325-0-325 PT too much V for Spitfire build?

[jcat5503]

This too much voltage for 2 el84's?

The PT is in an 18 watt clone at the moment, I'm thinking about doing a spitfire build with the parts.

I measured 325 before the 5y3 and about 367 after the rectifier

Should I invest in a new PT

I'm guessing I'll need a new OT as well? The current one only has an 8 ohm out, and the cabinet has two 16 ohm speakers in it wired to parallel..
So I cant do the 16 ohm out to 8 ohm load to get the 4k can I?

Also, any mod ideas for a spitfire build?

[jaysg]

build in power scaling...vvr

[jcat5503]

That seems like a good idea!

Is there any way I can make the VVR myself instead of buying one from Dana, etc.. ??

Trying to save a little cash as usual...

With a VVR dimed it's not enough to burn through the output tubes really quickly is it?

Also, can't i install a resistor instead of a choke??

[tubeswell]

You can zener down the B+ with one (or more) 5W axial 9V-15V zeners (in series) between the PT's high tension winding's centre tap and the ground return. Put the cathodes (banded ends) of the zeners point to ground. Mount the zeners on tag terminal strips away from the chassis. If you just stick a big resistor in the B+ it will drop the voltage alright, but you will also starve the power rail of current

[KT66]

367 is only 4% higher than the 350 the Spitfire schem shows, I don't think that's a big deal. Since the trannies worked ok in your 18W seems like they are ok for EL84s. You could just change the dropping string to hit the preamp voltages.

2 X 16Ohm in parallel presents an 8 Ohm load to the OT, so that should be fine as well.

Dana's VVR for cathode bias amps is only like $30 bucks :

http://hallamplification.com/main.html?src=%2F#2,2

Mark Huss posted the circuit below recently as an alternative to using the zener trick - I think that if there were a pot in place of R1 and R2 that it would be a variable power scaling type thing. I know I have seen a schem like that but can't find it now.

[FunkyE9th]

With a VVR dimed it's not enough to burn through the output tubes really quickly is it?

You can modify the VVR so that when it's dimed, it defaults to a lower voltage.