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Written Sept. 2002
1st Rev. Oct. 2002
2nd Rev. Nov. 2002
3nd Rev. Dec. 2002

(A Vacuum Tube "Multimedia" Amplifier)
by Fred Nachbaur, Dogstar Music ©2002


It's been said that "Necessity is the mother of invention." This could be extended to, "New tube circuits depend on the tubes you have on hand." In my case, it seems that I've accumulated an inordinate number of 6X8 and 6X8A triode-pentode hybrid tubes; although this device is similar to the common 6U8 and 7199 tubes commonly used in audio circuits, the 6X8 has the disadvantage of having both cathodes internally tied together. Perhaps that's why I ended up with so many of them; no-one else could figure out what to do with them either!

I therefore set out to find an application. The target project was a small stereo amplifier for a friend and fellow musician's computer system. I had an ideal enclosure, decided on a simple 6AQ5-based push-pull topology, and found some output transformers from a fellow enthusiast on I had exactly six tube socket positions available; therefore, the preamp and phase splitting functions had to be accomplished using a single tube for each channel.

With a 6U8 or similar tube, the pentode portion could be used as a preamplifier, and the triode as a cathodyne ("concertina") phase splitter. However, the 6X8 with its pesky common cathode precludes such a circuit. Time for some experimenting.


Push-pull output stages require two separate input signals, of approximately equal amplitude but opposite phase. In other words, as the one signal is increasing (becoming more positive), the other one must decrease (become more negative). The out-of-phase signal is therefore simply an inversion, or mirror-image, of the in-phase signal.

Let's have a look at one of the oldest phase splitter designs, a simple amplifier circuit with a rather fancy name: the "Paraphase". This is really nothing more than a low-gain common-cathode amplifier stage; the fancy name comes about from its function, rather than any circuit details per se. The schematic below shows a workable paraphase using the venerable 12AX7 twin triode:

Classic Paraphase
The Classic Paraphase Circuit

The in-phase signal is applied from the plate of the preamplifier, V1A, to the grid of the top output tube, via coupling capacitor C1. The grid resistor to this output tube is split into two sections, R2 and R3, forming a voltage divider (3:1 with the values shown in the schematic). This divided-down voltage is applied to the grid of the paraphase stage, V1B. This stage gets its grid bias from a portion of the cathode resistor, R6.

The AC plate load of the paraphase stage essentially consists of the parallel combination of the plate load resistor R5 (100k as shown here), and the grid resistance of the lower output stage R4 (330k), or about 77k. Note that this is approximately 3 times the value of the cathode resistor R7 (27k). Under the assumption that open-loop stage gain is quite large, and that therefore cathode voltage will approximately equal applied grid voltage, it follows that the voltage on the plate will be about 3 times the grid voltage, or about the same level as the voltage on the plate of the preamplifier but with the opposite phase.

An example of a commercial design using the classic paraphase is the Lafayette circuit used as the starting point for the "Li'l 4x4" project.


Now let's build a paraphase by using a grounded-grid topology instead of common cathode. The input to the phase inverter is applied at the cathode, allowing us to use the 6X8 with its internally joined cathode connections.

Cathode-coupled Paraphase
The Cathode-Coupled Paraphase Circuit

As before, the in-phase signal appears at the plate of the preamplifier stage (the pentode section of the 6X8) and is applied to the grid of the top output tube. However, unlike the classic design, a shared cathode resistor provides the coupling between the preamplifier and the phase inverter (the triode section of the 6X8).

Note, however, that the inverted output signal at the plate of the triode will be of considerably less amplitude than the in-phase signal. With the 6X8 and the component values shown, this will be about 1/3; this is why the plate load of the pentode preamplifier stage is split into two sections, giving a 3:1 attenuation of the in-phase signal in order to match to the amplitude of the inverted signal.

Note also that the shared cathode resistor introduces some local feedback to the pentode stage, so overall gain is only about 10 (20 dBv). Still, this is enough gain for many applications; in the circuit shown, only about 0.7 volts RMS is required for full output. Given that today's sound cards can output up to several volts from the line output, it would certainly be adequate.

The circuit also has a certain simple elegance; the entire preamp and phase splitter section consists of only five resistors and three capacitors (the two coupling caps and a screen bypass). If desired, part of the plate load of the pentode preamplifier stage can be made adjustable, such that AC balance can be adjusted exactly (perhaps by adjusting for zero AC voltage on the shared cathode resistor of the output stage).


For the target application, it was deemed desirable to have additional gain in order to offset the insertion loss (about 10 dB) of a simple "Big Muff" style of single-pot tone control, to give the end user some degree of control over frequency response. So the Cathode-coupled paraphase was modified slightly, as shown in the schematic below:

Enhanced Cathode-coupled Paraphase
The "Enhanced" Cathode-Coupled Paraphase Circuit

All we've done here is to sample part of the in-phase output signal at the grid of the top output tube, and applied it to the grid of the paraphase stage. Most of the coupling is still via the cathode as before, but by sampling a very small portion (about 1:33) of the in-phase signal, the gain of the paraphase is increased by over 10 dB.

