Rocket with 6G6 output, need cathode bias resistor value?

[loctal]

Hello group.

I am finishing a rocket with two 6G6 output tubes. I can not find any info on the correct value for the cathode resistor. The plates will be at about 250 volts.

The max anode dissipation for the 6G6 is 2.75 watts and the max screen dissipation is 0.75 watts.

I will use a 10 watt 1000 ohm adjustable pot if no one has any ideas.

I have built a plug in bias meter that is able to read how much current is flowing through each tube.

Any suggestions would be appreciated.

Thanks.

[Structo]

http://www.mif.pg.gda.pl/homepages/fran ... /6/6G6.gif

I found this.
http://www.mif.pg.gda.pl/homepages/fran ... /6/6G6.gif

But the specs say 180v is max plate voltage.

[Fischerman]

Maybe try the links at the bottom of this page.

[martin manning]

Data sheet here: http://tubedata.milbert.com/sheets/049/6/6G6G.pdf
What will the screen voltage be? If it's near the plate you can use the triode connection curves in the datasheet linked above. For 250V Va-k and 70% of the max plate dissipation I get 2.75* 0.7/250 = 7.7 mA. From the triode curves at 250V and 8 mA Vg-k is about -23V... Va-k will really be about 227V then, so 7.7*250/227=8.5 mA; let's call it 8.5 mA at 227V, Vg-k =-19V. Total current through the cathode resistor is 8.5* 1.17 = 9.9 mA (using the ratio of screen and plate current for the operating point given in the datra sheet), so you need 19/9.9=1.92k Ohms for one tube or 0.96k Ohms for both. That's my guess, anyway.

[loctal]

Thanks for showing how to work through the math Martin.

It looks like a 1000 ohm pot will work.

[martin manning]

No problem. I'm interested in hearing how it turns out.

[loctal]

I have a report on the 6G6 output tube rocket that I built last week.

I used a variable 1000 ohm cathode resistor. The voltage was higher than I expected on the amp. The plates had about 290 volts and the the cathode had about 22 volts. The current through the 6G6s was about 11mAs at idle and about 17mAs each at full output. This is all off the top of my head. The tubes are being operated beyond thier design.

The output transformer is from Musical Power Supplies. 5 watt 22K primary with 4,8 and 16 ohm secondarys.

Overall, the amp sounds like a rocket. It is not nearly as sweet and chimey as my EL84 rocket or the 6K6 rocket that I posted about two weeks ago.

I built the 6G6 rocket for a friend and he has it now. He dialed the cathode to 400 ohms, hooked an 8 ohm speaker to the 16 ohm tap of the output transformer and tried 6K6s in his rocket. Matt reports that the amp sound better with the changes. The amp should be putting out about 10 watts in this configuration. Way past the rating of the current output transformer. In the future, the amp will receive a different output transformer. Merit A-3128 with 8000 ohm impedance at 14 watt output.