Thanks Nickrose but this question is not dependent on any particular amp.
IMHO, its still a very very basic question about the use of a common 1/4" mono to stereo adaptor for combining
1) a mono output of the TF (placed in a generic series effect loop)
with
2) a mono dry signal (derived from the effect loop send, and, therefore, bypassing the TF)
for return to amp via the mono effects loop return:
Are there "electronic" considerations, voltages or phase issues, for example, with the TF (or any other line level device) in such a simple implementation that makes this solution problematic?