There are a few interesting aspects to this modified circuit:
  1. The parts count is exactly the same as the pure cathode-coupled circuit described earlier.

  2. The final topology begins to resembles the classic paraphase, but does not require the additional coupling capacitor

  3. The gain of the pentode stage is actually increased also. To understand why, consider first what happens when a signal is applied to the grid of the pentode. An increasing (more positive) grid voltage causes an increase in plate current, and therefore an increase in current through the common cathode resistor. This represents a negative feedback term, since it will tend to decrease the grid-to-cathode voltage.

    Now consider what happens when we inject part of the in-phase output signal into the grid of the paraphase. This will be out-of-phase with the input signal, and will therefore cause a decrease in shared cathode current. It therefore opposes the negative feedback element, and actually represents a positive feedback term. In fact, if the injection ratio is made too high, the circuit will actually oscillate!

  4. Given that there's no such thing as a free lunch, what's the cost? The answer - increased harmonic distortion, since we've essentially cancelled out the negative feedback formerly introduced by the shared cathode resistor. Essentially, we've restored performance to be virtually identical to the classic topology originally considered; the difference is that we've met the design goal of being able to use tubes with shared cathodes. At the same time, we retained a relatively high gain of about 40 (32 dB) per phase, or 80 (38 dB) plate-to-plate.
A similar approach could be used for dual triodes with shared cathodes, such as the common 6J6. I'll leave it to you to work out the details.

A more rigorous analysis of the circuits presented here, along with measured harmonic distortion behaviour, may follow at a later date.


Here's the complete schematic of the little tube multimedia amplifier, fleshed out with the power supply and input tone-control circuitry. Again, the parts on hand dictated the power supply topology; the relatively high power transformer voltage dictated the choke-input filter. (Even though the presence of C9 makes it look like a capacitor-input filter, its low value makes it have a neglible effect on output voltage. It was added as a tweak to help reduce 60 Hz. harmonics.)

The Tube Multimedia Amplifier
The Final "Multimedia" Amplifier

How does it sound? In two words, Just Ducky. The amplifier has the warm, gutsy sound characteristic of tube amplifiers with little or no global feedback. The tone control circuit puts a mild dip in the midrange, which tends to enhance that warmth - especially when used with efficient speakers such as the old theatre speakers that the end user plans on using with the amplifier.


For the Marantz retrofit project I expanded this paraphase idea a bit further. The circuit shown below sports the following refinements:

Changes as of December, 2002:

The circuit as published originally ended up being somewhat unstable, taking out the 10-ohm cathode protection resistors apparently at whim. As near as I can tell, the amplifier would break into ultrasonic oscillation under certain conditions. The following changes stabilized it:

The Improved GE/CC Paraphase
The Improved GE/CC Paraphase

Here is the power supply for the amplifier. Note how the voltage quadrupler circuit allows the use of the original power transformer designed for the solid-state amplifier that this circuit replaces. The winding that formerly serviced the preamp/ tuner sections now provides the low-current negative "B" supply for the preamp tubes, and the bias for the output tubes.

An added transformer salvaged from another old solid-state stereo provides the low-voltage DC for the pilot lamps and the preamp tube filaments, allowing the original low-voltage line to be reserved for the TT15 filaments. Another winding on this added transformer powers the tone-control preamps and tuner section of the receiver, further reducing the demand on the original transformer.

Power Supply
Power Supply

As mentioned earlier, this circuit shows a noticeable amount of 2nd harmonic content in the shared output tube cathode current. Since this represents a common-mode term, most of it will be cancelled out in the output transformer. However, it's interesting to note that a spectrogram shows this circuit's response to be a bit atypical for push-pull amplifiers.

Harmonic Distortion
Harmonic Distortion

This plot was taken at the 1/2 voltage point (1/4 power), which I've deemed to be fairly representative of where the amplifier will be operating most of the time. Note that while the THD figure of 0.34% is quite good for such a simple circuit, the unusual thing is that the most significant term is the second harmonic, at approximately -50 dB.

It's therefore no surprise that the amplifier has a sonic "signature" very much like a good single-ended design. I can only surmise that this is due to the inherently un-symmetrical nature of this phase-splitter, combined perhaps with slight differences in transconductance between the two valve sections. Better matching between sections and perhaps real-time distortion monitoring whilst adjusting the symmetry pot would probably reduce this even further.

Intermod Distortion
Intermodulation Distortion

Here is a plot of intermodulation distortion, using a test signal of 60 Hz. at 0 dB, combined with 7350 Hz. at -6 dB., and gain set such that the total of both components produced 5 watts at the output (1/2 of maximum power). No big surprises here, the highest IMD product is at a reasonable -40 dB. This is about 6 dB better than the otherwise similar "Li'l 4x4", which sports about the same amount of global negative feedback. The improvement is probably partly due to the better output transformers, and partly because of more linear output tubes.

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