Some basic question on the data sheet of 6L6GC

[Alan0354]

Hi I am quite new in tube power amp design. I am just reading the data book by RCA I got in the late 70s. I have a few question about the bias of the tube:

1) In the book describing Push Pull Class AB1, the plate voltage is 450V, the G1 bias is -37V and zero signal plate current for the pair is 116mA and max signal plate current is 210mA.

a) Does this mean if you ground the cathode and put -37V at the grid, you'll have quiescent current of 58mA through each tube?
b) By 210mA max, one tube is off and the other tube is conducting all of the 210mA?
c) The max power dissipation of one tube is 30W, It this only govern with W=IV? Also, in this case how do I calculate the power dissipation. Do I use half of 216mA=108mA times 450V = 0.108A X 450V= 48.6W? That exceed the 30W!!!

2) In the data book, at the Class A1, Class AB1 and Class AB2 have bias less than -37V. But you look at all the Fender amp, the bias is about -51 to -57V. This means Fender and other Guitar amps are bias way deeper into Class AB or approaching Class B!!!! I biased mine to -37V and I like it better!!!

3) Why in the data book, the Class AB2 is biased at -22V, plate voltage is 360 and the zero plate current is only 72mA.
a) What is Class AB1 and AB2?
b) Why you get less current even biased at a less negative voltage ( -22 vs -37). Is this only because of the lower plate voltage?

Thanks

Alan

[Enzo]

The RCA book is a great book, it has a lot more than the data pages. Look up front in the chapter on tube applications, class of operation is discussed in detail. The difference between class AB1 and AB2 is grid current flowing.



The higher the plate voltage the more it attracts electrons from the cathode, so yes, lower plate voltage will have lower current, so a lower bias voltage will control the same current.

Those Fender amps with higher bias voltage also do not have a screen grid sitting 50v lower than the plates.

[exclamationmark]

The '1' or '2' prefixes (ie, AB1 and AB2) denote whether the tube is operating with or without grid current. For example a class AB2 amp will not go into saturation once the tube is driven positive, and instead the current will keep rising (up to limit, obviously). This is usually achieved by direct coupling it to some sort of follower circuit. I would assume the voltage is lower for a class AB2 amp so you could make the load line steeper to take advantage of the 'extra' grid curves that appear on the datasheet once the grid is driven positive.

[pdf64]

A discussion on another forum got me thinking about the implications of AB1 compared to AB2.
The key thing is the ability of the stage prior to the power tube to force its grid above Vg-k = 0.
Given that, the power tube can conduct more current; so does that mean that the power tubes don't actually saturate in AB1 (rather it's the driver stage having it's output 'clamped' by the g-k diode junction)?
If so, what does saturation mean in AB2?
I've got a feeling that it's meaningless anyway, the classes of operation just to act as an aid to understanding, and that the the tube itself can generally always be persuaded to pass more current, never actually saturating, the issue being that the tube would go beyond its limiting values at some point.
So the VB+, load value and drive potential of the previous stage (ie class) are chosen to obtain the desired power output, within the limiting values of the tube.
Does that sound reasonable?
Pete

[exclamationmark]

Actually that's a good point. It doesn't technically saturate in AB1, instead the driving tube usually gives up first and the voltage is clamped (or the capacitor instantly discharges into the power tube grid causing blocking distortion). You CAN actually saturate a tube, in that you can reach the cathode emission limit, but in doing so you would pretty much vaporize the grids or melt something. I've messed around with 6as7 dual triodes, and gotten something like 1+ amps (!!) out of paralleled triodes at 48v by driving the grids hugely positive. The grids on these things are designed to dissipate quite a bit of heat (I'd guess in the range of 2-3 watts), and svetlana actually published a data sheet with the tube going into saturation some time ago. But it's not something you could achieve (or want to) using audio grade tubes.

[Austin]

Many people use 6as7 and 6080 tubes for audio, mostly output transformerless amps but some people use them normally. I think they are cool tubes.

Sure it is more challenging to implement but isn't a2 ab2 b2 more efficient and more powerful with less distortion than a1 ab1 b1? Most tubes don't have curves listed for class *2 but is there any reason you couldn't safely do it anyway?