Electronic – How to calculate impedance for reverb tank in a guitar amp

I am designing one pre-amplifier

if you are designing a pre-amp: then there are a couple of things you need to take in consideration

• Hi-Z vs Lo-Z
• Balanced vs Unbalanced
• Line Level vs Mic Level

Usually (but not always)

• Line Level inputs are Hi-Z (can be both balanced or unbalanced)
• Mic Level inputs are Lo-Z (and usually balanced)
• -20dB or -40dB pad's may be used for situations where line level is plugged into a Mic level input
• Balanced inputs can be XLR or TRS
• Sometimes someone plugs in an unbalanced Jack into a TRS input (you need to wire correctly for these situations)

(I just find it handy to have some DI boxes ready, knowing your outputs/inputs, and wire accordingly when the situation requires... much easier than catering for every possible combination)

can some one tell me how the guitar inputs differ from line inputs ?

Generally speaking Guitar Outputs are "Line Level" "Unbalanced" "Hi-Z" If the Guitar pickup/electronics are "active" (e.g. there is a battery or so)

If it is a Guitar without active components, then it is likely to be "Mic Level" "Unbalanced" and can be either "Hi-Z" or "Lo-Z" depending on pickup/built-in-mic.

Some "effects" used with guitar may increase even over Line Level, which means you need to attenuate, which can be done either by DI or on the Mixing Desk by "pads" or "attenuators" (terminology differs from brand to brand)

... my 2 cents ...

[EDIT] As this question is migrated from sound.stackexchange.com to electronics.stackexchange.com, this answer is given from a "sound production perspective" without going into further electronics design. While I do have some electronic skills, I would not edit this answer further as I feel I would not be skilled enough to advise on electronic subjects like designing an audio-pre-amp to use with Pro-Audio applications. Please take that into account while you consider voting or commenting on this answer.

The first tube is an anode gain stage bootstrapped with the signal taken on the cathode of the next stage. This is a positive feedback that tries to keep the current flowing trough R5 constant. This turns R5 to a virtually huge impedance load as seen by U1A. This is good because the ECC83 has a high internal impedance (rounded to 60k) so it needs a high load not to loose gain.

Since the current flowing through the tube becomes nearly constant, it compensates the fact R3 is not decoupled which results in a dramatic raise of the stage amplification. Using this bootstrap method you can bet it will give a gain between 80 and 100.

The direct coupling between the anode gain stage and the cathode follower is also a good thing in a guitar amp since, depending on the bias of the cathode follower, its grid may steal current from its neighbour causing nice H2 distortion.

Maybe a solution for being able to switch between high gain and low gain, would be a switch in the bootstrap loop of the second pair of ECC83. The filters behind the first couple of triodes will decide the output level of the amplifier.