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RCA 56X2 Output Transformer - Printable Version

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RCA 56X2 Output Transformer - Raleigh - 04-12-2013

I'm working on an RCA 56X2. After completely recapping it (except a couple of micas) and replacing most resistors, it worked well for a while, but then the primary of the output transformer opened.

By chance I found a junk set in the props storage of a local theatre company and brought it home in hopes of using its output transformer for my set. The junk set worked for about a minute before the same thing happened. So now I have two chasses with open output primaries.

Because I had two transformers fail identically I'm reluctant to buy an antique transformer, even if I can find one. I'd like to try to find a modern replacement. Can anyone recommend a replacement? Radio Daze "improved" their web site so I can no longer see any specs for what they sell, only "transformer -- audio chassis mount" or the like.

The final amp tube is a 35L6. Schematic is here: http://www.nostalgiaair.org/Resources/994/M0014994.htm

I think the primary DC resistance is 225 ohm, though it's hard to read. There is also a 12 ohm tap that the B+ goes through, presumably to help smooth out the ripples. The secondary is 0.8 ohm.

No doubt a modern transformer wouldn't have the 12 ohm tap, but I could probably find a choke to use in its place.

I don't know enough about transformers to understand the specs, other than matching the DC resistances and dimensions. Mouser has a Hammond 146S that matches pretty closely. Primary = 220 ohm, secondary = .7 ohm. The primary has a center tap, which obviously I wouldn't use. They have 2 in stock for $18.46. (Radio Daze lists the same part number for $16.75.) Would this be a good choice?

Any other suggestions?

RE: RCA 56X2 Output Transformer - BrendaAnnD - 04-12-2013

Is it the output primary opening up, or the choke section? Either way, I would check the cathode current of the 35L6 and make sure it isn't drawing excessive current. For this purpose you can just sub in a cheap filament transformer (either 6 or 12 volt secondary will do, you don't need hi-fi for the current check).

Transformers for 50L6/35L6/50C5/35C5 are just about universally interchangable, and I'm pretty sure I have, if not a new one, then a good used one in my stock I could send you. Dimensions are more critical than most other considerations in this case.

RE: RCA 56X2 Output Transformer - Raleigh - 04-12-2013

The primary of the output is what opened on both radios. The choke section is pretty close to 12 ohms on both. When the radio is working, the total B current through the 1200 ohm resistor R11 is a little under 20 mA. With the output transformer open, the B current through R11 goes up to about 40 mA, which burns out the 1W resistor.

I'm afraid I don't have the filament transformer you recommended. However, I did measure the cathode voltage after the transformer opened. It was a tad below the value on the schematic, about 5 volts.

Does your transformer have about the same DC resistance as the original (225 primary, 0.8 secondary)? I'm not at all sure how much difference it makes.

My original has tabs that go through slots in the chassis and then are twisted to hold it into place. Whatever I use I'll probably need to drill holes and bolt it in place.

The dimensions of the original are about 1.7 inch wide X 1.3 inch tall X 1.3 inch thick. The twist tabs are 1.7 inch apart, but it looks like the hole spacing on any replacement transformer would be a little wider than that. Not a problem in this radio. There is a little room for a larger transformer.

What do you think about the Hammond 146S transfomer? One thing that concerns me is that the max DC current is 2 mA. Does it seem like the plate current of the 35L6 would be more than that?

I actually have an RCA 65U radio/phono that uses a 50L6 output tube. The transformer part number is different, as are some component values. the circuit is much the same, except the bypass capacitor goes between plate and cathode instead of across the output transformer. I've been reluctant to pull it to try it out simply because the part number is different. I'm not sure if that transformer works. It's unrestored, and I haven't got around to it yet. Schematic here: http://www.nostalgiaair.org/Resources/062/M0015062.htm

RE: RCA 56X2 Output Transformer - BrendaAnnD - 04-12-2013

Like I said, transformers for these tubes are pretty much universal swap-out. DC resistance isn't much of a determining factor, as different manufacturers used different cores, different wire sizes, etc. which add up to different DC resistances. Impedence is what is more critical, and all the transformers used with the L6/C5/B5 series of tubes in AA5's are designed with the proper impedence.

What confuses me is that you are getting cathode current through the output tube with the transformer open. In this case, there should be no current at all (no plate voltage).

If the Hammond transformer is only 2mA max DC current, it will not work. The plate current through that tube is much higher.

RE: RCA 56X2 Output Transformer - Raleigh - 04-12-2013

Brenda,

I made a bunch of measurements to be sure I'd told you right about the cathode current. I do have about 22 mA of cathode current even with no plate voltage, but the screen grid is at about 77 volts (spec 100 V.) Could the screen grid be drawing some current from the cathode, even with the plate near zero?

