Liquid Ss
WARNING Safety first! Building your own tube amp can be a rewarding experience and it is nice to know how to bias your amp. Working inside a tube amplifier can be dangerous, tube amps have lethal voltages that can kill you and a little common sense can go a long way when working around dangerous voltages. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure or get yourself killed.

If you print out the Schematic you can then highlight connections that you make in the circuit.

An amp is a collection of smaller circuits, the heater circuit, the high voltage circuit, the power supply, some Led circuits and the signal path. If you tackle one circuit at a time it will make it simple and easy to diagnose faults. You Don't need to build the entire amp before you power it up. I have posted a 20min YouTube Video on the Matchless Hotbox Page showing the amp being powered up from when I first start putting it back together.

First thing you need to do is build your power supply, I use a 12 volt transformer removed from cheap or broken powered computer speakers. Dr Good or a typical amp will be powered directly off the mains voltage. To isolate the builder as much as possible from the mains voltage Liquid Ss is powered by a 12-12.6 volt AC power source.

Now you can use this 12 volt supply to make an LED circuit, you can put a switch in the circuit, you will need to put a 470 ohm resistor in series to limit current flow through the LED or it will pop. Connect one leg of the LED to a heater pin or one of the 12 volt leads, Connect the other leg of the LED to the 470 ohm resistor. Then the opposite lead of the resistor connects back to the other heater pin or 12 volt lead.

A 12AX7 has two 6.3 volt heaters and a 12CA5 tube has one 12 volt heater. For 12 volt heaters (Liquid Ss) the12AX7 heaters are wired in series, the heaters are joined internally at pin 9 so if you connect one wire from the power source to pin 4 and the other wire from the power source to pin 5 you will have them connected in series. With the tubes in and powered up you will have 6.3 volts AC between pin 4 and pin 9 and 6.3 volts AC between pin 5 and pin 9.

For 6.3 volt heaters like the Hotbox the 12AX7 heaters are wired in parallel, pin 4 and pin 5 are tied together with one wire from the power source connected here and the other wire from the power source connected to pin 9. With the tubes in and powered up you will have 6.3 volts AC between pin 4 and pin 9

The tag strips will support the filter capacitors and the rectifier, put them in position and drill mounting holes. Tag strip(1) will be where the negative leads from the capacitors are connected. I have woven a piece of wire on this tag strip to connect all the pins together. The #1 lead connects to the negative pin on tag strip(3) that the rectifier is mounted on. The #2 lead will connect to the sleeve/ground on the output jack. Tag strip (2) is where the positive leads from the capacitors connect. The positive end on the capacitor is the end with the black insulated cap, the negative end is the silver metal cap.

Manufacturers of 12CA5 tubes recommend a maximum of 130 Volts at the plate (pin7). The output voltages from the power transformer will vary between similar transformers, mains voltages will vary and current draw from similar tubes will vary.
To lower the plate voltage increase the value of the voltage dropper resistor R10 (100r).
If you are only getting say 90 volts at the plate and want to increase the plate voltage you could reduce the value of R10 or remove it altogether.

The total current for the B+ can be calculated by measuring the voltage drop across this resistor.
If 14.2 volts is drop and a 330r resistor is used.
Current = Voltage / Resistance Ohms Law Calculator
14.2 volts / 330r = (.043 amps or 43 mA) - Total Current Draw.
14.2 volts x .043 amps = 0.61 Watts - Minimum resistor power rating.

To keep the Screen grid (pin6) under 120 volts, aim for around 105 volts, R12 (680r - 2k) resistor is used. From Node(A) to Node(B) across R12 flows the Screen and Preamp current, If there is .0085 amps(8.5 mA) flowing and a 4.7k resistor is used, around 40 volts will be dropped.

4700 ohm x.0085 amps = 40 Volts.
40 Volts x .0085 amps = 0.34 Watts Minimum resistor power rating.

Build the amp in two stages, first build and sort the power supply, then wire up the signal path. Always have a load on the output transformer.

The rectifier is mounted on Tag strip (3). On the top of the rectifier you will see the markings for - & +, and S & S. The negative leg of the rectifier will go to the pin on the tag strip that bolts to the chassis. The positive leg of the rectifier connects to the pin on the tag strip where Node(A) connects. The two Sides marked S & S is where we connect the Toroidal Transformer secondaries, so connect these two legs of the rectifier to two pins of the tag strip that are beside each other.

