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-   -   Lifting centre tap for B+ delay (https://www.diyaudio.com/forums/tubes-valves/360235-lifting-centre-tap-delay.html)

hariadi88 15th September 2020 11:42 PM

Lifting centre tap for B+ delay
 
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 16th September 2020 12:26 AM

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

rayma 16th September 2020 12:44 AM

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

dgta 16th September 2020 12:59 AM

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 16th September 2020 01:12 AM

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 16th September 2020 08:18 AM

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 16th September 2020 08:36 AM

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 16th September 2020 09:25 AM

Quote:

Originally Posted by 6A3sUMMER (https://www.diyaudio.com/forums/tubes-valves/360235-lifting-centre-tap-delay-post6343055.html#post6343055)
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 16th September 2020 10:44 AM

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

dgta 16th September 2020 04:59 PM

Quote:

Originally Posted by Merlinb (https://www.diyaudio.com/forums/tubes-valves/360235-lifting-centre-tap-delay-post6343130.html#post6343130)
... 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.


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