Hey all, here's another one of my wacky ideas that may or may not work. I would like to build a B+ power supply for testing purposes. I've had several times I would like to power something with a known good power supply. It would need to have enough current capacity to power something large like an amp or TV. Yet, be able to reduce for a table radio.
My initial thoughts:
1. I'd need a power transformer with enough current and voltage, large enough to power a lot of 6.3v tubes and a couple 5V tubes (maybe).
2. good filter network
3. various taps for different voltage take offs
3. some type of protection
I'd want some type of bleeder resistor to keep the filter caps from holding a charge. Probably a relay to shut off the the HV when not in use (or if I see smoke
)
One question tho. I would need to make it variable B+. How to do this? If I use a variac, then that also cuts down the filament voltages if I need to use them. I would think some type of variable resistor that would be heavy enough, but with no load or a light load, how well would this work? I'm open to suggestions.
I have a variable power supply that was built for the Navy to service radios. It is a dual supply 0-350 and 0-150 both supplies a full floating and can be put in series or the 150 supply used as bias.
It consists of two plate transformers and two filament transformers supplying 6.3 vac and 5 volts respectively. The voltages are controlled by power stats one for each supply. There is a single dual range voltmeter and a switch to take either reading. I use it with a fused milliammeter.. In the past I have used it to completely replace the power supply in a 1949 Zenith console. That helped me to detmine that the radio was fine and that only the power supply needed restoration. The supply uses two 5R4's potato mashers though it was configured with 5U4's...
I have two mother rack mount DC supplies that are regulated, seldom used heavy old school late 40's One of which was built the Lincoln Laboratory... FWIR I have hand drawn schematics for both.
Similar power supplies can be built from old BW tv transformers, separate filament transformer and a power-stat from modified plans in a early 60's era ARRL Handbook. These are "meat and potatoes" supplies one of my favorits is a full wave supply using the 5U4 and two FWIR 6DE4 dampers giving a high and low range DC.
If you shop around for the iron and are handy with chassis one could be built for some $50.
GL
Chas
I found a power trans that might work, although not as heavy as I want. It's about the size of a large ac radio trans. It has some pretty heavy 6.3v leads, looks like a 750v HV, but the center tap is internally grounded. I took the cover off and verified this. Looks like I could clip the ground lead, solder a wire on and run it out. I did a quick bridge hookup with the HV to the AC in, - to the trans case and + to the meter (nothing else). But something must be wrong as it overloads the trans. It's a bridge unit that I have used in other apps, but was in a hurry and probably didn't hook up right. I may keep looking. Found a few on epay from organs that would have lots of power. I have a really cool trans from an old TV that has the rectifier tube socket right on top. Just got to find it tho. A schematic would be interesting, Chas. I could use a couple transformers with the filament on one and the HV on the other controlled with a heavy variable resistor or something.
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A primary resistor will be a heating problem...
IMHO that transformer is rather small for a plate transformer. But if that all you have.
Do you have goals to meet for maximum DC current and voltage?
IMHO 100-125ma and 350-400 volts would cover most everything except a large TV... chas
No, it's not very big. Probably ok for radios or a small TV. I'm eyeing one from a big Baldwin organ that should be sufficient. Lots of tubes and taps. I'm still looing at options. A small variac might be the best way to control things with some kind of voltage divider. If you have a diagram I could look at, that might be helpful. PM me please when you get a chance.
I found this on the other forum posted by "Rich". It looks like it has at lease some variability to it.
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I did my test by stealing power from a Leslie amp using a variac. Upon a suggestion from a forum member on the other site, I found this on eBay for a reasonable - not cheap price, but saves me the hassles of putting something together. I may not be able to power a large TV, but it has enough juice to test about anything I need to. It was already recapped, so I don't even need to work on it.
Here's my solution:
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I would not go for a stepless-variable supply using a tube as the regulator. It is an LDO equivalent in the tube world and will dissipate a lot across the regulating tube. Meaning, your current sourcing ability is severely limited.
I would use a tapped transformer. Better yet, and isolated variac (truly stepless-variable). Then rectify and filter.
And then use a couple of smaller transformers for the filament volatges.
Thanks for the input guys. I found a big ole transformer from a mid 50's Admiral TV. Got lots of current, lots of voltage and a really cool rectifier tube socket right on top. I might save this one for an amp project tho. It's really neat looking. She was putting out 600v dc off the rectifier output with a filter cap clipped on. No load. I could use a separate trans for tube filaments and run direct, with the B+ run off a small variac. Be heavy tho. Probably be a built in part of my service bench. Future project...