18w EL84 Cathode Bias question

[Greg_L]

They probably intended to insert 1k but inserted 10k by mistake.

No I put a 10k in the screen supply before the 100r screen grid resistors just as a test to see how far the screen voltage would drop.

[Greg_L]

By coincidence I was working on a similar amp for a customer. Not a Mojo kit, B+ settles around 330V. Output tubes were rattly well-used EH EL84's, and biased about 10% beyond their suggested 12W plate power limit. 160 ohm substituted for the 125 ohm Rk, new pair of JJ EL84, Pp now 11.8W. Amp sounds great, crossover starts just at clipping. So folks, don't be afraid to sub the cathode resistor, run those outputs a bit cooler and within their suggested limit. FWIW on occasion I've used 180 ohms in a similar situation. With your higher B+ I wouldn't be afraid to go there, maybe even 200.

Good discussion, good graphs, to all correspondents.

Thanks for the input.

Yeah the amp sounds great and is seemingly happy overall with the bias running the tubes just under 100% plate dissipation. My plate voltage and screen voltage is a little high, but maybe I'll just let it ride. As usual I'm nitpicking because I read things and get obsessed with trying to make things perfect. I don't have any training or electrical education. I'm just a guy learning as I go.

I haven't scoped the output yet. I want to, I will, I'm wondering what kind of mind games those results will play on me.

[Dave H]

No I put a 10k in the screen supply before the 100r screen grid resistors just as a test to see how far the screen voltage would drop.

Sorry, I must have been thinking of post #25 when I posted that.

[Leo_Gnardo]

As usual I'm nitpicking because I read things and get obsessed with trying to make things perfect. I don't have any training or electrical education. I'm just a guy learning as I go.

Two things to keep in mind. From Enzo: "This ain't rocket science, just guitar amps. Don't need to build to NASA specs." and from Duke Ellington: "If it sounds good, it IS good." Folks please pardon me if the quotes aren't 100% accurate, the meaning is clear.

[Greg_L]

Two things to keep in mind. From Enzo: "This ain't rocket science, just guitar amps. Don't need to build to NASA specs." and from Duke Ellington: "If it sounds good, it IS good." Folks please pardon me if the quotes aren't 100% accurate, the meaning is clear.

I couldn't agree more, and I do try to keep those ideas in mind when I'm nitpicking.

For me though the nitpicking is usually educational. I know I don't really always need to do it, but by doing it I always learn stuff.