Bias 6L6WGB?

[GuitarNorton]

I have a quad of Philips ECG 6L6WGB's I'd like to try in my Ganesha to see what they sound like.
What would the Bias range be for this tube?

[Old Tele man]

6L6WGB implies a 23W plate dissipation (Pd) rating, so, if you follow the "70%-of-Pd" rule-of-thumb and assume plate voltage (Vp) of 450V:

Ip = ( 0.7*Pd / Vp ) = (0.7*23W / 450V) = 16.1W/450V = 0.0358 ~ 36mA

NOTE: replace the "example" 450V above with the actual plate voltage of your amp!

[KLB]

The Ganesha runs the plates around 530-550v.
This is probably too much for a 6L6-WGB.

Since the plate volts don't drop as you lower the wattage, you better check with Dave Zimmerman about the safety of this.

[John Phillips]

The previous two answers show exactly why you should not try to bias an amp 'by current' without knowing a lot more about it.

First, you must know (not assume) the plate voltage to have any chance of telling whether the tube will even take it, let alone bias correctly at some particular current/dissipation.

And second, the 70% "rule of thumb" is questionable anyway. It's an upper limit, not a target.


The right current is above the point where crossover distortion occurs on a full-signal clean tone, and below the point where the tube ratings are exceeded when driven fully into distortion. Sometimes this is a small range, sometimes large. Often - with well-known amp/tube combinations - you can pick a figure which will be in the right range without having to measure anything else directly.

Another problem is that if you're using the common plug-in bias probes, the current you measure includes screen current (usually a few mA). Actually, this is a good thing in some ways as it reduces the real plate current slightly at any given measurement, hence making the setting a little more conservative - but most people don't seem to know this, let alone account for it. Either way, quoting bias figures to +/- 1mA, or anything like it, is inherently flawed.

Sorry for the lecture.

[Old Tele man]

the "assumed 450V" was for "example" usage...notice the later statement about using the "actual" plate voltage number.

if plate current (Ip) is determined from a cathode measurement you are 100% correct (but I did NOT specify where or how measured); however, if plate current was measured by the "shunt" method, you're not correct.

however, having both agreed and disagreed with you (in the same sentence no less!) it should be noted that EVEN though the cathode measurement method isn't 100% correct, it is, however, valid because it predicts a conservative (ie: "safe") recommended idle current!

...and, ironically, I tend to agree with you (can you say vascillation? ) about the dubiousness of the 70% number. After MUCH literature research, I've only been able to substantiate the "AVERAGE"-value (2/PI ~ 64%) mentioned by Bias-Rite.

[GuitarNorton]

Thanks John, your posts are always very informative, I always learn something from them.

And thank you Dave for the post and the call, top notch support from you as always!