Freebird - 2 Watt 6AQ5 Guitar Tube Amplifier - Cathode biased Beam Pentode tube.

6AQ5 Guitar Tube Amplifier You Tube Video


The 6AQ5 Datasheet shows the heater current draw is 0.45 A at 6.3 Volts.
It Shows the Maximum Plate Voltage and Dissipation is 250 volts and 12 Watts.

To supply power to the heater circuit, a transformer, rated to at least (12AX7 heater filament)300 mA + (6AQ5 heater filament)450 mA + Led's or bulbs is needed.
Allowing 1000 mA at 6.3 volts, 6.3 volts x 1 A = 6.3 VA is needed.

Using an under-rated transformer, the transformer will at best get very hot with a big voltage drop under load. An over-rated transformer will stay cool and handle the load much better.

The 6AQ5 Datasheet shows operating information for a class A amplifier operating at 180 volts and another operating at 250 volts.

Operating at 180 volts the 6AQ5 plate will draw (Maximum-Signal Plate current) 30 mA and the 6AQ5 Screen will draw (Maximum-Signal Screen current) 4 mA.

To convert the 6.3 or 12.6 volts to 180 volts to power the tube plates and screen (B+), a transformer, rated to at least (12AX7 plate current draw) 2mA + (bleeder resistor current draw) 0.5 mA + (6AQ5 plate and Screen current draw) 34mA is needed.
Allowing 50 mA and 180 volts for the B+, 180 volts x 0.05 A = 9 VA.

To supply power to this amplifier, a 6.3 or 12.6 volt transformer, rated to at least (heater circuit) 6.3 VA + (B+)10 VA = 15.3 VA is needed.

The 6AQ5 Datasheet shows the (Load resistance) of 5500Ohms (5.5K), This is very similar to an Output Transformer typical for an EL84 (5K)(5Watts), So an EL84 Output Transformer will work a treat.

Sorting and wiring the Transformers
` This Transformer is rated to 3 A at 12 volts (36 VA) so can easily supply power to this amplifier. Connect a 2.5mm plug to the two 12 volt secondary taps of this transformer. It's AC so it doesn't matter which tap you connect to the centre pin on the plug. The two red wires are the (240 volt) primary taps, connect them up to the Active (Brown or Red wire) and Neutral (Blue or Black) wires on the power cable and solder the earth to the case. Put the transformer in an enclosure and your done.
This is a Creative power supply salvaged for an old pair of computer speakers and will work perfectly to supply power to this amplifier.


The two transformers below are rated at 20 VA so either could be used to supply power to this amplifier. Each has two sets of secondary taps that could be connected in series to double the voltage or connected in parallel and maintain the voltage. The E-core transformer has two 12 volt secondary taps and the toroidal transformer has two 15 volt but they can also have 6, 9, or 24 volt taps.


Because the 6AQ5 heaters use 6.3 volts, using 6 volts to power this amp will work best. If a 12 volt supply is used the voltage would need to be reduced to power the 6AQ5 heaters. If you were to use a 12CA5/FX5 tube(12.6 V heaters)(Liquid Ss) then using 12 volts to power this amp will work best.
The MT2086 Toroidal Transformer has 240V primary taps and two 15V secondary taps, Red-0V Black-9V and Yellow-0V Orange-9V. Connecting the secondary taps in parallel, Red-0V to Yellow-0V and Black-15V to Orange-15V, and applying 240V to the primary taps, will induce 15V on the secondary taps.
The voltage induced at the secondary taps is determined by the input voltage and the transformer turns ratio.
With 240V on the primary taps and 15V on the secondary taps, the turns ratio is 240 /15 = 16:1.
If 16V is applied to the primary taps, 1 volt would be induced on the secondary taps.

The turns ratio can also be reversed, applying 1 volt on the secondary taps (now the Primary taps), will induce 16 volts on the primary taps (now the secondary taps).
With the E-core Transformer we are applying just over 12 volts to the 15 volt taps of the toroidal, 12 volts x 16(turns ratio) = 192 volts. Should we see 192 volts at the toroidal transformer output? No Transformer is 100% efficient, there will be losses, an average transformer will be 90-95% efficient.  192 x .92 = 176 volts.



