03 July 2012

The Second Shoe Drops

Octal CCDA Rev. A
The Noval CCDA has a brother. When I laid out the new version of the noval CCDA, I also brought the Octal CCDA PCB into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ and heater power supplies and to add a second set of output coupling capacitors.

CCDA B+ Power Supply
The old CCDA PCB held only one power supply RC filter per channel. The new Rev A boards hold a pi filter after the rectifiers (C7 & R17 & C8) and then each channel gets its own RC filter (R9 & C5||C4).

Resistor R17 can be replaced by a choke or a tube rectifier, such as I did with my Unbalancer project, which bestowed a slow turn on and made use of the 5Vac winding that would have otherwise gone to waste.

CCDA Heater Power Supply
The old CCDA PCB's heater power supply circuit was a modest affair. The Rev A PCB holds a much more flexible heater power supply, which can turn 6.3Vac into regulated 12Vdc when its reservoir capacitors and rectifiers are configured as a voltage doubler.

The preferred power supply voltage is 12V, even with two 6.3V tubes. Why? Using 12Vdc as the regulator output voltage greatly unloads the regulator and increases the dropout-voltage threshold. Wait a minute, you cannot use 12Vdc with 6SN7s! Sure you can, just place the heaters in series. Of course, a 6Vdc (or 6.3Vdc) heater power supply can be setup (as long as all the tubes used have 6.3V heaters). Bear in mind that a 6.3Vac secondary cannot create a 6.3Vdc regulated output. Why not? Rectifier losses and the low-voltage regulator, although an LDO design, must have some extra headroom to operate. A 6Vdc or 6.3Vdc heater power supply requires at least an 8Vac heater winding.

Three jumpers allow the heaters to be strung together like Christmas lights or placed in parallel. To place the two tubes heater elements in series, use jumper J2; in parallel, J1 & J3.

Octal CCDA Tube Types
I know that 99% of Octal CCDA users will opt for the 6SN7 as the tube of choice. And it is a good choice. There are other types worth checking out, such as the 8SN7, 12SN7, 12SX7, 18SN7, and 5692. I have been extolling the use of oddball heater voltage for a long time. A decade ago, you could buy a first-rate 8SN7 or 12SN7 for a few dollars, whereas the 6.3-volt version of the exact same tube sold for $50 to $100. No more. The word has gotten out and although 8SN7 tubes are still cheaper than 6SN7 tubes, but not by nearly as much.

The 12SX7 is great tube and it sells for about $30 today. It was designed to work under low B+ voltages, as found in aircraft. Building a relatively low-voltage octal CCDA, with a B+ of 120Vdc, would make a fun project. A 120Vac secondary will rectify up to about 165Vdc, which can then be reduced to 120Vdc by the RC filters on the PCB. The big advantage that this low B+ voltage offers is that the power-supply capacitors are much greater in capacitance at a 200V voltage rating, rather than at 400V.

The Chinese brown-base 6SN7 is fine little tube, which is why it is used in so many $5,000 tube preamps and power amplifiers. At $10 each, they are a bargain.

A 6SN7, unlike the 12SN7, requires more B+ voltage to sing. I used a 240Vac secondary, which rectified up to about 330Vdc after the rectifiers, which fell to 260Vdc at the tubes. Since the triodes in the CDDA circuit see half of the B+ voltage, each triode sees 130V from its cathode to its plate. With the 22k plate and cathode resistors that I used, the plate dissipation is only 0.75W and the idle current for each triode is a tad less than 6mA.

I used the same positive feedback arrangement that I used in my Tetra Sans PS and Noval CCDA, so the signal gain came in at +23dB (or 1:14). And once again, I am hearing the same spritely sound that the Noval CCDA with positive feedback produced. And once again, this CCDA is dead quiet.

The only thing that I don't like about the octal version is the height of the tubes, which makes placing this CCDA in a 3-inch tall enclosure impossible. On the other hand, this CCDA would look great with the tube protruding from the chassis. How is that possible? I have placed redundant regulator pads on the bottom of the PCB, so all the parts, save for the tube sockets, can be placed on the bottom.

The Rev. A Octal CCDA PCB and kits are available at the GlassWare/Yahoo store now.

Next Time
More tube circuits, more PCBs.

//JRB