CIRCUIT FEATURES

THE POWER TUBE:

The power tube, V2 is sometimes referred to as the output tube. V2 is the final stage of amplification and its purpose is to amplify for power (voltage x current) where V1 (EF86) and V2 (12AX7) were focused on voltage amplification. The signal enters the power tube at pin 5 (grid) and leaves the power tube via pin 3 (plate). It then goes to the output transformer (OT) which is mounted on the backside of the chassis.

Like with the guitar’s pickup, magnetism can be used to generate electricity in a coil. You can also do the reverse and pass electricity through a coil and generate magnetism. The amplifier’s output transformer uses both of these principles to pass alternating current (AC) from its primary (input) winding to the iron core as magnetic flux and on to the secondary (output) winding as alternating current.

THE OUTPUT TRANSFORMER:

The output transformer’s windings are really just two wire coils wrapped around an iron core. The input, or primary winding uses electric current flowing through it to generate a magnetic field or flux. This magnetic field fluctuates with the guitar AC signal voltage and is captured by the transformer’s iron core. The captured magnetic flux flowing through the core generates a voltage and current in the secondary winding. You can alter the voltage and current from primary to secondary by changing the ratio of coil wraps from primary coil to secondary.

The output transformer’s primary takes in a high voltage, low current signal (high impedance signal) and puts out a low voltage, high current signal (low impedance signal) through the green wire to the speaker jack and on to the speaker.

The alternating current audio signal flows through the speaker’s voice coil which generates a magnetic field. The voice coil is simply a single wire wrapped into a coil as shown below. The magnetic field created by the voice coil is either attracted to or repelled by the speaker’s magnet. Positive voltage generates a repulsive magnetic force and the speaker coil and cone moves outward away from the speaker magnet, negative voltage generates an attractive magnetic force and pulls the speaker cone inward.

The speaker cone alternates between moving outward and inward as the signal voltage alternates between positive and negative. For every guitar string movement there is a corresponding speaker cone movement.

This magnetic attraction and repulsion moves the voice coil and speaker cone back and forth to create air pressure waves that our ears perceive as sound–the sweet sound of electric guitar. When the speaker cone moves outward a positive air pressure wave is created and when the cone moves inward a negative (low pressure) wave trough is generated. 

These air pressure waves move our ear drums in and out. The ear drum movement is translated into neuron activity which is sent to the brain where pleasure is created, thus electric guitar + guitar tube amp = pleasure 😉

THE POWER SUPPLY CIRCUIT:

Once again, I do not believe in building amps without good ventilation (inside combo cabs or head cabs) nor do I want to have really hot tubes send upward rising heat into a chassis filled with electrolytic capacitors and other sensitive components. Voltage and heat are the two killers of electrolytic caps, and voltage and heat are in abundance in tube amps.

By design, my open well sized (14” wide x 11” deep) chassis provides the best ventilation possible without going overboard. The spacing of the tubes and the components is optimized to allow good heat ventilation and use short wiring runs. And, I also use a NOS GZ34 rectifier tube that has a “slow startup” capability to prevent the first electrolytic filter capacitor in the power supply to get a large spike in voltage when first turning on the amp. The slow startup function takes a period of time before voltages become large and this alone prevents stress in the circuit components over time.

These 2 things plus the use of long-life high quality Mundorf electrolytic capacitors (audiophile grade and not low cost guitar amp grade) provide as much protection as is possible within this amplifier design without resorting to the use of an even larger chassis and huge film capacitors with high enough voltage ratings. Also with the GZ34, expensive NOS tubes benefit substantially by allowing voltages to climb gradually before the amplifier can generate its normal signal levels.

The electricity for this amplifier comes in via the high quality IEC module (Furutech FI-03) located in the rear corner of the amp. It then travels to the on/off switch where electricity to the power transformer is controlled via this switch. There is no need for a standby switch (particularly in this amp) and inclusion of one actually is detrimental to the amps longevity (check Google for more information). The GZ34 rectifier also really helps to alleviate any problems that might occur if using a “fast start” rectifier tube (i.e. typical 5A4) as an alternative. Even though the NOS GZ34 costs a large amount of money, it should last for a very long time and possibly the life span of its owner so it is money well spent.

The power transformer has the black pair of wires for the 120 VAC power entry and it has 3 pairs of secondary wires: 1 pair provides the 5 VAC needed to power the rectifier tube, 1 pair provides the 6.4 VAC needed for the heater circuit in the 3 tubes, and 1 pair of wires that provides the 350VAC wire that powers the amp via its connection to the first power supply filter capacitor.