Biasing the Vibro-King "by ear"
Fender recommends using this "by-ear" approach to adjust the bias on a Vibro-King:
Thanks for your inquiry and your interest in Fender Musical Instruments.
On this model, the Bias doesn't need to be measured to be accurately set. To set the Bias without removing the chassis, set the Volume knob between 1-1/2 to 2, and adjust the Bias knob (located on the under side of the chassis, next to the power cable,) so the amp produces the maximum hiss. Strum a full chord and let ring.
Now, turn the Bias control back just to the point that the sound of the guitar loses warmth and sustain. Then increase it slightly. This will ensure minimum crossover distortion and the best sound.
John Shannon Consumer Relations
This approach works because the Vibro-King, unlike the majority of tube guitar amplifiers, doesn't use a negative feedback loop around the power stage. Let's see whether I can clarify this...
A negative feedback (NFB) loop attempts to keep the output stage operating linearly despite how the output tubes are biased. The effect of the bias setting in an amp with a NFB loop (i.e. not a VK) only becomes apparent when the feedback loop can't do its job. This happens at very low volumes when the output stage is biased cold and at very high volumes when the output stage is biased hot.
What does that have to do with setting the bias by ear, and why shouldn't you set the Vibro-King's bias using a meter? Patience, grasshopper...
There's no harm in setting your output tubes to idle at a specific current so long as the person calculating that idle current is aware of the operating voltage on the tube. In other words, you need to know both the tube type and the amp type in order to be able to specify a recommended idle current. Anyone who tells you something like "6L6s should always be biased at 35ma" is missing a very important piece of the big picture and giving out bad advice.
Keep in mind that any recommended idle current setting is a rule of thumb and not an absolute requirement. Tube circuits work fine over a wide range of operating conditions; small deviations in idle current will not produce dramatic differences in sound. Also, those settings you strive for with your meter will drift over time as the tubes age and will change in proportion to the line voltage. My point is that it's unnecessary and self-defeating to try to set idle current with too much precision; if you set the idle within ten or twenty percent of the expected value your amp should be fine. I have to chuckle whenever I see someone report that they've adjusted their idle current to three significant digits...
As a rule of thumb, lower idle currents will give you more "sterile" clean sounds, later breakup and more headroom; your tubes will last longer at lower idle currents. Higher idle currents will give you warmer cleans, earlier breakup and less headroom; you'll burn (literally) through tubes faster, too.
At extremely cold bias settings you'll find that your amp starts to sound awful as a quiet note decays. At extremely hot bias settings the plates of your output tubes will glow cherry-red or even orange as they fail to dissipate the excess heat. Tubes are really rugged and can tolerate this kind of abuse for short periods, but they will self-destruct if it goes on for too long.
I buy tubes that are close to the same spec - not "matched", just close. I've been using Mesa/Boogie tubes. They're selected for a very narrow range of operating conditions that happens to be compatible with the middle of Groove Tubes' range. That way that I can pop in a replacement and start playing right away if an output tube fails. So long as the amp still sounds good (and it always does so long as the tube failure hasn't taken out a screen resistor) and the tubes don't show any redplating, I don't go any further than that; I don't tweak bias either by ear or by meter.
If I was changing tube brand or type in my VKs, then I'd set bias using Fender's procedure. I wouldn't bother to meter the idle current unless I happened to have the amp on the bench for some other reason. And even then I'd make the measurement just to satisfy my curiousity. If the amp sounds good and the tubes aren't redplating then the bias setting is OK.
I have never used a bias-testing jig myself, although I certainly believe they're the safest approach for someone not experienced in vorking with vacuum-tube electronics. I measure the resistive drop from the OT primary center tap to each plate and calculate the current using Ohm's law. But of course that method requires access to the guts of the amp.