The Octal Line Preamplifier, part 2 : Power Supply

Hi!

As promised in the first article about the Octal Line Preamplifier I will present the power supply schematic today.  Similar to the Octal Phono stage, there are two possibilities. One using a choke and a very simple one using only resistors and capacitors for the smoothing.

Both power supply options are very similar to the power supplies of the Octal phono stage. They use the 6BY5 as rectifier tube. The 6GL7 can be AC heated. Since we are dealing with much higher signal levels as in a phono stage, no DC heating is necessary and still the preamp is hum free. This is the schematic of the version with choke input filter as is used in the first version which I built:

Very straight forward, a hybrid rectifier bridge with four UF4007 diodes augmenting the 6BY5. I used two of them in series in each leg (4 total) to increase peak inverse voltage capability. With choke input filters some very high peak inverse voltages can occur at turn on.

The rectifier feeds the input choke, a 40Hy unit, followed by a 47uF electrolytic cap. Two RC sections with 500 Ohms and 47uF each provide sufficient filtering for hum free operation. The 100k resistor serves as bleeder resistor. Together with the two other 100k bleeders, one in each channel of the signal section, critical current is always guaranteed even if the signal tubes are unplugged. This avoids a steep voltage rise which could otherwise occur in case the tubes are unplugged and the PSU is turned on.

The rectifier tube and the signal tubes are heated from separate windings. The 6GL7 heaters are referenced to ground via the two 100 Ohm resistors.

The 6AX5, which I presented last month, could be used as an alternative to the 6BY5.

If you want to save the cost of the choke, use this simpler PSU:

The same rectification scheme, but only RC sections are used for smoothing. This version needs a lower secondary voltage to get the desired B+. Also only two UF4007 are needed. In this schematic signal tubes and rectifier share the same heater winding. Therefor the heater winding is elevated to a positive voltage by connecting one side to a tap of a voltage divider between B+ and ground. This is not very elegant and separate heater windings would be preferable. But I used such a scheme already with the Octal phono stage and it works fine. This second option is not tested, so some minor adaptions might be necessary if you build it.

Here a view of the backside of the preamp showing the rectifier and the connectors:

Best regards

Thomas

Tube of the Month : The 6AX5

Hi!

The tube of last month was a directly heated rectifier suitable for power amps. This month I am presenting another rectifier, but an indirectly heated and rather small one, usable for small amps or preamps. The 6AX5.

The 6AX5 is a glass tube on an octal base, hence it is also named 6AX5GT. It has a common cathode and two plates in one bottle which makes it usable as a full wave rectifier with a power transformer with center tapped secondary. Of course it could also be used in a bridge configuration together with two individual diodes, either silicon or tube, for example 6AX4.


The pinout is pictured on the left. Based on it's electrical parameters it would be usable in power supplies designed for the 6BY5 unless the cathodes need to be separate. An advantage of the 6AX5 over the 6BY5 is the lower heater current of 1.2A versus 1.6A. As such it would be perfect as an alternative in the power supply of my recently shown version of the Octal Preamplifier. With it's octal base it would fit right into the theme of that preamp. See the datasheet for all technical details about this tube. This is a great little rectifier which seems to be rarely used nowadays. Availability is good and prices are low.


The 6AX5GT can be found from all the major tube manufacturers. Here for example from Tung-Sol:

Two versions from Sylvania. One in military packaging and the other in the commercial Sylvania box:

RCA:

General Electric:

I have seen two construction styles of the 6AX5. One has two cathodes (internally connected) :

And the other a single (larger) cathode in the center between the two plates:

Let's have a closer look at both these versions. First the one with two cathodes. Removing the glass:

Cutting off the top mica disk allows a look into the plates:

A close up showing the heater wire in the cathode sleeve:

Removing one of the plates:

A close up of the cathode:

Glass removed from the other type:

Bending away one of the plates reveals the cathode:

A great rectifier which deserves to be used in audio applications.

Best regards

Thomas