2kv Amp Tune Cap relay controlled? Experiment

[Naysayer]

Will this this 12A IDEC relay survive being connected to the Tune cap?
I've seen longer Tune leads on factory amps.
Tune cap is a 2-section affair where 2nd section only for 80-40. I need a way to switch the second section in & out. Tune cap is a punishing location but the lead lengths are short and I can bypass coil control leads to ground. I have a box of these relays so It's an experiment.

 

[Captain Kilowatt]

Spacing looks too small for my likings. It will likely arc. Reactance voltages get stupidly high on the lower bands. What kind of plate voltage and power output are you looking at?

 

[nomadradio]

Vacuum relay is your only hope. Reminds me of the 160-meter conversion I did for a customer's SB-220. The sample was ample. Won't try that again. Found a few vacuum relays surplus that made it practical. Or nearly practical. Won't need to have a high current rating, but will need a breakdown-voltage rating at least as high as the Tune control. Typical air gap on a conventional relay might not be sufficient.

Cheap russky vacuum relays were a common item on fleabay ten or more years ago. Trouble was they were SPST. Never could figure out which type number was normally closed or normally open. A proper pair, one of each would take the place of a single SPDT relay.

The era of cheap ex-soviet RF parts will probably draw to a close soon if it hasn't already.

73

 

[Naysayer]

An experiment for sure and 80m only as the rest of the bands ok with only 1 section of the tune cap. Yeah, the Tune cap gets brutal but, I'll post what happens when I attempt to run it into DL.

 

[Naysayer]

Relay connected. 12v to relay coil bypassed with caps to ground at toggle switch. The coil wire is Audio shielded cable covered in RG8x vinyl jacket. Later today try it.

 

[nomadradio]

Um, the vinyl jacket on a coax cable is not all that RF-friendly. Factories use 27 MHz RF to melt vinyl-type plastics. The center-conductor dielectric is polyethylene, a much more RF-resistant recipe. After all, the RF is supposed to stay inside the shield braid, not run on its outside surface.

The original D&A Phantom linear had twelve tubes, and used a relay to switch the driver's plate circuit. The High/Low feature would redirect the output of the 4-tube driver stage direct to the antenna for Low side, putting the 8-tube final stage on standby. Turned out that the black phenolic resin of the relay was perfectly stable with only the 50-ohm RF voltages, but would break down with a thousand Volts or more of plate-circuit RF on it. We replaced it using a RF relay with a fiberglass insulator for that repair. Until we couldn't get them any more and used a different plan that just got rid of that third relay. D&A learned their lesson and revised the Phantom to use only two relays, selecting Low side from the power supply and using ten tubes, not twelve.

So long as the insulation holds up, you're good to go.

73

 

[Naysayer]

On point as always Nomad, no substitute for experience.
Single pole vacuum latching would be efficient,

I presumed the Soviet model I put in the plate circuit was 26v because it was Soviet. From a vacuum part search I found that many Western RF switches also use 24-26v coils. (https://www.surplussales.com/Relays/REVacRel-3.html).

Currently I'm on the well-worn path of finding a voltage source for yet another relay. Amp will sound like a clock soon. So many different voltages, it's driving me nuts, From datasheet, I can get away with only a 1/2" hole for mounting near Tune cap. I have a lot more respect for the designers of decades old, sweep tube amps some of them were very clever when it came to sourcing power (employing a neon bulb as a regulator comes to mind).

I may soon be posting a dramatic picture of an RF-melted relay,

 

[Naysayer]

It took more time than expected but relay is holding up under zsac key-up. Amp still on bench so I'll try fully connected soon. Recognize that relay is NO when not on 80 and the never under power when contacts make/break. It only connects 2 sides of the air variable Tune cap and stays closed. I'll eventually replace it w/ a NO, SPST latching vacuum but I had a few these relays left from the T-R setup so I'll resume moving forward.

While I was mucking about with relays, I began cleaning up the wiring by removing several bullet connectors and a small transformer so it's getting neater underneath. There will be more to come.
neil NYC