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Wednesday, September 11, 2013

The Octal Line Preamplifier, part 2 : Power Supply


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



  1. Thomas, cool little preamp.

    the schematics say ‘UF1007’. I thought ‘hey, why not 4007?’ but in the text you say it _is_ the latter.

    maybe you want to update your drawings—clearer for your intended audience.


  2. Hi Peter,

    yes UF4007, typo in the schematic. I updated them. Thanks for noticing!


  3. I saw the 6BY5 cathode connected to one of the heater leg. Is there a reason for that? Do we want indirectly heated cathode?

    1. The 6BY5 is indirectly heated. This connection keeps the heater to cathode voltage in spec