Spec's for Mod Transformer?

[WA1HZK]

I have an old buzzard rig under restoration that uses a Thordarson Mod Transformer # T21M65. The label says 300/500 watt. Does anyone have the hook up data on that one? The rig has a pair of 4-250's in the final. I was thinking of driving it with a ranger for RF & audio. Maybe triode connect a pair of 813's for the mod deck.
Whadda you think?
Thanks
Keith

[W7XXX]

A pair of class B 813T's would be perfect in my opinion. I am biased toward them because they aren't biased. Simple to construct and nice clean audio.

Do a continuity check on the terminals and draw out a schematic. Then if you have a variac or 10 volt AC source, then do a ratio test. Voltage ratio equals turns ratio. I use 10 vac as it is easy to figure. You will be stepping down so put the 10 vac on the secondary for easier calculation. Then !2 vac read on the primary would then be 1.2:1, 14 volts 1.4:1, etc.

Calculate your class C final load resistance, then figure the ratio with the 813's. In triode connection with 2000-2500 volts on the plate, if I remember right, they run about 18k-21k p to p.

Here is the only specs I can find. The 21 series is apparently not multi tap. In light of this, I have to guess the 300 watt rating is continuous and the 500 watts is peak since there are no taps for connecting coils parallel. They don't show the 65.

http://www.bunkerofdoom.com/xfm/mthordarson/thordarson1.htm

[k4kyv]

Regardless of class C load impedance, if you are using a common power supply for modulator and final, you need a turns ratio of somewhere between 1.2:1 and 1.4:1 total primary to secondary, for full 100% modulation capability, plus a little additional headroom.

If it is a fixed ratio transformer, once you run the voltage test as described above to figure out which taps go to which winding, the problem now is to locate the proper taps for the +HV and plate connections.

On the secondary side, you want to make sure the correct tap is used for B+ and that the correct one is used for class-C plate feed.  If the primary is split, you want to make sure you strap together the correct two taps to form the midtap, and connect the modulator plate leads to the other two taps.

This is important, since the insulation at the primary midtap terminals and secondary B+ terminal may not be designed to carry the full +HV plus audio signal voltage, which is what appears at the modulator tube plates and class-C plate feed.  Connecting the transformer up wrong could cause insulation failure (especially in a decades-old transformer with deteriorating paper insulation that may have absorbed moisture over the years), destroying the transformer.

Even if the insulation holds up at modulation peaks, the capacitance to the frame of the transformer may be excessive at the primary midtap and secondary B+ terminals, since these taps are intended to carry DC only, and not any signal voltage.  The wrong connection might result in a degradation of the transformer's frequency response.

[KE6DF]

I have a PDF of the 1949 Thordarson catalog which lists the 21M65.

It specs it at 500 watts and calls it a multimatch transformer and says it has two secondaries that can be connected in parallel (with a 640 ma rating) or in serial (with a 320 ma rating). The primary has a 320ma rating.

There are a couple pages of tube and driver transformer recommendations that might help you figure out the pinout.

I can't seem to cut out a few pages and attach them.

The catalog is a bit over 4MB. It's a pdf.

How about I email you the whole catalog?

Just post me your email address.

If anyone else wants a copy, also send me an email address.

It looks like a good fit for an 813 modulator.

Dave

[KE6DF]

Well, I should have looked first.

I found your email address in your profile.

The catalog is on it's way.

If you didn't get it or something let my know and I'll try again.

Dave

[Steve - WB3HUZ]

More info here. Scroll down a little.

http://amfone.net/AMPX/108.html

[Opcom]

here's the only data I have onit. Maybe the connections are not too hard to figure out.

[KL7OF]

Don....I'm confused about your description of the connections on the mod transformer.....Are some connections designed to take more voltage/ current than others?   All the mod transformers I am familiar
with have identical (outer) insulated terminals for all taps....Is the internal insulation at the taps different somehow?

[KE6DF]

Well, Don is the real guru, but here are my thoughts on the subject.

The size of the terminal insulator on a transformer doesn't necessarily indicate the voltage rating -- except in so far as a very small terminal probably won't be used on a very high voltage connection.

The manufacturer might use all the same terminals just for manufacturing efficiency -- easier to stock one part.

