Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups

Biasing w Cathode Resistor Method

Discussion in 'Amp Tech Center' started by ocduff, Aug 12, 2017.

  1. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    A question that's been plaguing me. Not which is best but which does one TRUST as yielding ACTUAL bias?

    In staying honest and having no money on the game as far as a prescribed method of biasing a tube amp - I can say that different methods yield different results.

    Transformer resistance method requires a digital multimeter that uses several decimal places. Useless for those who don't have a FLUKE etc.

    Transformer Shunt is considered accurate but dangerous and again, doesn't yield exact results as above.

    Which isn't to say there are reasons for this potential disparity but again - this is not about reasoning behind the disparity but that the disparity exists.

    NOW - we get to Cathode Resistor Method. This takes your power tubes Cathode voltage and converts it into milliamps. Basically, it moves some decimal places for you

    BUT - the big issue I have with this, not to mention it yields vastly different readings than above 2 methods, it relies on basically taking ones Cathode voltage reading and saying that is the bias reading (assuming one can move a decimal place in their mind). Big name commercial plug in tube testers use this method.

    HOWEVER - nowhere have I ever seen someone suggest that (let's pretend the Cathode Resistor method didn't exist) by taking the Cathode voltage one can accurately obtain the bias reading. It's confounding!

    FURTHER - why then should we persist in believing then that all bias and dissipation levels should give us some universal standard reading that we believe is right, when in actuality they all give different readings.

    That's is - just taking a Cathode voltage and applying it to calculate 50% dissipation is not going to give us the same reading as Transformer Shunt Method.

    So then why do we persist in believing that all methods should thus be applied to preconceived notions of "correct" bias - an issue that seems lacking in logic and clarity given the amount of time spent discussing and changing and biasing tubes?

    Now - I have 20 years experience working on amps - I have the tools and knowledge. I represent more than just the average hobbyist but if I'm doing something wrong then EVERYONE below my skill and knowledge level is too. So this suggests yet another issue.

    Until the industry standardizes a method we cannot refer to some arbitrary chart or calculator to ascertain what exactly our "bias" is.

    Now - this hasn't only been something I've addressed as my NOS tube stash runs low. I don't care what modern production tubes run at - they willgo microphonic or fail before they wear down. But in regards to NOS tubes it suddenly becomes an issue to me.

    It's deceptive to oneself to accept one method as correct "just cause" when in fact the tubes could be under or over biased. Or are they? How will we ever really know?

    I know there are methods of removing an OT transformer lead and some say THIS will reveal true bias but yet again this compounds the issue. If we aren't getting the same readings with different methods then the whole thing negates itself.

    But the Cathode Resistor Method - which I like because of its ease of use, not to mention safety aspect for myself and amp - makes the least sense to me. To "bias" an amp with Cathode voltage? AND adjusting bias based on the same "chart" as say, transformer Shunt method?

    Yields two totally different readings.

    What am I missing? And if I am missing something, then the whole world of hobbiests and professionals too are missing something as well.


  2. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    The GOAL is to get "correct/accurate" numbers. Thus, the METHOD is only as important as to how well it supports getting that GOAL.

    IMPORTANT point to remember -- especially with differing beam- and true pentode-tubes -- is the plate-to-screen current "split" ratio. A plate-measurement measures only Ip, while a cathode-measurement measures the SUM (Ip+Is) of BOTH the plate- and screen-currents; which, while typically a small value, is NOT trivial (roughly about 20:1 for beam tubes and 10:1 for true 5-grid pentodes at idle).

    So, just how "accurate" do want/need to be?:

    • CATHODE-resistor bias method = SAFETY, simple in-circuit, but slightly high (5-10%) readings. what most amp manufacturers use in their amps.
    In-line PLATE bias methods = DANGEROUS, complex, exact plate-current readings.
    Plate BYPASS/SHUNT method = DANGEROUS, complex, semi-exact plate current readings.
    Last edited: Aug 12, 2017
    ocduff likes this.

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I have measured the same amp using cathode resistor voltage drop (subtracting 5.5% for screen current), transformer shunt and transformer voltage drop and came up with numbers within 1 milliamp so I wouldn't say they're "completely different results".
    moosie, ocduff and RLee77 like this.

  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The ear is the best tool, imho. Then, one would want to know through measurements and calculations where the ear has caused the operator to set the bias....for future purposes. OF course, some experience might be advisable when using this aural method. I have set bias on amps for players with their ears making the decisions....knowing that the bias adjustment was in a safe range.
    telemnemonics and RLee77 like this.