AC comes out of the Toroidal Transformer secondaries, the two orange (or blue wires), at Node(A) where the first filter capacitor connects it is rectified to approx 130 volts DC loaded(tubes in and connected) or (160volts unloaded). At Node(B) where the second filter capacitor connects, resistor R12 will drop the voltage to approx 100-120 volts DC. Node(C) supplies the B+ to the plates on the preamp tube Via R3 and R4 the plate load resistors. R13 controls the voltage here.

Connect the two twelve volt windings from the Toroidal Power Transformer in series by joining the red and white wires together. (Or yellow and black), depending on what transformer Jaycar has in stock. The yellow (or red) wire will go to the centre pin on the on/off switch and the purple (or orange) wire will go to the centre pin of the 2.5 mm power jack. There are pictures of wiring the other transformer is in Stage III build. To complete the primary circuit the outside pin of the 2.5 mm power jack will connect to the outside pin on the on/off switch.

The red wire from the Output Transformer connects to the plate of the power tube. The blue wire connects to Node(A). The yellow wire connects to the tip of the output jack and the black (Common) wire connects to the sleeve/ground on the output jack.

A Bleeder resistor R11 (270k - 390k) is added from Node A to ground. This will bleed the charge from the caps and stop a charge building up when the amp is switched off. If Node A is at 120 volts, 120 volts will be dropped across this 390k resistor.

120 volts / 390000 ohms = .0003 amps Current Draw
120 volts x .0003 amps = 0.036 Watts Minimum resistor power rating.

The Hotbox heaters are wired in parallel for 6.3 volts. The Liquid Ss Heaters here are wired in series and will be powered from a 12 volt AC source. The yellow twisted wires are connected to the centre pin of the 2.5 mm power jack and the centre pin on the on/off switch and are going to pins 4 & 5 of the preamp tube, from there the red & black twisted wires are going to the pins 3 & 4 of the power tube.

R16 & 17 two 150r resistors that connect from the heater pins to ground form a Virtual centre tap to reduce heater hum.

To measure the voltage when the amp is switched on, and check the high voltage filter caps for residual voltage when the amp is switched off. First put the (-) lead of the Multimeter to ground. Use the other lead (1 hand only) - it's the current across the heart by way of the arms that kills you, to Probe the positive side of high voltage filter caps at node A, B, & C.

How to discharge the capacitors - The filter capacitors can retain a charge after the amp is shut off. To drain the capacitors to make sure your amp is safe to work on you could use a jumper cable consisting of alligator clips on the ends of a 1k 5watt resistor. Connect one end to ground, then the other end to the positive side of the high voltage filter caps or install a bleeder resistor which will drain the capacitor automatically, after the amp is turned off. Don't depend on this resistor, always measure the voltage in the capacitors before working on the amp.

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: 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.

The numbering of the tube socket pins looking from the bottom of the socket starts from the gap and goes clockwise.

WARNING Pay Attention to polarity on Cathode Filter Capacitors. They may catastrophically fail if voltage is reversed. The capacitor will short circuit following the loss of the dielectric material, and with sufficient current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst. Polarity is very clearly indicated on the case. A bar across the side of the capacitor usually with an arrow being used to indicate the negative terminal. The negative terminal lead of an electrolytic is shorter than the positive lead also.
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The 39k Grid stopper resistor connects between the tip of the input jack and pin 2 of the preamp tube socket. Larger values reduce the signal going to the grid and roll off more highs. Smaller Values will Strengthen the signal.  The grid stopper resistor when combined with the capacitance between the triodes grid and cathode forms a low pass filter. This low pass filter has a cut-off frequency well above the audio range, filtering out radio frequencies that could cause parasitic oscillation in the stage, while leaving audio frequencies intact.
A 1 meg metal film resistor connects from the sleeve of the input jack to the tip of the input jack. The preamp plates are connected to the high voltage B+ at node C through R3/R4 two 100k Carbon Composition Plate load resistors.
Carbon comp resistors have their resistance vary with the voltage across them, making the signal gain non-linear, adding harmonic distortion.

R1 the Grid leak resistor sets the input impedance of the stage and provide the ground (zero) voltage reference to the grid, while preventing the audio signal from being shunted to ground. They are usually up to 1 meg in value, if you used a 1k resistor most of your signal would use this path to ground. The higher the value, the less signal goes to ground.