The 12 volt E-core transformer output is connected to the secondary taps (now the Primary taps) of the toroidal Transformer, the toroidal Transformer primary taps (now the secondary taps) are connected to the multimeter.
A full wave bridge rectifier uses four individual rectifying diodes connected in a closed loop to turn AC into pulsating DC.
176 volts AC is 176 volts RMS. RMS voltage stands for Root Mean Square and is 70.7% of the peak voltage.
The peak voltage is 1.414 x RMS voltage

An Electrolytic capacitor smooths the output of the rectifier and holds the voltage close to the peak voltage.

Switch the power off and the resistor will bleed the charge from the capacitor.

Mount all the Hardware, Tube sockets, Transformers, Jacks etc. It may be easier to mount the tag strips that hold the filter caps before the toroidal is mounted.

Electrolytic capacitors come in different capacities and voltage ratings as well as axial and radial configurations. For better filtering and bass response I like to use 100uf for the first filter cap, usually I use a 100uf/250 volt rated axial cap, but for Liquid Ss stage III a shorter radial cap is used to make room for the extra preamp tube and for Freebird a higher voltage rated radial cap is used.

The negative lead on the radial capacitor is shorter and marked with a line down the side of the capacitor, this lead is connected to the lug on the tag strip that is bolted to the chassis. A black insulating disc covers the positive end of the axial capacitor, the positive leads are connected to the other lugs on the tag strip.

Soldering is easier when the parts and soldering tip are clean and tinned. The Nos tag strips are dirty so sandpaper is used to clean them, then flux and then they are tinned.

The AC leads on the rectifier are marked with a backwards S, these leads are connected to the two insulated lugs that are side by side on the tag strip. The negative lead is connected to the lug that is bolted to the chassis and the positive lead is connected to the last lug. Copper alligator clips are used to draw heat from the soldering iron away from sensitive parts.
Light bulbs and Heaters are made of a thin section of tungsten wire called a "filament". The tungsten filament has electrical resistance and as a resistor it develops a voltage drop.

Connected in parallel on the left side is two Light bulbs and a 42 ohm resistor. Connected in parallel on the right side is three Light bulbs. When circuit elements are connected in parallel, the voltage across each circuit element is equal. The two sides are connected in series, with 12.5 volts across the circuit and a multimeter measuring the voltage drop across each side. When circuit elements are connected in series, the current is equal in all circuit elements.

A 12AX7 and these Light bulbs will draw 150 mA at 6.3 volts.  6.3V/.15A = 42 ohms. When Heated the resistance of a 12AX7 heater filament or these Light bulbs is around 42 ohms. On the left side only the 42 ohm resistor is conducting. On the right side only one Light bulb is conducting. With near equal resistance on each side there is near equal voltage drop on each side.
With all Light bulbs conducting there is a bigger load and current draw, dropping the supply voltage to 12.2 volts. Still with near equal resistance on each side there is near equal voltage drop on each side. The approximate resistance on each side is 42/3 = 14 ohms.
The measured current draw is 465 mA. 6.1V/.465A = 13.12 ohms. So the actual resistance on each side is closer to 13 ohms.
On the right side with two Light bulbs conducting the resistance is 42/2 = 21 ohms. On the left side the resistance is 13 ohms. With a larger resistance on the right side there is a larger voltage drop. The measured current draw is 360 mA. 7.9V/.36A = 22 ohms. so the actual resistance on the right side is 22 ohms and on the left side 4.1V/.36A = 11.5 ohms.

Each Light bulb on the right side is drawing 180 mA at 7.9 volts. Each Light bulb on the left side is drawing 120 mA at 4.1 volts.
A 12AX7 heater filament and these Light bulbs operate properly drawing 150 mA at 6.3 volts. Side effects from running them outside of correct voltages range from inefficient to detrimental.