I have seen mil spec transformers with modest sized ceramic terminals used for a mid range B+ (around 1kv) and the same terminal used for the 6.3V filament winding which carried a working voltage max spec of 200 volts. Obviously a smaller terminal could be used for the 6.3v winding but the manufacture wanted to standardize. I also have a big commercial transformer rated at 3450V where all the primary connections (two 115v windings) use the same big ceramic terminals as the B+. Clearly smaller terminals for the primary would have been OK if the manufacturer wanted.

As for the different ends of a high voltage transformer like a modulation trany, the voltage spec of one end of a winding might be less because they put a thinner layer of insulation between that winding and the one below it. Then they put on the winding and put a thicker layer of insulation between the winding and the next one. Also, the insulation between each layer of the winding, and between the first winding and the core might be of differend thicknesses depending on planned working voltage.

One example of this is the common microwave oven transformer.

Most are intended to work in a 1/2 wave rectifier circuit with one side of the secondary grounded. The other side has the full 2K volts.

I have one where you can see the grounded end of the secondary coming out of the windings, ending at a soulder lug attached to the core by a small screw. So I could un-ground this terminal and try to use the transformer in a bridge circuit.

But, you should see what thin insulation they use on this gounded primary wire. And it comes out right next to the core from deep inside the transformer and is only insulated by very thin insulation on the wire itself. I suspect if I un-grounded it I would have a arc-over to the core. So I use that transformer for a door stop.

So anyway, microwave transformers are an extreme example of where the different ends of a winding have different voltage capabilities.

Dave

[W7XXX]

Don....I'm confused about your description of the connections on the mod transformer.....Are some connections designed to take more voltage/ current than others?   All the mod transformers I am familiar
with have identical (outer) insulated terminals for all taps....Is the internal insulation at the taps different somehow?

Steve, I think what Don is saying is if the primary (also applies to secondary) has 2 separate coils, then it is important to get the correct connections. IE, the inner most winding near the core tap would be the plate of one modulator tube, then the end of that winding would be B+ and the start of the next winding would also be B+ and the end of that winding or the top or furthermost from the core tap would be the other mod tube plate. If the inner most winding start tap was hooked to B+ instead of the plate, and the tap at the end of the highest winding was hooked to B+ instead of the plate, then the insulation could break down. The insulation isn't different, but the wrongful hookup could cause currents that could arc thru the insulation and burn the winding in an expedited manner. Hope to catch you on 75 again soon. I am usually on in the AM 0500-0700 on 3875. Sometimes jump in the 3885 NW group. No one on any lower freqs I have xtals for. Anxious to try 3820 again.

[k4kyv]

The start turns on the  winding are separated from the core or underlying windings by  the insulation.  The finish turns may be at the outer layer.  Therefore, voltage appearing at the start turns may be more prone to breaking down the insulation than voltage appearing at the finish turns.

Some transformers are would symmetrically so that there are actually two bobbins, mirror images of each other, laid side-by-side on the core, and both halves of the split winding may begin with "start" turns next to the core.  Since the turns at the taps of a split winding that are supposed to be strapped together to form a midtap are designed for very little voltage difference between them, the manufacturer may not have put much insulation between them since it wouldn't be necessary if the transformer is wired up properly.  Allowing the full output voltage of the winding to appear between those two taps would likely blow the transformer.

As added protection, I usually don't ground the frame/laminations of a modulation transformer, modulation reactor or filter choke, but mount them on insulation.  Grounding the core puts a totally unnecessary strain on the insulation between the winding and core.  I ground the transformers/chokes only when I know for sure they are working well below their maximum rating and have a very high safety margin.  The insulation used in old transformers is usually some kind of impregnated paper.  Paper may deteriorate with age, as well as pick up moisture, and lose some of its insulating properties.  A transformer built in 1943, rated at 10,000 volts breakdown, may not even withstand 5000 volts in 2008.

[KE6DF]

As added protection, I usually don't ground the frame/laminations of a modulation transformer, modulation reactor or filter choke, but mount them on insulation.  Grounding the core puts a totally unnecessary strain on the insulation between the winding and core.  I ground the transformers/chokes only when I know for sure they are working well below their maximum rating and have a very high safety margin. 

I've heard this suggested many times, and it makes sense, but wouldn't it be better to go one more step and connect the case to the high voltage.

For example, for a modulation reator, connect the case and core to the B+ from the power supply.

The modulated output terminal (connecting to the final tubes) would still swing under 100% modulation from 0 to 2xB+, but the case and core would then be right at the mid point which seems like a good thing.