Coupling Capacitors C1/C2 with a value from .01 - .033uf are wired between the stages and block the high voltage DC source but allow the AC signal to pass through to the next stage. The Coupling capacitor combined with the Grid leak resistor form a high-pass filter, a high-pass filter is an easier path for any frequency above the cut-off frequency, while any frequency below the cut-off frequency will be attenuated. If we were to double the size of the Coupling capacitor or the Grid leak resistor, we would halve the cut-off frequency (reduce it by an octave) allowing more base frequencies through.

On Liquid Ss or a fender Champ the pot wiper (centre pin) to the pot ground leg form a variable Grid leak resistor, at full volume the grid leak resistance will be 1 meg. Turn it down so the grid leak resistance is 500k and we double the cut-off frequency reducing the base. Turn it down so the grid leak resistance is 250k and we double the cut-off frequency again and reduce the base further, and so on. Giving a treble boost at low volume.

The preamp Cathode resistors R8/R9 (2.4k) are wired from cathode pins to ground and will set the bias on the preamp tubes.
Lowering the values increases the current, warming the bias.

The Cathode bypass capacitors are wired in Parallel with the Cathode resistors, they effect the low frequency cut-off point and will reduce or eliminate the effects of cathode current feedback. A capacitor blocks DC but allows an easy path for AC, any AC signals on the cathode are bypassed to ground, so AC current does not flow in the cathode resistor and the DC bias voltage remains unchanged. The capacitor smooths out changes in cathode voltage, helping to hold the cathode voltage constant and preventing cathode feedback.

A capacitor allows greater current flow at high frequencies than it does at low frequencies. If the Cathode bypass capacitor value is small (partially bypassed), Matchless Hotbox uses (.22uf), then only high treble frequencies will be boosted while lower frequencies will not. The stage will have maximum gain at high frequencies and minimum gain at low frequencies, producing a treble boost. A fully bypassed stage will have maximum gain at all frequencies. The capacitor must be large enough, to smooth out the lowest frequencies of interest. Doubling the value of the cathode capacitor halves the cut-off frequency. Every frequency above the cut-off will be boosted giving control over the gain between lower and upper frequencies.

22uF boosts almost all bass frequencies, which can sound dark. Lowering the value rolls off bass frequencies which cuts low end mud and can tightens up a flabby bass. 1uF and 5uF are used in some high end amps, .68uF are used in some Marshall amps, The Bitmo kit uses 3.3uf or removes them altogether and Fromel uses a 2.2uf

The power tube Cathode resistor (62r-100r) will set the bias on the power tubes.
Lowering the values increases the current and warms the bias.
A Cathode Capacitor with a value from 100uf-200uf is wired in Parallel with this Cathode resistor.

Tube specifications will vary slightly between different manufacturers
Here are Data sheets for the TungSol 12CA5 and TungSol 12FX Power Tubes.

12CA5 Miniature Beam Pentode

Heater........................................12.6 V/0.6 A
Plate Voltage...................................130V Max
Plate Dissipation ..........................5.0 W Max
Grid No. 2 Dissipation................1.4 W Max
Plate Current..........................31 mA - 37 mA
Grid No. 2 Current....................4 mA - 8 mA
Grid No. 1 Peak AF Voltage...........4.5 volts
Cathode Bias Resistor............................100 r
Load Resistance............................3.5K - 4.5K
Power Output..............................1.1 W - 1.4 W
Total Harmonic Distortion ...................6 %






Control Grid, Grid No. 1






Control Grid


Screen Grid, Grid No. 2


Plate (Anode)

12FX5 Miniature Beam Pentode

Heater........................................12.6V/0.45 A
Plate Voltage..........................................130V
Plate Dissipation..........................5.5 W Max
Grid No. 2 Dissipation...................2W Max
Plate Current..........................................30 mA
Grid No. 2 Current ............................10 mA
Grid No. 1 Peak AF Voltage...........3 volts
Cathode Bias Resistor.....................62 r
Load Resistance................................3K O
Power Output ..................................... 1.3 W
Total Harmonic Distortion ............ 8 %..

