... A couple of the PT's I have are 380-0-380 (according to the schematics of where they were taken...one reads 385 sitting unloaded on the bench).
What can I build from this beginning? ...
... Just not seeing any common guitar amps with voltages this high out of the PT...
How did the organ from which these came use the PT? What was the power supply arranged like, and what output tubes/OT did the organ use?
That's your first best guess on how to make use of them. The mistake I made when I was beginning in this stuff was thinking I would amass a bunch of random "tube amp parts" and then make something coherent out of them. Instead, the random parts often sat on their own, because certain elements go together to make a working whole.
With the right tube rectifier that PT will put you right in the +450 ballpark. Lots of 6L6, EL34, KT88 designs can work with that.
Maybe, maybe not.
380v * 1.414 = ~537vdc; we'd have to lose 87v across the rectifier to get down to 450v at idle. That
seems like a lot of voltage drop, which will get even larger (sag-ier) when playing unless the amp is drawing fairly constant B+ current (like class A). The filter caps still need to be rated for 550v and then some, given no voltage drop until the tube start pulling some current, and because you'll need to allow for high line voltage.
You might look to a lossy tube rectifier to drop this voltage, but rectifiers often act like a certain amount of series resistance: they drop less voltage with lower current draw and more voltage with higher current draw. What happens if the lossy rectifier you'd like to use won't drop the voltage unless you pull enough current to pop the rectifier?
You might look at a choke-input power supply (meaning also, look to see if the "organ donor" had a choke-input supply). Output voltage from these is Volts RMS * 0.9, minus whatever voltage the rectifier and choke DCR drop. Filter caps after the choke still need to be rated for Volts RMS * 1.414, because the conversion factor for a choke input supply again only applies once current is flowing.
So choke-input might look like 380v * 0.9 = ~340vdc. For the choke to filter effectively, you want a constant minimum current draw. Given the relatively low output voltage you might as well also shoot for Class A, in which case the mode of operation and the hefty choke filtering are mutually-supportive to create a rock-solid and stable B+. The choke has to be rated to support the B+ current of the entire amp at maximum output power (such an organ probably had a choke as big as the OT and/or PT).
At this point, if the original organ's plan is unknown, there is a LOT of recursive/iterative design to be done figuring out "which output tube," "what idle current for Class A," "what bias voltage," "How much voltage is left across the tube after cathode bias," (and cathode bias is to be assumed for a Class A output stage at these voltages) "how much voltage dropped across the rectifier and PT winding resistance," etc.
Once all that is sorted, a choke of a certain size (especially current-handling capability) will be dictated. And so you hope you have one on hand. If the organ used the PT and used choke-input filtering to supply a reasonable B+ voltage, then the organ had one. Did you steal it too? Do you still have it around? Can you find it?
And so you see why it's often better to keep the entire PT, power supply, output section, output tubes (and maybe phase inverter) intact when you rob from old equipment. All these design issues have already been sorted out and the parts work together well.
... I've read elsewhere of 5E3's being built with PT voltage this high.
Probably not true 5E3 Deluxes, meaning a likely move up to 6L6's and/or different power supply configurations, etc. At that point, it's probably just "5E3" in name, or using the tweed Deluxe's simple preamp. The 5E3 schematic & layout don't help much with their lack of voltages, but between the
5C3 Deluxe and
6G3 Deluxe, we might guess the PT was between 310-0-310v and 330-0-330v, to yield ~360vdc after a 5Y3 rectifier. The 5Y3 probably drops 75-80v along the way, but can't support a d.c. draw much about what a pair of 6V6's in Class A at these voltages demand. You could go up to the beefier 5AR4 or 5U4, but rectifier voltage drops are then lower...
It comes back to another vote for using the iron as the source circuit used them.
Topic: What to build with old iron (Read 3659 times)