Tube radio Power Supply Building

So I acquired a BC-342-N WWII receiver. The original rectifier unit was removed and the previous owner made a ton of mods to the radio, this included making a home brew power supply. The power supply design is solid, the construction not so much. There are some major safety issues and wiring rats nest in the current supply. I have decided to keep the supply a solid state, and enhance on the filtering, and safety of the original builders design. Some basic info on the radio requirements.

Originally it had a 3 stage transformer, with a 5W4 Rectifier. The outputs were 250v dc after the rectifier, 12v ac and 6.3 volts ac for the tube filaments. The 12v side was to power the dial lamp, the original lamps were replaced with #47 bulbs and hooked up to the filament windings.

The current power supply has a HUGE transformer, it does not have a current rating on it. It has a 230 primary and 115 secondary. It is hooked up backwards to net 230v ac on the output. It also has a small 6.3v 10A filament transformer.

So my plans are to keep the same basic design, and enhance it with a common mode choke on the AC mains, fuse, and 2 Y2 line bypass caps. I will replace the solid state rectifier and the filter capacitors. I plan on keeping the PI filter output on the secondary's. Apparently from my research on this design, it is a very good performer for low loss and good ripple rejection, and is common place on high dollar amps. It seems to not be used much because of the cost of chokes that are able to handle the power requirement. I will add a bleeder resistor for the filter caps, and a volt meter and Ammeter. I also planned on adding a main power and filament power lamp on the front panel. I was also considering of adding a filament switch so I could keeps the heaters on if I wanted to.

I built a schematic on the power supply and the TR/RX relay I am planning on building for this station. I would greatly appreciate any feedback or criticism on the design, particularly any safety issue's that can be seen. I am not an expert on this stuff and before I got into radio's I did not feel comfortable wiring in a light switch at home! Things do definitely change.

This will be the first supply I have built or rebuilt, so I want to take as much precautions as I can.

Oh and some of the components were not in my program so I had to Improvise!
Thanks



Link to fullsize schematic

Looks like lamp2 will be on when main power is on. I think you need to move SW2 "above" the lamp for it to function the way you want.
ETA––
ETA-2
Revised to incorporate individual heater ON switch

I will fix that!

See edit 2

OK I fixed it on my schematic. Where I made that mistake I was trying to keep the line caps in front of any load and after the switch. I would rather not have the capacitors working 24/7 while plugged in. I guess I will have to question the idea of having a filament switch in there. I wanted to be able to burn the filaments to regetter the old tubes, and if I want to shut the radio down for a half hour and come back I do not have to worry about drift as much.

I guess I will work that out as I go.

I will post a picture of the innards of this supply tomorrow.

Three switches. Main power (before the caps), HV and Heater after the caps. Homebrew equipment needs lots of switches and the old "jewel" lamps.

The plan is old looking round analog meters, jewel lamps, and black wrinkle finish!

This will be part of a novice station from the 50's so I need it to look authentic

EATETA: That may work fine, it makes it look important with all of the switches.

ETA2: Damn trying to type in the dark, I gotta hit the rack!

Looks good, there are a lot of 110 filters that integrate the input caps and chokes you could use avail in the junk box, maybe old PC PSs, etc.

I like you have 2 redundant gnd wires from the PS to enhance safety.

Here's a link...

http://www.ohio.edu/people/postr/bapix/BC342.htm

There's all sort of quality old equipment on ebay that goes for little with complete PS's that can be salvaged for parts.

A hi qual sealed .mil tube tester transformer went the other day for $7 IIRC.

Compact and all the windings are brought out to mini porcelin terminals.

I have a bunch that I have robbed from computer power supply's, that is why decided to incorporate it into the schematic. The line voltage here is very noisy and we get a ton of electrical storms.

It's not uncommon to install 0.1 uF caps across each diode in the bridge rectifier, as protection against transients.

Why are you adding a common mode choke? This is a linear supply - there isn't any high-frequency switching going on, so there shouldn't be much (if any) noise on the AC line that makes it past the power transformer and filter caps.

Interesting, this is one of the reasons I posted this here. I have never dealt with HV supply's before, so this is a learning experience for me. It makes sense and I will look into it. I have plenty of .1 600v orange drop caps that would work good for that.

As far as the common mode choke, I guess because I robbed it out of the power supply and figured what the hell I will throw it in there. Basically anything I can do to make the supply smoother and reduce the noise that I have laying around I am willing to do.

If the common mode choke isn't going to make much difference then I can omit that from the design. I can always us it on another project.

ETA: New Schematic

You need another set of meters on the filiment. Ooo, how about metering the input voltage and current too!
ETA–– your transformer wiring is off..lemme correct it..wait one..or two..
ETA-2

You must have a switch on the high voltage output or you will reduce the useful life of all the tubes in the receiver. If the high voltage supply immediately turns on, then the filaments on the receive tubes will not be hot enough to "boil" the electrons off the cathode. The high voltage supply will generate an electric field strong enough to rip electrons off the cathode anyway, which damages the cathode. A tube rectifier takes time for its own filament to get hot so all the tubes warm up in the same time period.