To Bias the 12FX5 Power tube we need to find the Plate current, the Cathode current will include both the Screen current and the Plate current.
So if we subtract the Screen Current from the Cathode current we will get the Plate current.
Ground the (-) lead of the Multimeter, Use the other lead (1 hand only) to Probe the plate (pin7) and cathode (pin1) on the power tube socket and node A & B from the Schematic

To get the Cathode current measure the voltage drop across the cathode resistor and divide this by the value of the cathode resistor.
Cathode Voltage (2 volts) / Cathode Resistor (62 ohms) = Cathode Current (.032 amps).

Probing the Plate (pin7) at 100 volts. Probing the Cathode (pin1) at 2 volts.

Probing Node(A) at 125 volts.

Probing Node(B) at 85 volts.
From Node(A) at 125 volts to Node(B) at 85 volts, the Screen and Preamp current flows through R12, with 40 volts dropped across a 4.7k resistor.
40 Volts / 4700 ohm resistor = .0085 amps.
40 Volts x .0085 amps = 0.34 Watts. Minimum resistor power rating.

The Kit uses a lower value 1k resistor at R12 to increase the voltage at Node(B)
The Current will still be .0085 amps
.0085 amps x 1000 ohms = 8.5 volts. so we will drop 8.5 volts
8.5 Volts x .0085 amps = 0.072 Watts. Minimum resistor power rating.

From Node(B) to Node(C) we have the Preamp Current and a Voltage drop of 3 volts over 4.7k resistor R13.
3 Volts / 4700 ohms = .0006 amps.

Screen and Preamp current (8.5 mA) - Preamp current (.6 mA) = Screen Current (7.9mA)

Cathode current (32mA) - Screen Current (7.9mA) = Plate Current (24.1mA).

To find the plate dissipation multiply the voltage drop between the Plate and the Cathode, by the current that flows between the plate and cathode.
Plate (100 volts) - Cathode (2 volts) x Plate Current (.024 amps0 = Plate Dissipation (2.35 Watts).

Most 12FX5 tube Manufactures Recommend a Maximum Plate Dissipation of 5.5 watts. (should not be exceeded)
Exceeding this value may dramatically shorten the life of the power tubes.
No two power tubes are exactly the same. Some may move more current than others.
Increasing the value of the Cathode resistor will lower the Cathode current, Cooling the power tube.
Decreasing the value of the Cathode resistor will raise the Cathode current, Warming the power tube.

I Swapped out the 12FX5 tube to Bias the 12CA5 and got these Voltages,
Plate (80 volts), Cathode (2.48 volts), Node(A) 123 volts, Node(B) 98 volts.
Cathode Voltage (2.48 volts) / Cathode Resistor (62 ohms) = Cathode Current (.04 amps).

From Node(A) 123 volts to Node(B) 98 volts, we drop 25 volts over a 4.7k resistor.
25 Volts / 4700 ohm resistor = .0053 amps Screen and Preamp current.
Screen and Preamp current (5.3 mA) - Preamp current (.6 mA) = Screen Current (4.7mA)

Cathode current (40mA) - Screen Current (4.7mA) = Plate Current (35.3mA).
Plate Voltage (80 volts) - Cathode Voltage (2.48 volts) x Plate Current (.035 amps) = Plate Dissipation (2.71 Watts)

A resistor could be connected from the B+ to the screen to measure the screen current directly.
If a 1k resistor it would make calculating the screen current easy. 4.7 volts / 1000 ohm = (.0047 amps)

Liquid Ss Parts List

Coupling Capacitor (2).022uF 250v
Cathode Capacitor 1uf, 2.2uf 50v
Cathode Capacitor 100uF 25v Nos Sprague
Filter Capacitor (1)100uF, (2)22uf 250v

Pull Down resistor 1M/39k Vishay Dale
Plate Load resistor (2)100k Allen Bradley
Grid Leak resistor 470k Metal Film
Cathode resistor (2)2.2 Carbon Comp
R10 resistor 2W Vishay Metal Film
Bleeder resistor 330k 2W
R12/R13 resistor 1k, 4.7k 1W
R14 resistor 62r - 100r
R16/R17 resistor (2)150r 1W Vishay

12CA5/12FX5 NOS USA Power Tube
Tube Sockets  9-pin & 7-pin
Terminal strips (3)4 Lug,
Grommets (3)13.5mm,
Potentiometer Alpha Audio,
Jack (2)Mono 6.3mm,
SPDT Toggle Switch,
Bridge Rectifier Single-Phase,
Power Panel Jack DC 2.1mm,
LEDs (2)Diffused & 475r resistor,