300b lovers

I ask again this technical question again. Which rectifier is better: full wave or bridge?

My driver stage power transformer has three taps: 250 0 250 and supports both - full wave and bridge. Full wave affords use x2 times bigger DC current. Why do many people prefer a bridge rectifier over full wave?

They’re different.

* Full-wave uses two rectifiers and only one half of the secondary at a time. Each active half-secondary switches back and forth 120 or 100 times a second.

* A bridge uses four rectifiers and the entire secondary all the time. The current through the full secondary switches direction 120 or 100 times a second.

That’s the most basic level. To a first approximation, full-wave transformers need twice the voltage for the full secondary, since only half the winding is used at a time.

In more depth, if there is a first cap following the rectifier(s), the rectifiers only conduct for very brief intervals (a few milliseconds) when the cap recharges. These brief, high-current pulses can shock-excite the half-secondary, causing ringing from the abrupt cutoff of the unused half-secondary. The shape of that shock is controlled by the on-off ramp of the diode, and if there are charge-storage effects from a solid-state diode.

The rest of the time, after the first cap is recharged, the diodes are switched off, and the amplifier is powered from the charge in the first capacitor. This cap is steadily discharged until the next pulse comes along and recharges the cap all over again. This charge-discharge cycle happens 120 or 100 times a second.

There's a tradeoff in sizing that first capacitor. If you double the size, the voltage sags half as much ... but the inrush current is also doubled, too. If you really overdo it, the inrush current will be so large it pulls the circuit breaker.

A "choke-fed" supply has the diodes directly feed a special inductor rated for very high voltages. The diodes remain "on" for most of the AC waveform. But ... the special inductor has to tolerate a significant voltage kickback when the diodes do cut off ... this in turn can create ringing and possible voltage breakdown of the windings in the choke.

All of these supplies create very large current pulses that radiate into the air and are are transmitted back down the power cord of the amplifier, which turns the cord into an antenna radiating a 120 or 100 Hz pulse train, with harmonics extending throughout the audio band,

A general rule-of-thumb in power supply design is to minimize the "loop area" of the most powerful transmitters, which are the loop created by the transformer secondary, the diode array, and the first filter element. Minimizing the wire length, tightly twisting these wires, and keeping them as far away from the input circuit as possible is highly desirable. This is why running AC power to a front-panel switch is undesirable from a noise perspective.

There’s a simple emulator called PSUD that lets you play with different power supply topologies and circuit values, and lets you scale the load to a real amplifier.

Which rectifier is better: full wave or bridge?

Full wave is preferred if you are planning a bipolar (+ and -) power supply. If only a single pole, a bridge does have the advantage of the power transformer not needing a center tap. That advantage is because a center tap is never truly centered, so the output from the transformer applied to a full wave is slightly different with each half of the AC waveform. This means the diodes are making slightly different current spikes as they commutate (turn on and off). 

That might not make much of a difference, since the best way to snub the circuit (to kill the swept resonance that occurs when the diodes shut off) is to snub the rectifier with a resistor and capacitor in series across the input to the diodes. In case its not clear, the source of 'diode noise' is really the power transformer inductance, interacting with the capacitance in the junctions of the diodes. For this reason, if you use a semiconductor rectifier setup, you really want to keep the leads from the power transformer as short as possible.

If you do that right, you can make the nasty silicon rectifiers perfectly silent; obviating any need for a tube rectifier (who's main advantage is low 'rectifier noise'). I put that last bit in quotes since the transformer is what is causing the noise, reacting to the rectifier.

In the old days it was common practice to put a 0.01uf cap in parallel with semiconductor rectifiers. This does nothing and might actually make the problem worse. Any diode junction, if you plan to snub the rectifier itself, must use a resistor in series with a small capacitance to properly snub the rectifier. But usually you can have great success with just a resistor and small capacitance at the input to the rectifiers instead, since snubbing the transformer is putting out the match rather than the forest fire.

Hi @lynn_olson ​​​​@atmasphere ,

Thank you a lot for detailed answers!

I have another question:

In balanced push-pull self bias output tubes is not a big issue because the cathode resistor and capacitor are out of the main signal path. But what to do in my case with a 300B SET. 

Now I use self bias with 50uF AN Kaisei NP bypassed by 4750uF Nichicon and two Ohmite Gold 10W resistors in parallel.

So the question is what is the best solution for SET? Fixed bias? Is fixed-bias reliable and safe? What is important to know for building a negative power supply for fixed bias?

To complete the current loop around the 300B (or any other power tube), there has to be a low-impedance path from the cathode to the ground side of the filter cap of the B+ supply.

This can be (A) fixed-bias operation with the cathode at ground, or using something like a 10-ohm current-sense resistor. This requires an adjustable low-noise -80 volt supply, and a meter across the current-sense resistor. Adjust the minus supply until you have 70 to 80 mA flowing through the 300B. The plate voltage also has to be reduced by 80 volts or so, since you’re not dropping 80 volts across a cathode resistor.

A potential source of instability for the fixed-bias circuit is having a regulated minus supply while the main B+ supply is unregulated. Small variations in AC power voltage can result in large changes in bias point. The solution is either have a tracking feature for the minus supply, or regulate the main B+ supply, which is not cheap. In other words, when both ends of the tube are controlled by plus and minus supplies, they should track each other, or be fully regulated.

The (B), self-bias option is dropping 80 volts across a 20-watt cathode resistor, and bypassing it with 200 uF of film capacitor. B+ will be around 425 to 480 volts at the top of the output transformer primary. Sonics will be strongly affected by the quality of the film capacitor since it is in the main current loop of the 300B. Adjustment of current flow is not necessary since the cathode resistor provides a degree of negative feedback for the DC current flow through the tube.