Kit6550 Soft Start

[Revoli]

My kit6550 has always liked a dramatic start at switch on. It’s not good for my nerves and can’t be good for the amp and I suspect has shortened the life of valves and other components. It would seem a soft start circuit might help. Can anyone guide me on this. I note some posts talking about this but no real detail or examples. Am I right and what’s the best way to do this…..

Many thanks

[Greg.]

Do a forum thread search using keyword ‘Thermistor’. Several threads will come up that you should find helpful, all worth reading for your solution.

[Revoli]

Thx. That’s got me in the right direction. I am now building a long list of mods I should probably have done years ago. I am sure it will result in me having some questions to check my sanity and safety later….

[John Caswell]

Try at least one CL60 Thermistor or similar about 3A rating, or preferably two in series to reduce the inrush current. Fit them after the fuse, ideally on a tagstrip/board with long leads and, contrary to the advice I usually give, make sure they are mechanically sound, just in case the solder melts.

John

[Greg.]

See also this FAQ thread.

http://www.world-designs.co.uk/forum...ead.php?t=3859

[davebms]

Hello all .. Stolen from another forum ! My amp has run with this mod for many years. no boing on start up




I tried this yesterday and it seems to be useful ~

RS part No. 216-1422
NTC Inrush Limiter 2.5 Ohms
(May also be described as Type S237 thermistor 2.5 Ohms)

Connect it in series with your rect. valve heater supply on the pin opposite the one where the HT normally comes out (that is, NOT pin 8 [usually])

Warm up delay is supposed to be 90 seconds. Well certainly nothing seems to happen for a bit then the cathodes start glowing. I also tested it by heating one lead with a soldering iron and the resistance went down to 0.1 Ohm. After about a minute or so.

Crude but ingeniously simple. I wasn't too sure if it was going to work at first since these things are intended for mains usage, but no, it works. Image The valve is a U54. (In this case.)

RS page:
http://uk.rs-online.com/web/search/sear ... &R=2161422

Dsve

[Wieslaw Lipowski]

Why do you delay the heater by as much as 90 seconds?

In this case, the anode voltage will appear fully in the cold valve much earlier.

In my opinion, the heater should not be delayed, just the plate/anode voltages.


​

[bob orbell]

Hi Wiesiek, Dave is suggesting to put a NTC in series with the 5U4 heater supply, the two ECF80 valves will come on as normal ( solid state rectification ) , but the HT will now be delayed by 90 seconds or more, sounds a bit too long to me. This device should be connected as John has posted, but 2.5 ohms is too small a value for that purpose, the way forward is to follow the link from Greg, and Johns advice, failing that two new rectifier valves. Bob

[davebms]

hello all,, i think all the valve heaters will be on warm up as only one 6.3 volt 11 amp heater winding for all so NTC only slows HT the HT comes up pretty slowly. so no Boing on start up,, for me it works // some books etc some recommend heater warm first,, my old guitar amp on stand bye left heaters on all day if forget to press go !! an article i read a while ago recommended pre heat NOS valves 3 to 5 mins,, i think a lot off how to treat /look after valves
is peoples opinions,, but as long as we enjoy them all good

Dave