Power filtering question

[mort]

Just trying to get my head around how the capacitors function in a power filtering section. What would be the electronic effect of raising or lowering the uF values of the capacitors in the following section. I don't wish to change them, just to understand what the effect would be.

[JHow]

Valve wizard has a pretty good article on smoothing here...

The Valve Wizard

[mort]

Valve wizard has a pretty good article on smoothing here...

The Valve Wizard

Just read through a chunk of that. Thanks, looks like a good resource for me. It's a bit thick for me due to my green streak, but I am getting some of it already. What is HT ?

[tedmich]

oops, Merlin is a Brit, so HT is, I believe, "High Tension" or B+ = high voltage

[JHow]

oops, Merlin is a Brit, so HT is, I believe, "High Tension" or B+ = high voltage

Yep. Hence the "valve" wizard, instead of tube. Could be worse... at least he's not the "tappet" wizard.

[tedmich]

Yep. Hence the "valve" wizard, instead of tube. Could be worse... at least he's not the "tappet" wizard.

magliozzi humor! Merlin is great and hangs out over at ax84.com pretty frequently, and here a bit too!
http://music-electronics-forum.com/users/611/

[mort]

Another couple questions:


What is this little section called, with the single diode, cap and resistor?

How would using higher or lower value caps in this position effect the circuit?

How would this diode need to be spec'ed, and should it be the same as the rectifier diodes?



Thanks in advance.

[Jazz P Bass]

That, my friend, is the negative voltage bias supply.
The 6L6 output tubes are running in a grounded cathode configuration.
So the grids need to be biased at a lower voltage (hense the negative supply)than the cathode.
Changing that cap will do nothing, tone wise.
There is not any 'tone' in the power supply.
Maybe stiff or saggy would be better terms that could be applied.

[mort]

I happen to have a 33uf @ 100V cap lying around. Is it safe to use in this position?

Also, how does the diode work faced in this direction? I thought a diode wouldn't allow voltage to pass through with the polarity positioned this way.

[soundmasterg]

I happen to have a 33uf @ 100V cap lying around. Is it safe to use in this position?

Also, how does the diode work faced in this direction? I thought a diode wouldn't allow voltage to pass through with the polarity positioned this way.

A 33uf 100v cap would work just fine, though it won't really gain you anything. What the bias supply circuit is there is a half wave rectifier. Half wave rectifiers are noisy and don't regulate very well, but in a bias circuit where there is almost constant and very low current draw, it doesn't matter. You can experiment with values there but don't go any larger than 100uf in the bias supply as it takes a larger cap like that too much time to charge to keep up with the instant demands of the power tubes it is supplying with a negative voltage.

A power supply at this point is an AC signal before it goes through the diode. AC means that the waveform pulses back and forth from positive going to negative going 60 times a second in the US. (120V 60 Hz, 220V 50 Hz in Europe) The diode acts as a one way switch. In a normal setup, this one way switch allows the positive going signal to pass but blocks the negative side, but in the bias circuit, you need a negative voltage as your output, so the diode AND cap are hooked up backwards. The diode being backwards allows a negative signal output, and the cap acts to filter the ripple to ground. Ripple is from the signal switching 60 times a second and is noise, but you can envision it as a small AC signal riding on top of the DC voltage that was created by sticking your diode in there. Going to a larger cap allows more of this noise to get shunted to ground, but if you go too large, then it takes too long to charge the cap up and your bias supply is too slow to adequately perform it's task of supplying a negative voltage to the power tubes as they need it.

Personally I wouldn't bother replacing those caps unless they are bad, and anywhere close to the stock value is fine as long as the voltage rating is sufficient.

Greg