We have discussed tube rolling and biasing in the ZMA section of the forum.  One discussion ended up on a tangent about how high we can bias the ZMA without risk of damage.  Anyway, it was an interesting journey down the path of mA and power transformer temperatures.  

I for one tend to bias on the higher side of whatever tube I am using.  I feel with the ZMA I get better results when the tubes are pushed a little harder.  However, I never really knew what that meant with respect to the tubes longevity.  I knew the specifications on the Hammond power transformers Steve uses in the ZMA.  I also used my Fluke IR thermometer to take a ton of reading of the transformers as the bias changed and time “on” increased.  So, I was comfortable knowing I wasn’t going to break anything on the amp.  

Some of the tubes I have, like many others on this forum, are DAMN expensive!  If I spend $500 on a quad of matched output tubes, my hopes are I’ll get a few years out of them.  While the discussion I mentioned in the ZMA section was focused on the amp, I really had no idea on how any of it related to the tubes.  Was I using and abusing my precious valves?  That is what motivated me to achieve a better understanding of tube architecture and the various parameters and rating of each tube type (KT88, KT66, EL34, etc., etc.).  I had just purchased a digital bias meter with four 8-pin octal tube socket probes to use on my vintage Harman Kardon Citation II amplifier.  The built-in meter on this amp is pretty crude and notoriously inaccurate.  The digital bias meter I have produces readings down the the tenth of a mA and is very precise.  

I recently had one of my Western Electric analog bias meters go out on my ZMA.  It was an anomaly as they were recently upgraded to the latest and greatest a few months ago.  It happens.  Problem was, I could not swap out my current output tubes because there is no way I could re-bias tube #4 due to the faulty meter.  That is when I decided to bust out the handy dandy BiasPro by Asharp Fretworks and continue on my tube swapping journey.  



Besides being able to swap my output tubes despite the rigor mortis in meter #4, I wanted to be more strategic with the biasing of various tubes on the ZMA.  Through my research I discovered a formula to assist in determining logical bias settings based on percentages of maximum current  for a particular tube type.

It is based on three things:
1) The tubes type’s plate dissipation rating in Wattage
2) The plate voltage at the output tube socket, measured by a purpose-
    built tool
3) The percentage of maximum current you desire to bias (65%, 70%
    75%, etc.)

The formula is:  (Plate Dissipation/Plate Voltage) X (% of maximum bias)

In the case of my 7027A tubes, they carry a plate dissipation rating of 35W.  I used my BiasPro tool, equipped with a plate voltage/cathode current combo probe which allowed me to precisely measure the plate voltage at the socket of the output tubes.  In my case it read 386 volts.  There is some slight variances of a couple volts depending on when you take a reading, but that is normal.  So, we will use 386.  Lastly, I chose to run them at 70% of maximum to take advantage of running them high to leach out every bit of dynamics without needlessly jeopardizing the well-being of my precious tubes, especially these NOS 7027A’s!

EX:  35/386 X .70 = 63.47mA.  Rounded up, 64mA bias setting.  

This exercise was interesting to me because I thought I was running them really hot at 60mA prior to using these measurements and formula.  I was able to bump them up to 64mA without fretting.  I ran these tubes for 8 hours yesterday and my power transformers measures approximately 131 F, which is well under their maximum rated operating temperature.  

This can be applied to any tube type (KT66, 5881, KT88, EL34, etc…not make/model) as long as you know the plate dissipation.  Here is a screenshot of a chart with some commonly used tubes, but you can find the rating for any tube with little effort via Google.



Prior to applying this method, I really had no sense of what a bias setting of 52mA vs. 62mA on my ZMA meant beyond what my ears detected.   Sure, one can “tune” based on the numbers represented on the meters, and at the end of the day, what it sounds like is a major consideration and for some, the only consideration.  However, if you have a hefty amount of Benjamins invested in your output tubes, and you are not interested in spending endless amounts on money replacing them when they burn out prematurely, then you must consider the impact on these bias settings as it relate to tube longevity.  

That being said, the more conservative recommendations by Decware will likely not put you in a situation where tubes suffer an early death, but you also want to make sure you are getting the most out of your amp and too conservative of settings could very well rob you of this experience.  The motivation to push the envelope in a thoughtful, controlled, non-reckless manner is something I highly suggest.  This is not meant to use measurements as the sole means to achieve better sound quality, but rather a method to ensure you are running the gamut as you adjust and listen and eventually decide on what sounds best.   In my opinion, following this methodology or one similar, is a better way to approach the end goal knowing you are less likely to leave a stone unturned.    

Yeah, they seem to be quite popular/necessary in the guitar amp community.  As a matter of fact, the builder of my BiasPro meter - ASharp Fretworks - is all about guitars.  I am not a guitarist and therefore have never owned or operated a guitar amp but I assume that when it comes to driving tubes into distortion, this biasing can be quite necessary to get the desired results.  

I'm with you on tuning the bias by ear.  There is no substitute.  This just helps one better understand the range in which a tube type could/should be run on their particular amp.  For someone not familiar with bias values, the large difference in range between a 6L6GC and EL34 won't make any sense at all.  And without knowing the plate voltage, there really is no way to know how much load is being placed on the tube.  And, I do like knowing if I could push any of my output tubes harder, and by how much without running the risk of burning them out in 6 months.  That is what this tool and method has done for me.  I am not at all worried about biasing lower, because that rarely produces the best sound for me.  It was when I got my 7027A NOS tubes that I started to concern myself with getting the best sound AND insuring I can make these tubes last as long as I can.  They were very expensive, but beyond that, I purchased the last two sets of quads from Watford Valves.  When these tubes die, I imagine that will be the end of my 7027A run, unless the new production tubes happen to perform well which is my hopes.  But, I don't want to think about that right now  

Do you have an idea of what KT88's you are thinking about?  I assume the Gold Lions?

Unfortunately, the lowest my KT88’s will bias is 76mA.  So, I have to turn the bias adjustment as low as it will go to get it down to 76mA.  They are running hard and high at that setting but that is the way it is which is the main reason I was carefull to monitor the power transformer temps.  They sure sound good though.  

Yeah, I remember you acquiring those tubes.  I have two sets of KT88’s and both bias just under 80mA at their lowest setting.  I also remember Steve having similar experiences with some of his.  As a matter of fact, he said if we wanted to run KT88s as our primary tube type on the ZMA, he could modify the amp so they bias lower.  However, if he did that, the KT66s and others similar tubes will not bias correctly.  That is what makes me think the KT88s - even with the lowest setting - will generally run to the outer edges of allowable bias before crossing over into power supply overheat territory.   I have not run enough makes/models of KT88s to know if some could bias into the 60’s on the ZMA.