Fixed AND cathode bias?

[martin manning]

Martin,yes 40ma at 300v's would be 12 watts.I dont know what you mean by "published curves",but I suspect the "chart" you are looking at is stating the tube is good for 14watts max,but the curve it is showing is a "typical operation" example?

Stokes, the chart I referred to is from JJ's web site, The link is in post #10, but it's here:
http://www.jj-electronic.com/pdf/6V6.pdf
This just looks like an error to me; a 12W line labeled 14, but then at the low Va-end it seems well below 12, so I'm still a bit puzzled.

I'm kinda new to this game, still collecting gear and books.

Bruce/Mission Amps' post over in the current "Bias is Bias" thread
http://music-electronics-forum.com/s...ead.php?t=6259
gives some great insight as to how much variation one might expect to see in the same part number, even from the same manufacturer. Also interesting that 50-60% of max dissipation is acceptable in many cases.

MPM

[Old Tele man]

This just looks like an error to me; a 12W line labeled 14, but then at the low Va-end it seems well below 12, so I'm still a bit puzzled.

...you've got sharp eyes, I never noticed that! you're correct, it looks to be mis-labelled, for example, the horizontal 60mA line crosses the vertical 200V line right at the 14W Pa label, but would actually only be 12W Pa.

[stokes]

Although it doesnt state it,I am still going with the notion that the tube is a 14 watt max tube and the chart is showing a "typical opertion" condition.Or it is a misprint as you guys think.
OTM,now you are really confusing me.In your first reference to the 6L6GC example you seemed to be inferring that the AB1 numbers you were qouting (87% max diss) were hot.Now you are saying AB1 was considerd "cold".I'm probably missing something there,and I really dont want to get into arguing math,I am sure you've done your homework on that front,so I'll concede,90% idle diss has nothing to do with class A,it works for me,even if the numbers disprove it,I'm too old to go back to math class,cant teach an old dog......etc,etc..you win.

[Old Tele man]

OTM, now you are really confusing me. In your first reference to the 6L6GC example you seemed to be inferring that the AB1 numbers you were quoting (87% max diss) were hot. Now you are saying AB1 was considerd "cold". I'm probably missing something there, and I really dont want to get into arguing math, I am sure you've done your homework on that front, so I'll concede, 90% idle diss has nothing to do with class A, it works for me, even if the numbers disprove it, I'm too old to go back to math class, cant teach an old dog......etc,etc..you win.

...I apologize for not being more concise; both postings refer to 2 x 6L6GC Class-AB1 circuits, but the RCA-example has an 87% idle wattage (58mA at 450Vp), while in the second posting I describe Leo's later brown/black tolex-era mid-range 40-45W amps (Pro, Concert, Super, Bandmaster, Vibroverb, etc.), which, by current standards and tastes, were idled rather cold at about 55% idle wattage (about 35mA at 450Vp).

FWIW -- the real differentiators between the RCA and Leo amps are (a) the OT Zpri values: 5.6K-ohms (RCA) and 4.0-4.2K-ohms (Leo), and (b) their output distortion levels: 1.8% (RCA) and about 5% (Leo, at rated power into resistive load).

...and, surprisingly, the PI output drive signal levels to the 6L6CG's for both amps are identical: 35Vg.rms (RCA for 55W) and 35Vg.rms (Leo for 40-45W).

[stokes]

Okay,so if I got this right the RCA example,you cite is right out of the book,yes?And of course Leo's is obvious.So it kind of confirms part of my point,we are obviously getting at it two different ways,and that point is, that in guitar amp world there are a lot of liberties taken and the parameters are stretched at times.And as you point out,the book almost always goes for the lowest distortion levels,while we dont.Choice of OT's makes a world of difference as to the parameters the tubes are working within,blah,blah,blah these are all things we agree on,so,dont laugh yet,using the mathematical examples as you do gets us to the book values,which would seem to be useless in guitar amps,(okay,not totally,but stick with me here),okay,now you can laugh,I am going to say that in the case of a SE guitar amp idling the power tube at 90% dissipation will get you in class A "territory".Sorry,but I am a thick headed old Irishman.And this all started how?I gotta go back and read the first post over again.

[loudthud]

The plate curves on the JJ 6V6 are for triode operation so they aren't much use for predicting bias with a pentode connection. The spec sheet lists the max plate dissipation in triode mode as 10W so why would they draw a 14W line? I'm happy to see the published some sort of spec sheet though.

(side note) O'Connor writes in one or more of the TUT books that you should still put a resistor in series with the screen grid when connecting tetrodes as triodes. This is probably a good idea in guitar amps to limit screen current since screen current spikes up when the tube is driven hard.

[Old Tele man]

(side note) O'Connor writes in one or more of the TUT books that you should still put a resistor in series with the screen grid when connecting tetrodes as triodes. This is probably a good idea in guitar amps to limit screen current since screen current spikes up when the tube is driven hard.

...+1, also, GEC always recommended use of at least a 100-ohm screen grid to plate resistor when operating any of their "KT-" tubes in triode mode...something that was seldom specified by USA tube manufacturers.

[martin manning]

The plate curves on the JJ 6V6 are for triode operation so they aren't much use for predicting bias with a pentode connection. The spec sheet lists the max plate dissipation in triode mode as 10W so why would they draw a 14W line? I'm happy to see the published some sort of spec sheet though.

Yes, now that you mention it those are triode-shaped characteristics, aren't they... duh.

Anyway, I went ahead and e-mailed the folks at JJ. The response came back that their 6V6-S is a 14W max Pa tube, and the line on the chart is "approximate." So there you go. I guess.

MPM