  5. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    Robrob - that's possible of course, but is that universally true in every single amp? Because I have not found that to be so. Either way this is all - myself included - somewhat anecdotal information.

    My findings are - that ultimately I get different readings with each type in different amps. Some may match up, but more often they don't.

    I'm just saying - if readings are 10% off between two common biasing methods (that could hypothetically ALSO mean two "matched tubes" read 20% apart) and then we are using published voltage/milliamp parameter that applies to one method and not the other, then ultimately we are left with the job not done correctly.

    I'm not out to question anyone's expertise or to question methods but to raise issue with something that bothers me - given these different readings from different methods how can we bias an amp at 70% dissipation and thump our chests and say we done good when it's actually at 80%? OR 80% on one tube and 60% on the other?

    THAT'S my reality. And with the sheer amount of money that exchanges hands yearly for techs to replace and bias tubes - man I'd want to be able to explain my logic if my feet were held to the fire.

    Which they NEVER are in a tech world - they just cover over the issue with more technical speak when in actuality there isn't a problem. If every bias tester is giving readings that are 10% off and we are sending folks back on the road with false bias readings, that's not good service.

    But I'm off topic.

    If the pro's say I can bias my dear Tung Sol's at 449v and 28ma (50%) in my 5F6-A with Cathode bias Resistor Method and I can bet the life and longevity of the tubes on that method, despite any other reading I get from other methods, then I will sleep better.

    Over thinking this? Maybe. But Tung Sol isn't answering the phone to take my orders anymore.

  6. alnicopu

    alnicopu Friend of Leo's

    Oct 3, 2009
    Thats what I was thinking. No earthshattering difference. Also, i thought you added back a couple ma. because screen current isnt included in the bias resistor method. Doesnt going by cathode current alone yield a slightly colder bias?

  7. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    I want to know who the Audiologist is for any person who "tunes by ear" and can discern 5%THD (ROTFL)
    LeicaBoss likes this.

  8. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Mar 1, 2010
    Kent, OH
    Dude. You are way overthinking. And you seem to have some angst down in there as well.

    However one measures bias, all it is is a reference. A place to start. Every amp and tube is different. Why would you expect there to be 1 perfect end-all-be-all method?
    JD0x0 likes this.

  9. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    So perhaps this 5.5% thing or adding a couple ma could account for the difference beteeen shunt and Cathode Resistor Method.

    Could someone explain this to me please? Because this is news to me.

    Perhaps this is the pertinent info missing from most Cathode Resistor Method instructions and thus, the discrepancy I'm seeing.

    Thanks everyone for tuning in and responding. Super knowledgeable group here - been years since I've been active on here and I have read many of your posts over the years and your sharing your knowledge is invaluable.

    As I said, with new production tubes "close is good enough" was always my thinking but afternoon losing a minty JAN
    6L6WGB (likely from too high a plate voltage) it just got me a bit intent on doing it "right".

    When it comes to my amps I want all to be right in the world. I feel I'm in good company here on that.

  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    ocduff, just curious......but why 50%? And....being totally serious here....have you ever biased to that 50% point, listened....and then raise the bias to 55%...and listen....and then raise it to 60%...and listen....
    IMe, there are discernible aural differences there.....and this is not a mathematical exercise but rather is an aural endeavor. There is not one certain bias point that is 'correct' for every ear and every application. I find it interesting to hear different bias points and let the ear decide.
    ocduff and RLee77 like this.

  11. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    Oh yes - have absolutely done this. My '63 Vibroverb runs are 30% dissipation so the bias vary term will work.

    Run my tweed Deluxe at 100%. My 5E8-A at 65% with matched 1956 GE 6L6GB. Run my 5F6-A at 50% w TS 5881 because more than that just gets too agressive.

    My point is really to understand - and not just accept - the rationale behind some of this stuff.

    If there's different biasing methods, all yielding for me - different results (regardless of your results) how helpful are these methods?

    That's all.
    Wally likes this.

  12. Kevin Wolfe

    Kevin Wolfe TDPRI Member

    Jun 13, 2017
    Lakeland FL
    If your bias point is in the middle of the load line, at idle, (i.e. center biased) your valves should be able to handle the swing in voltage while playing. Is this not correct?

  13. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    Thank you for your input. But this merely articulates the reason for my question to begin with.

    You seem like the "why put the window in level when the house isn't level and the land isn't level"?

    I think we are just too far apart to agree! :)

  14. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    Getting back on topic as I've been long winded:

    Robrob or others, can you please clarify this 5.5% (or adding ma to a Cathode Resistor Method reading).

