The Howler Monkey Push Pull 6AQ5 Guitar Amp

(C) 2012 G. Forrest Cook

Introduction

This amplifier is a push-pull design which features a pair of 6AQ5 beam power tubes driven by a 6J6 dual triode, all of which are 7 pin miniature tubes. It is the amp that I designed after my popular Squirrel Monkey one-tube amp. The only thing this amp has in common with the Squirrel Monkey amp is the choice of an electrical wiring box for the chassis and a monkey in the name. The 6AQ5 output tubes were commonly used for single ended audio output amplifiers in radios and televisions, they are plentiful and inexpensive on the surplus market.

The 6AQ5 output tubes are described in the RCA receiving tube manual as being lower voltage versions of the larger 6V6 beam power pentodes. The maximum undistorted power output from this amplifier is about 5 watts, which is plenty loud for a practice amp. The amp works well driving either a 10" or 12" guitar speaker. A big advantage of the lower power 6AQ5 output tubes is their ability to reach plate starvation, with it's inherently pleasant sounding distortion, at much lower volume levels. This can be an advantage for recording, and for those who wish to preserve their hearing. There are several military versions of the 6AQ5 including the 6005 and the 6094, the latter is a 10K-hour tube with ceramic spacers and thick glass.

The 6J6 dual triode is ideally suited for use as a long-tailed pair phase inverter since the two cathodes are tied together internally. The preamp and tone control recovery amps use a common 12AU7 dual triode, higher gain 12AT7 and 12AX7 tubes can be substituted for the 12AU7 if more gain is desired. The 12AU7 produces a cleaner and softer sound than the higher gain tubes. If one wishes to build the amp entirely with 7 pin tubes, two common 6C4 triodes can be substituted for the 12AU7. Another combination of 7 pin tubes that should work nicely would be to use a higher gain 6AV6 triode for the preamp stage and a lower gain 6C4 triode for the tone recovery amp. 6AV6 tubes tend to be microphonic, so you may need to hand-select a quiet tube.

Warning

This is a fairly high-level project. It takes advanced technician skills to construct the ampifier circuitry. Also, there are plenty of lethal high voltages inside of this amp including 120 VAC and 250 VDC. 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

Power On/Off
High Cut
Bass
Treble
Volume
Gain

Theory

The guitar input jack feeds into preamp VT1a, one of the 12AU7 triodes. The preamp output drives a Baxandall tone stack, which feeds the second 12AU7 triode tone recovery stage. The tone recovery triode feeds the volume control pot. The high cut switch places a low pass filter into the circuit after the preamp stage to roll off the highest frequencies of the signal.

The output from the volume control pot drives VT2a, half of the long-tailed pair phase inverter circuit. The common cathode in VT2 is pulled low through a current regulator circuit. The current regulator is simply a LM317L variable voltage regulator wired as a constant current circuit, it is set to limit the current to 3.5mA. The current regulator allows both halves of VT2 to run at the same quiescent current, while maintaining excellent balance of the output levels. One often sees a resistor in place of the current regulator in this circuit. With a resistor, one has to choose either constant gain or equal tube current, but not both at the same time. Max Robinson's article on phase inverters inspired this design, it is also similar to the phase inverter in the Fender twin reverb amp.

The two out of phase signals from the phase inverter stage drive the grids of the two 6AQ5 pentodes. The 6AQ5 pentodes are wired in a push-pull configuration and drive the output transformer, a P-T291 standard replacement type. The 6AQ5 pentodes are configured with a fixed-bias cathode arrangement.

The amp features an adjustable negative feedback (Gain) control that feeds some of the speaker signal back to the input of the phase inverter. The gain control adjusts the amp's compression and linearity and adds to the range of sounds that the amp can produce. When the gain control is at the maximum resistance, the amp is essentially running with no negative feedback, which should please all of the Fender 5E3 fans out there. The gain control could also be labeled a "presence" control.

