New Rev B. CCDA PCB
The GlassWare CCDA 9-pin stereo PCB: This FR-4 PCB is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through heavy 2oz copper traces. In addition, the PCB is lovingly and expensively made in the USA. The boards are 5.5 by 6 inches, with five mounting holes, which helps to prevent excessive PCB bending while inserting and pulling tubes from their sockets.
Each PCB holds two CCDA (Constant-Current-Draw Amplifier) line-stage amplifiers; thus, one board is all that is needed for stereo unbalanced use (or one board for one channel of balanced line-stage amplification). The CCDA board makes building a standard-setting line stage amplifier a breeze. This assembled board with a chassis, volume control, selector switch, power transformer, and a fistful of RCA jacks is all that is needed.
Introducing an Old Friend: The CCDA
I have resurrected an old trusted circuit that is tube-stingy, the CCDA line-stage amplifier. CCDA stands for constant-current-draw amplifier. The CCDA consists of a grounded-cathode amplifier directly cascading into a cathode follower. So what; what's so special about this obvious pairing? Its special status lies in the details.
The circuit’s constant current draw feature is highly desirable, as the signal amplification will not alter the amount of current being sourced from the power supply. Consequently, by not perturbing the power supply, the design of the power supply is greatly simplified and the need for expensive parts is lessened, parts such as big reservoir and exotic bypass capacitors.
In order to achieve constant-current draw, in the CCDA circuit each triode splits the B+ voltage, so each sees the same cathode-to-plate voltage; and each works into the same load resistance with the same idle current. Thus, total current draw for the two stages equals twice that of either stage individually.
Now here is where the constant-current draw enters the picture. Both the grounded-cathode amplifier and the cathode follower are in voltage phase, but not current phase. For example when the grounded-cathode amplifier sees a positive going input signal, its plate current increases, which increases the voltage developed across the plate resistor, which in turn swings the plate voltage down.
This downward swing is then relayed into the cathode follower's grid, whose cathode follows its grid, reducing the voltage across the cathode follower’s cathode resistor, which in turn causes the cathode follower to decrease its current conduction to the same degree that the previous stage's current increased, as they share the same load resistance. Thus, the net current draw from both stages remains unchanged, although the cathodes and plates might be swinging hugely.
Why make a CCDA PCB? Since we are facing some bleak economic times, using fewer tubes can only help. Besides, the CCDA sounds much better than you might imagine. And there are projects that only require a simple-but-good tube line amplifier, say, the frontend of an SE power amplifier or adding some tube magic to DAC.
Redundant Solder Pads: The CCDA board holds two sets of differently-spaced solder pads for each critical resistor, so that radial and axial resistors can easily be used (radial bulk-foil resistors and axial film resistors, for example). In addition, most capacitor locations find many redundant solder pads, so wildly differing-sized coupling capacitors can be placed neatly on the board, without excessively bending their leads.
The boards hold two coupling capacitors, each finding its own 1M resistor to ground. The idea here is that you can select (via a rotary switch) between C1 or C2 or both capacitors in parallel. Why? One coupling capacitor can be Teflon and the other oil, or polypropylene or wax and paper…. As they used to sing in a candy bar commercial: “Sometimes you feel like a nut; sometimes you don't.” each type of capacitor has its virtues and failings. So, use one type of coupling capacitors for old Frank Sinatra recordings and the other for string quartets. Or the same flavor capacitor can fill both spots: one capacitor would set a low-frequency cutoff of 80Hz for background or late night listening; the other capacitor, 5Hz for full range listening. Or if you have found the perfect type of coupling capacitor, the two capacitors could be hardwired together on the PCB, one acting as a bypass capacitor for the other.
Power-Supply-Decoupling Capacitors: The CCDA PCB provides space for two sets of capacitors to decouple both CCDA gain stages from the B+ connection and each other. This arrangement allows a large-valued electrolytic capacitor and small-valued film capacitor to be used in parallel, while a series voltage-dropping resistor completes the RC filter. (As an option, in place of the series resistor, an off-board choke can be used for each channel.)
Tube Selection: The CCDA is quite flexible, as a CCDA can be built using many different tubes. For line-stage amplifiers, the 6CG7, 6DJ8, 12AU7, 12BH7, ECC99 are the obvious choices, but other twin-triodes can be used. For example, a 12AX7 will yield a gain close to 50 (up to 70 with bypassed cathode resistor), which would be suitable for a microphone preamp (with an input transformer perhaps); a 6H30 could withstand a high idle current and deliver a low output impedance that could drive long capacitance-laden cables. In other words, the list of possible tubes is a long one: 6AQ8, 6BC8, 6BK7, 6BQ7, 6BS8, 6DJ8, 6FQ7, 6GC7, 6H30, 6KN8, 6N1P, 12AT7, 12AU7, 12AV7, 12AX7, 12BH7, 12DJ8, 12FQ7, 5751, 5963, 5965, 6072, 6922, E188CC, ECC88, ECC99… The only stipulations are that the two triodes within the envelope be the same and that the tube conforms to the 9A or 9AJ base pin-out. For 95% of line-stage amplifier applications, however, the list is fairly short: 6CG7, 12AU7, 12BH7, and ECC99. Do try to think outside the 6 and 12 volt tube box. For example, NOS RCA 6CG7s sell for about $30, but the same tube with an 8V heater can be bought for $5.
PCB includes 16-page user guide