C e r r e m has quite a few versions of Ed's rig and some have cabinet loads and some have dummy loads and whatever. Whether some of it is right, who knows because he contradicts himself.
Transformer is a 4:1 step down transformer. 10k secondary on amp 1's side and 600 ohms primary on amp 2's side
P1 is a 250k voltage divider pot (from Jose load box)
R1 is optional and is around 680 ohms
R1 stops the P1 pot value ever going to 0 ohms.
Basically the Jose load box transformer steps down the voltage from speaker level and the pot fine tunes the voltage to line or instrument level. The output impedance is the pots resistance value either side of the wiper in parallel which is around 62.5K for a 250k pot at a middle setting. So 62.5k goes into a 500k-1M amp input and hardly any signal voltage loss occurs.
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The output impedance of a load resistor is basically whatever value it is, so running a long cable from a load resistor is ok for both the Jose load box and a resistive dummy load because the impedance is low like 8 ohms and then it goes into a voltage divider pot or the Jose stepdown transformer with a voltage divider pot and as long as the pot's value is designed ok then the output impedance of a 250k pot is the same (62.5K) as above and this 62.5K goes to a 500k-1M amp input.
The pot can divide down the speaker voltage to any level including line and instrument level.
