Stereo to mono bridge/phase inverter for AVA ST70 amplifier

Thanks to all for a very helpful response! It's nice to know that there are still people who are willing to share their knowledge without getting paid! Again, thank you very much!

I have 2 AVA ST70 amplifiers that I want to use as mono.

@rommel If the ST70s have the mono/stereo switch, put them in mono.

Then parallel the outputs by connecting the 16 Ohms together if you are using 8 Ohm speakers, or the 8 Ohm taps together if using 4 Ohm speakers.

The Common connections should be jumpered together as well.

This is known as 'monostrapping' which is different from bridging. You can do this with a tube amp but for the most part not with solid state.

The instructions for this are found in the original Dynaco ST70 manual:

Monophonic 70 watt amplification
For monophonic operation, the two channels of
the Stereo 70 may be paralleled. For this, only
one input cable is connected to either the left or the
right input (but not both), the input s witch is set to
"mono", and the outputs are connected together.
An insulated wire jumper should be connected from
"c" to "c" on the screw terminal strips. Another
insulated wire jumper should be connected from
"8" to "8" if a 4 ohm loudspeaker is used, or from
"16" to" 16" is an 8 ohm or 16 ohm loudspeaker is
used. When using the 16 ohm loudspeaker in this
fashion, the amplifier's maximum power output is
not as great because of the impedance. mismatch,
but there is no loss of power when using a 4 ohm
or 8 ohm loudspeaker as specified above. The
maximum power available into a 16 ohm loudspeaker
is still 55 watts, however, and there is a compen-
sating factor. The distortion, as levels below max-
imum, will be reduced by this type of connection.
What happens is that the operating conditions are
shifted from maximum power output to minimum
distortion. This same operation can also be
achieved, of course, by connecting an 8 ohm loud-
speaker to paralleled 8 ohm taps, or a 4 ohm
loudspeaker to paralleled 4 ohm taps. Note that
whenever the two outputs are paralleled, only one
input cable should be used, the other input socket
should not be used, and the switch is on "mono"

https://skyfiaudio.com/products/electronics-service-labs-esl-bridging-amplifier-device

If you are DIY handy, use a transformer is an easy way to get the job done.

Sorry, don't know about the phase inverter, but to get the equivalent of a pair of monoblocks, I'm running a single channel from two different Dyna/VTA 70 amps.  I just removed the output tubes from the unused side, and also switched to a SS rectifier.  They sound pretty amazing.