RCA Mod iron info

[W8ACR]

One of the items I picked up at the W0ZUS auction was a large RCA modulation transformer. The info on the transformer is as follows: Type XT 2568, pri 10,300/2580, sec 3750. There are only three primary terminals, so I assume that this transformer is designed for a primary impedance of 10300 ohms and a secondary impedance of 3750 ohms. But what is the 2580? Also, the secondary terminals are labelled "S" and "F". What does this mean?

Thanks,  Ron

[W7TFO]

Operating Volts, Start & Finish.

73DG

[W8ACR]

Thanks Dennis. Does anybody have any experience with this transformer? Does anybody know its original application?

[Opcom]

part number does not match the BTA-250L manual. so it isn't that one..

[KC4VWU]

Doesn't match the BTA-500R part # either. Is it an open frame or enclosed?

Phil

[W8ACR]

Here are some pics

[W7TFO]

Methinks BTA-1L.  Late 40's, 833's by 833's.

I babysat one from KWKW in Pasadena back in the 70's.

73DG

[KM1H]

I dont think it would be 833 audio, that P-P spec is for 3000V and 1600-1700W. The iron says .83KVA.  More likely 810's at around 2KV for 750W of audio.

Carl

[W7TFO]

Both the RCA and Gates of the era running 833a's had Ep around 2500, and 750-800 W for the modulator output.  An easy life, more about tube hours and reliability than what those bottles could do.

73DG

[KM1H]

I agree but its the P-P spec thats of concern here, its way high for 2500V 833's unless they were trying for a lot of processing and high positive peaks. I have a BT-1K version here and 750W of audio would be pushing it.

Weight comparsions could also be used.

[WB3JOK]

The info on the transformer is as follows: Type XT 2568, pri 10,300/2580, sec 3750. There are only three primary terminals, so I assume that this transformer is designed for a primary impedance of 10300 ohms and a secondary impedance of 3750 ohms. But what is the 2580?

Well, since 2580 is almost exactly 1/4 of 10300, and impedance is proportional to the square of the turns ratio, I would guess that the third terminal is a center tap between the Start and Finish (i.e. at 1/2 the turns)? Checking the DC resistance would likely confirm this... just a thought.

[W7TFO]

Stock mod trans for Gates BC-1 series is rated @ 750W w/833A's. 

It never gets that close, if run properly.

73DG

[KE6DF]

With the impedance ratios stated, it seems like it was intended to run with the modulators at a higher voltage than the RF side.

Pretty high step down ratio if you are trying for 100% modulation with a single supply.

Doesn't the bc-1 use a common power supply for both?

[W7TFO]

Yeah, but we are talking about some RCA iron.

73DG

[Opcom]

Whatever it is, I believe it is the same as the one in the Tucker which has no label. It looks exactly the same.

It makes sense for 2500V, but was the unit ever run at 830VA? What was the real plate input at carrier? Surely not 1600W! That 830VA was for headroom/asymmetrical peaks, maybe?

[k4kyv]

I'd say the RCA 1 kw.  The primary impedance is almost exactly the recommended plate-to-plate Z of a pair of 833As.  The  lower Z figure would be the impedance for each side of the primary winding (per the turns  squared rule). The rig ran a common power supply.  Some of those old RCA xfmrs were notorious for too much step-down.  They were designed for 100% modulation and nothing more, and apparently RCA didn't care about "headroom".  Those transmitters were built before broadcast transmitters were allowed by the FCC to run in excess of 100% on positive peaks. A 3:1 impedance  ratio (1.7:1 turns ratio) is pretty common in older broadcast iron. I believe Steve, WA1QIX, used one of those RCAs in his plate modulated rig back in the mid 70s.

I have a large one here, came out of a 250w transmitter, designed to use without a reactor, so it's about the same size as a KW transformer used with a reactor. The impedance ratio on that one is approximately 15K to 5K. (BTW, if anyone is interested, I'll let it go cheap, pick-up only. Contact me.)  The way to get more  headroom, or modulate beyond 100% positive, is to run separate power supplies and higher voltage on the modulator, or else use a common power supply and a large network of voltage dropping resistors to reduce the voltage to the final.

