A Question about Amplifiers

[unit_399]

I bought this amp online last week . It was advertised as a Palomar, but was actually a TX.

Back in the day, TX amps were the best custom built solid-state amps out there. The real base bias circuit provided lots of clean power. Plus their mil-spec RF power transistors made them practically bulletproof. These boxes were unique in that the heat sinks were on top where they should have been on all amps. The seller advertised this one as working 100% and said it had the OEM RF120 power transistors. He was asking $150 OBO. I offered $100 and he accepted.

On the bench it did a little over 200W PEP modulated 100% @ 13.8vdc with 4 watt dk drive.

I have 2 questions :

1.) Does anyone (Hey, Nomad) have any solid info on these power transistors ?? Years ago, the builder told me he bought several hundred of these at a surplus electronics store in SoCal. Made by a company called "RF TRANSISTOR Co (???). Supposedly mil-spec and from a parts overrun on a military equipment build.

2. Also I was looking the amp over and noticed the output cap between the board and the antenna SO-239 connector. (The builder used exactly the same cap on his TX600 models.)I got to wondering how leads this small (less than 20 ga.) could pass that much rf power. Wouldn't they be current-restrictive ??. The value of this cap is .01uf. I was wondering if replacing it with five .002uf caps in parallel with their lead wires twisted together would allow for more output power. But before tearing into it, I thought I would run it by you guys and see what you think.

Thanks for your help.

- 399

 

[nomadradio]

I recognize the transistors. Those came from a now-defunct outfit called "RF Power Limited". Was run by a guy named Kevin, a real expert on the subject of bipolar RF transistors. We serviced VHF/UHF land-mobile FM radios 35-odd years ago when he was in business. Any time I needed to replace a police radio's final or driver with an odd manufacturer's "house" number on it, Kevin knew what to sell me. He had his own "house brand" transistors, used the prefix "RF" on them. Don't know what the original factory number on the "RF120" would have been.

The slogan on Kevin's sale flyers was "Over a billion Watts in stock". If I ever dig up one of those, I should scan and post it.

The amplifier is 100% Steve Canfield's baby. Has his trademark four-bead common-mode DC choke feeding DC to the collectors.

Not sure if Steve bought these in bulk from Kevin, the "RFP ltd" guru or if someone installed them as replacements. The 1986 date code won't clear up that question by itself. I have a derelict TX800 here with 1994 and 1995 date codes in it, so I guess he was at it for a fair while.

Kevin sold "RF Power Limited" to Richardson Electronics some time around or shortly after 1990. Never heard from or about him since. For all I know Richardson could have continued selling parts with the "RF" prefix in the years since. No data on that question

Yeah, the output-blocking capacitor seems sketchy. The mass of metal inside the capacitor is more important than the size of the lead wires. Just the same, if it gets hot it's too small. And if not, there's no problem to cure.

Putting multiple capacitors in parallel divides the current between them and decreases series resistance. The capacitance value isn't critical, so long as the reactance is well under 1 ohm at the operating frequency. The more the merrier, for this part of the circuit until the capacitor gets large enough to begin acting like an inductor.

You got a real score, there. Fewer and fewer of Steve's amplifiers out there with every passing year.

73

 

[unit_399]

Nomad -
Thanks for the RF120 info. My TX600 had 120s in it when I bought it new, so I think they were OEM in all the TX boxes.
When I saw the amp advertised, I jumped on it. I met Steve back in '86 when I bought my TX600. Never knew his last name. He was living in Riverside CA at the time. I've run my 600 for over 30 years (base and mobile) and it's never missed a beat. 73s.

- 399

 

[kopcicle]

https://www.worldwidedx.com/members/steve-tx-amps.48519/