Dr Good 12 Volt AC - 1 watt 6AK6 Pentode Power tube amplifier.

Using an old cigar box as an enclosure all the parts are mounted starting with the tube sockets.
The input jack is mounted close to the preamp socket, the output jack close to the power tube socket.

A #6 lug was mounted using the screws on each tube socket with a ground bus wire connected between these two lugs. The 12 volt supply negative lead is connected to this ground as well as the filter caps negative leads and pins 4 on the preamp and power tube sockets (heater filaments). Also connected to this ground bus is the input and output jack ground pins, the pot ground pin and the three cathode resistors and bypass capacitors.



The power adapter supplies 12.4 volts. This will work perfectly to supply the two 6.3 volt preamp tube heater filaments, if they are connected in series.
The power tube is a single 6.3 volt heater filament and will draw around 150mA.
A module was used to convert the 12.4 supply voltage to 6.0 volts.

For The High Voltage Power supply I am using is a Nixie 1363 power supply. A miniature flyback boost converter that operates from 2 to 16VDC input, with an output of 150 to 200VDC set by a single resistor to ground. No input reverse polarity protection is provided. The Enable pin must be driven at all times, it may not be unconnected when power is applied to the module. The value of R-adjust can range from open, which will supply 150V, to short-to-ground, which will supply 200V.

I tried a few resistor values and list the power output. 68k (164 Volts), 34k (172 Volts), 22k (178 Volts), 10k (187 Volts).


A 12 volt lead from the power jack connects to the centre pin on the toggle switch.
The brown wire from the toggle switch connects to pin1 (Voltage in) & pin3 (Enable) on the power supply.
The green wire connects the ground pin on the power supply to the ground bus at the negative lead of the first filter cap.
The red wire connects pin 11 (High Voltage out) on the power supply to the First Node, node(A) at the positive lead of the first filter cap.
Pin7 (R-adjust) is jumpered to pin8 (ground) with a 10k resistor to give 187 volts at the first node.

Node(A) supplies power to the plate of the power tube via the red wire to the output transformer.
The blue wire from the output transformer connects to the plate of the power tube.
The negative lead of the first filter cap (that supplies power to the power tube) is grounded at the cathode of the power tube.

A 390k bleeder resistor in parallel with the first filter cap will bleed the charge from the filter caps and stop a charge building up when the amp is switched off.
An 0.1uf capacitor is placed in parallel with the first filter cap is for high-frequency decoupling.

(cathode voltage) 9.1 volts / (cathode resistor) 560 ohms = (cathode current) .0162 Amps
(cathode current) 16.2mA - (Screen current) 2.2mA = (Plate current) 14mA
(plate voltage) 181 volts - (cathode voltage) 9.1 volts) x (plate current) .014 Amps = (Plate Dissipation) 2.4 Watts

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

6ak6.gif.png (32282 bytes) 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.