Typical DC input to a pair of 833As running a KW output is about 1400 watts. The V-A rating on the name plate probably indicates some built-in safety margin, as well as accounting for a transmitter running lower efficiency (DC input/rf input to antenna). The 833As could easily handle 1600w DC input, plate modulated.

Extremely unlikely it was designed for a pair of 810s.  The 750w class B audio rating for 810s as listed in the ARRL Handbook is the ICAS rating (intermittent commercial/amateur service).  The CCS  rating (continuous commercial service) is much lower. In broadcast service a pair of 810s modulated by another pair typically run at 250w to the antenna, with a maximum of about 1600v DC on the plates.

I use a smaller RCA transformer from a 250-watt xmtr in my HF-300 rig.  Mine was designed for use with a reactor, and has a much better turns ratio (1.55:1). I have run as high as 1 KW DC input to the final, modulated up to 150% positive peaks, and it not only didn't crap out; it didn't even get warm to the touch. RCA rated their transformers very conservatively.

On the secondary, you want to use the Start terminal for the +HV, and the Finish terminal for the modulated HV line to the final.  "Start" means where they started winding the coil, next to the core, and "finish" means where they finished up the winding with the last turn on the coil. The Finish end of the coil is less likely to arc over on modulation peaks, and Start has more capacity to ground due to its close proximity to the frame.  Wiring it backwards (+HV to the Finish and modulated DC from the Start) is more likely to cause a crap-out, and the high frequency response may affected by the capacitance to ground. (This may not be strictly true with some transformers that are wound symmetrically with each winding split into two or more sections, but to be on the safe side the S/F polarity should always be observed).

[W2VW]

The transformer pictured looks exactly like the mod iron which came out of WHLD near Buffalo. The transmitter was an RCA which used 833s modulated by 833s with a common supply.

According to the paperwork left at the station the transmitter was put in service in 1937.

The Raytheon RA-5 which was at the same site was installed in 1947 and had RCA 1:1 mod iron.

[KM1H]

Extremely unlikely it was designed for a pair of 810s.  The 750w class B audio rating for 810s as listed in the ARRL Handbook is the ICAS rating (intermittent commercial/amateur service).  The CCS  rating (continuous commercial service) is much lower. In broadcast service a pair of 810s modulated by another pair typically run at 250w to the antenna, with a maximum of about 1600v DC on the plates.

I wasnt even looking at the ICAS ratings since the "typical" CCS ratings fit that transformer well considering the 830va rating which would be input and not output. PP load and va fit perfectly for CCS 810's at 2000V.

http://tubedata.tigahost.com/tubedata/sheets/049/8/810.pdf

[k4kyv]

I wasnt even looking at the ICAS ratings since the "typical" CCS ratings fit that transformer well considering the 830va rating which would be input and not output. PP load and va fit perfectly for CCS 810's at 2000V.

http://tubedata.tigahost.com/tubedata/sheets/049/8/810.pdf

According to the above sheet, reprinted from the RCA TX tube manual, typical 810 CCS rating in class-B modulator service:

DC plate voltage 2000 

Plate-to-plate load impedance 11K

Maximum signal power output 590 watts


The 830 va rating marked on the transformer refers to its audio (output) power handling capability which, with a sine wave tone, would 100% modulate a final running 1660 watts DC input.  At 60% efficiency 1660 watts input yields 1000 watts rF output, pretty typical of a class-C final after tank circuit, ATU and feed line losses are taken into account. My Gates, which also runs a pair of 833As modulated by another pair, originally ran 2600 plate volts @ 0.575A to the final, 1495 watts input for 1 KW out.  At 590 watts maximum audio output, a pair of 810s in the modulator would fall substantially short.

The RCA 1 KW transmitter, originally manufactured in the mid 30s, used a pair of 849s in the modulator, until the 833(A) was introduced in the late 30s.  The 849 at 2500 plate volts in class-B modulator service fits the transformer pretty closely.

http://scottbecker.net/tube/sheets/111/8/849.pdf

[KM1H]

The transformer doesnt look like it was built in the day of 849 relics.

The 849 wasnt even an RCA developed tube; they purchased them from Westinghouse who introduced the tube in 1927 strictly for audio use, and did the merchandising in the late 30's likely as they didnt have anything comparable. Im surprised they even used it in a transmitter.

In 1945 Amperex brought it back to life as the 849A/H but it was really a new tube.