It can be done, but you will be restricted to 16 Ω and 8 Ω speakers, along with (possible) sonic degradation. 4 Ω speakers are not allowed.
A technique known as bridging allows any stereoblock, with a common ground between channels, to be converted into a monoblock, safely. If this schematic is close to the amp in question, what follows below will work.
Build a mono buffered control center whose O/P impedance is VERY low. The center's O/P is associated with a Sowter Model 1475 transformer. The 1475 allows you to drive the stereoblock's I/Ps with 180o out of phase signals. Connect a 16 Ω speaker to both 8 Ω taps and an 8 Ω speaker to both 4 Ω taps. Do not connect to the ground positions. Each section of a bridged stereoblock "sees" only 1/2 of the speaker load. The 20 Kohm pot. and C18 are removed from the signal path. Be prepared to adjust the value of R22, should the 1475 "ring".
In the heydays of tube amplification it has been very common to simply parallel both channels of a stereophonic amplifier, i.e. to connect both inputs and speaker outputs each. Tube power amplifiers usually don't have very low output impedances, so no problems should impend. Full power requires half the speaker load impedance, though.
Best regards!
Best regards!
The problems with this approach is that both sides differs, and even if a slight
amplification difference one channel has to work much more, causing more
distorsion. This is elevated with negative feedback.
For PA this is acceptable, for hifi it isn't.
More complex schenarios ( as eli suggested) depends on external phase
inverters/transformers and bridging the speakers.
The one and easy solution to mono use of a stereo amp is to have each
channel feed it's independent speaker system.
Some folks have success in simply connecting sections in parallel, but other folks get heat and bad sound. Bridging is 100% safe, as it sums the voltage swings of the 2 sections in the load. The closer the 2 bridged sections are to being truly identical, the better the result produced the composite. Meter matched NFB resistor pairs and very careful, individual, tuning of the phase compensation caps. contribute to achieving a good result. The fact that no 2 transformer specimens are identical requires the custom tuning of phase compensation. The NFB resistor pairs control closed loop gain and the need for identity should be self evident.
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