reverb complement with 4 extra preamp tubes and 6v6's.

    So, for a project that gets me a "new amp" for the cost of a new OT, a couple tubes, and swap in a 10 watt resistor that I bought for this very purpose over a year ago, I'm looking at this: just get a 6.6k Deluxe Reverb OT (1750H) and use 6L6/5881 with any rectifier, and 6V6 with a 5y3, and not worry about an impedance switch or completely losing the classic 5e3 crunch by going overboard with a 30-40 watt OT that would maximize the bigger tubes. Which I don't particularly want.

    Thoughts?
     
  14. peteb

    peteb Friend of Leo's

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    Those are interesting numbers to look at but I don’t think it is a straight across comparison for looking at how the bias affects the tone.


    When the rectifier is changed, the response of the amp changes as well as the B+. When the power tubes are changed between different tube types and outout levels there will be changes to the tone as well.




    I think the general rule is a higher bias current will bring on distortion at lower volume levels and maybe increase the harmonics. A colder bias will cool the sound down and maybe cause crossover distortion. I think this is true across all types of tube amps, single ended, pushpull, cathode and fixed bias. Maybe not.



    I am interested in finding evidence that a cooler bias might produce more clean rated volume.




    I once changed the bias in a champ from the stick bias 470R to I think a 1K bias resistor. It didn’t really clean things up like I hoped for, but it did not seem to lower the power out either, which I expected. I switched it back right away because I didn’t want to miss out on anything, but if my memory serves me, there was nothing wrong with the cooler bias.




    An engine that has a faster idle is not any more powerful than an engine with a slow idle.




    Pick out a value, change out one resistor, then you will know. The best bias is subjective and a personal decision.
     
    Last edited: Dec 7, 2019 at 11:47 AM
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