The Squirrel Monkey One Tube Guitar Amp

(C) 2010 G. Forrest Cook

The finished Squirrel Monkey amp and power supply boxes.

The Squirrel Monkey being prototyped, this mess actually worked.

Introduction

After building the full-featured Hammonator 2RVT and Lil' Tiger amps then the simpler Spartacus amp, your author decided to go in a completely different direction and design a minimalistic class-A guitar practice amp, the Squirrel Monkey. What makes the Squirrel Monkey interesting is the choice of a three element 6AF11 Compactron tube which packs a high gain triode, a medium gain triode and a power pentode into one envelope. The amp was originally called "the Chimp" in reference to the Fender Champ, but that name has been used for other amps, so enter "the Squirrel Monkey", a diminutive primate. The Squirrel Monkey amp is essentially a "Champ in a bottle".

The Squirrel Monkey is a simple design with only two controls, volume and tone (high cut). The amp only outputs 0.8W of audio power, but that is loud enough to work as a practice amp or a recording amp. The tone is surprisingly nice and the hiss is very low. The 120VAC input power was measured at 13 Watts. The Squirrel Monkey was constructed in two sections, the power supply and the amplifier circuitry. Each of the sections was built onto a pair of common 4"x4" electrical utility boxes. By adding a multi-pin output connector, the power supply can double as a test supply for other vacuum tube projects.

Warning

This project uses high voltages including 120VAC and 200VDC. The project should only be taken on by someone who has experience working with high voltage circuitry. The power cord should always be removed when working on the amp, the circuitry is designed to discharge the capacitors when power is removed, but it's always a good idea to short out the electrolytic capacitors before working on the amp.

Connections

Power Input - grounded 120VAC
Guitar Input - High Impedance
Speaker Output - 8 ohms

Controls

On/Off
Volume
Tone

Theory

The guitar input jack feeds directly into class-A preamp VT1a, the single high gain triode section in the 6AF11. The preamp output feeds into the volume control pot, and that drives the power amp circuitry, VT1b and VT1c.

VT1b is the first gain stage in the power amp, it includes negative feedback input on the cathode and a high cut R-C network on the plate to lower RF sensitivity. VT1b feeds the output stage, pentode VT1c. Both VT1b and VT1c are running in class-A mode. The pentode is cathode biased to the tube's optimal 24mA by the 160 ohm resistor. The 180K resistor from the transformer output to the VT1b cathode adds a small amount of negative feedback for stabilizing the amplifier. The 3nF capacitor across the output transformer keeps the output stage from oscillating in the RF region.

The tone control is somewhat non-standard, it involves an adjustable high pass network within the power amp's negative feedback loop. The negative feedback inverts this function to make an adjustable high cut filter. The tone control's response is quite useful, musically.

There are a lot of Fender 5E3 fans out there who think that negative feedback is a bad thing in any form. From my experiments, I have found that the secret to using negative feedback is to use a minimal amount. That greatly reduces the production of bad-sounding distortion, while preserving the good-sounding distortion and punchy presence. Bad-sounding distortion is visible on an oscilloscope as square waves and/or radio frequency oscillations, while good-sounding distortion is visible as the soft rounding of the tops of the waves as a result of plate starvation.

A number of different junk box output transformers were tried in this circuit with minor variations in the performance. A 3.5K to 8 ohm transformer produced the most power output of the transformers that were tested. Since this is a relatively low power amplifier, it is not nececessary for the output transformer to be perfectly matched to the tube, although the volume will be loudest when the match is ideal. Transformers with input impedances from 2.5K to 10K should work and a common 5K to 8 ohm transformer would be a good choice if you cannot locate a 3.5K to 8 ohm unit.

The power supply section uses a standard 6.3VAC/2 Amp filament transformer for lighting the tube filaments up, the two 220 ohm resistors cancel out any filament-induced hum. If you use a filament transformer with a center-tapped output, ground the center tap and eliminate the 220 ohm resistors.

The high voltage transformer uses a bit of trickery that has been seen elsewhere on Internet amplifier designs. A 28VAC/1 Amp (1/2 Amp would work) transformer with dual primaries is rewired so that one primary winding became a secondary winding and the 28V secondary voltage was added to the 120V secondary to produce 148VAC. This feeds into a bridge rectifier to produce the high voltage DC, which is around 210V with no load. A more common 24VAC dual primary transformer should also work fine here.

The high voltage DC is filtered through a capacitive/inductive pi filter to remove hum and produce the B1+ supply. The B1+ supply runs at around 180VDC with the tube plugged in. The B1+ supply is further filtered through an R/C filter to produce an isolated B2+ supply for the first two gain stages, this runs at around 175V. The 2M resistor discharges the capacitors when power is removed.

Parts

The project was built almost entirely with junk box parts that were scrounged from old radios and TVs. Most of the parts for this amplifier project can be purchased from mail order electronic suppliers such as Mouser, Digi-Key and Jameco. The 6AF11 tube and duodecar (12 pin) Compactron socket can usually be found on eBay for reasonable prices.

The output transformer, power supply choke coil and high voltage capacitors are available from Antique Electronic Supply or other amplifier parts sources. Jameco.com part 101864/XFR2424Q should be suitable for the power transformer. Antique Electronic Supply sells the Hammond P-T154M 2H, 200mA choke and Hammond P-T1750F output transformer, which should both work for this project. Mouser.com is a good place to find diodes, resistors, jacks and potentiometers. See The Strat Monger for other parts suppliers.

Construction

The amplifier and power supply sections were built onto cover plates of standard 4"x4" electrical utility boxes. A chassis punch was used to cut out the 1-3/16" hold for the 6AF11 compactron socket. It would be a good idea to add more physical protection around the vacuum tube such as U-shaped rack handles or a top. Numerous other holes were drilled to mount the transformers and a few terminal strips. Most of the amp wiring was done between the tube socket and the terminal strips in a fairly high-density manner.

Three chassis knock-outs were removed on the front and back of the amp's utility box chassis. Pieces of double-sided circuit board were cut to fit into these spaces and holes were drilled into the circuit board pieces for the jacks and controls. Aluminum pieces would work just as well here. Standard Romex style cable clamps were used on the power supply box for securing the power input and output wires.

Beginners would be advised to use a roomier enclosure. An old 5-tube AM radio would make a spiffy box for this project. The speaker would probably need to be upgraded. A car stereo speaker would be a good choice since they are small and are designed to handle more power than a radio speaker.

Use

Plug the amp into a guitar speaker. Plug an electric guitar into the input. Tweak the knobs for a good sound. Play the guitar. Enjoy the warm tube sound. The simple design produces very little hiss. This is a fairly high gain amplifier and it can overdrive quite easily. If you want a clean sound, be sure to keep the volume control fairly low. Turn it up for more fuzz. If you liked the Squirrel Monkey amp, check out its successor, the Howler Monkey.

This amplifier also works well as piece of test equipment. It can be used to trace signals through amplifiers or other audio equipment. If you intend to use the amp this way, add a 50nF (0.05uF) capacitor in series with the input to prevent any external bias voltages from reaching the grid of VT1a.


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