Marshall 50w Jmp 6550 bias consensus

[matt h]

(deleted)

[David Root]

By which you mean the 6550 screen is beefier than the EL34 screen so can draw more current without wimping out?

[matt h]

(deleted)

[Gaz]

Dude, screen's not gonna die with 1k limiting resistors at 400v.

[matt h]

(deleted)

[Leo_Gnardo]

DA SCREEN BOSS DA SCREEN!

6550's are not just 35/25th scale (40% bigger) EL34's.

HAH!

IT'S THE BLIMP FRANK, THE BLIMP!

Sure 'nuff 'n yas I do. . .

You're right matt h! No matter how blimpy they look...

[Gaz]

I'd encourage you to look back to a fairly recent thread about this very issue with 6550s and/or KT88s. I'd also repeat what that thread says and encourage you to look at what's going on in the screen curves when the plate hits the knee area.

Oh, I understand, and have spent a lot of bench time measuring screen dissipation. That fact of the matter is that this particular example is not dangerous, because by the time the power amp starts to clip to voltage will have sagged to a more than safe level. If you are reall over taxing the screens anyway, lowering the bias a little is not going to help, or the best solution.

And FWIW, KT88s have a 8W screen dissipation, not 6W like the 6550. I've done plenty of fear-mongering myself when it comes to screen grids, but I encourage everyone to measuring screen dissipation in their particular amp because B+ sag plays a huge factor in as well as the particular screen grid resistor arrangement.

[drasp]

Belated THANK YOU for all the extra info in this thread. . . Still running @ the bias #s I mentioned originally, may bump up to ~45mA & see if I notice a difference.

[Jana]

I like to bias tubes from Doug's Tubes.