[KL7OF]

[  The finish turns may be at the outer layer. 

OK....So I have a mod tx with 4 terminals on the secondary...The ohm meter shows it to be a continuous winding with a tap on each end and two taps  equally spaced in the middle....How to tell which which end tap is to be the start (B+) ??

[k4kyv]

That may be difficult if there is no clue to go by. 

If all else failed, I would try using a capacitance meter and measure the capacitance of each tap to case.  The end showing more capacitance would be a good candidate for the B+ tap.  Another thing to try would be to inject audio at about 3000~ into the primary, and measure the audio voltage to ground at each end of the secondary winding, with the entire winding  floating free from ground.  There will probably be a small voltage at both ends, but the tap with the most voltage would be my candidate for the +HV line to the final.  If both the above tests points to the same taps as B+ and plate line, that's how I would hook it up.

If it is one of those "universal" modulation transformers like the UTC VM-5 or CVM-5, they are wound symmetrically, so if  the secondary windings are connected in series, there is no proper B+ and plate tap.  But you still have to determine which is the end of the windings and which are the midtaps for the  split winding.

Sometimes, despite everything you try, you are left with a crap shoot.  If all else fails, I would try connecting each winding each way, and preferably at reduced plate voltage, drive the modulator with a variable audio signal generator and see if connecting it one way gives a smoother and  better high-end frequency response than connecting it the other way, and use the connections that give best performance.

One more thing.  Often when a balanced winding is  split, the two terminals that are supposed to be connected together to form the tap are reversed in sequence.  As an example, if the terminals are lined up and numbered sequentially 1-2-3-4,  2 and 3 would be strapped together to form the midtap.  But continuity could appear between 1-3 and 4-2 instead of 1-2 and 4-3 as you might expect.  This is important to know if you are metering each tube separately, or in the case of a  driver transformer, you are using separate grid bias adjustment on each tube.

It might be worth waiting until you could get more information on the transformer before firing it up, if you think there may be any possibility of finding it, versus just trying it each way to see if it works or smokes.

[W7XXX]

[  The finish turns may be at the outer layer. 

OK....So I have a mod tx with 4 terminals on the secondary...The ohm meter shows it to be a continuous winding with a tap on each end and two taps  equally spaced in the middle....How to tell which which end tap is to be the start (B+) ??

On many multitap single coil secondary xfmrs I have messed with the taps on the secondary are usually on the high side or plate side. Just a simple resistance reading will show this. I have a known (by spec sheet) small UTC as an example ... using the B+ tap as 0 the next tap reads 65, and the next (the high side) tap reads 109. The difference between taps 2 and 3 is 44 ohms and the difference between tap 1 (B+) and 2 is 65 ohms, agreeing that taps 2 and 3 are plate taps as indicated on specs sheet. This is not always true so beware.

Take an ohm reading on the secondary and see if this gives you any clue. Some smaller multitaps have taps for 300-500 ohms these are found on the low side and would show a low resistance. If you know the xfmr has a 500 ohm tap, then this might be a clue. As Don said if not sure better not to guess and burn a good mod xfmr.

[KL7OF]

Mx Mod tx is a (I'm guessing by the size, it's big) 500 watt model....SOLA brand   marked MOD    .Three taps on the primary.....it is a continuous winding.....4 taps on the secondary...also a continuous winding...Here's what I did....I put it in a mod deck with triode connected 813's.....hooked the modulated B+ from that deck to push pull 100th's final deck.... Lowered the plate voltage to 1000 volts and injected  a 1000 hz tone into the speech amp.....I monitored the output on my scope and on air monitor for these tests....The B+ was was tested on both end taps of the secondary and the results were the same...It works very well...I tried the two intermediate taps and the ratio is wrong as I was not able to obtain 100% modulation   The primary (with three taps) was hooked up normally with the B+ on the center tap and the platesof the mod tubes to the end taps...I then cranked the B+ upto 1900 volts...(that's all the rig has)  and it works well...Nice power output with 100% positive peaks just hitting the base line once in awhile...I'm gonna run it just like this....

[WA1HZK]

Thanks guys. This will help on figuring it out. An actual schematic would be nice but that was probably only supplied with the original packing. It's interesting to note that the iron cost almost 100 bucks in the 40's! A rig using that iron would probably be a top of the line old buzzard rig.
Keith