Firefly Power transformer question

[kkillebrew]

I have a triad transformer I'd like to use for a firefly build. It's rated at 230v 110ma and center tapped (two separate windings) but I'm actually getting 142vac across each winding so it's really 284vac. Problem is I'm getting +390V without load with full wave and +195V full wave CT grounded. The 12au7 is rated for 300 volts on the plate so I'd either need to cut down the B+ or use the +195, which I fear is too low.

Suggestions? Can I use a power resistor in front of the pi filter? is +195 too low?

[Wilder Amplification]

Plate voltage and B+ voltage aren't one and the same thing. Once the valve is in and biased, it's gonna draw current through the plate resistor, which is gonna cause the plate resistor to drop the plate voltage.

Moreover, plate voltage is relative to the cathode, not relative to ground.

[kkillebrew]

In this circuit the 12au7 is the output tube in push pull configuration so the 22K output transformer center tap is all there is between the b+ and the plates. It's cathode biased at about 3.3% of the plate voltage or 13v in this case, which still leaves 377v unloaded.

I guess the question is how much will the 3 watt amp load the power supply, and what is a reasonable safe plate to cathode voltage for the 12au7 (actually a 5814). AND is it a good idea to try to load the B+ with a power resistor in front of it. Or should I just run off 195v.

[Wilder Amplification]

OK I see now. And the DCR of the OT primary isn't near enough to knock the plate voltage down on its own.

For that yes you can use a resistor but your voltage will sag under load via doing things this way. I'd consider running off of 195V and expect to get about 2.5W clean output.

[kkillebrew]

OK I see now. And the DCR of the OT primary isn't near enough to knock the plate voltage down on its own.

For that yes you can use a resistor but your voltage will sag under load via doing things this way. I'd consider running off of 195V and expect to get about 2.5W clean output.

Yep. A 4.7k 3w resistor does the trick knocking down the voltage, BUT the amp sounds better clean and overdriven at 195v b+.

I realized almost immediately after finishing the amp that it's not my cup of tea. It's a real one-trick-pony. The overdriven 12au7 power tube sounds great emulating those big marshall sounds but breaks up way too soon, and what little clean power there is sounds flat, almost solid state like. I'm probably gonna change the OT and stick in a single EL84 output stage, keeping the casdode drive stage to overdrive the front end when I want distortion.