help on the rca bta1r

[w5tmg]

does anyone know how to calculate the original mica frequency respondant caps in this old rig to get it up to 75 meters around 3.890. thnxs

[KA3EKH]

I have a BTA-1M on 160 and that’s a whole different animal then the 1R being the M uses 833 tubes and think the R uses 4-400 so think you will get it there. Got any pictures? Did you already have it on 160? Going to run it at full power?

[w5tmg]

ok heres the deal. i got it and the guy had started on a convertion for 75. he pulled half the coils out of the inductors, and rearanged the micas. i put it back together as stock as i could. i never got it to key on 1450 or 1.885, with correct plate current. its got low plate current and low output. im tryin to just get it on 160, on the % meter all the readings are correct, plenty of drive and everything else is correct to, except for the cathode current reading on the pa. there suppose to be 55 to 75%and they read 30%. i know this has something to do with the pate and screen rolling off together and if the plate isnt rite the cathode wont be rite...i hope this hasnt totally confused you and i appreciate the reply... tracy w5tmg

[KA3EKH]

Ok, this is what works for me and am cretin just about everyone will disagree and point out the error of my ways.  I wanted to keep as much of the original tank from the transmitter as there was that includes the original plate tank coil and the sample coil that feeds the modulation monitor (L6 and L7) also I removed one of the two tubes I had in the PA because I had no desire to run the thing at full power and also I reduced the AC to the plate transformer down from 240 to 120 volts to try to keep from melting plates and other damage while I was working on this. Keep the B+ voltage for the drivers and oscillator all the same but cut the plate voltage while trying to get the PA to work. I only wanted to go to 160 (1.885) so it was fairly easy. The size of the plate tuning capacitor (C21) had to be reduced down from 100 pf to 25pf, was surprised on how small that was but also remember that my transmitter uses a 833 in the PA and that has a tone of internal captaincy but if you're going up in frequency and keeping the same tank coil you need to reduce capatance. Using a low plate voltage of around three hundred volts and a sensitive wattmeter was able to find the sweet spot on the tank coil (L6) by trial and error that turned out to be 22 turns down from the C23 side of the coil. Output loading capacitor (C23) turned out to be huge! I started out with four .001 cap in series, then three and ended up with just one. The transmitter acted just like what you described with low output and low plate current although I did see a pronounced plate current dip even with the low output. Do not know why the output loading capacitor is so large but that gives me a good output tank on 160 that depending on where I set my plate voltage will allow me to run anywhere between two to five hundred watts with 40 to 50 dB of suppression of second and third harmonics.
Think the key to getting everything working correctly may be to get your transmitter working on 160 first and make cretin everything is good there and then start working towards 80 The basic design for any 4-400 broadcast transmitter tank will be the same and there got to be a ton of people who built tanks for their Gates and Collins stuff and if you can get a copy of their tank circuit you don’t have to re invent the wheel, biggest problem you may find is  that the tank coils may be too big for 80 but I don’t know that for a fact myself. The other thing I would recommend is reduce your plate voltage as much as possible until your tank starts working properly, it's easy to cook tubes. I am impressed that the 833 bottles I have are capable of being turned cherry red and still come back and work again but 4-400 are another story. Here is a copy of what my finished tank looks like but remember it’s a 833 and been downgraded to just one tube and is on 160

[k4kyv]

I parted out a BTA1-R several years ago. The main tank coil has a lot of turns, and is tuned with variable brass slug that slides up and down inside the coil.  Not the most efficient method of varying inductance IMO.  In fact, that coil was the source of some of the coil stock I used to make my 160m antenna coil, shown  in the current thread on polystyrene.

I would consider revamping the whole RF section to a configuration more typical of an amateur transmitter, using a pi- or pi-L network, a multi-position bandswitch with fixed taps on the coil for each band, and vacuum variables (or bread slicers), keeping the inductances fixed for each band.  Variable inductance and fixed capacitance in the tank usually results in the resonance point being slightly off the plate current dip.

