[Joeztech]

For some time I have wished I had a utility tube type power supply to use in testing vintage radios. I recently found a nice old military type power transformer that I used to power a modified BC-348 receiver many years ago. it is a rather hefty unit that is 3in. X 3.5in. X 6in. and weighs about 6Lbs. It is hermetically sealed and has a total of 10 round ceramic insulated terminals with solder slots on the bottom along with 8-32 3/8in. mounting studs on it.

I did connect to 122VAC and here are the voltages I measured:

Measuring Instrument = Fluke 8600A digital multimeter recently calibrated.
Terminal Definitions:
1 – 2 = AC primary, 4.48 ohms DC resistance
3 - 4 – 5 = HV secondary, 3 - 4 = 96.8 ohms, 4 - 5 = 103.7 ohms, 3 - 5 = 200.5 ohms DC resistance
6 - 7 = 5VAC secondary, 0.13 ohms DC resistance
8 - 9 - 10 = 6.3VAC secondary, 8 - 10 = 0.14 ohms DC resistance
With 122VAC input these AC voltages were measured across secondary windings:
3 to 4 = 271VAC (4 is the center tap of the HV winding).
4 to 5 = 271VAC
3 to 5 = 544VAC
6 to 7 = 5.3VAC (probably a 2 amp capacity)
8 to 10 = 6.9VAC (probably a 5 to 6 amp capacity)
Looking at Chicago Transformer catalog information, the case style seems to be type B hermetically sealed and potted unit. There are two 1.125 in. indented circles on diagonal corners of the top of the case. I never did find any stamped part numbers or manufacturer & date code information on this transformer. There was no printed schematic of the transformer, although someone had scratched it in the paint on one side before I acquired it.

I have a 5AR4 rectifier tube and a 1.75H 200mA, 62 ohm choke and 500VDC rated electrolytics and some octal sockets on order to use with it. I have on hand some 4-pin and 5-pin sockets from Antique Electronic Supply. I can make the choke an item that can either be in circuit or out of circuit using banana jacks and plugs in case the radio I work on has a field core speaker. In those cases the field core will be the choke. The electrolytics can have an isolated plus and minus side so that a divider resistor can be used to provide the necessary negative bias for output stages if needed.

I should be able to get at least 270VDC from this or more depending on rectifier circuitry. I could use silicon rectifiers for higher voltage applications, perhaps a bridge arrangement and another set of electrolytics to handle even higher voltages if needed. I have a fair number of banana jacks and plugs on hand to use. I might find a suitable DC voltmeter to add to the chassis to monitor B+ while working on the radios.

There is room on the chassis I am using to add a Variac if desired. I need to find one of those. Our contributor, Brenda, sent me one that will handle up to 90 watts or so. I might want to go higher for more hungry radio chassis.

I am going to have to run some load tests of the B+ output to see just how much current it can handle safely. I do remember that using it with the BC-348 it never got really hot to touch. I had built a separate power supply and audio output stage using a 6AV6 and 6AQ5 at the time it was used with the BC-348.

I can also use the supply to breadboard experimental tube circuits. Lots of fun ahead!

Joe
KC5LY

[Radioroslyn]

Hi Joe
You aught to put your 5ar4 up on ebay. Some nuts are paying 25-100 bux for em. Unless you need it for your Valiant.
Terry N3GTE

[Arran]

  The voltages are an important consideration, but you didn't mention anything about current ratings for this transformer. From your description it's a light to medium duty transformer even though it is inside a potted can. The typical radio power transformer has a high voltage winding somewhere between 300-0-300 and 350-0-350, 271-0-271 is rather low considering that this is with no load across the winding.
   I would guess that since this transformer has a proper 5 volt rectifier winding then the H.V winding is probably rated at 125 ma or something since the voltage is around 271 volts with no load. You could use a 5AR4 but it's probably over rated for this application, a 5Y3/5Y4/80 will handle it quite nicely. Just remember that whilst you may almost double the output voltage by using a silicon bridge rectifier and omitting the center tap you will also reduce the current output of the supply by half verses a conventional full wave setup.
Regards
Arran

[Joeztech]

  The voltages are an important consideration, but you didn't mention anything about current ratings for this transformer. From your description it's a light to medium duty transformer even though it is inside a potted can. The typical radio power transformer has a high voltage winding somewhere between 300-0-300 and 350-0-350, 271-0-271 is rather low considering that this is with no load across the winding.
   I would guess that since this transformer has a proper 5 volt rectifier winding then the H.V winding is probably rated at 125 ma or something since the voltage is around 271 volts with no load. You could use a 5AR4 but it's probably over rated for this application, a 5Y3/5Y4/80 will handle it quite nicely. Just remember that whilst you may almost double the output voltage by using a silicon bridge rectifier and omitting the center tap you will also reduce the current output of the supply by half verses a conventional full wave setup.
Regards
Arran

Arran;

Yes the power transformer does have its limitations. There is also the voltage doubler approach using silicon diodes that could be used. There are even full wave voltage doubler topologies that can be employed if I need to go much higher. However, for most of what I want to work with the transformer should be adequate. I don't plan to be working with circuits that need large amounts of current or high voltages. This is just a way to use what I have on hand for now. Later on I might look for a transformer capable of higher voltages and current capacity. I still need to build the supply and test its load capacities. As you said the fact that it is potted does not necessarily mean it has enormous current capacity. TBD!

Joe

[Alan Douglas]

Sounds perfect to me. For testing, you don't need high voltage. For intermittent use you can draw a little extra current without heating the transformer much. If for some reason you did need more voltage, you could plug in a pair of silicon diodes in an old tube base temporarily.

[Joeztech]

The JJ 5AR4 was purchased from Antique Electronics Supply. It is a new one and was $15 - not costly. I don't go for boutique rectifier tubes by specific manufacturers. I also have an old JJ 5AR4 powering my Dynaco ST-70 amp and it does a fine job. In this application it will likely be coasting. $15 does seem high but I have to remember that due to inflation the cost of tubes, like everything else, has gone up X10 since the early 1970s. Such is life.

Joe

[codefox1]

You can do just as well with 10 cent diodes, in series if you want to, add some beefy elecrolytics then a fuse then and then a old fashioned ammeter properly protected with diodes, and you're set. Sell the Tube.

[Joeztech]

So far, after wiring up two 12AX7 tubes as line, driver & phase inverter and using two 6V6GT tubes running at 32mA each the transformer does not even get warm. However 32mA is high for 6V6GT tubes, so I increased the cathode resistor for the push-pull 6V6 pair to 390 ohms and dropped the current per tube to 26.5mA where the whole arrangement runs quite cool. B+ is running about +330V at the first electrolytic cap and +325 at the second electrolytic cap which is after the choke.

Power transformer is quite cool to the touch even after an hour or so of operation.

Joe