soft start circuits

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neilwight

Well-known member
Joined
Apr 13, 2005
Messages
49
anyone know of or have schematics regarding building a simple one of these.

i've just got a valve compressor coming but all the tubes take a big hit when powered up, they all flash like light bulbs.
it would be nice to build a small circuit modifier that releases power into the system over 10 seconds or so up to the 240v the power switch passes and then basically then doesnt come into play again until next power on.

also while i'm on. hows the best way to clear any thump when powering up or off my GSSL. ive not built it yet, just ordering parts but am keen to integrate this. the power switch will just be a regular SPST type though i can easily replace for another kind if this will make this easier.
ive heard something regarding caps but surely this cant be just placed across the connections, it would surely flow voltage then regarldess of switch position....unless it sits between switch connection and wire out.

any thoughts will be most appreciated.

thanks,
neil
 
hmm, maybe this should be in the drawing room. aplogies if so and please feel free to move mods
 
Oh boy. Ya know, you could build something, but I hate to see you waste a lot of time and money and research on something that is going to break over time. Tubes flashing means there is definately something wrong. Something way beyond a soft start problem.

I hate putting excess baggage in the pwr supply because it always fails. Relays, logic chips, stuff like that dosent belong around 300 VDC. Since the pwr supply is responsible for 90 percent of all failures, you want to keep it as simple and bulletproof as possible. Especially if Frank Sinatra is doing a session, because he get's pissed when something goes wrong. (wait a minute, he's dead, isn't he?)

Did you try a web search?

An indirectly heated cathode in a tube diode is the way to go. Like a GZ34.

Or, if you want the cadillac of soft start, get the GZ34 And the gas regulator tube aka Fairchild 670 pwr supply and you will be in soft start heaven and the darn thing will out live your Grandmother.






I mean Grandchildren!
 
How about just adding a standby switch? This way when you power on your device the tubes have a chance to heat up before you apply the high voltage to them. This will extend tube life. It's a simple circuit. Here's an example:

bassman-5f6-a-schematic.gif


Scott
 
thanks for that....the solution looks like it can be achieved pretty straighforwadly....which is nice.


any ideas on damping a power on/off thump in solid state circuits?
 
I've been seeing a filament flash (orange) on power up on a recent project as well. They're 12ax7's with the filaments running on 12vdc, from a 7812 reg. Is this harmful to the tubes? This is the first time I've run filaments off of a regulator, and haven't seen this type of flare before.

craig
 
Well, if you want a ramp up heater regulator check out page 14 of this datasheet:
http://cache.national.com/ds/LM/LM117.pdf

You could also think about using constant current heater instead of constant voltage.
http://www.tubecad.com/july2000/page6.html

These are some interesting ideas. but I don't know if you really need any of them.
 
Constant current regs will limit the surge current for a cold filament to be sure. However, as the tube ages and the filament resistance goes up it will hasten the tube's demise by putting ever more power into it (until the voltage compliance of the current source is reached).

The approach I like the best is a constant power regulator, which can be approximated by a circuit that synthesizes an output resistance equal to the nominal operating temp filament resistance. This was described briefly in a recent issue of audioXpress.

James Boyk scoffed at the need for one of these, so the idea has promise imo ;-).
 
Put a big power resistor (value depends on total filament current) in series with the filament supply, and short it out with a set of relay contacts connected across the resistor. Control the relay coil with a time delay circuit, easily made up with an RC circuit and a transistor driving the relay coil.

The resistor limits the current through the filaments until the time delay relay shorts it out, applying full current to the filaments. As an added feature, you could switch the B+ with an additional set of contacts enabling the B+ after the warm up delay period.
 
Thanks Brad for that lm117 ramp-up circuit link. Somehow I've never noticed that one before.

I've been doing some searching around the net, and the concensus I find seems to be that the brief flare is no big deal.

craig
 
In the older TV they had 120V strings of tubes in series.
They were a light bulb when turned on.

I do not think is is an issue.
Preamp tubes are so cheap and I think the sonics
and the added parts will degrade the sound do not waste your time.

Now if you want to go the constant current route.
A simple LM317 current regulator works great.
Actually it will give the best sound. I do not know why.
But I learned this from those audiofool people we love to trash here.
He uses a LM317 for each tube.
 

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