5e5a spare parts build

[mbailey]

I posted about what to build with some spare parts I had & decided with some suggestions from you guys on a 5e5a. I got a few questions I thought someone might help me with. first I'll list what I'm building with...
a power xformer pulled from an old organ chassis I measure 380-0-380 @ the secondary unloaded it also has a 12v, 6v & a 5v tap, no bias tap. judging from the amount of tubes on the chassis it should have plenty of filament current.
The output xformer is from an ultralinear bassman ten 70w I don't think I will use the UL taps unless someone can give me a good reason.
I have a choke from a Dual showman
I ordered a 5e5a board from Watts & ordered 4 22uF 500v filter caps.
I'm building this in a recycled Dual showman head that was given to me.
first does anyone see a problem with what I got so far?
second since I have no Bias tap should I cathode Bias or generate a bias supply
third what rect. tube should I use?
any suggestion or ideas will be appreciated!!

[luthierwnc]

You're going to need to knock that voltage down a lot -- a 5U4 and/or a big zener on the centertap. Shoot for 410 to 440 VDC at the first node. Other than that, it ought to work. Pretty heavy for a Pro though.

You can just pull the bias tap off one of the HV legs like a Marshall.

Skip

[mbailey]

how much difference is there between loaded and unloaded voltage. with a 5u4gb I get 397vdc from pin 8 to gnd?

[tubeswell]

how much difference is there between loaded and unloaded voltage. with a 5u4gb I get 397vdc from pin 8 to gnd?

Load it up and see. With the rest of the tubes in there, the voltage should drop quite a bit.

[luthierwnc]

That's quite a drop. 760 volts center-tapped would be a lot higher if we were talking about a new guitar amp trannie. The wildcard is those old organ transformers are built to light up a lot of tubes but without much HV draw. If you are under 400 at no load, there's no telling how much lower it will go. I agree that at this point you should just make it and decide on your rectifier when you spark-test it. You might end up putting some diodes in there.

The caveat is that if the transformer is so weak you need to prop up the volts, make sure it is still running cool. After you leave it on for 10 minutes, it shouldn't be hot to the touch.

For laughs, run the Duncan Power Supply program with the known parameters and toy with the internal resistance on the PT.

Cheers, Skip