April 25, 2016



Current Project:

Hand Built 15 Watt Tube Amp (Based on Fender BJIII).

Examination of PreAmp Stage:


  • V1B is one Triode of the 12AX7 Vacuum Tube.
  • R1 is the Grid Leak Resistor, it provides Gnd reference to the PreAmp input and also allows stray electrons that build up on the Grid to leak to Gnd.
  • R2 is the Grid Resistor which prevents RF from entering the PreAmp- it forms a High Pass Filter with the input Capacitance of the Grid.
  • R3 is the Plate Resistor. DC Voltage is delivered to the Tube Plate via R3 current limit.
  • R4 is the Cathode Resistor which sets the Bias for the Tube. Grid must be Negative with respect to the Cathode for normal operation, R4 achieves this by raising the Cathode above Gnd (1.34V in this case). NB Grid is ref 0V via R1 therefore Grid is negative wrt Cathode.
  • R5 is the Volume control Pot.
  • C1 is the Cathode bypass Cap. Allows AC signals (above Fc) to bypass R4 so the Bias is not affected.
  • C2 is the Coupling Cap- Blocks the High Voltage DC and passes the amplified AC signal subject to HPF frequency response.
  • C3 allows higher frequency signals to pass un-attenuated to the next stage.



BJ III PreAmp Section only

LTSpice Model of BJ III Pre-Amp Stage:

Built this Spice model of the all important Pre-Amp stage.

Blues Junior PreAmp Schematic Spice Model

LTSpice Simulation Results :

Vin 10mV Sine wave 10Hz to 1KHz

Vout response

Gain of approx 50 at 1KHz as per actual schematic info.

Blues Jnr PreAmp results to 1KHz large


Click here for a better view of above image:


Build of Power Supply:

Mains Transformer Hammond 290 XEX

Power Supply

Power Supply

PSU Schematic

PSU Schematic



Mains Transformer











Build in progress:

Ordered Chassis and some fixturing from Ampmaker.com

Ordered Components from Mouser.com (Selected mid range good quality components).

Output Transformer Hammond 1760F.

NB I have lots of great Links for all things technical related to Tube Amps – use Contact page if you want any recommendations.




Phase and Output stages

Phase and Output stages

Output Transformer

Output Transformer






Powered up and passed smoke test!! –



Heater Glow

Heater Glow


Sound Check

Raw Amp Sound -No Pedals, No Effects

Very little Hum or Buzz


Testing the PreAmp Gain

  • Scope Channel 1 is the Input 20mV pp Sine Wave @ 1KHz from the Function Generator
  • Scope Channel 2 is the PreAmp Output at C2.
  • Output is just over 1Volt pp so PreAmp Gain is approx 50 as per Spice simulation.
  • Note the Output Sine Wave is shifted in phase by 180°

Bench Setup

CH1 20mV/Div. CH2 200mv/Div.














Next Step – Speaker and Cabinet

Speaker ordered from Watford Valves UK

Celestion Greenback GM12 25W 8 Ohm

Celestion Greenback GM12 25W 8 Ohm