    Perhaps this is the disparity I have seen.

    Lastly, I have an electrical engineer audio designing friend who I have asked - "how does measuring Cathode voltage tell us anything useful in regards to the same bias parameters used with Trans shunt method?"

    His answer:

    "It doesn't."

    With him we walked through several methods of biasing to sort an amp out - and we settled on Cathode Resistor Method to bias that amp (as known matched tubes were measuring not matched with other methods).

    I've found the same with other amps - some methods produce similar results, others not so much.


  15. LeicaBoss

    LeicaBoss Tele-Meister

    Oct 16, 2015
    New Jersey
    Outside observations.

    I sense a few assumptions that don't feel right to me. First, that "correct bias" is an absolute and that a single, perfect standard must exist. Second, that various methods of measurement should agree.

    I would consider ditching these thoughts entirely. Accept that what you observe is how the system behaves and attempt to discover why, rather than insisting that thebsystem shouldn't behave the way you observe it to.
    RLee77 and ocduff like this.

  16. dan40

    dan40 Tele-Holic

    Aug 19, 2015
    Richmond Va
    A couple of things that may cause a known matched pair of power tubes to not read a match in your amp would be a slight imbalance in the two halves of the OT winding and/or your 1 ohm bias resistors on the cathodes are slightly different values. Both of these would cause a matched pair of tubes to read slightly different. Not accounting for the included screen current when using the "cathode" method will also yield slightly different results than when measuring directly from the plate.
    ocduff likes this.

  17. Snfoilhat

    Snfoilhat Tele-Holic

    Apr 8, 2016
    Oakland, CA
    The cathode is the source of all the electrons we're interested in when we talk about current, right? Not only that, but 'bias' is by definition a relationship between grin and cathode (in the types of circuits being discussed), so the EE's downplaying cathode resistor voltage drop--which is an easily measured proxy for cathode current-- seems weird

    Anyhow, most of those electrons appear at the plate, and some of them instead end up on the screen, and both plate and screen have associated currents, which are estimated right on the data sheet (highlighted in green).


    Class A one tube 315V supply idle screen current / (screen current+plate current)
    2.2 mA / (2.2 mA +34 mA) = 0.061 = 6.1%

    Class AB two tubes 285V supply idle screen current / (screen current+plate current)
    4.0 mA / (4.0 mA + 70 mA) = 0.054 = 5.4%

    The percentages change with supply voltage but you can at least see how the current flows roughly proportionally in plate and screen. This thread seems to be focused on bias as it influences idle plate dissipation, so you handle screen current and screen dissipation separately. Robrob's (5.5%) and OldTeleman's corrections (20:1) take it out.

    That's how I understand it, FWIW
    Last edited: Aug 12, 2017
    clintj, robrob, ocduff and 1 other person like this.

  18. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Mar 2, 2003
    Lawndale CA
    I think what you're missing is the fact that numbers make NO sound. All those precise pieces of numerical data you are worried about do not matter. Only a range matters - then it's up to your ears.

    A bias setting is nothing but a guideline - a "safe zone" setting to ensure you don't over stress the power tubes - you need your ear for the "tone" part of it.

    Setting bias by numbers is a terrible way to get the best possible tone from your amp. You HAVE to use your ears. The goal is to get ht best tone in a safe setting range. NOT to solve an equation.

    Equations sound like crap.
    RLee77 likes this.

  19. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yep. There is no magic about dialing in precisely 70% that will guarantee your tubes are safe and the sound will be best.

  20. ocduff

    ocduff TDPRI Member

    Nov 17, 2003
    Man, a guy asks a question on a forum of like minded individuals and some insist it isn't a valid question.

    Guys - there is an entire INDUSTRY based on the notion that one can correctly bias an amplifier. Hard earned money is exchanged hands over it. Chapters of BOOKS are devoted to it. Guru's tout one method as accurate and the rest junk.

    Was I the one who wrote those books or went on forums and suggested others were over biasing their amps or doing harm to their tubes or amps because of incorrect biasing?


    If I've gone against the grain in asking a serious question in the pursuit of meaningful and honest discourse and no one can provide an answer then the problem must either be with me or biasing itself, right?

    So I'll go warm myself before a fire of tube manuals and tech literature, and rock in the fetal position while I count the hundreds of hours I've wasted reading and trying to understand something that in the end, was basically something that these authors - if pressed - reveal that to some extent, is about as accurate as celestial navigation.
    telemnemonics likes this.

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.