Discussion in 'Amateur Radio Amplifiers' started by G3YRO, Dec 14, 2016.
Could have told you that.
Yes . . . should have done the maths BEFORE trying to delay throwing the shorting switch !
The standard "soft start" relay-switched scheme in most commercial amplifiers (including my Ameritron) are <<1 second delay. I think it's really about 100 mS maximum, as you really can't even hear the relay click -- it's almost but not quite instantaneous with power on.
2-3 seconds would be much too long.
If the HV filter capacitors are each below about 300uF, a step start is wasted money.
In an SB-220 it certainly is. Probably in the TL-922 also. It uses eight 200uF filter capacitors and a pretty hefty switch.
The 922 has some weak links, but that's not one of them.
I use a 300 watt light bulb. When the bulb stops dimming I throw a switch to energize a relay to short out the bulb. Got lots of room in my short rack cabinet.
Well I replaced the resistors . . . and now hit the mains switch on the wall followed immediately by the Shorting switch . . . probably about half a second delay. Seems quite happy with that.
You can tell it's worthwhile, as you don't get the big switch-on thump that you do without it.
I gather it's not just about saving the HT Circuit . . . but reducing the big current surge into the filaments of the 3-500Zs.
At some point I may wire in the resistors and a shorting relay INSIDE the Amp . . . but it's really no big deal to do this manually, as you obviously only turn it on ONCE ! (and I don't always use it . . . only when DX-ing on 160m)
The step-start circuits are mostly to save the power on/off switch. The tube filaments don't need it, and unless they used really wimpy rectifiers they shouldn't need it either.
The big thump is the transformer magnetizing, and that doesn't damage anything.
The funny part about step-start circuits is they are set up for inrush at power on only, and not at turn-off...which is when the power on-off switch can be subjected to inductive kickback and actually see more stress than at turn-on.
You may well be right . . . I used my FL2000B for 35 years without any soft-start circuit, and never any problems either.
It does make you wonder about a lot of these suggested mods . . .
Because I plug my Amp straight into the switched wall socket right next to where I sit it's just as easy to turn it on and off with that switch, rather than the one on the Amp.
Those switches are designed to switch 13 amps . . . but if they do fail, you can replace the whole switched socket for about £2 in 5 minutes !
Yes, good idea.
Of all the "commercially built" ham amps, Henry had the best idea for switching, with most of them: They used a DPDT 240V 20A circuit breaker as the on-off switch in many models. Very unlikely to fail, but if it ever did, a quick trip to any big box store would result in a replacement for very low cost, and they'd drop right in.