A few extra components in the circuit deserve explanation. The 100nF capacitors on the cathodes of VT1a, VT1b and VT3/VT4 improve the frequency response of the electrolytic capacitors that they are in parallel with. The 4.7M resistor on the grid of VT1b eliminates scratchy sounds from the bass control by presenting a constant but tiny connection to ground. The 3nF capacitor on the negative side of the current regulator prevents an oscillation in one of the phase inverter triodes. Lastly, the 10nF disk capacitors on the output tube screen grids and output transformer center tap stabilize the circuits by bypassing high frequency signals to ground.

In the power supply section, the transformer high voltage winding is sent to a center tapped full wave rectifier consisting of two 1N4007 diodes. The high voltage DC is dropped through a totem-pole array of resistors and capacitors to produce the voltages used in the amp. The 6.3VAC filament supply is rectified by four 1N4002 diodes and filtered by a 470uF capacitor to produce around 7.5VDC. The DC voltage drives an amber LED through a 470 ohm current limiting resistor.

There is an 0.4 ohm resistor in series with the filament winding on the power transformer, this resistor is used to tweak the filament voltage to exactly 6.3VAC. Without it, the tubes tend to run a bit hot. You may want to adjust or eliminate this resistor to compensate for your local power grid voltage. The two 220 ohm resistors across the filament winding reduce the amount of hum that the amp picks up from the filaments.

Parts Sources

The 6AQ5 and 6J6 tubes are almost always available on eBay for reasonable prices. The knobs, power transformer and output transformer were purchased from Antique Electronic Supply. The jacks, potentiometers and resistors were purchased from Mouser and the electrolytic capacitors were purchased from Jameco. Everything else came from your author's junk box. None of the parts should be too difficult to find from the above companies or other sources.

Construction

A 70 square inch electrical switch box was used for the amp's chassis, a piece of sheet metal was cut and drilled to make the bottom plate. Three pieces of wood were cut to form a C-shaped wooden case that is attached to the bottom sheet with four screws. Stick-on rubber feet were applied to the corners of the bottom sheet to prevent scratching. Inexperienced builders are advised to use a larger box since wiring is quite tight with this enclosure.

The jacks, power switch, LED and control potentiometers were all mounted in the center of the box knockouts. The moving side of each box knockout was secured to the chassis by drilling a small hole next to the knockout and inserting a small 6-32 screw and nut. After the knockouts were secured, the holes in the center of each knockout were punched and drilled. An alternative to doing all of this work on the knockouts would be to fashion front and back plates out of aluminum or double-sided printed circuit stock. Knock out all of the plugs, screw the two plates to the chassis and drill the jack and potentiometer holes in the plates. The power cord was secured to the box with a standard romex cable clamp.

The tube sockets were all mounted inside of the knockout holes on the top of the box. The 9 pin tube required drilling two small 4-40 screw holes in the chassis. The 6AQ5 tubes were installed in 1/2" knockout holes by using a jeweler's rat-tail file to make semi-circular holes for the mounting screws. The 6J6 socket was installed in a small piece of aluminum, the aluminum plate was installed into a 3/4" knockout hole. Flat washers were used on the bottom of the 7 pin socket mounting holes to insure a good physical connection to the box.

A number of three and five pin terminal strips were screwed into the bottom of the box The internal wiring was done between the tube sockets, terminal strips, jacks and potentiometers. It is a good idea to wire the filaments with twisted pair wiring, do this before wiring the rest of the circuitry. All of the amp's ground connections were tied together with pieces of bare #22 gauge tinned bus wire. This type of ground is not quite as good as a star-ground system, but the amp's hum level is very low.

In the amp's layout, the connections from the transformer's high voltage winding to the rectifier diodes was too close to the treble control's contacts. This caused hum to be picked up and amplified. This problem was fixed by putting a wide piece of braided copper wire between the high voltage AC signal and the tone control. The braid was covered with plastic tubing and was grounded on both sides. If you build the amp in a larger box, it would be a good idea to move the transformer and power supply components farther away from the sensitive preamp section.

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 and howl away. Enjoy the warm vacuum tube sound! The Fuzznikator push-pull tube distortion circuit makes a nice companion for this amp.


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