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What would you do?

Captain Kilowatt

Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
17,352
12,427
823
61
Nova Scotia,Canada
I have a quandry on my hands. A lot of you have heard me mention that I have an RCA 1 Kw commercial AM broadcast transmitter that I had hoped to convert to 80m and use my TS-820S as the VFO. The plan was to first find out what was wrong with the modulator and then rebuild the output section and retune for 80m.No small task as the entire PA tuning section has to be stripped and rebuilt from scratch. Next I need to devise a PTT and antenna change over circuit and run it remotely as the TX is in the basement and the shack is in the den directly above the TX.I would also need to monitor plate current and ideally grid current as well.The more I look into what I have to do the bigger the project becomes and all this for a single band plate modulated TX. I am starting to think about stripping all the components and using them to build a linear amp that I can use for all bands and all modes, not just AM.The pros of the TX are beautiful audio with a 750 watt carrier which is legal here BTW. That is the legal limit in Canada for AM. There is also the ego factor of saying "The rig here is an RCA BTA-1S modified for 80m with a pair of 4-400's modulated by another pair of 4-400's." I would also be the only station in the entire region to have such a transmitter on the air.The cons are a LOT of work and it will be single band and single mode.At this point it looks like the modulation transformer may be toast but I have another transformer I can make do with by making a few component changes and running no more than 500 watts carrier.Getting all the PTT and T/R sequencing is going to be a chore. The pro's of an amp are all band and all mode. I can still use it on 80m AM with my Heath DX-60 as an exciter/driver.I can build the amp with four 4-400's in the final that will loaf along at the 2,250 watt pep Canadian legal limit for SSB. The transformer is a Peter Dahl rated for 3700 volts at 1.25 amps CCS forever and the rest of the power supply components are very heavy duty as this thing was meant to run 1 Kw AM carrier 24/7 at 100% modulation.I have a pair of 1000pf Jennings vacuum varible capacitors to use in the tune/load positions.I also have a 42 inch high rack cabinet to mount everything in but it will be a heavy beast when it is finished. I figure the power supply alone will weigh nearly 200 pounds with the plate transformer,filament transformer,screen power supply,filter choke etc.Having come from a broadcast engineering background the transmitter has a certain appeal but I'm not sure if it is the best idea or not. TX or amp. What will it be? :confused::confused:
 

Since you have the background to do it, go for the all mode unless you are just obsessed with AM.

My .02, I don't like much of what I've heard of the "AMers" on 80m...they put a brick down on the key and go so long they overrun their mandatory ID interval before the next guy even gets to speak. Just not my style.

You can take your now monster all mode and run on AM just dandy and still be well ahead of the herd. At least with my 'bit of everything' operating, that would be a lot more useful.
 
I am not obessed with AM but I do like it and there is an AM net here in the Maritimes region on Sunday mornings that attracts a lot of former and present broadcast engineers as well as some former military comms guys. I am sure they would be tickled to have me put the RCA on the air and to tell the truth it does have appeal to me as well if for nothing other than the bragg factor. :laugh: That is why I am shifting back and forth on just what it is that I want to do. To be honest it is probably the only way I can see in the foreseeable future to own an amp capable of our legal limit but it almost seems a sin to scrap the transmitter. If I was not a former broadcast engineer it would be no problem,build the amp. There is just something that keeps me from wanting to scrap it however. Old habits and ties are hard to get rid of.
 
Hopefully I'll get there with the know how to build my own amps. Just shaking the dust off my electronics and getting back into it. Been fun so far.
 
QRN, It's my opinion that while the 'brag factor' is certainly nice, a one band, one mode device is only 'good' for one thing. What happens if/when that 'brag factor' isn't a factor anymore, or you become interested in some other mode for whatever reason? It's also a fact that 'AM' just isn't a generally used mode on the amateur bands, and if you should ever decide to sell it, you have a very limited market. I really think that converting that thingy to an all mode, all band, sort of amplifier would probably be your best bet. The 'flip-side' of all of the above is that if that's what you want to do, then do it! Who sayz you can do the 'other' conversion later? If I were that 'thrilled' with 'AM' to start with, I think that's what I'd aim for. But, since I'm not, I keep thinking about the fact that, "Cain't use it here!", thingy. Glad it's you and not me!! - 'Doc (Not much help, huh?)
 
Well it looks as if the decision may have been made for me. After some carefull,and I do mean very carefull, poking around the plate voltage supply it appears that the modulation transformer may be toast. The TX was tripping the plate current overload relay when the HV was turned on and I could see a bright green flash from somewhere inside the cabinet.:cry: The TX had poor audio when I got it but it would at least put a signal on the air,actually into a dummy load on 730 KHz. The audio would hit 100% neg but only around 50% pos. and was distorted with a lack of highs. One day it just refused to come on and this is when the arcing first occured.The actual flash was coming from a spark gap across the high voltage filter choke.I cleaned and adjusted the spacing which made no differance. I then disconnected the HV leads going to the four 4-400's and again no differance. Next I lifted the HV lead going to the center tap of the modulation transformer and the problem with the arcing stopped. The transformer is showing a couple thousand ohms to the case but at 3100 volts that is a lot of current.The modulation transformer is at least 8 inches by 10 inches by 14 inches high and must weigh 100 lbs. Not exactly something one has in his junk box.It now looks like I have a dismantling job ahead of me followed by a major construction project. In one way I am saddened by the prospect of scrapping the broadcast TX but on the other hand I am delighted with the prospect of building a legal limit plus :drool: amplifier. Having four 4-400's in the finals should let me work across town shouldn't it?
 
