> When you slap 6550s into a Fender, at those plate voltages, about how much
> negative grid bias is required, and where do you set the idle plate
> current? Any mods required to the bias supply? Did you change the 470
> ohm resistor on the screen grids?

I used a pair of 6550Cs in my Twin chassis (the only thing left that is
original are the power and reverb transformers, and the power supply--
even the case is gone). I was surprised at how well they complimented the
preamp tone that I was using at the time.

I left the screen values at 470-ohms; B+ was only 460V, and bias was
-53V just as Fender wired it. No adjustments were necessary for the 6550s
or 5881s. Each tube draws a unique yet appropriate, safe and toneful
current. Reliability of Sovtek 5881s and Svetlana 6550s in this circuit
has been road proven over the last two years.

In models with only a single pair of original power tube sockets, an
auxiliary filament transformer will be required. Models like the Twin,
Showman or 100+ Bassman can accept A SINGLE PAIR of 6550s, with the other
pair of sockets empty!

> I think Svetlana makes a couple different 6550 versions. Anyone know what
> the differences are?
> Tonally there are no differences. Svetlana has swithced over to the 6550C
exclusively, as far as I know. You might want to check with them for up
to date info. The only technical difference between the A, B and C
versions was the gettering.

If you do use 6550s in up-side down chassis', spring retainers are a
must!

Happy playing!
Kevin O'Connor

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