The other problem I observed with the BTA1-R was the layout of the RF tank circuit components.  The RCA design engineers seemed to just throw the components in the box wherever they thought it was convenient, and use whatever length of connecting wire it took to get from point A to point B.  I'm sure that was OK for the AM broadcast band and with a little care, can be made to work on 160m, but by the time you get up to the 3.5-4.0 mc/s region, those long leads are apt to become problematic.  Don't even think of trying to get it to go to 40m.  OTOH, redesigning the output circuit along conventional HF transmitter lines and using a proper driver stage or appropriate RF exciter, it should be easily possible to make it work bandswitching all the way from 160m to 10m, much in the manner of the pair of 4-400s/4-250s in the KW-1, Desk Kilowatt and GPT-750.

[n2bc]

Don is right, even 40M is tuff with the stock huge layout.  I ended up completely disassembling my RCA and building the whole thing into a standard rack.  Covered 160 thru 10 and instead of living in the garage, took it's place with the other junk in the basement shack.

I did get it to go on 160 and 80 quite easily, but that involved reworking the driver and converting the PA output to a standard PI net with vac variables.

Some pix below.  Reworked driver / Reworked PA tank / Completely 'repackaged'

73, Bill  N2BC

[KA3EKH]

What are the values that you used for the plate tuning and loading capacitors? Also what was the value of the plate blocking capacitor? I do not understand why the plate blocking capacitor is so small in the 1M I have (C15) but it works great. I have thought of building up new tank for 80 and maybe 40 but know that’s a stretch with the 833 and the stock layout.

[w5tmg]

ive been tryin to use the old inductors, and it just aint workin out. i just pulled them out and im gonna use a simple pi net work... and about youre tryin to get that rig on 40, good luck, i think that you'll find the leads will be to long unless you rebuild the whole thing... ive got w0zus rca bta1r2 and it on 40 but it was completely gone threw. pretty interesting rig... thnxs for the help. oh and the blocking cap it called for was 620pf, ill have to rethink that on this one, ive found the .001 work real well...

[k4kyv]

Those old broadcast rigs were designed to run 24/7 on a single frequency, year after year.  It didn't matter that it might have taken half a day or longer to find all the right taps on the coils to get everything coupled properly and in resonance, but once accomplished, those settings should hold for ever or until major change are made in the circuit.

But amateur operation is a totally different proposition (unless you work SSB in one of the 75m "groups" that purchased THEIR frequency from the FCC back in the spark days to work the same 4 or 5 guys night after night, and would never QSY for any reason, come hell or high water). I have never been satisfied with a non-frequency agile transmitter, and want mine to be capable of operatinhg every band from one end to t'other.  With the old BC transmitters, especially if you want to work multiple bands, the best solution would be to strip out the original RF section entirely and use as many of the original components as possible to start from scratch, using more conventional long standing ham radio transmitter construction techniques.

[KB5MD]

I have an RCA BTA 1R1 that works 160-80-40.  I will post pics tonight of the tank circuit. If you pull out all the old output network and replace it with a pi-output..."it ain't that difficult."

[KB5MD]

more pics of inside BTA 1R1 160-80-40     Top pic is of shafts for caps extending to counter and knob on front top.
 Box with large crank and counter dial controls the big Gates roller inductor.  The vacuum cap is 300pf 5kv and the loading cap is a 5 section receiving type.  There is a mica cap next to the loading cap, it is in parallel with the loading cap.

[w5tmg]

thnxs doc for those pics and thxns to you don for putin up with me all morning, always enjoy talking with ya...

[k4kyv]

... the loading cap is a 5 section receiving type. 

That just reminded me, I'm overdue for my regular appointment to kick myself for leaving four each 900/900 pf 2500v solid brass Cardwell split stator variables in the Ethiopian scrap yard back in 1967, because I was too damned cheap to pay the shipping costs to send them back to the states.  Probably would have cost me less than $50 shipping, plus the $2 or so I would have paid to the scrap dealer for them.