Time delay circuit
« on: February 17, 2010, 07:57:11 AM »
I'm looking for a circuit to mute the tube preamp output on powerup for a couple of seconds.
Searching with google gives alot of circuits. I don't know which one to choose.
I want simplicity, not so much ms accuracy. What I have is 12vdc coming from a 7812.
Help is needed.
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip


Re: Time delay circuit
« Reply #1 on: February 17, 2010, 08:29:04 AM »
I tried this and it works...dunno if it's the best way though, just trial and error science.

A cap in series with a relay coil. Cap conducts while charging and powers the relay coil. When full it does not conduct any more. 1000uF to 1500uF gave me a couple of seconds with a 20V relay.
One thing to keep in mind though. The cap needs to be discharged when you power the unit off. Otherwise this trick wont work if there's not enough time between two sessions. Maybe an extra pole in the on/off power switch to short the cap with a resistor or so....

EDIT: can you post a couple of circuits that you've found? Would be interesting to see ...
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan


Re: Time delay circuit
« Reply #2 on: February 17, 2010, 08:45:01 AM »
What I have is 12vdc coming from a 7812.
Help is needed.

But you also have heaters for the tubes, right?

If you happen to have regulation there, you could upgrade that with a slow start, and have it open the output relays.

I have this kind of safety latch for some of my tube project B+ lines. slow start heaters are controlling a relay that protects B+. If heaters die for some reason, B+ will be disconnected also.


Re: Time delay circuit
« Reply #4 on: February 18, 2010, 02:00:39 AM »
Urei LA4 has a 555 delaycircuit. Or just a 100-220uF over the relaycoil and a serial resistor.
Thomas "TK" Kristiansson
"The Sound Is In The Iron"


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