Can someone shed light on the bias adjustment in the Fender Blues Junior

[nigelwright7557]

Originally Posted by Zung

Gentlemen!

It's OK to play russian roulette with your own life by messing with the hot end of the tubes (anode), but it's not OK to talk other people into it.

If they are that inexperienced they shouldn't be messing with valve amps anyway.
Always turn off amp before working on it and discharge B+ through a resistor.
I always add a resistor and LED to my valve amp designs.
Don't touch until the LED goes out.

[Enzo]

Note the schematic calls for -10.7v for bias. It doesn't specify tube current. Like many makers They know that there is a wide range of acceptable idle current. They determined that with the bias set at -10.7v, then most EL84s that get installed will be within that range.

[JMFahey]

^^^^^^^ That.

[PRR]

> range of acceptable idle current.

On a similar amp in hi-fi service, there was a slight preference of idle current. Best sound was with about 17 Watts dissipation in the 12W plate. Turned down to a safe heat, there was remarkably little change of sound, even quite "cold".

I like thermal biasing. Use your IR thermometer. Both of a pair should be similar. Ideally you have the same tube type in a chassis which can be properly instrumented, so you can get a temperature for "good" heat.

But back through most of the golden age we did NOT obsess about bias.

[Zung]

Originally Posted by PRR

...
On a similar amp in hi-fi service, there was a slight preference of idle current...

That's an understatement:

• Setting the idle current is the final goal: too low and you have cross-over distortion, too high and you have red plates
• Setting the bias voltage (Vg) is a mean to achieve the goal. With the wide variation in the current production tubes characteristics, setting the current is safer.

[Enzo]

too low and you have cross-over distortion, too high and you have red plates

True, but in between those extremes is a wide valley of pleasant living. We might find say 20ma is too cold, and 50ma is too hot, but in between settings of 30ma, 35ma, 40ma will all be fine.

[Gnobuddy]

Originally Posted by Enzo

True, but in between those extremes is a wide valley of pleasant living.

With tube guitar amps, my ears agree with Enzo. Bias seems far less critical than you'd think from all those Internet forum posts.

A long time ago, I used to build solid-state power amplifiers for myself, and with those, bias really was quite critical. The "sweet spot" was really narrow - and if you plotted the summed transconductance of both transistors (in the push-pull pair) versus bias current, as Douglas Self did, you could see that this was the case. Bias a little too cold, and you have too little transconductance through the crossover. Bias a little too hot, and you have too much transconductance through the crossover region.

By comparison to transistors, tubes change transconductance quite gradually with change in current. As a result, the sweet spot seems to be much wider.


-Gnobuddy

[PRR]

> too low and you have cross-over distortion, too high and you have red plates

Right, but... 6BQ5/EL84 at 325V, you ALWAYS have crossover. Until you turn it hotter than about 16 Watts dissipation. But from 6W to 12W there is not a lot of change of low-level crossover. And when SLAMMED, it's crossover no matter how you bias it.

If we really gave a bent pick about crossover, we'd run these tubes down closer to 250V where the idle current can be a large fraction of the peak current without melting.

[Enzo]

But these are not hifis, so we accept some crossover and other offenses.

So just my opinion, but if you really hear a difference, and 68.23% dissipation is a real thing for you, then more power to you. But for the most part I think 10-turn pots and such for bias, as if it is some super-critical adjustment, is just overkill. Your mains voltage is not stable anyway, and so all your internal voltages will not be stable either.

[Pano]

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