Time delay circuit

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.

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 ...

ChrioN said:

What I have is 12vdc coming from a 7812.
Help is needed.

Click to expand...

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.

Google "LM555 time delay relay"

Here's one
http://www.aaroncake.net/circuits/relaytim.asp

Urei LA4 has a 555 delaycircuit. Or just a 100-220uF over the relaycoil and a serial resistor.