Gates 250GY Iron

[Barrie]

Anyone have the specs on the Gates 250GY iron?

Several years ago I was given the plate transformer, modulation transformer, modulation reactor, swinging choke and the smoothing choke from that transmitter.

These pieces have just been sitting around, but now I have a project that I may be able to use some or all for.

Any help appreciated!

73, Barrie, W7ALW

[WQ9E]

Barrie,

The Peter Dahl catalogs are still online and they are a good source for specs on this iron:  http://www.pwdahl.com/dahlcatalog/bctrans111103%5Bg%5D.html  I do have a BC-250GY and the manual so if you need additional hookup info I can find that for you.

Rodger WQ9E

[Barrie]

Roger:

Thanks for the info.

I found all the specs excepting I don't understand the modulation
transformer information.  It says the primary is 7500 ohms and it says that (possibly) the secondary is 5000 ohms?

73, Barrie, W7ALW (trying to relearn AM)

[WQ9E]

Barrie,

That sounds correct.  The BC-250GY uses fairly low voltage on a pair of 810's (push pull) modulating another pair of  810's in the final.  Note that the modulation transformer in the 250GY (like most broadcast rigs) requires the use of a modulation reactor and high voltage blocking capacitor so that the plate current doesn't saturate the modulation transformer.  Although this is pretty conservative iron you will still need to use the modulation reactor and blocking capacitor as in the original design.  The original Gates design uses an 813 driver for the 810 finals and a pair of 6L6's to drive the 810 modulators.

I can scan and email the original BC-250GY schematic to you in the next few days if that will be helpful.

73, Rodger WQ9E

[Barrie]

Roger:

Thanks again.

Yes, I'm well aware of the use of a modulation reactor, or at least I was 15 years ago.  I built a transmitter at that time using a push-pull pair of 450TLs, modulated by a pair of 304TLs (AB1), using a modulation reactor.

I plan on rebuilding that transmitter.

However, the rig I'd like to build right now is a push-pull pair of VT-127As (similar to the 100TL), modulated by a pair of the same tubes (again, if I can, AB1).  I don't know how well that modulation transformer would match-up in that situation.

I guess age must be creeping up.  I have trouble remembering things I once knew.  I've been hitting the books, but there is very little information in the books about AB1 modulation.  And, I don't have the curves for the VT-127As, so I can't calculate easily.

73, Barrie, W7ALW (vanity call, but also my original call from 1950)

[WQ9E]

Hi Barrie,

Sounds good and that must have been quite a rig you built before!  Here is a link to a data sheet on the 100TL that may help you:  http://frank.pocnet.net/sheets/088/1/100TL.pdf  but it looks like you should be in the ball park for impedances with the Gates iron.

I have the mod transformer out of an old Westinghouse rig (I believe it was an MW2 built for/used by the Navy) and it will also require an external modulation reactor.  Just one of several homebrew projects awaiting some time.

73, Rodger WQ9E

[Barrie]

Roger:

Do you know how much RF input the 250GY modulation transformer will actually handle?

It would seem that it should handle at least 350 watts input, but could it be more?

I'd really like to run the VT127As at about 500 watts input.  In that case, the 250 transformer is not likely to work.

73, Barrie, W7ALW

[WQ9E]

Barrie,

I will look at the manual when I get home tonight to see what the typical input was calculated to be under normal operating conditions but I would imagine you should get away with 500 watts input to the final without problem.  Since the Gates is rate for 250 watts out it would have been designed for at least 375-400 input allowing class C efficiency plus losses in the output network.  Since this was also a continuous duty rating with broadcast audio you shouldn't have any problem with developing and fully modulating 500 watts input in amateur service.  Weight/size wise the Gates transformer is considerably beefier than that in my Viking 500 and of course the Viking 500 mod tranny also has to carry the DC plate current for the final through its secondary.

[WQ9E]

Well,  the Gates manual doesn't offer a lot more information.  The manual states the final will typically run at 246 ma @ 1350 volts to produce the rated 250 watts output.  The PA and modulation overload relays in the transmitter are set to trip at 300 ma and the manual advises removing the plate connection from one of the 8008 rectifiers to reduce voltage (and nuisance tripping) during initial setup.

Around 400 watts input might be a good compromise goal for your rig which should be easily within the capabilities of the iron you have without the risk of stressing it severely.  You probably would still be safe at 500 watts in but since the difference would be insignificant on the receiving end probably better safe than sorry. 

If you need scans of any information out of the Gates manual let me know.

73, Rodger WQ9E

[Steve - WB3HUZ]

It more an issue of working voltage. Most 250 watt BC rigs ran 1500 volts or so on the final and modulators. If you keep your B+ in this range, a few extra milliamps through the secondary or reactor isn't going to make much difference. But if you want to run your B+ over 2 kV, you may exceed the insulation breakdown voltage and wipe out your nice mod tranny.