6L6 watt dissapation at idle

[kiteman]

I have a pair of GT 6L6 CHP blackplates in my Bogner Alchemist. These tubes are rated 30 watts but the amp is 40 watts (20 watts per tube).

Do I set the bias at 70% of 20 watts because of the amp or 70% of 30 watts because of the tube?

Right now it's at 14.8 watts idling (425 pv * .035 mv) so I think I did this right.

 

[tunghaichuan]

I have a pair of GT 6L6 CHP blackplates in my Bogner Alchemist. These tubes are rated 30 watts but the amp is 40 watts (20 watts per tube).

Do I set the bias at 70% of 20 watts because of the amp or 70% of 30 watts because of the tube?

Right now it's at 14.8 watts idling (425 pv * .035 mv) so I think I did this right.

Usually tubes are biased at 70% of the maximum dissipation, which is 30 watts. So you want the tube to idle at 21 watts. 21/425= .049A or 49mA. And that is per tube.

Here is a handy chart:

http://www.webervst.com/tubes1/calcbias.htm

Scroll down the the 6L6GC settings. According to Weber's chart, you're bias would be hot at 49mA so you don't want to bias the amp any hotter than that.

Some people like the tone of the amp biased hot, but the tubes wear out quicker. This is why a lot of new amps come with cold-biased tubes, they last a long time so there is less of a warranty issue.

 

[kiteman]

Thanks Tung but does that make my amp put out 60 watts?

 

[tunghaichuan]

Thanks Tung but does that make my amp put out 60 watts?

Bogner's official rating is 40 watts. That's probably with a 1khz sine wave put into the amp measured right before the onset of clipping. IOW, it will do 40 watts clean. If you push the output section, you may get 50W or so. A few factors come into play: the current capacity of the power transformer, the size and ratings for the output transformer and the lower plate voltages.

 

[kiteman]

Bogner's official rating is 40 watts. That's probably with a 1khz sine wave put into the amp measured right before the onset of clipping. IOW, it will do 40 watts clean. If you push the output section, you may get 50W or so. A few factors come into play: the current capacity of the power transformer, the size and ratings for the output transformer and the lower plate voltages.

Whew, there's more I need to learn. I haven't measured the plate voltage but from researches I got 425/426 the most and one says 475 so I went with 425. For sure in 20 watt mode the plate voltage is 325 because they all said that.

You think it's better to set the bias in 20 watt mode and get 70% of 15 watts?

The transformers looks pretty big in this amp.

 

[tunghaichuan]

Just out of curiosity, how are you measuring and setting the bias?

Whew, there's more I need to learn. I haven't measured the plate voltage but from researches I got 425/426 the most and one says 475 so I went with 425. For sure in 20 watt mode the plate voltage is 325 because they all said that.

Before you change anything in the bias, you need to know the exact B+ voltage. It is critical. If you bias the amp so that the tubes are drawing 50mA each, at 475v you're running almost 80%, which is too hot, IMHO. Your tubes won't last long.

You think it's better to set the bias in 20 watt mode and get 70% of 15 watts?

I would set the bias in the 40W mode as 1) it is the normal operation of the amp and 2) I'm not sure what the circuit is doing to get down to 20W.

 

[kiteman]

The amp has two blue square pots and a 3 pin test plug on the output tube board. I used my meter set to 1000 mvdc and biased them to .035 using the test plug. It has 1 ohm resistors to the left and right pins.

I did researches for the bias on the alchemist and everybody (including the Line6 tech) says .035 mv. I think that's with the original tube, Ruby 6L6GC MSTR. I think that's a 30 watt tube too.



The first time I set the bias I had the meter set wrong (set to ma ac) and I biased it too hot. I noticed it when I tried it out and shut it down quick. Duh.

 

[kiteman]

Ok, I got it biased by reading the plate voltage and increment the bias 1 at a time. The plate voltage was 428 at .035 so when I got to .049 the voltage was 420.

420 x .049 = 20.58 watts

Good enough? :dance

EDIT: Oh yea, it sounds gooood now. :dude

 

[TradAmpGuy]

When it come to biasing an amp, the key is not to exceed the plate dissipation rating of the tube. Your 6l6s are rated at 30w (of plate dissipation) so you want to shoot for 70% of that... max. To figure that you need to know the plate voltage so that you have a target current draw to shoot for. Anyone who just gives you a number (ie 35mA) as the magic number isnt taking the plate voltages into account. 35mA might be right, might be too hot, might be too cold. As you saw, as you increase the idle current, the plate voltage decrease due to voltage drops in other areas of the power supply. So it is adjust, measure, calculate and adjust some more. Also, you dont have to hit 70%, that is just the recommended max dissipation. If you are going for more headroom, cleaner or thinner tones, you can idle the tubes colder. The sound you want is the right idle setting for your amp as long as the plates arent glowing cherry red that is Once you have a number you like, it does make it easier to dial in a new set of tubes though.

Hope this has helped.

Brian

PS. Plate dissipation and output power are not the same thing. 6L6 amps:

Tweed Super: 28w
Tweed Twin: 40w
Tweed Bassman 45w
Fender Supersonic: 60w

 

[kiteman]

I got it at 60% now and I like it better. :dance

422 x .043 = 18 watts. Not so hot now.

 

[Radioboy950]

I got it at 60% now and I like it better. :dance

422 x .043 = 18 watts. Not so hot now.

That seems to be a great middle ground lots of people are happy with , Kite.
Pretty much where are I always ran mine...60-65%.

I found the Weber Bias Rite to be one of the best tools out there, especially if you like to tweak amps and swap tubes. A toggle switch gives you both the plate voltage and the mA readings on the tubes.

So for example, the amp reads 422 volts.
30 Watts divided by 422 = .071 or 71mA.
.071 X .60 = 43 mA.

I found that great for almost all 6L6s I tried. HOWEVER, I will say that one variety in particular, the SED =C=, went from great to spectacular around 70%.

Tubes are good...ummkay.

 

[kiteman]

I've read that some tubes are not happy unless you run 'em at 70%. My 6L6s are Chinese made so I didn't want to push my luck but at 70% it sure has some warmth. I sacrificed that little warmth going 60% but still great tone so I'm happy and I'm sure the amp is happy too.

I may get those winged Cs soon, I've been hearing a lot about 'em. Right now I want to get an NOS tube for V1 in my amp and get that creamy tone. I have a JAN 12AT7 in the Defender h5 (with Mullard EL84) and it's got a creamy tone. I swapped a lot of tubes and settled on this combo.