When calculating dissipation of 6L6GC's for biasing?....

[Gutch220]

For doing bias calculations with JJ 6L6GC tubes, I've always used 30 for the dissipation watts, but I've also seen/heard people use 25w. So I wanted to hear some feedback.
What should you use? Which one is correct?

There's like a 20% difference of mA when using each so I'm assuming it would matter since using 30w at 70% is higher current then 25w at 70%

25w?
26w?
27w?
28w?
29w?
30w?

From JJ website, they say 30w

 

[Kuga]

I was told here (MF)6L6GC at 60%. Yeah they are 30W plates disipation.

 

[Matthews Guitars]

Check the data sheet from the manufacturer of your specific tube type to be absolutely certain.

Not all 6L6 tubes are the same. There are variations.

 

[Gutch220]

I was told here (MF)6L6GC at 60%. Yeah they are 30W plates disipation.

really? 60%?
I've always went by the notion of 60-80% so 70% is right in the middle. Is there something I'm not aware of with 6L6GC tubes?

 

[william vogel]

really? 60%?
I've always went by the notion of 60-80% so 70% is right in the middle. Is there something I'm not aware of with 6L6GC tubes?

No, there’s nothing you’re not aware of with 6L6GC. They are 30 watts dissipation. I bias them between 60-70% every time. I find that if the plate voltage is lower (420), they require less dissipation to reach full clean rms output with no crossover distortion. Like all tubes, when the voltage is on the high end, it requires hotter bias to achieve the clean sine wave without crossover distortion. I check power output, waveform and sound and adjust them around until I get the most of everything without pushing them really hot. I don’t worry much about trying to get them to last 10 years because power tubes are consumables.

 

[Pete Farrington]

Anode dissipation limiting values require the context of the rating system used to derive that limit.

Unfortunately modern valve manufacturers don’t publish that info.
But historically the 30W limit of 6L6GC was under the design maximum system; its equivalent design centre rating might be about 26W.
Whereas the 25W limit of EL34 was under the design centre system; its equivalent design maximum rating might be about 29W.
70% of a valve’s design centre limit is about the same actual idle dissipation as 60% of its design max limit.
I suggest that in the typical fixed bias AB1 guitar amp circuits, it’s probably best not to idle either EL34 or 6L6GC above 18W.

 

[Gutch220]

Anode dissipation limiting values require the context of the rating system used to derive that limit.

Unfortunately modern valve manufacturers don’t publish that info.
But historically the 30W limit of 6L6GC was under the design maximum system; its equivalent design centre rating might be about 26W.
Whereas the 25W limit of EL34 was under the design centre system; its equivalent design maximum rating might be about 29W.
70% of a valve’s design centre limit is about the same actual idle dissipation as 60% of its design max limit.
I suggest that in the typical fixed bias AB1 guitar amp circuits, it’s probably best not to idle either EL34 or 6L6GC above 18W.

So, I'm assuming the limiting values on their website, like this one, are the "absolute maximum", or is it one of the other two?

 

[Pete Farrington]

The 6L6GC type standard has a 30W design max anode dissipation limit.
eg see https://frank.pocnet.net/sheets/093/6/6L6GC.pdf

https://frank.pocnet.net/sheets/049/6/6L6GC.pdf

Modern manufacturers are somewhat obliged for their product to conform to the type norm. Otherwise it’d be misleading to sell them as being 6L6GC or whatever.

The 6L6GC was introduced after North American manufacturers switched from design centre to the design max system. So the only published info for them shows design max ratings.
But some valve types that were current around the time of the transition (from design centre to design maximum around 1957/8) had info published for both systems.
eg 6L6GB

https://frank.pocnet.net/sheets/049/6/6L6GB.pdf

https://frank.pocnet.net/sheets/127/6/6L6GB.pdf