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Any experience of Avondales Soft Start board

lilolee

lilolee

pfm Member
I know I could email Les, but somebody here may be able to help.

First off I may be wanting to use it where it's not supposed to go. I am a valve man and basically want one switch on my amps to power them up. at the moment it has 2. One for the LT (valve heaters) the other for HT. I wanted to put the SS board on the 240v line to the HT rectification, so it came up slowly.

Bottom line can the SS Board handle 240v and what would the max wattage rating be for it.

BTW the amps aren't some little 8watt jobbies, but 120 watts worth of 4 x 6550 valves.

Any help welcome.
 
Hi there. The soft start module isn't really designed for the purpose you describe and may not be strictly necessary.
Where soft start scores is when a large toroidal transformer is connected instantaneously to a mains supply. Consequently, the toroid exhibits a high inrush current characteristic and pops a mains fuse especially if there are many microfarads connected to the secondary winding.

Valve amps usually have stacked transformers not having a high inrush and therefore have an inbuilt soft start characteristic.

Having said, the low resistance of the heater filaments when cold may present any transformer with an adverse load or a heater filament may become temporarily overloaded before the operating temperature is reached. In this instance, soft start may have some benefit in prolonging the life of valve heaters by reducing the shock load at switch on.

The SSM1 module takes 1.5 seconds to switch as much as a 1KW load.

Hoping that sheds some illumination.
 
Les

Valve amps are a bit different. It is necessary for the HT voltage to rise slowly, to prevent "cathode stripping". So although stacked transformers wont produce a huge inrush current, the HT voltage will still rise too quickly (before the cathode has had chance to heat up) and reduce the life-span of the valves in question.

BobMax

PS. I guess lilolee would want a delay of about 60 seconds before the HT line reaches a maximum. 1.5 seconds is way too quick.
 
Agreed Bob thanks.
I addressed only the LT side of the equation where the low resistance of the heater chain might cause premature failure of one of the filaments at power up. You are of course quite correct in saying that HT should not be applied until the heaters are 'well alight' but thats not within the scope of the soft start module.
 
Lee

I guess you are modifying an existing amplifier and need a bolt-on addition?

If designing from scratch though, you would be better off putting this sort of "slow rise" circuitry into the feedback network of the regulator. There are lots of standard designs you can use.

If you don't already have it, I would recommend you get the book Valve Amplifiers by Morgan Jones, 2nd Edition Newnes, which is superlative. It's also very useful for solid-state junkies, hence the reason I bought it.

BobMax
 
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