Biasing with time constants biasing capacitors filters

W

walters

New member
I have seen different amps with different Biasing capacitors and filtering

some are 10uf or 20uf or 50uf and then 150uf has alot of time constant, its takes along time to stablize the DC voltage but filters out the noise and ripple

The biasing block diagram is :

1.) Transformer tapped at 100V AC
2.) goes to a Biasing Diode which rectifies it to DC
3.) now goes to the biasing filters caps

When i put my oscilloscope probe down to measure the biasing voltage or waveform , mostly i just see pure NOISE, i don't see a straight line at all like it should be

It should be a straight line on the oscilloscope between -35 volts DC to -70 volts DC but at a straight line

ON the oscilloscope you have to trigger the scope to LINE 60hz right when you measure the biasing voltage?

The DC biasing noise or ripple can leak into the cathode input or screen inputs , mixing with the AC signals

How can your filter out the Biasing better? use 500uf or use alot of capacitors in parallel?
 
Re: Biasing with time constants biasing capacitors filters

by time to stabilize the DC, do u mean warmup time? Leaving an amp on standby to let the caps finish getting full charge? This is very interesting! I dont have a degree or anything in electrical stuff, but it definitely interests me.
 
Re: Biasing with time constants biasing capacitors filters

NO the biasing filter caps are different than the power supply caps filters two different things
 
Re: Biasing with time constants biasing capacitors filters

paging the one dudes who have the website that keeps being offered for your questions.
 
Re: Biasing with time constants biasing capacitors filters

walters said:
The DC biasing noise or ripple can leak into the cathode input or screen inputs , mixing with the AC signals
Click to expand...

Someone in your general vicinity might have passed gas, the ripple showing on the octopuscope.
 
Re: Biasing with time constants biasing capacitors filters

joelap said:
I've given up.

Actually, the reasoning behind all that is kansas. Time constants are much greater in kansas, which cause emf interference often referred to in the hi-fi domain by the german name "sheizer hosen" (sic). My guess is you're in kansas. If you move, the residual noise would be much less. Also worth checking into is if your oscilloscope is set to t.a.r.d mode. If your oscilloscope has that function, turn it off.
Click to expand...

HAHAHA. That one actually made me laugh out loud!
 
Re: Biasing with time constants biasing capacitors filters

walters said:
NO the biasing filter caps are different than the power supply caps filters two different things
Click to expand...
Not it's not two different things.. it is exactly the same thing but the bias supply is typically a half wave rectifier so you'll see more of the ripple effect in your scope if it is a decent one.
The time constant of the bias supply is not really relevant, except to say that it might take a couple seconds longer to hit max negative voltage.
The bias supply has no external load and it's own circuit draws minimal current so once the filter cap(s) are charged to peak -DC ripple, that is pretty much where it will stay.
Yes, from the low impedance side of the power tube's grid load resistors there will be some residual AC signal swinging on the bias supply rail from driver stage but those AC signals will also be absorbed in short order by the filter caps and shunted to ground.
 
Re: Biasing with time constants biasing capacitors filters

missionamps said:
Not it's not two different things.. it is exactly the same thing but the bias supply is typically a half wave rectifier so you'll see more of the ripple effect in your scope if it is a decent one.
The time constant of the bias supply is not really relevant, except to say that it might take a couple seconds longer to hit max negative voltage.
The bias supply has no external load and it's own circuit draws minimal current so once the filter cap(s) are charged to peak -DC ripple, that is pretty much where it will stay.
Yes, from the low impedance side of the power tube's grid load resistors there will be some residual AC signal swinging on the bias supply rail from driver stage but those AC signals will also be absorbed in short order by the filter caps and shunted to ground.
Click to expand...

Hi Bruce! It's cool that you're sharing your knowledge with us all and I'm really glad to see you posting here. But watch out for this guy. I'm not sure what game he's playing - but it's a game.

Lew
 
Re: Biasing with time constants biasing capacitors filters

Lewguitar said:
Hi Bruce! It's cool that you're sharing your knowledge with us all and I'm really glad to see you posting here. But watch out for this guy. I'm not sure what game he's playing - but it's a game.

Lew
Click to expand...

Indeed!

Please don't feed the troll...
 
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