"Spunky" Tube Amplifier

My interest and fascination with inexpensive "radio" valves using series- connected line- operated heaters didn't stop with the completion of "Li'l 4x4". At the same time, my lovely wife was a bit envious of that little rock-speckled box that sits in our room. So what better project than to build her very own tube guitar amplifier?

"SPUNKY" Shaza Jane
by Fred Nachbaur, Dogstar Music
©2001

The resulting project sports up to about 8 watts of output power, which when connected to an efficient speaker is plenty of power for small rooms. For larger rooms, the unit is easily mic'd into the house PA system.

Overview

"Shaza Jane's 'Spunky'" bears a strong resemblance to "Li'l 4x4", cosmetically, structurally and electronically. However, there are some significant differences, first of which is its approximately doubled output power. This is accomplished by using two paralleled pairs of 35C5 valves in push-pull, instead of the single pair of 30A5/HL94's.

The 35C5 has an inately "warmer" sound than the 30A5/HL94, without being brassy like its cousins the 25C5 and 50C5. This warmth is augmented by not incorporating any global feedback (i.e. including the output stage tubes and transformer). In fact, there is not even any local feedback (except for a negligible amount introduced by very low-value cathode resistors, used for measuring cathode current during setup.) The absence of negative feedback around the output stage also gives much softer saturation characteristics, for that warm tube fuzz-distortion. However, the amplifier sports enough system gain to allow for a much harsher "lead" distortion if so desired.

Like the 30A5/HL94, the 35C5 lends itself very well to fixed grid biasing, and somewhat higher than usual plate voltage. Individual bias controls for each output valve allows adjustment for clean, symmetrical output even with significant variances between tubes. Crossover distortion is not a problem with this design, even though the output tubes are operated solidly in class AB₁ mode (20 mA cathode current each tube, at 160-200 V plate voltage). As with "Li'l 4x4", the screen voltage is zener-regulated for best linearity. The overall result is about double the output power from each tube (as compared to their typical use as class A amps in table radios), while remaining well within their maximum ratings.

Unlike "Li'l 4x4's" long-tail-pair phase inverter, "Spunky" uses a more traditional "cathodyne" or "concertina" phase splitter. Similarly, the driver stages are single-ended rather than push-pull. This reduces the number of tube sections needed, and continues the more essential design approach used for this project.

Spunky's tube complement is as follows:

9JW8 (pentode/triode) - input stage (pentode) and second amp (triode)
12AU7 (dual triode) - third amp, phase splitter
4 x 35C5 (beam pentode) - PPP output stage

You've probably never heard of the 9JW8, or even of its 6-volt equivalent the 6JW8. Don't feel bad, I wasn't familiar with it either. For this project I wanted to use a pentode/triode pair, since I like pentodes as input amplifiers and wanted an extra triode section for an added gain stage. I considered the 7199, but it would not have worked well in my line-operated heater scheme. Besides, I tend to shy away from tube types whose price is inflated by "manufactured demand". (So don't expect to see a 300B design from me any time soon!)

The next likely candidate was the 9U8 (9 volt @ 0.3A equivalent to the 6U8). This type of tube is commonly used in later stages, and has a solid reputation as a reliable, sturdy device. However, as an input stage for a high-gain amplifier it's absolutely awful; the sound isn't bad, but the microphonics are unbearable. Having the chassis anywhere near the speaker caused a horrible screetchy feedback to occur at higher gain settings. So the 9U8 was thrown back into the pool, perhaps to be caught again at a later date for a driver/phase-splitter application.

I then tried several different pentode/triode tubes, most of which even had the same pinout. I hit the jackpot with the 9JW8. This tube was designed for use in television horizontal circuits, but is also a phenomenal audio tube. Microphonics are very low, and it sports good gain and a clean, warm sound. I consider it anywhere as desirable a valve as the premium 7199.

You might also note that we have a one more stage of amplification as compared to the Li'l 4x4. This added stage is desirable because the two triode stages (the 9JW8 triode section and half of the 12AU7) are relatively low-mu (low gain), and because having that added stage gives us enough gain to allow local cathode feedback on all three preamplifier stages. The result is that the signal remains clean and crisp throughout, with enough headroom to cause overdrive distortion only in the PA stage.

Having a bit of extra gain available also allows incorporation of a simple tone control, of the single-control bass/treble variety. Values were chosen as a tradeoff between insertion loss (about 8 dB) and usable control range. Combined with the tone controls on the instrument itself, this affords a wide range of possible guitar tone settings.

Finally, a compression scheme similar to that used in the "Li'l 4x4" extends the amplifier's usefulness even further. For the sake of simplicity, values were chosen for a "best response" compromise, with a simple in-out toggle switch provided on the front panel. The switch does little at low volume settings; however, as output power approaches maximum, the gain of the input stage is progressively reduced. This can give the apparency of being considerably louder, without going into distortion. When distortion is achieved at very high volume and input level settings, it has a restrained, almost "dry" quality as a result of compression.

Shaza Jane's "Spunky" happily installed in her case
alongside her "Merton" Les Paul - style guitar


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