'69 Marshall Head 6550 to EL34

[NannerPuddin]

I have a '69 Marshall 50w head, it has 6550's in it.  I know how to convert to EL34s, what I wonder is if I do this, what will I need to do if I want to use 6L6GCs from time to time?  Will a rebias do it, or do I need to change some resitors?

Thanks,

Doug

[jojokeo]

The harder part is converting to the EL34's not the other way around since you need to connect pin 1, figure a lower OT reflected Z, things you know... as for quick biasing help - http://www.webervst.com/tubes1/calcbias.htm
As usual there's some things you can do or not depending on how anal you want to be about it for sound & performance : compromizes.