A 6AQ5 heater filament operates properly drawing 450 mA at 6.3 volts. Two 12AX7 heater filaments and a light bulb or 42 ohm resistor in parallel will also draw 450 mA at 6.3 volts. So I will connect these two elements in series in this amp and be able to power it with 12.6 volts.
It's very difficult to connect more than one wire to either the power jack or a toggle switch so they are connected together in series and then to the two insulated lugs of a tag strip. Then any Toroidal taps or Heaters and LED's can easily be supplied 12 volt from these two lugs on this tag strip. The grounded lug on this tag strip is connected to the ground from the other two tag strips, the output jack ground and the negative leads from the filter capacitors.

The AC output from the Toroidal is connected to the two insulated lugs that are side by side on the tag strip. The positive output from the rectifier is connected across a dropper resistor to the first node on the voltage supply rail. A bleeder resistor is connected between the first node and ground. The red wire from the output transformer is connected to the first node, the blue wire from the output transformer is connected to the power tube plate (pin 5) and the secondary taps are connected to the output jack. The power tube Screen grid(pin 6) is connected to the second node.

The 12AX7 heaters are wired in parallel with the light socket and in series with the 6AQ5. The heaters are pins 3 and 4 on the 6AQ5 socket.
The Cathode at pin 2, is connected by the cathode resistor to ground.
The Control grid at pin 1, is connected by the grid leak resistor to ground and provides a zero volt reference to the grid for biasing.
With no tubes in there is no load on the circuit. The multimeter common is connected to ground so any measurements taken will be relative to this point. Plug in the power and just like the earlier test you can see around 244 volts at the rectifier output.

The tubes are in and there is an 8 ohm load on the output to protect the output transformer. The multimeter common is connected to one of the 12 volt supply nodes so any measurements taken will be relative to this point. Plug in the power and you can see zero volts here, with heater pin 4 directly connected here we will also see zero volts at pin 4. Across the heater filament to pin 3 we can see just over 6.3 volts. Pin 3 on the 6AQ5 is directly connected to pin 9 on the preamp socket, so these two pins will be at the same voltage potential. Then across the preamp heaters there is 12.3 volts. A 6 volt difference in voltage potential across the preamp heaters. This is directly connected to the other 12 volt supply node so it will be at the same voltage potential.

The multimeter common is connected to the first node, so any measurements taken will be relative to this point. Plug in the power. All current supplied to the tube plates and screen grid flows from the rectifier to the first node across a 100 ohm dropper resistor. The Voltage drop across this resistor is 3.48 volts. 3.48 volts divided by 100 ohms equals .0348 amps or 34.8 mA.

Only the current supplied to the screen grid and preamp tube flows across 3.3k resistor R12 . The Voltage drop across this resistor is 8 volts. 8 volts divided by 3300 ohms equals .0024 amps or 2.4 mA. with no preamp tubes connected yet, the screen alone is drawing 2.4 mA.

The multimeter common is connected to ground and with the tubes in there's a load on the circuit. At the rectifier output there's around 196 volts, so we have dropped around 48 volts with a load on the circuit. If we measure the first node we know the voltage will be around 3.5 volts under the rectifier output voltage and the second node voltage will be 8 volts under the first node voltage.

The cathode voltage is 9.2 volts, divided by cathode resistor value of 270 ohms will give the cathode current of 34 mA. The cathode current will include both the screen current and the plate current. So if we subtract the 2.4 mA screen current from the cathode current we will get the plate current of 31.6 mA.

To find the plate dissipation multiply the voltage drop between the plate and cathode by the current that flows between the plate and cathode.

With the Plate at 185 volts the voltage drop is 176 volts, times 31.6 mA plate current equals 5.6 watts Plate Dissipation.
The 6AQ5 datasheet give the maximum ratings as plate voltage of 250 volts and Plate Dissipation at 12 watts so this tube could be pushed much harder if you wanted to.
The 12AX7 is a nine pin miniature, twin triode tube.
Twin triode means it has two separate tubes inside one glass envelope.

Each triode has three electrodes:
At pin 1 is the Plate or Anode.
At pin 2 is the Control Grid.
At pin 3 is the Cathode.