In modern tube equipment, when running a solid-state rectified power supply, you either build in a soft-start circuit (which just delays turning on a relay or high voltage FET switch on the high voltage supply), or you put in a toggle switch. If using a toggle switch you allow the heaters to warm for a minutes or two before turning on the high voltage supply. In tube guitar amplifiers, this is usually labeled the "standby" switch.

This is why I wanted to have a switch on the filament transformer. The revised design I moved the Y2 caps before the switches, allowing me to kick on the filament power for a couple of minutes prior to switching on the HV supply. I do not like the fact that the line bypass caps will work all the time, but it is the only way to work it. I also do not plan on having the radio plugged in all the time. The original builder only put one switch on this thing!

I ended up going with 47uf 450volt caps, they were exponentially cheaper than the old value of 40uf.

That is great information.

Thanks to everyone on this thread I think I will have a very nice power supply!

It looks like my bridge rectifier is not going to be sufficient. The original diodes that were used were closer, but still not sufficient. Doing some more studying the original bridge has 1N2484 diodes in it. They are rated for 750ma and a PIV of 600 volts. The replacement rectifier I had has a 8 amp rating and 400v PIV. I was unaware that the calculation for the PIV was 2x RMS measured at the first filter cap!

With the transformer I have I should be getting about 244 volts ac on the secondary, and of course a little bit of voltage drop through the diodes. If I used AC output and calculated the PIV, 244 x 1.414 x 2= 690. Based on that I need a diode with a little higher PIV. The transformer is rated for 200va, or about 140 watts, so my maximum current at 122v is 1.14. So the transformer is doubling the voltage and cutting the current in half. So my max current on the secondaries is 570ma. IF I have calculated this correctly I need a diode with at least a 600ma current rating and a PIV of 700 volts, and in reality a little more for wiggle room.

So based on this it looks like I can use some 1N4007 diodes, and I happen to have a bunch of these here in the junk boxes.

Or am I far off on this?

Your 1N4007's should work fine. Typically, they're rated for 30 amps of surge current, so they should be able to handle the momentary surge due to charging up the filter caps when the transformer is first turned on.

Sometimes it's good practice to put some sort of inrush current limiter after the diodes and before the caps.

The choke will help some.

PTC resistors are cheap and might be a solution. Digikey or ebay.

Example...

http://www.ebay.com/itm/Inrush-Current-Limiter-Power-Thermistor-2-5-Ohm-6-5A-x4-/170809614491?pt=LH_DefaultDomain_0&hash=item27c50be09b.

Just search 'inrush limiter'

I find cheap time delay relays on ebay that would also save a lot of hassle.

But instead of putting a sw in the filament ckt, just turn the PS off...

I'm not sure there's going to be enough electron emission from an unheated filament with the impedences in series w/ the plates for currents and voltages this small.

An old ARRL manual or RCA tube manual might answer this question.

I think this issue is more important with mercury vapor rectifiers and higher voltages.

Interesting,
It seems to be one of the things that does not HAVE to be done, but with these old transformers the biggest stress seems to be the current surge after initial power on. I would imagine also with an inrush limiter in the circuit one could use some larger filter caps and not have to worry about the surge stress on the diode's and transformers. Thank you for that link, I think I will get one, it is not going to hurt.

So I had planned on building this thing on a chassis, but the height of T1 is will not clear the front panel and it makes it tight on what I want to do. I was hoping to make it really clean looking, but for the sake of making things work for now I have had to reuse a bunch of stuff.

The original rectifier, fiiler caps and choke were mounted on a plate, and bolted to the top of the transformer. I have decided to reuse the plate. I was taking the old diodes off of the insulated standoffs and one of them crumbled from the heat. The original 2 section cap had an octal socket it plugged into. I removed the octal socket, but came up with a solution to my problem.

I built the rectifier bridge into the an octal socket plug I had for the globescout. I added a terminal strip to mount the 2 capacitors to, and put the choke back into the original location.


As you can see the box next to the transformer is a Conduit box 6x6x4 that I used to make my dim bulb tester. The transformer is big.



You can see where the original rectifier was. I will now be able to remove that side of the plate. I plugged th transformer into my dim bulb tester with the rectifier unplugged, and turned on the power. The bulb did not make any light, so the transformer windings are in good shape. After about 5 seconds there was a very light hint of dark orange in the center of the 40w test bulb, so some very small leakage in T1, nothing bad. I then took some readings on the pins.

214.5v AC out of the secondaries
198.1v DC through the rectifier bridge made from the 1N4007 diodes
6.1 Volts of ripple through the diodes, no filter caps installed.

So it will be more than adequate to run this radio.

ETA:

Forget the numbers above, I had a bad reading or something. With full main power I am reading.
230v AC Secondaries
330v DC our of the rectifier. I sure hope the filter caps and the choke bring this down a little, I need to be around 250v.

ETA2: So acording to what I am reading the voltage will drop under a load about 10-15 percent, that still seems to be high for my requirement.

