Moving a Gates BC1T transmitter to 75M (3885 kc)

[w4ax]

I have a new to me Gates BC1T that is in nice condition. I have searched AMFONE and Google and I see where many people have done the transition to 75M. However, I can’t find any information on how they retuned the tuned circuits. I can of course measure the inductors and do the math but I am hoping for a starting point. To move from 1 Mc to 4 Mc I would take 50% of the inductance and 25% of the capacitance as a starting point and then dial it in. Will you please share the changes you made? I did see an excellent discussion on this forum about converting the oscillator. The YouTube conversion video is pretty light on details.

Second question. My Gates is missing L4 the first choke in the 650V LCLC power supply. The manual or schematic don’t list the value of this choke or current. If you know the value will you please share that information. I did the math and I’m thinking around 10H at 350ma.

Thanks in advance for any assistance.

73,

Mack
W4AX

[KD1SH]

Your math was right on. The choke is a UTC CG-104, 10H, 350ma, 90 ohm DC resistance, hipot tested to 5KV.
Here's a picture of one from Worthpoint (sold): https://www.worthpoint.com/worthopedia/utc-cg-104-big-power-supply-choke-350-2023149425

I have a new to me Gates BC1T that is in nice condition. I have searched AMFONE and Google and I see where many people have done the transition to 75M. However, I can’t find any information on how they retuned the tuned circuits. I can of course measure the inductors and do the math but I am hoping for a starting point. To move from 1 Mc to 4 Mc I would take 50% of the inductance and 25% of the capacitance as a starting point and then dial it in. Will you please share the changes you made? I did see an excellent discussion on this forum about converting the oscillator. The YouTube conversion video is pretty light on details.

Second question. My Gates is missing L4 the first choke in the 650V LCLC power supply. The manual or schematic don’t list the value of this choke or current. If you know the value will you please share that information. I did the math and I’m thinking around 10H at 350ma.

Thanks in advance for any assistance.

73,

Mack
W4AX

[w4ax]

Thank you very much for confirming the choke value.

Vy 73,

Mack
W4AX

[KD1SH]

No problem. I've got the entire complement of Gates BC1-T iron down in my shop, and the guy I bought it all from was nice enough to mark each one with its designation. I just went down there and looked at it.

Thank you very much for confirming the choke value.

Vy 73,

Mack
W4AX

[KA3EKH]

I did a RCA BTA-1MX some years back and only went to 1885, both the Gates and RCA use 833 tubes in the PA and as modulators and the 833 have a ton of capacitance that tends to make it hard to build stable output tanks for high frequencies. 
Bests advice would be get the transmitter working on its original frequency first, second remove all the HV divider resistors that were used for low power operation if installed  and all the remote control stuff that had been added over the years and third and most important be extremely careful with the side panels or front and back door open. There are plenty of ways of killing yourself with the plate voltage that it runs at.
Main reason for me writing this is it gives me time to tell one my favorite 833 stories, back in the seventies when in high school had a part time job at a radio station and remember being called out in the middle of the night because the engineer on duty did some cleaning on the Gates BC1 and afterwards the transmitter instantly blew the barker when the plate was applied. Turns out he installed one of the 833 tubes in the PA backwards, they have a key but you can do it anyway. The grid and plate were reversed. But the best part of the story was that after we discovered that the tube was backwards we put it back in the right way and there were no ill effects to the tube that had three thousand volts dumped on its grid!

[w4ax]

I’ll bet that did bring out hidden passions! I’ll have to be certain I don’t make the same mistake. The project is coming along nicely.

In my 50+ years as a ham I’ve worked around a lot of HV including a current rig that uses 5kv at 3A. However, I always appreciate a caution from anyone. Thank you.

Mack
W4AX

[KA3EKH]

The old first generation BC1 had that huge pot on the side of the transmitter that was exposed with the output of the plate supply on it, that’s how they varied output power. Along with having a voltage divider built out of huge wire wound resistors for low power operation, when you ran the transmitter in night mode they just stuck a bunch of resistance in the HV line to reduce power. The old power control was right there out in the open with the side panel removed, just begging you to touch it!
Back when I was in school had my Ham ticket and that led into twenty or thirty years of working on broadcast transmitters. The old analog television were the worst being you would have 30 kV beam supplies and everything on the base of the klystrons tubes floated above ground. All that said don’t think I ever received any bad shocks from a transmitter, High intensity obstruction lights are another story.

[W1TAG]

On the other hand, the old WTAG-FM in Worcester, MA started with a 1 kw GE rig in the 44 Mc band. It had a pair of 810s in the PA.  When they had to move to the present band, the 810s wouldn’t budge. So they switched to 833As, and they worked OK at 102.7 and later 96.1 Mc.
There was a photo of WPI Professor Newell holding an 833A in front of the converted rig, and the photo was published in GE News. I’ve always wondered why he was point to the tube with his middle finger. Maybe the conversion had some unpleasant memories.

John, W1TAG

[KA3EKH]

The oldest FM transmitter that I ever worked with was some nineteen fifties Gates thing that had a pair of 4-400 tubes running in push pull in a tank circuit, unlike every FM transmitter built after that that used a cavity circuit. Horrible transmitter being the amplifier would never stay in balance and was at best maybe forty or fifty percent efficient. Also tended to produce massive amounts of AM noise and other garbage. The difference between coaxial tubes and cavity amplifiers and all that old Lumped Components junk was night and day.
Think the 833 was the ideal tube for the AM Broadcaster and all those transmitters with the big open tank circuits and can see where in the world of high-power amplifiers, at least low frequencies why that tube was in use for at least fifty years and is still used in some of the high-end audio amplifiers today. Not may other pre-WW2 tubes around today that can say that.
On your Gates are you planning on running at 1 kW or reduced power? I ran my RCA at 375 watts and that was plenty for 160, at that power level I just used one 833 in the RF and kept the two in the modulator. Biggest problem on 160 was when you called CQ you would have a strong signal out to other stations but people would come back to you running there Icom 756 running twenty watts of AM and you would have to work to copy them.