I read about this in The Vox Story : A Complete History of the Legend (Guitar History, No. 4) David Petersen, Dick Denney / Paperback / Published 1993 Amazon: $21.95 (back ordered). Introduction by Brian May. available at Amazon.com.
To avoid "intermodulation distortion", as he calls it, he modified a tape echo so that the direct guitar signal goes to one saturating tube amp, the first echo goes to a completely separate saturating tube amp, and the second echo goes to yet another saturating tube amp.
Some people try putting echo in the amp's effects loop. This does avoid confusing the amp's preamp distortion stage with a jumble of direct and delayed signals, but this approach still confuses the power tubes (and the output transformer and guitar speaker) with too complex a signal. If you want to preserve clarity of power-tube saturation, echo must go after *all* types of distortion, clipping, saturation, whatever -- or, as Brian May does, each delay signal must remain separate and be sent to an isolated, dedicated saturation path.
These same placement principles hold for delay-derived effects -- time-based effects such as reverb, chorus, flanging, and auto-harmony. Reverb works in amps clearly only if no significant saturation occurs after reverb -- that is, if the power tubes aren't saturating audibly. Heavy reverb followed by heavy power-tube saturation cannot yield a clear type of saturation, whereas heavy reverb *after* heavy power-tube saturation sounds much clearer, more musical, and more real. Van Halen uses the immediate sound of a saturating amp in one channel, and in the other channel, a natural-sounding, reverbed version of that same signal -- as with the reverb that a concert hall adds, *after* the amp has already established its clear saturation tone. A cranked stack in a concert hall is echo-after-saturation and sounds natural, while echo-before-saturation sounds garbled and unnatural and unmusical, useful only as an occassional, distinctive special effect.
The incredible trouble that Brian May goes through to avoid intermodulation between the echoed signal and the direct signal, as the input signal for the saturating power tubes, should drive home the importance of avoiding echo-before-distortion/saturation. His method is a little impractical, and most people should simply run time-based effects through the mixing board's effects loop, or at any rate, between the mic and the final (non-saturating) monitor amp and monitor speakers.
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