Lifting centre tap for B+ delay

[hariadi88]

Hi,

If I have power trans with full wave rectifier, is it possible (and safe) to put switch between centre tap of secondary and ground to delay the B+?

I know ther is more proper B+ delay circuit out there, but I prefer the simplest one (albeit manual).
Any simpler way? Use Thermistor instead of switch?

Thanks in advance

 

[audiopro]

Guitar amp manufactures have been doing that for decades. It's called a standby switch.

 

[rayma]

Right, but still use a quality switch rated at a reasonable AC voltage.

 

[dgta]

Tube PA amps did the same, although some didn't just open the circuit.They had a resistor across the switch so when open (stand by) there is a high resistance from the CT to ground. I believe this is a better arrangement.

 

[hariadi88]

Thanks guys,

Btw, what is the resistor value around? a few kOhm? or hundreds Ohm enough?
Would the full current draw pass through this resistor, ie need high rating?

 

[rahew]

Hi



If it helps my Racal RA17 receiver uses a 10 k 10watt resistor in standby (About 370v DC across it in standby)
Robert

 

[6A3sUMMER]

rahew,

I often forget details, as do we all.

P = E(squared)/R

370VDC and 10k = 13.7 Watts!

A 10 Watt resistor will smoke!
Or at least melt the insulation off a wire that is too close.
Prevent Forrest Fires.

 

[Ketje]

rahew,

I often forget details, as do we all.

P = E(squared)/R

370VDC and 10k = 13.7 Watts!

A 10 Watt resistor will smoke!
Or at least melt the insulation off a wire that is too close.
Prevent Forrest Fires.

Only in the case if the 370V positive is shorted.
I think 10W will do fine.
33k / 5W is another possebility.
Mona

 

[Merlinb]

I've used 47k to 100k 2W without problems. Depends how much current the amp sucks during the 'standby condition', of course.

 

[dgta]

... Depends how much current the amp sucks during the 'standby condition', of course.

And that depends on the resistor you place across the standby switch

Last PA amp I looked at was a 50W amp and it had 3k@5w across the standby switch.

 

[6A3sUMMER]

He said: "(About 370v DC across it in standby)"
It = 10k 10 Watt.
I took him Literally.

 

[Ketje]

He said: "(About 370v DC across it in standby)"
It = 10k 10 Watt.
I took him Literally.

Well, he is right, at open circuit that is
Mona

 

[petertub]

Hi,

If I have power trans with full wave rectifier, is it possible (and safe) to put switch between centre tap of secondary and ground to delay the B+?

I know ther is more proper B+ delay circuit out there, but I prefer the simplest one (albeit manual).
Any simpler way? Use Thermistor instead of switch?

Thanks in advance

Delaying B+ is overrated and might be damaging to the power tubes.
What do you want to solve with this ?

 

[hariadi88]

Hi,
I saw the voltage at turn on goes up to 500V and settles down to 400V at the first cap (as intended) in 30 seconds or so. That exceed the capacitor rating of 450V, and I don't have higher rating capacitor.

 

[6A3sUMMER]

Get capacitors that are rated for 500V or more.
Some are rated for 550V surge and 500V operational.

You may have to stack capacitors in series, with leakage balancing resistors across each one. You need 2X the capacitance for each one.

Keep in mind, if the output tubes ever die, the B+ will rise to 500V, just like they do at start up.
To quote the capacitor . . . Ouch!

And ask yourself, what happens if you, or someone else forgets to put the standby switch in standby before powering on the amp?