Dr Good 240 Volt AC - 1 watt 6AK6 Pentode Power tube amplifier.

The switches are power and standby, The chicken Head nob is preamp bias control with the sweet spot being between 1.5-2k, lowering the value warms the bias.
The black dial is the volume, the white Jack is the input, the metal Jack is the output.



The Power Transformer connects straight up to the 240 volt mains.
There is a black wire connected from the 0 volt tap on the primary side of the power transformer to the neutral terminal of the AC inlet.

From the active terminal of the AC inlet is a red wire going to a fuse holder, from the fuse holder across a 10 ohm resistor to the power switch.
Then from the power switch to the 240 volt tap of the power transformer.

The 10 ohm resistor is there so I can measure the current, it is not necessary otherwise.
I connect a multimeter to each side of the resistor with the amp powered up I drop 0.46 volts across this resistor

Current = Voltage / Resistance,     0.46 volts / 10 ohm = 0.046 Amps (46mA).     Ohms Law Calculator
Power = Voltage x Current, 0.46 volts x 0.046 Amps = 0.02Watts. Minimum resistor power rating.
VA = Voltage x Current,  240 volts x 0.046 Amps = 11VA Minimum transformer power rating.

The black wire connects back to the earth terminal of the AC inlet and connects to the ground lugs on all the tag strips. The Diodes are connected to the high voltage taps and form a Full Wave Bridge Rectifier, where they meet at the yellow wire we have DC.

On the 6.3 volt tap I have soldered one leg of an LED, the other leg is soldered to a 500r resistor, and the other leg of the resistor is soldered to the 0 volt tap. The resistor limits the current flowing through the LED. 6.3 volts / 500r = 12.6mA. If I wanted the LED brighter I could add another resistor in parallel to make a 250r resistor.  6.3 volts / 250r = 25.2mA.

An effective way to reduce heater hum is to create a virtual centre tap by soldering two 150r resistors onto the 0 and 6.3 volt heater taps, then connect them to ground, or even better an elevated voltage potential, like the cathode pin on the power tube.  Current draw 6.3 volts / 300r = 21mA.

The yellow wire connects to a standby switch wired in before R11, the bleeder resistor, so when it is on, it doesn't isolate the bleeder resistor from any of the filter caps. From the standby switch across R10 the dropper resistor, (500r) 2 x 1k resistors in parallel. With the circuit loaded, (heaters and plates connected), there is 10 volts dropped over R10, the 500r dropper resistor. The total HT current draw is 10 volts / 500r = 20mA.
The control grid has a big influence on how much current flows in the 12AX7 Tube,

As the voltage at the grid goes more negative the current through the tube decreases until no current flows and the anode current is cut-off. The grid voltage may continue to swing more negative, the tube will remain in cut-off and the output waveform will be clipped. Depending on the circuit a 12AX7 will cut-off anode current when the grid is around - 4 volts, the grid may go to - 8 or - 12 volts, but current stopped flowing in the tube at - 4 volts, and the output waveform started clipping. Biasing a tube closer to cut-off Distortion results in less power dissipation and is called cool biasing.

As the voltage at the grid goes more positive the current through the tube increases until maximum current flows. Grid current Clipping occurs when we try to drive the grid beyond 0v.  Electrons being drawn from the cathode get attracted to the grid rather than the anode causing a forward grid current into the grid. This current causes a voltage drop across the resistance that is in series with the grid, making it hard to drive the grid beyond 0v.  It is the grid signal that is being clipped, while the tube amplifies what appears on its grid normally. The grid voltage may continue to swing more positive, no more current can flow and the output waveform will be clipped. Biasing a tube closer to Grid current Clipping results in more power dissipation and is called warm biasing.

To set the bias point of the grid to - 2 volts relative to the cathode. the Control grid is held at 0 volts via the grid leak resistor and 2 volts is applied at the cathode via the cathode resistor (Cathode Biased).

An AC sine wave swings both positive and negative relative to the bias point, so if the signal is 3 volts, on the up-going signal the voltage at the grid will be 1 volt and on the down-going signal, the voltage at the grid will be at - 5 volts. 4 volts peak to peak is enough to swing a 12AX7 from cut-off Clipping to Grid current Clipping.
The two green wires from the preamp socket go to the cathode resistors and the two blue wires go to the plate load resistors.

The tag strips with two lugs and a ground lug lend themselves well to connecting up the radial filter capacitors.
The negative leads on the caps are soldered to the ground lug and then you could connect four plate resistors if you have two preamp tubes.



With a 470r Cathode resistor the plate is 201 volts and the cathode is 9.1 volts,
Cathode current = 9.1 / 470 = 19.4mA, - Screen current 2.4mA = Plate current 17mA
(201 volts - 9.1 volts) x 17mA = Plate Dissipation 3.25 Watts

With a 560r Cathode resistor the plate is 206 volts and the cathode is 9.7 volts,
Cathode current = 9.7 / 560 = 17.3mA, - Screen current 2.2mA = Plate current 15.1mA
(206 volts - 9.7 volts) x 15.1mA = Plate Dissipation 2.96 Watts

With a 760r Cathode resistor the plate is 210 volts and the cathode is 11.3 volts,
Cathode current = 11.3 / 760 = 14.9mA, - Screen current 2mA = Plate current 12.9mA
(210 volts - 11.3 volts) x 12.9mA = Plate Dissipation 2.56 Watts

Maximum Plate Dissipation for a 6AK6 is 2.75 Watts.
With the 470r and 560r Cathode resistors the amp exceeds Maximum Plate Dissipation.
R10 the dropper resistor is increased to 560r to lower the voltage

6AK6 Miniature Beam Pentode   Showing the Suppressor grid tied to the Cathode, some tubes have an internal connection.

Heater ....................................6.3 V/0.15 A
Plate Voltage .....................................180V
Plate Current.....................................15 mA
Max Plate Dissipation ......................2.75 W
Cathode Bias Resistor .....................560 r
Grid No. 2 Current ............................2.5 mA
Load Resistance .................................10K
Power Output (Max) ..........................1.1 W
Total Harmonic Distortion ..............10 %		 Pin1  Control (Grid 1)
Pin2  Suppressor (Grid 3)
Pin3  Heater
Pin4  Heater
Pin5  Plate (Anode)
Pin6  Screen (Grid 2)
Pin7  Cathode

 


The 12AX7 twin triode nine pin miniature tube.
Twin triode means it has two separate tubes inside one glass envelope.

Each triode has three electrodes: plate, grid and cathode.
At pin 1 is the Plate or Anode.
At pin 2 is the Grid.
At pin 3 is the Cathode.
The other triode is pins, 6-Plate, 7-Grid, and 8-Cathode.

There is a heater filament between pins 9 and 4, and another between pins 9 and 5. The heater circuit is often omitted from circuit diagrams, it is not considered to be a ‘working’ electrode as it plays no part in the audio circuit. To wire the heaters for 6.3 volts (heaters in parallel) you tie pins 4 & 5 together.