The other triode is pins, 6-Plate, 7-Grid, and 8-Cathode.

For correct biasing the control grid needs a zero volt ground reference, at pin 2 this is provided through the two input resistors. A 33k resistor connects the input jack tip to the grid and a 1 meg resistor connects the input jack tip to ground. The Control grid at pin 7 is provided a ground reference by a yellow signal wire through the gain pot to ground.

With the power on there is 182 volts at the second node, and 177 volts at the third node. The plate load resistors are connected from the voltage supply to pin 1 and 6. At the plate's there is 131 volts and 136 volts, dropping 41 - 46 volts across each 91k resistor. 45 divided by 90 equals .5 mA. The same current the flows through the plate load resistors will flow through the 2k cathode resistors. The cathode resistors are connected from pin 3 and 8 to ground. 0.5 mA times 2k = 1 volt. So there will be around 1 volt at the cathode.

The AC Signal comes in at the input jack to the control grid on the first triode, it's picked up at the plate and sent to a voltage divider(gain pot). A capacitor is used to block the DC plate voltage but allow the AC signal to pass. The AC Signal is picked up at on the centre pin of the gain pot and sent to the control grid on the second triode, it's picked up at the second plate and sent to the control grid on the power tube.
There is a heater filament between pins 9 and 4, and another heater filament between pins 9 and 5.
Each heater draws 150 mA at 6.3 volts and can be wired in series to draw 150 mA at 12.6 volts or in parallel to draw 300 mA at 6.3 volts.
To calculate the minimum VA rating of a transformer to supply this current, multiply volts by amps. 0.15 A (150 mA) x 12.6 volts = 1.9 Volt-Ampere

The Plate of the 12AX7 also draws a current and can operate on anything from 12 volts to 300 volts, as the voltage at the plate increases so does the current draw.
The current draw for a 12AX7 is very small, at a plate voltage of 180 volts each plate will draw around 1 mA. 180 volts x 0.002 A (2 mA) = 0.36 VA.

A bleeder resistor will bleed the charge from the filter capacitors and stop a charge building up when the amp is switched off.
This resistor will also draw a current. Assume 180 volts across a 390k (R11) bleeder resistor.

Current = Voltage / Resistance Ohms Law Calculator
180 volts / 390000 ohms = .0005 A (0.5 mA) Current Draw
180 volts x 0.0005 A = 0.09 Watts (minimum resistor power rating) = 0.09 VA (minimum transformer rating).

To supply power to a 12AX7 tube, a transformer, rated to at least (1.9 VA(heater draw) + 0.36 VA(plate draw) + 0.09 VA(bleeder draw) ) = 2.4 VA is needed.

6AK6 Miniature Beam Pentode Showing the Suppressor grid has an internal connection to the Cathode.

Pin1  Control (Grid 1)
Pin2  Suppressor (Grid 3)
Pin2  Cathode
Pin3  Heater
Pin4  Heater
Pin5  Plate (Anode)
Pin6  Screen (Grid 2)
Pin7  Control (Grid 1)

Freebird 6AQ5 Parts List

NOS USA 6AQ5 Power Tube
Tube Sockets 9-pin & 7-pin

2 Watt 130r Vishay Metal Film Dropper resistor
2 Watt 390k Metal Film Bleeder resistor
2 Watt R12 3.3k, R13 4.7k, Metal Film resistors
2 Watt 270r Cathode resistor
1M Vishay Metal Film Pull Down resistor
39k Vishay Dale Grid Stopper resistor
(2)91k Allen Bradley Plate Load resistors
470k Metal Film Grid Leak resistor
(2)2k Carbon Comp Cathode resistors

Alpha Audio Potentiometer
W10G Bridge Rectifier
(2)0.022uf Mallory Coupling Cap 0.1uf, 2.2uf Cathode Caps 100uF 25v Sprague Cathode Cap (1)100uf, (2)22uf, 250v Filter Caps

Terminal strips 4 & 3 lug
SPDT Toggle Switch
DC 2.1mm Power Jack
(2)Mono 6.3mm Jacks