UTC S-21 Modulation Transformer Ratings

[wa2pjp]

Just picked up a new UTC S-21, does anyone know the what the max voltage that can be used with this transformer? I know it's good for 115 watts of audio. I was thinking about a 4-125A modulated be 811's with about 1700 volts across each, but don't know if this transformer can handle it. Thanks

[KE6DF]

I  have never seen a statement by UTC on max voltages,

but they did put a list of recommended tubes and operating conditions in their catalogs.

So you can look to see which is the highest voltage they recommend for any tube.

Looks like it's 1250V.

I think 1700 would be pushing it -- especially if, like many people these days, you plan to run over 100% modulation (which results in higher peak voltages).

Plus with 60 year old dried out insulation you would be risking zapping your transformer IMHO.

Something like using it for a pair of 807s at 750VDC sounds much safer.

Attached is a page from an old UTC catalog.

[wa2pjp]

I found the same thing looking through thier catalog, so I guess I knew that, but wanted to ask anyway. Thanks.

[WA1GFZ]

Joe how about 4 807s, or a pair of 6080s ?

[wa2pjp]

Hey Frank,

I have a number of NOS 4-125a's and 811A's that I have for spares for the other rigs and was just trying to use the mod xformer. Maybe I'll just go with a lower voltage.

73's
Joe

[KE6DF]

811a's would probably work just fine with that transformer. They would put out 115W+ at any voltage from 750 - 1250 VDC.

[WA1GFZ]

Hi Joe,
I'm in the same boat. I have some 4-125s and 813s with a few 811s. I would love to build a little rig. I have a chance to buy a beat Valiant that could be modified.
It has a bunch of extra holes in the back but the front isn't too bad. Look at the specs on the 4-125. I bet a pair of them would get the Z low enough for you running at 1200 volts. I've run 811s at 1200 volts at RF. I bet a pair would be ok at zero bias.

[N3DRB The Derb]

I have one, if I can ever get around to wiring it up and getting the driver tubes it'll be ready to go.

Try for a tube that will make around 125 watts of power at anywhere between 750 to 1000 volts. 809's would be good, plain old 811's would be good, zero bias at anywhere lower than around 1100.

I'm going to use Hytron HY30's which will do 135~145 watts @750 volts. Thats the upper limit. I have all the iron for the 750 v supply already. Just havent gotten to it.

[WA1GFZ]

How about the Thordarson T11M76. I have one and wonder what voltage it can handle? HUZ site shows connections but no ratings.

[KE6DF]

Well I suppose  you could use the same method to see the intended tube applications of the 11m76.

Here are some pages from a Thordarson catalog showing intended operating conditions for various tubes and mod transformers.

Looks like about 1000VDC is the highest for the 11m76.

Looks like the pages are too big to attach. If you give me and email address, I send them.

Dave

[W1AEX]

How about the Thordarson T11M76. I have one and wonder what voltage it can handle? HUZ site shows connections but no ratings.

Dave's suggested method of looking at the intended tubes is probably the best way to go. My archived Thordarson catalogs don't give any voltage ratings either. I have the big brother to your TLLM76 (TLLM77 - 300 watt) and it has never complained when I run 1500 volts through the primary and 800 volts through the secondary, and I do that frequently. I never bothered with mounting it on stand-offs, it's bolted right down to the modulator deck.

[N3DRB The Derb]

I used the M77 in my first HB rig and beat it down with 1500 volts to a pair of 805's. Never complained once. I had 370 watts going through it.

[KE6DF]

I used the M77 in my first HB rig and beat it down with 1500 volts to a pair of 805's. Never complained once. I had 370 watts going through it.

Looking in the same 1942 catalog, the highest voltage tube spec for the M77 is 1750, but most specs were for 1500V.

Obviously the higher the power level of the modulation transformer, the higher the voltage spec at which it's designed to work.

In the old days when these transformers were designed, the typical modulator tubes at the 300 watt level were 805s, 838s, 211s, 203Z, ZB-120, etc. Most of these old triodes were usable up to 1500 Vs so that was a typical max safe working voltage.

Now for the 500W+ modulation transformers, people used tubes like 810s, 822s, HF-200s, etc. which were at the 2KV+ level.

My thinking, and the gurus can correct me if I'm wrong, is that we should err on the side of lower voltages for a few reasons. One, people now days go for high modulation percentages and therefore peak voltages are higher. Two, old dried out insulation may be more likely to break down. And three, if you blew a transformer back then you could just order a replacement.

Dave

[N3DRB The Derb]

the early S series xfmrs with hot rod louvers and the wrinkle teal finish are totally open to the wx. the later gray ones are potted and at least they have that going for them. I like teh louvers but unless they are had from a good environment it's more than likely they wont make it. Of course the proof is hi potting it and seeing if it breaks down.

My S21 was NOS in the box so I have some confidence in it, but I'm gonna use the lowest voltage I can.

[KE6DF]

The site admin types may move this post to the ebay section, but I'm posting it here because of the relevance to this thread.

Here is a nice looking T11M77 for sale:

http://cgi.ebay.com/1930s-THORDARSON-MULTI-MATCH-AMP-TRANSFORMER-300W-NIB_W0QQitemZ130355710691QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item1e59ce4ae3

[N2DTS]

I had one of the thordarson transformers in the 30K1 at 2500 volts for years without problems.
I had a pair of 100TH modulators and a 4-400 in the 30K1 andran it at 2500 volts for 350 watts out.
I sold it with the thordarson, but have another (thordarson).

From memory, the thordarson sounded ok, and better than the cvm-5...

Brett

Well I suppose  you could use the same method to see the intended tube applications of the 11m76.

Here are some pages from a Thordarson catalog showing intended operating conditions for various tubes and mod transformers.

Looks like about 1000VDC is the highest for the 11m76.

Looks like the pages are too big to attach. If you give me and email address, I send them.

Dave

[WA1GFZ]

looks like the 11m76 would be good in a Valiant

[N3DRB The Derb]

The CHT series is pretty good iron. I know I gave my M77 all kind of hell with the BJB crap and never had any problem.
Orders of magnitude better than any of the UTC S-series iron, but not a fair comparison. CHT was the best thordarson could make, and the S-series was built for budgets and intermittent service.

I like em both. Just gotta be aware of the deal.