Tube idle setting

[new2Krell]

I am curious what others do with regards to setting bias for plate current. I consider myself fortunate to have power amps with fixed bias (adjustable) in my 50 watt EL34 mono amps. There are guidelines, basically a range (3mA), given by the manufacturer, within which the plate current should (can) be tweaked. The range (26–29mA) is fairly small and the figures are relatively low. My tubes run at just over 500v and it equates to a fairly low tube idle of around 53 – 59%. I have had the amps (A/AB) for over 35 years (never owned any other tube gear) and would like to know how these settings compare with other equipment, be they adjustable or non-adjustable bias.

 

[retired farmer]

interesting I will look mine up I think mine are to be set a bit higher on a canary ca160

 

[retired farmer]

 

[new2Krell]

Thanks for the input RF. I have tried to find what voltage the CA160's run at, but have been unable to do so. If they run at 500v as my powers do, they would certainly be biased hotter, 70%. But, if the 160's run at say 400v, that would put them in the same ball park as the EL34's in my amps - around 56%.

 

[retired farmer]

Thanks for the input RF. I have tried to find what voltage the CA160's run at, but have been unable to do so. If they run at 500v as my powers do, they would certainly be biased hotter, 70%. But, if the 160's run at say 400v, that would put them in the same ball park as the EL34's in my amps - around 56%.

Not sure if this helps but my amp in ultra linear is rated at 140 amps and triod at100 amp. With eight power tubes a side. Out put power would give you an idea no?

 

[new2Krell]

Not sure if this helps but my amp in ultra linear is rated at 140 amps and triod at100 amp. With eight power tubes a side. Out put power would give you an idea no?

I guess you mean watts not amps?
The number of tubes is irrelevant, and the actual voltage any amp runs at is not particularly relevant either (though there may be good reasons for designers to choose higher or lower voltages to run their amps at - I am not an electronics expert!). As for tubes they all have their designed power output and are biased according to this, EL34's are rated at 25 watts.
I can only speak from what I have learned looking after my amps (which run at just over 500 volts) during the 30 plus years I have owned them . The manufacturer gave a range of 26-29 milliamps when biasing the tubes, this equates to 52-59% (of tube rated output).
26ma idle current = 0.026A. 0.026A x 500v = 13watts. 13 watts = 52% of 25watts (EL34 rated power).
If the amplifiers ran at 400v, the calculation would be different to achieve 52% of course - idle current would be 32ma (0.032 x 400 = 12.8).

I don't know why the bias range quoted is only 52 - 59% in my case, as EL34's can be biased up to 70% (35ma at 500v).
Whatever the reason, I have always assumed there were technical reasons and stayed within the recommended range. Fortunately, I find the amps also sound best when biased at the lower end of the range too so an added bonus that the tubes really run quite cool and last longer.

A few years ago I bought a couple of useful devices (pic) to allow biasing all four tubes at the same time. At the flick of a switch (well two) you can read plate voltage and current for each tube. It certainly beats messing around inside the cases with probes!