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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).
http://www.freewebs.com/valvewizard2/ampwithLEDs.jpg