If I can find the time, I'm going to try the output transformer from my RCA radio/phono this weekend. It's a little different, but it's close enough that it should work unless it decides to die like the two others.

I'm afraid my understanding of impedance versus resistance is rudimentary. There doesn't seem to be any rhyme or reason to the impedance specs in the Hammond transformer data sheet. The values are all over the map. How would I know what is the proper primary/secondary impedance to get?

RE: RCA 56X2 Output Transformer - Mondial - 04-12-2013

You can get the recommended output transformer impedance from the 35L6 data sheet. At 110 VDC plate voltage typical of an AC/DC set, the recommended transformer primary impedance (load resistance) is 2500 ohms.

And yes, even with no plate voltage, the screen grid will still draw current, which is why you are still seeing current in the cathode circuit.

http://www.hebertech.com/view/1698-tung-sol-35l6-gt.pdf

RE: RCA 56X2 Output Transformer - BrendaAnnD - 04-13-2013

Forgetful me... I've been working on sets with directly heated triodes for so long that I'd forgotten about the screen grid. That being said, the screen should draw only a small percentage of the plate current. 22mA is quite hot for a screen grid. Have you tried another tube?

RE: RCA 56X2 Output Transformer - Radioroslyn - 04-13-2013

Also check your cap from the plate the ground on the output tube. If it is bad it will take out the primary.
GL
Terry

RE: RCA 56X2 Output Transformer - Raleigh - 04-13-2013

On this radio there isn't a cap from plate to ground. There is a cap (.02 mf, 700 volt) across the primary of the output transformer, including the tap that is used as a B+ choke. I used two .04 mf caps in series to make the higher voltage than the 630 volt caps I am using.

There are variants of this radio that do have a cap from plate to ground, but not this one. There also is not a cathode bypass cap, just a self-bias resistor.

As I understand from Mondial and the tube data sheets he linked to, I need a transformer with 2500 ohms primary impedance. Now, per the schematic the primary voltage is 125 V, so plate current is a tad under 100 mA. So I need a transformer that will handle 100 mA on the primary?

The total B+ current for this radio is about 20 mA when it is working properly. With the primary open as it is now, it is about double that.

What about the secondary? What impedance should I look for?

Is DC resistance irrelevant?

I'll measure the cathode current again with another tube, but I probably won't be able to do it until tomorrow evening.

Thanks to everyone for their suggestions.

RE: RCA 56X2 Output Transformer - Arran - 04-14-2013

You can use a replacement without the extra tap, you may have to add an extra resistor ahead of the pi filter or use a larger value choke resistor but these sets are usually not that fussy. If the set is cooking output transformers then there has to be a circuit defect like a bad bias resistor a very leaky paper cap, I don't think it fried the second transformer because it was used. Just get another from another AA5 or 6 with a similar output tube 35L6, 50L6, 35C5, 50C5, it doesn't matter as long as it fits mechanically.
Regards
Arran

RE: RCA 56X2 Output Transformer - Mondial - 04-14-2013

Looking at the tube data sheet, the normal plate current is about 41 mA, so your transformer needs to have at least this primary current rating. This is confirmed from the schematic where it shows the cathode voltage at 5.6V. Using Ohms law, this gives a total cathode current of 5.6/120 ohms, or about 46 Ma, which includes a few mA of screen grid current.

The schematic shows a transformer secondary impedance of 3.4 ohms, so your replacement transformer should have a 3 to 4 ohm spec. So what you are looking for is a transformer with a 2500 ohm primary and a 3-4 ohm secondary with a 40+ mA current rating. The actual DC resistances are not really significant if these specs are met.

RE: RCA 56X2 Output Transformer - codefox1 - 04-14-2013

Any old tube output transformer from a similar set will be fine. You will not be able to hear the difference if you are off a bit.

RE: RCA 56X2 Output Transformer - Raleigh - 04-14-2013

Brenda, I confirmed with two other tubes that the cathode current is around 22 to 23 mA. These measurements were made on my junk chassis, which has not been recapped. Since I removed the output transformer from my recapped set, I can't make measurements on that one.

I measured all the voltages that are listed on the schematic, also on the junk set, which worked for a short while before the output transformer went out.

Arran, the two transformers failed in two different radios. Just by chance I found a junk set (same chassis, different cabinet) that I fired up to make some measurements on. It wasn't that my recapped radio fried 2 transformers. It was very odd that both failed in the same way, though.