I've owned half a dozen tube rigs (receivers, transmitters and transceivers) over the years - None of them had any special provisions for (1.) reducing inrush current while the capacitors are charging, or (2.) not applying HV DC until after the tube filaments warmed up. It's true that both of those precautions were pretty commonplace on linear amplifiers - but not for most other ham gear.

"330v DC our of the rectifier. I sure hope the filter caps and the choke bring this down a little, I need to be around 250v. "

I think you will find the voltage will go up once hooked to the capacitors. Not alot, but the opposite of what you want.

I did a test last night with a 100uf 350v cap last night, and measuring the output through the choke the voltage dropped from 330 to 320, and the ripple dropped to .568mv ac. But that cap was before the choke, I am expecting the output on to C2 to rise back up. I will find out tomorrow when I start burning some more lead.

Ok so I made some changes, and wired the caps and choke in.

I have a solid 320v DC output. I am going to look at the tube specs and see if this will work, I am thinking it will be ok because it is not much different from the original supply. But I want to be sure.

Anyone have a creative way of dropping the voltage on this supply without having to find some very large resistors?

I believe the correct way to change the voltage is to use different taps on the transformer secondary. Of course that's not available on all xfmrs.

My Heathkit HP-23 did it that way at least. 275 or 350 volts depending on which tap you used.

Well, you could disassemble the transformer, and remove some of the turns on the secondary.

Taking apart a big transformer is pretty straightforward, assuming it isn't potted. The core on most large transformers consists of E and I-shaped pieces of stamped steel - Basically, you pry out a few of the I-shaped pieces, and then wiggle out an E-piece. After you get one removed, it creates enough wiggle-room to remove the rest. That leaves you with the primary and secondary windings on a paper form.

If you're lucky, the secondary windings will be on the outside of the form, allowing you to remove some of them without disturbing the primary windings below them. Based on your DC voltage measurements, you'll need to remove about 20 percent of the secondary turns to get you back in the 250VDC output range.

Ok the problem may have already been solved for me, but I am not sure.

The rig itself has been heavily modified by a ham, there is a spot where a capacitor went, it now contains an 0C3 regulator tube. Now here is the interesting question. The B+ comes into the radio, and to a terminal strip. The B+ has about 5 wires coming off of it, capacitors, plate's, and to the lonely 0C3, it passes through the 0C3 and then to ground. There is a resistor on the jumper section of the tube, and it appears to be about a 10 watt, I cannot see the value of it.

It appears to be setup line a zener diode and bypassing potential to ground? My knowledge of the 0C3 is none, I have looked at some data sheets but that is about it.

Any ideas?

ETA: Ok now I am confused.

The 0C3 wiring,
B+ comes into pin 7
Ground Pin 2
Resistor jumper pin 3 and 5
One wire off of pin 5 that goes up to the RF deck.

Something tells me that I am going to scratch my head a lot on this one, and learn a ton.

ETA2:
The plate voltages of the tubes are about 250 max, I found a conversion manual that talks about an 0C3 to the HF oscillator to stabilize it. I think I may have to rethink things here. I have a spare 5u4GB tube I may have to see if I can use it, I will have to put it onto a chassis and not use the original enclosure, that sucks.

I am really going to have to think this through.

It might be reasonable to choose a different xformer.

There are tons of them around on ebay, old equipment, receivers, old TV's, etc.

The xformer I mentioned earlier for a quality tube tester has many taps and ceramic standoffs and IIRC sold on ebay for $7. Heck of a deal.

I think something like that would be purfect.

Alternatively you could put an appropriate small 'filament', bell, or other transformer in series on the primary side of yours and wire in as a buck to reduce or as a boost to increase output voltage.

This is a very simple and quick fix. Ask...

The reg tubes are a solution, but you need to drop the voltage a lot and the heat dissipation will be, well, warm.

Also, ARRL handbooks contain a wealth of info on PS building.

Here's one from 1936 that's got mostly the same info as later ones.

http://www.tubebooks.org/Books/arrl_1936.pdf

Page 283 suggests if you use a choke input supply instead of the cap input like you're now using, there may be advantages and lower OP voltage but don't remember.

Why not disconnect the input cap and see?

I may pull all of the tubes except for the oscillator tube and the regulator and see how much of a drop the regulator is putting into the B+ supply. The guy who modified this thing had more knowledge when it came to tubes than I have, so I have to assume that the modifications were done for a reason. If all else fails I may just pickup a 5 volt transformer and use the 5U4GB regulator tube. The voltage drop on the rectifier plates is rated at 50v per plate, and from the testing I have done on the Globescout that seems to be right on, right at a 100v drop on the rectifier tube.

I can get another transformer, but finding one in the voltages I need prove to be challenging. Although some of the guys with more experience on this particular radio say it will run on as low as 150 volts on the plates, and in fact may be more stable at the lower voltages. The stability is not a concern since I think that has been covered with the OC3. So one of the options I was looking at was looking at is getting an Isolation Transformer, and just putting 122 volts into the rectifier. It should keep the voltage above 150, eliminating the need for a filament transformer for the rectifier tube and keeping my voltages lower in the process.

Thanks for all the help guys, I am learning a lot here, just got to figure out all the mess on this radio.

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