Niegative bias voltage, rectifier.

[trem]

Can you guys tell me if I am understanding this correctly. In the bias circuit below, I assume the negative half of the wave goes from the tranny tap through the 470R and then the diode. Does it then charge that 25/50 cap before it heads for the 10k pot? Then I assume that 25/50 cap will continue to charge with the incoming negative wave and then does it discharge through that 15k resistor and back through the 10k pot to provide the smoothed DC for the TWO 220k bias resistors. Is that how that 25/50 cap works?
Thank You
http://www.el34world.com/charts/Sche...SMAN_AA864.pdf

[Wilder Amplification]

Can you guys tell me if I am understanding this correctly. In the bias circuit below, I assume the negative half of the wave goes from the tranny tap through the 470R and then the diode. Does it then charge that 25/50 cap before it heads for the 10k pot? Then I assume that 25/50 cap will continue to charge with the incoming negative wave and then does it discharge through that 15k resistor and back through the 10k pot to provide the smoothed DC for the TWO 220k bias resistors. Is that how that 25/50 cap works?
Thank You
http://www.el34world.com/charts/Sche...SMAN_AA864.pdf

The transformer center tap and the 70 volt bias tap continuously charge up the filter cap on each negative cycle via the 470R resistor and the backwards bias rectifier. The bias pot and the 15K that's in series with the bias pot draw a very small amount of current from the filter cap while the center tap and 70 volt bias tap continuously replenish the charge that is drawn from it. As the pot and 15K resistor draw current, it causes the capacitor to discharge until the next negative cycle comes through to replenish the charge.

Think of the filter cap like you would your car battery while the 70V bias tap is your alternator.

[tubeswell]

Everything Jon said, plus the 10k pot and the 15k series resistor form a voltage divider to adjust the amount of negative voltage that is bled back to ground (so that you can adjust the amount of -ve bias voltage going to the 220k grid load resistors/grids).

[trem]

My 15k is actually 39k, to allow enough negative voltage. So biased at 35 mA there is a 50 volt drop across that 39k. That is about 1.28mA. So that current is supplied by the discharging 25/50 cap and also smooths out the ripple voltage from the diode? I guess I have a rope around this now. I don't know how the chassis can decide where to send the current. I suppose that is what makes a ground scheme so important, and also why ground loops occur.
Thanks

[mooreamps]

Can you guys tell me if I am understanding this correctly. In the bias circuit below, I assume the negative half of the wave goes from the tranny tap through the 470R and then the diode. Does it then charge that 25/50 cap before it heads for the 10k pot? Then I assume that 25/50 cap will continue to charge with the incoming negative wave and then does it discharge through that 15k resistor and back through the 10k pot to provide the smoothed DC for the TWO 220k bias resistors. Is that how that 25/50 cap works?
Thank You
http://www.el34world.com/charts/Sche...SMAN_AA864.pdf

The simple answer is yes. There is very little current draw on the bias circuit, thus the filter cap will charge to full value of the voltage divider. Also, since the load impedance is so high, you can get away with just the half wave rectifier circuit as shown.


Actual values may vary depending upon the application. Mine are a little different since my preference is using combination bias for a push/pull power amp.



-g

[tubeswell]

I don't know how the chassis can decide where to send the current. I suppose that is what makes a ground scheme so important, and also why ground loops occur.

The chassis is at ground potential, so current within the ground return is theoretically non-existent.

(I say 'theoretically', because there are minute currents within the ground return because there are minute microrises in ground potential at different locations within the ground return - and these can cause ground loop hum in amps - but that is a side-issue).

Current needs voltage/potential (and resistance path to a different potential) in order to manifest.

In my mind I compartmentalise the various bits of the circuit and assign them tasks. So I see the (theoretical) situation with the bias supply circuit as follows.

Alternating current flows back and forth (between negative potential and positive potential) from the bias power supply (from the PT). The reverse-biased diode (which comprises the rectifier in this case) only permits the negative half of this AC pulse to pass on to the next part of the circuit.

The resulting sequence of negative (half-wave rectified) voltage 'pulses' are then filtered/smoothed with the reverse-biased cap, which alternately stores and releases 'negative charge' from the sequence of pulses, so as to result a continuous negative voltage potential at that point in the circuit

When you connect this negative voltage potential to the ground return via a conductive/resistive pathway, the energy resulting from the (negative) voltage then discharges back to ground potential through the (in this case - voltage divider -) pathway in the form of current. Once it gets 'back' to ground potential, the energy is expended, and ipso facto there is (theoretically) no current within the ground return at the 'bottom' of the voltage divider.

However at the 'knee of the divider (where the 10k pot wiper is in the schematic), there is a continuous negative (DC) voltage/potential. Anything connected to this, which does not have a resistive/conductive pathway back to the ground potential, will also be at the same negative potential as the knee.

Well that's my take on it anyhow

[Wilder Amplification]

The chassis doesn't decide anything. It's the polarization of the power supply that does that. The chassis is nothing more than a mere conductor that is referenced to one side of each power supply.

[Enzo]

Nothing "sends" current, current is drawn by a load of some sort.

Look at a battery and a light bulb. Until you connect the bulb to the battery, no current flows. It is only when you connect the bulb that current flows. In a power supply, the voltage is made available with current avaiable behind it.

Current only flows when there is a complete circuit. The ground is just part of that circuit path.

[Wilder Amplification]

Nothing "sends" current, current is drawn by a load of some sort.

Tell that to the guys who think that tubes make power and all you have to do is pull two of them to 1/2 it without compensating the load.

[mooreamps]

Can you guys tell me if I am understanding this correctly. In the bias circuit below, I assume the negative half of the wave goes from the tranny tap through the 470R and then the diode. Does it then charge that 25/50 cap before it heads for the 10k pot? Then I assume that 25/50 cap will continue to charge with the incoming negative wave and then does it discharge through that 15k resistor and back through the 10k pot to provide the smoothed DC for the TWO 220k bias resistors. Is that how that 25/50 cap works?
Thank You
http://www.el34world.com/charts/Sche...SMAN_AA864.pdf

The simple answer is yes, that is how it works ; on a Fender. The power transformers I get from Hammond have a separate pair of wires for bias. I still use a half wave rectifier, but I put the diode on the ground side of the secondary.

-g

[J M Fahey]

Tell that to the guys who think that tubes make power and all you have to do is pull two of them to 1/2 it without compensating the load.

That's (relatively) nothing.
Tell that to those guys who think that pulling one of their 6L6 in their 50W , two 6L6 amp is the way to go to turn it into a 25W amp.
I get that question at least once a month.

[Albert Kreuzer]

I still use a half wave rectifier, but I put the diode on the ground side of the secondary.

Why? Any special reason for this?

[mooreamps]

Why? Any special reason for this?

No special reason. Perahps I could say the coil helps isolate the switching noise from the diode, but I use the fast recovery diodes. I've also done this on one HV supply for my 813 project, but that was to keep the voltage stress off the part. In this case, 1.4 KV.

-g

[Wilder Amplification]

Why? Any special reason for this?

For the sake of being different. 6 in 1, 1/2 dozen in the other.

[Albert Kreuzer]

For the sake of being different. 6 in 1, 1/2 dozen in the other.