I'll have my 4-400 amp back on the air tomorrow. I can't wait!

I'm thinking about adding a 3rd tube so it "loafs along" at the USA standard legal limit of a California Kilowatt.

Sounds like you have a load of fun ahead of you. What will be your plate voltage? If you use the Dahl with a full wave bridge you'll have enough voltage.for a pair of 4-1000s! I'll bet you've got a nice Variac in mind or else you'll find out how long a set of 4-400s will survive 5200v!

They'll probably do fine, don't you think?

4800v under load, 1050ma, .635 efficiency, about 5kw input so 3500w pep with 300w drive that is. Yeah, I could hear that at least down the street a block or three.
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I'm running my Amperex 4-400Cs at 4600v and no hiccups. I'm not even using vacuum caps,

...yet.
 
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Well I may,just may, have been suckered. I plan to check something out later today after I get a few things done.If it ends up that all is well and I just had a stupid spell then I will look at convertint the TX to 80m. If all is in fact NOT well I will scrap it and build an amp.

CDX-007, the plate voltage will be around 3100. I will use all the parts as they are in the TX right now and use the plate transformer with a choke input supply with a full wave bridge rectifier.You get better filtering with lower capacitance with choke input and better regulation as well.As configured right now the TX only has 12 uF of filtering and is clean as a whistle. Also, with 500 mA plate current and peaks of 300 mA of mod current the voltage drops no more than 100 volts from no-load values. It is very stiff.The transformer is tapped as well to allow lower output voltages. The voltage would be extremely high,well over 5000 volts, if I were to use a simple capacitor input supply and finding caps. rated at over 5000 volts is very hard in the the value needed for proper filtering.I am not interested in running the tubes at absolute max ratings and prefer to run them conservativly in the name of longeviety and spectral purity.I also have no need to go beyond the 2,250 watt pep power limit I have here in Canada but having the headroom and extra plate dissipation is a comfort when it comes to running a 750 watt carrier plus modulation on 80m AM.

Time to dig out the old Tripplet VOM and see if I was suckered or not.
 
Good news and bad news. First the xmfr is NOT toast. I was suckered big time. :blushing: There was a branch circuit I did not se that was giving me the resistance to ground that I was seeing. Also I had a faulty safety interlock that was shorting the high voltage to ground all the time. Problem # 1 was stupidity and problem # 2 has been fixed. Now I just need to restore grid drive to the 6146 driver stage and I suspect that problem may be related to some rewiring I was doing earlier. I was replacing the unshielded wire from the oscillator to the 6146B driver grid. I guess I need to recheck things there. Oh well, at least the big expensive things look fine.
 
QRN, you should be ashamed of yourself! But as your friend, I'm going out of my way and help you out. First, box up the transmitter and send it to California. Because I'm helping you out, the least you can do is pay for the shipping. I will use my local resources to rebuild the TRANSMITTER and will take good care of it. I will treat it with the respect it deserves!:wub:

Make an amp out of it.......:angry::censored:

You can buy a SSB radio and a simple amp that will do more than a pair of 4-400s for next to nothing.

Also, as you know, if you need more output than two tubes can provide, you need bigger tubes. This has been the staple for the broadcast industry for decades!

I have to go......:( I don't think I can bear it any more......:cry:

;)
 
I guess I have good news for you Master Chief. I was just pumping a 750 watt carrier on 720 KHz into a dummy load mere moments ago. The HV problem is fixed and the loss of IPA grid drive is cured as well.So after all my hard work this past week or so I have worked my way back UP to square one.:D For those that do not know, a transmitter is not quite like an amp.It will not turn on unless there is the proper amount of grid drive. The big problem Ihad was that it would not stay on long enough to see if there was any grid drive. I had to assume there was not and work from there.Suddenly I had about 25% of what I should and the TX would hold on the air but just barely. A few tweaks of the oscillator to bring things back into alignment and everything is as it should be. Except for the gross imbalance in the pos/neg modulation peaks and grid currents in the two 2E26 audio tubes.Hopefully that will be cured soon as well as I downloaded some info from a fellow that had a similar problem with an RCA broadcast TX.After I get this beast working 100% on the original freq of 720 KHz I will then proceed with the frequency conversion to 80m and convert the final stage to accept RF from my TS-820S rather than from a crystal controled source feeding the driver.In effect I plan to bypass the driver stage alltogether.I plan to run the TS-820 in the tune mode with just a few watts output in CW mode to act as a frequency agile exciter so I am not limited to one freq. all the time. Yep it feels good to have worked my way up to stage one again. :blink:
 
Boy-a! You're hurtin' my back just TALKING about that thing!:D:drool: I helped get a Collins 20V out of a broadcast station last spring, and I hope to NEVER scuffle with something like that ever again!!!!!:thumbdown: It is STILL sitting in the back of a pickup truck (in a garage) in several pieces! Now I LIKE AM (but a little dab will do me), but the older I get, the less I want to fool with those old 1/2 ton transmitters!:(


73

CWM
 
The Boat Anchor Floats Again!!

OK, I finally figured out how to find this new forum again. The link to old posts gets a 404 error, will they ever be working again???????

I'm glad to hear you've got it on it's way to working as the transmitter you wanted it to from the beginning. I'll want to hear it on the air, let us know when / what freq. in advance and we''ll try to get the Elmac on freq.

73
 

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