What is the tube lineup? Also, are the heaters/filaments in series, or parallel?
Chances are good (but best verified by looking inside the radio), that the black is going to be a common chassis ground, yellow should be filaments (but best verified), and red should be "B+". (also should be verified as red and yellow may have been reverse of this).
: I would like to power up an old Silver model 800 HF receiver but have no idea what source or voltage to use. It has a (blk, red, yel) twisted triple out the back and I have not been able to locate a schematic for it. Can anyone help with information? Thanks in advance for your time. BCP