ARC-5 Output Impeadance

[NC2W]

After finishing my power supply, for the 80m ARC-5, I took some time to measure output into a 50Ohm dummy load. The current pair of 1625s seems to create about 40-50w.  Is this about right?

I've read that the ARC5 has an output impedance of 12-20 ohms. Does anyone have any better information? Does anyone use a 4:1 unun with it?

[W2JBL]

I run two ARC-5 stations, one mobile, the other fixed. Both are plate modulated "Navy" units. I get a good 100 watts out of them with about 800 volts on the 1625's. Having tried all the "internet" fixes for the low output Z, I found that by resonating the output coupling link I could get the rigs to deliver rated power for the 1625's, or close to it. I use an Aroco
mica compression trimmer (2000 pF max) soldered right accross the link, with the shortest leads possible. That said if you are doing the usual ARC-5/SCR-274 setup with 500 volts to the PA and get 40-50 watts you are doing well as is.     

[SM6OID]

Hi!
What kind plate and screen voltages do have run to see 40-50 W out?

[NC2W]

600V on the plate, 300 on screen, 218 on oscillator

[SM6OID]

Hi!

OK, with 600 V plate and 300 V screen, I would have expected a bit more power coming out of the box. (but I may be wrong)
218 V to oscillator is OK, but possibly could be increased to get more drive to the finals, I think it was W2JBL that told me so?

Let me see if I can check one of my transmitters later today.
Still have a few job related bits and pieces to take care of, plus a short one hour lecture.
Then I'm free to do some radio stuff!

Currently I run my command sets with a MD-7/ARC-5 modulator, but the transmitters are SCR-274N or Navy.
Obviously, they don't mind being plate & screen modulated...
I do have a BC-456E modulator, but it is not hooked up, yet.

[AJ1G]

My command sets are a joint Army-Navy mash up of 2 ARC-5 transmitters, a T-19 for 80 meters, a R
T-22 for 40 meters, and a SCR-274 BC-456 screen modulator with DY-8 dynamotor.  System also includes an ATA series antenna relay and a SC4-274 series 4 transmitter control box.  The entire system is powers from 28VDC.  The DY-8 puts out about 550V.

I use an output matching network described by Walt Hutchens, KJ4KV back around 1989 in Electric Radio in Uniform consisting of a parallel resonant circuit with two taps on the coil towards the cold end. The one closest to the ground side of the coil receives the transmitter output through a nominal 100 pf series cap.  The antenna is tapped off the coil a couple of turns above the transmitter input tap.  The circuit is essentially a resonant auto-transformer.  The series output cap is resonated with the roller inductor within the transmitter.  Careful adjustment of the parallel tank circuit and the roller inductor results in about 50 watts output on CW, and about 20 watts on screen modulated voice into a 50 ohm
dummy load or a nominal 50-75 ohm antenna. For voice operation, best modulation is obtained by tuning for maximum output in CW and then switching to AM.  Adjusting for maximum carrier output in voice mode will result in less than optimum modulation quality.  The adjustable coupling link on the transmitter should not be advanced beyond the point of maximum output when tuning up in CW mode.  If you can monitor your modulation envelope with an O-scope, placing the set in MCW if you have that capability, can allow some tweaking for the best modulation waveform, or you can inject a tone from an audio signal generator or simply sock it with some atomic yay-los.

I’ve tried feeding a nominal 50-75 ohm load from the transmitter via just a series 100 pf capacitor and the internal variable rotary inductor, but only can make about 30 watts output in that configuration.  I believe Mike, WU2D did an ER article on various output matching techniques which had similar results to mine.

The parallel tuned auto-transformer also provides a measure of attenuation for harmonic radiation, I recall getting an OO report for a strong 7 MHz 2nd harmonic when operating a command set on 80 meters as a JN without any downstream antenna matching network.

Chris, AJ1G Stonington CT