When I assembled my N5X kit, the engineer in me was worried that the circuit wouldn’t be within the range of ‘typical’ values provided with the kit by way of guidance. After the 1st switch on (well, 2nd or 3rd – I had a dodgy speaker cable which halted things for a few minutes) and taking all the necessary readings I was pleased to see my circuit was pretty close on all counts. I couldn’t just leave it at that and move on. I wanted to know more about what was going on in the amp and how it was operating. Hence a lengthy process of reading up on valve amps and getting to grips with the simple maths behind it all.
I used the following books for guidance: Guitar Amplifier Preamps and Guitar Amplifier Power Amps by Richard Kuehnel, and Designing Tube Preamps for Guitar and Bass by Merlin Blencowe. Below is a run through the amp from input to speaker (following the style here. I’ll apologise now for inaccuracies / mistakes – please comment or contact me if you spot any. I’ll focus on the basic maths and graphical methods to determine what is going on at each stage. There’s a lot left out, but I’ll update as I learn more.
Is a gamma. I think.
Preamp 1st Stage
This stage is used to amplify the guitar signal to a suitable level for the rest of the circuit.
Preamp 2nd Stage
This stage is used to further amplify the signal to drive the power amp. It also adds a level of overdrive to the signal if it itself is driven hard by the 1st stage.
As we have the capability to use both EL84 and 6V6 valves this section is split in to two.
For the EL84 valve and the ‘typical’ measurements:
Idle (Quiescent) Plate Voltage,
Idle Grid Voltage,
Using the scaled valve data to work out some currents at a few known (or assumed) grid voltages:
- Cut off is assumed to be at . From the charts you can read off (or interpolate) this to give .
- The DC operating point is assumed to be so .
- Using the Triode data, you can estimate the idle cathode current as .
From these values it is possible to estimate the cathode resistor, , that should be used:
The power dissipated at idle in the cathode resistor is
The Plate Supply Voltage, , is calculated using
where is the output transformer impedance.
The actual cathode resistor is 150 ohm, giving and .
The equation of the load line (in form) can be deduced from
- the inverse of the output transformer impedance as the gradient, , and
- the intercept calculated by