# Applying the Briggs Valve PDF Principles to the Marshall Circuit

Applying the Briggs Valve PDF Principles to the Marshall Circuit

On reading the Briggs pdf document, I thought I would try to relate some of the general nuts and bolts principles as I come across them or try to understand them, as laid out in Briggs’ comprehensive text, to the real circuits I have covered in my Posts but did not understand too well at the time, or have yet to understand better but have read about (long-tailed pairs, parallel power output stages and phase inverters etc. in the Guitar Amp Handbook by Dave Hunter).

The first explanatory example I have chosen (p36) should be useful for applying to any amplifier – valve or transistor – for affecting the bass/treble response in relation to the shunt capacitor C15 in the output stage of the Marshall Mercury:

Cathode “auto-bias” is common in valve amps, so is useful to understand. A seen in the Fender Champ also using a 25V, 25uF cap for the 6V6 output tube :

Explained in my English, rather than Briggs “older” lingo – this all means that as the grid voltage increases with the positive swing of the AC signal to be amplified, it causes more current to flow through the valve anode and cathode (fine so far – amplification), but this then means that a larger voltage drops across the cathode resistance (R15) which then simultaneously decreases the voltage difference between the grid and cathode – i.e. works in opposition to the amplification that is desired – but in phase (Briggs “degeneration”) as both voltages are positive going, as opposed to Negative Feedback (out of phase) principles.

By putting the shunt capacitor C15 of a suitable value for the audio range to be amplified, in parallel with the R15, this AC signal voltage component can bypass R15, keeping its bias current – so its voltage drop – constant, allowing linear amplification of these frequencies at the anode.

This implies that changing the value of C15 in any cathode biased circuit will change the amount of treble/bass component of the amplified signal at the anode (emitter) also. The gain 1 and 2 transistor stages of this circuit at C5 and C12 also eh?

This assumes of course that there is the relevant bass and/or treble components present in the signal in the first place, so the relevant values of capacitors chosen for the inputs and outputs to the bases of these transistors and from the emitters has to be correct also (C3, C6, C7 and C10, C13).

Note the guitar jack inputs determine the initial frequency conditions for the amp:

The low input allows more bass through to the transistor, and the high input is more trebly. I’ll record this amp soon so the input differences can be heard. You can see from the diagram roughly this effect, given that small capacitances pass higher frequencies. The low input shunts higher frequencies straight to earth via the small 2.2 nanoFarad cap, so only the bassier frequencies cause a fluctuation over the 100k resistor for input to the base of T1. The high input passes the higher frequencies to the input of T1, with lower frequencies being dropped across R1 (220K). This capacitor (C1) would be a good one to change first, to a larger value, to allow more bass to pass through the amp from the start.

Also note that in this Marshall circuit, the Anode load resistance is provided by the primary of the output transformer. There is mention of inductors used as anode loads on page 29 of Briggs:

The interesting part for me above is the less effectiveness as frequency goes down – given how trebly overall this Marshall is compared to other guitar amps. The Champ has a similar inductive load, so it would be interesting to compare component values between these models.
The ideal anode load inductor requires a low DC resistance according to Briggs, so that as much of the supply voltage as possible can be dropped across the valve, so it has larger possible +ve and -ve voltage swings across it from the available supply, allowing larger voltage amplification of the AC signal. The AC signal component drops across the inductor load (which is what was amplified from the gate) and stepped down (lower voltage, higher current) to the secondary winding to power the speaker.