PT question

[gui_tarzan]

I've got a handful of PTs taken out of various devices (Hammond organ, couple of unknowns, etc.) and the lowest pre-rectifier B+ I'm getting is between 370v-440v depending on the PT. This is too hot to power EL84 tubes from what I've seen so short of purchasing a new PT what are my options to get the voltage down to a usable level? The one that gives me 370-0-370 is the perfect physical size for a project I'm working on and I really don't have the cash to get something else right now.

[Chuck H]

A big zener diode or a string of smaller ones on the CT will get the voltage down by the zener voltage. You can also add a sag resistor after the rectifier to drop more and simulate a rectifier tube. Or, if the PT has adequate 6V wind rating or a separate 5V winding you can use an actual rectifier tube instead.

[pdf64]

Bear in mind that the voltage output at full load will be less than when measured open circuit, around 90-95%.
This is referred to as regulation.
It tends to be higher for B+ windings intended for tube rectifiers as they require a minimum series resistance in their plate circuit (to limit current), which is generally designed in to the B+ winding, eg by the use of thinner wire.
If using a tube rectifier, there should be scope to experiment with the different types in order to get a suitable dc VB+, eg a 5Y3 may be able to get it down to a suitable level.
Pete

[Mick Bailey]

PSUD will give you a pretty good idea of what to expect 'in circuit'

Are the PTs centre-tapped and where are you measuring the voltage?

A handy way of reducing B+ if you're using SS rectification is to use a choke and a very low value input cap - about 1uf - 2uf or so, then put the main filtering after the choke. I use motor caps for the input side.

[J M Fahey]

Short, crude answer: forget it, you're way too high for EL84.
With 370+370VAC you are too high even for 6L6/EL34 if you use SS diode rectification, go figure (523 VDC).
You are fine for big bottles using a tube rectifier or a *reasonable* series resistor to emulate it and add some sag.
But forget EL84.
Series diodes are a band aid, not open heart surgery.
Sorry.

[Enzo]

Don't use any input cap with that choke. Make it a choke input filter, and that ought to make about 333vDC from your 370vAC.

[J M Fahey]

Right.
Just that I doubt guitarzan has a suitable choke available and by own admission buying one is out of the question.
The typical screen suply choke found everywhere is way too small as a main choke filter.

[gui_tarzan]

Don't be sorry, that just means I can use this for something like a 6550/KT88 project later. I can handle disappointing news, I just try to find something good to take out of it. I have only begun salvaging parts from old equipment, I'll find the right size transformer for this yet. My next lowest one is only 200v-0-200v so that's too low. Plus it doesn't have a 5v winding.

Short, crude answer: forget it, you're way too high for EL84.
With 370+370VAC you are too high even for 6L6/EL34 if you use SS diode rectification, go figure (523 VDC).
You are fine for big bottles using a tube rectifier or a *reasonable* series resistor to emulate it and add some sag.
But forget EL84.
Series diodes are a band aid, not open heart surgery.
Sorry.

[gui_tarzan]

The choke I have in it right now is out of a Fender Bassman 100 and the only other one I have at the moment is even bigger (physically) than that one. Chokes aren't that expensive, I can handle purchasing one of those. By the way, I should have taken that 20 henry rating off that schematic. It was on there from the original I copied the power section from. I don't know what rating this one actually is, all I know is it's from my Bassman 100.

Yes, I know... I should know the value of what I'm putting in my amp. (ashamed and hanging my head)

Yikes! It's a 90ma, 4 Henry choke. I guess 20 was a bit off, eh?

[gui_tarzan]

Yes they are and I'm measuring from one lead to the center tap. Voltages across both "hot" leads are in the 740-880v range.

PSUD will give you a pretty good idea of what to expect 'in circuit'

Are the PTs centre-tapped and where are you measuring the voltage?