The Valve Wizard

Here I intend to keep useful bits and pieces, whenever they come up.

This graph shows how the frequency reponse of a typical ECC83 with 100k anode load and 1.5k cathode resistor, for different values of cathode bypass capcitor.

This graph shows how the gain of a typical ECC83 varies with anode resistor when loaded by various following AC load resistances.

Below: A useful chart for converting a fret / note to frequency (zero decimal places).

Below is a simple circuit for controlling multiple amp channels. Each momentary footswitch controls a latch (flip-flop). When a footswitch is pressed, the appropriate latch is set and its Q output goes high (5V), while all the other latches are reset via the diodes, so their Q outputs go low (0V). Whichever latch is "on" also lights an LED to indicate the active channel, and might control a transistor for driving the channel switching relay or LED/LDR, or whatever you're using. The capacitors remove switch bounce. If LEDs aren't required in the footswitch enclosure, then most of the circuit could be housed inside the amp of course.

If you still think logic circuits are the devil, the following circuit allows any number of channels to be controlled by arranging opamps as SR latches instead. It works in exactly the same as the circuit above, except that much higher supply voltages can be used. Some opamps can even drive LED arrays and small relays directly...

The graph below shows the absolute minimum required reservoir capacitance when using a conventional IC regulator, to avoid the regulator switching off at any time. It assumes a regulator drop-out of 2.5V. First, take your DC input voltage to the regulator and then subtract the regulator's output voltage from it, then read off the capacitance for the required output current. It is advisable to use a value which is somewhat greater than indicated, to be on the safe side.

The schematic below shows a simple preamp (albeit with some Valve Wizard tricks incorporated) and shows some fun ways in which LEDs can be incorporated, usually with no impact on the tone of the amp. The different applications are numbered:
1: In series with the anode. For normal values of anode resistor, this will have no effect on tone / operation. The LED will slowly come on as the valve warms up.
2: LED bias. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations. The LED will slowly come on as the valve warms up.
3: In series with a cathode load resistance/tail resistance. Although this will raise the cathode voltage by a tiny amount, it is usually insignificant in terms of normal circuit operation. The LED will slowly come on as the valve warms up.
4: In series with a smoothing capacitor. A rectifier diode is also added in parallel to allow normal operation of the cap. The LED will be off normally, but will light up as the capacitor discharges at switch off, providing a warning while the cap is still holding charge
5: A dual-colour LED in series with a smoothing capacitor. The green LED will light for a couple of seconds at switch on as the capacitor charges, and the red LED will light at switch off. Don't use this one on the reservoir cap though, the higher inrush current will fry the LED!
6: Ok, this one isn't an LED, it's a neon lamp used as grid-to-cathode arc protection on a DC-coupled stage. The neon will light at switch-on until the valve has warmed up, when it will switch off. (An LED can't be used here due to the high reverse-voltage during normal operation).