• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Simple Time Delay (no more excuses for not having one)

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#41
The thermistor would work on it's own (subject to the down sides i listed, and not protecting against on-off-on power failures), if it was eatting fuses any more often than it currently is I would probably add that as a stop gap until getting a better delay in place.

That other circuit sounds like it would also work, my only concern is that the capacitor isn't the only load involved- would the amp itself potentially open, or prevent the relay from closing? It likely wouldn't need to be 95% of output either, a lower value (half, a third?) would probably work just as well for protecting fuses and breakers.

Here's a calculator I came across for capacitor charging that would be useful to anyone wanting to use this circuit for soft starting purposes like i've suggested.
Charging a Capacitor

Now back to vacuum tubes...

How long of a delay do you think is required for tube heating? 20 seconds seems pretty long to me.
 
#42
cjmurphy87 said:
That other circuit sounds like it would also work, my only concern is that the capacitor isn't the only load involved- would the amp itself potentially open, or prevent the relay from closing? It likely wouldn't need to be 95% of output either, a lower value (half, a third?) would probably work just as well for protecting fuses and breakers.
Click to expand...

The circuit uses the rectified supply voltage as a reference. When the monitored voltage reaches 95% of reference, it activates. You can easily adjust the ref. voltage to make it activate lower then 95%. The reason I built it the way rather I did (rather then a resistor/zener for v reference) is so that it could be used on a very wide range of voltages. In my case it was systems ranging from 48-144v. A few resistor changes and it'll work at kv if you wanted.

cjmurphy87 said:
How long of a delay do you think is required for tube heating? 20 seconds seems pretty long to me.
Click to expand...

Some say 30sec to a minute. Some wait 15min. That'd be one big capacitor.
 
#47
Mine is much simpler, and works like a charm. 🙂

It consists of one relay and one resistor. Resistor is in series with primary of the rtansformer, relay is powered from filament rectifier. When all capacitors are discharged the resistor limits inrush current. As soon as all voltages on all rectifiers equally raise to the percentage enough to engage the relay it shorts the resistor.

Here is PCB, the smallest one. The rest is 400V and 800V B+ rectifier, and voltage stabilizers of 400V for preamp & driver, 270V for screen grids, -72V for bias, and +12.6V for filaments of small tubes. Boards were designed for slim 1U chassis.

An externally hosted image should be here but it was not working when we last tested it.
 
#49
lazzer408 said:
When the circuit is off, R2 gives Q2's base a path to ground causing Q2 to conduct and discharge C1 (when C1 has a charge). When the circuit is powered, Q2's base sees 12v which keeps the transistor off allowing R1 to charge C1.

I've adapted this circuit from an electric vehicle DC motor controller I designed years back. I used it as a throttle buffer so when you stomped the throttle to the floor, the motor would receive a slow rise in voltage to avoid jerky starts. When the throttle is released, the controller had to reduce the voltage as quickly as the throttle was released.
Click to expand...

Hi Lazzer!

Doesn't R1 Also Give a path to Q2's base?
If What i said is right so why is R2 needed?
 
#54
You may hate uprocs, but it's hard to disagree with a little circuit like this, it has power saving too...

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


'Course I could shrink it a bit if I go to a 6-pin PIC and put components on both sides.

Wait a while, turn on GP0
Wait a while, turn on GP1
Wait a while, turn off GP0

...no electrolytics to go soft... no question about it, it's gonna reset PDQ.

You didn't write your post on an analog computer, did you? Analog is great, but there comes a point where clinging to it just looks like an affectation.
 
#57
One thing to remember about all of the circuits running on DC is that the off delay will be dependent on amount of filtering (bulk storage caps) in your DC supply for the circuit.

Keep filtering to a minimum to minimize off delay if you are concerned with dropouts. The microprocessor based circuit may be more dependent as they often times have a more stringent supply requirement.
 
#58
TheGimp said:
One thing to remember about all of the circuits running on DC is that the off delay will be dependent on amount of filtering (bulk storage caps) in your DC supply for the circuit.

Keep filtering to a minimum to minimize off delay if you are concerned with dropouts. The microprocessor based circuit may be more dependent as they often times have a more stringent supply requirement.
Click to expand...

That's why many people run delays off the heater winding.

As with any DIY design, it's up to the builder how they use it or what changes they make so a design suit their needs.
 
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