Discussion in 'Amateur Radio Amplifiers' started by KC5SCK, Jun 17, 2017.
It's a 240-volt Supply, tube amp. Want to use main power switch to turn on. Did have a variac in line took variac out as voltage is low enough not to use variac 2500 to 3000 volt to plate.
If 240 VAC mains, you must use a 2PST switch, You must "break" both sides of the 240 VAC line.
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Further, the switch should be capable of handling not only the steady-state current but also, the inrush current. Most ham amplifiers use a capacitor input filter. That is, immediately after the rectifier, is a capacitor(s). At the moment you turn the power switch ON, the capacitors appear as almost a short circuit (there's a yuuge inrush current pulse). Switches don't like that and can quickly wear out.
The answer is either a humongous switch/relay, or limit the inrush current pulse. Typically, a resistance is connected in series with the ON/OFF switch. A brief time period after the ON/OFF switch is turned ON, a pair of relay contacts shorts across the resistance. This is how I modified my Heathkit SB-220 to have "step-start":
All new/added circuitry is shown in red. At moment the switch is turned ON, K1's contacts are open and R2 + R3 limit the current into T1. Relay K1 has a 24 VDC coil. D1 rectifies 120 VAC and R1 limits the current so that, after C1 charges up, K1 pulls in (about 1 second or so). K1's contacts then short across R2 + R3, applying full voltage to the transformer primary windings. Since I'll never operate my amplifier on 120V, I didn't bother configuring it for other than 240V.
DPST switch rated for both running current and switching current.
Henry used 240V 20A double pole circuit breakers as on-off switches in many of their amps, and that was a fairly good choice.
If you're starting from scratch, that works. Otherwise, a workaround as I showed is appropriate.
what tube choice may help too some tubes require a warm up time good stuff so far . I try to use high current relays to turn on plate transformers after .5kva with some sort of soft start . add on from there if I need a timer I use the timer to hold off soft start as to keep high currents off of the timer module and keep relays and complexity low . not hard just have to draw it out . prior planning goes a long way
keep in mind 2 piece amplifiers require a bit of wiring challenges for safety and proper metering
Why? Most equipment only breaks the live wire !
Including the swtiches on the mains sockets on the wall.
In the U.S.A. they are both Hot/Live.
Rodger, did you not know that when we use 240VAC here, it is 2 hot legs? I know it's inferior to your British methods, but, that's how it is. A DP switch/breaker/relay Is needed for safety over here.