Bridging stereo tube amp to one more powerful mono output

[rayma]

I'll take a look at that, could it be as simple as a jumper between the - terminals of channel A to channel B?

Here's the Stereo 70 manual section on paralleling channels.

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 switch 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" if an 8 ohm or 16 ohm loudspeaker is used.

 

[coolhandjjl]

Are you sure both phases of the DI output are actually driven, it is not terribly uncommon to present a single ended output on an XLR - handled correctly you still get the hum cancelling of a balanced input, but you do not get the 6dB additional output of balanced mode, and in this case one channel of that amplifier would be undriven - which would match the symptom you describe. DI outputs intended to be plugged into microphone inputs are heavily attenuated.

It's possible, I have never been inside that preamp. I was trying to test with with what I had on-hand rather than buying a $70 transformed just for testing purposes.

Yes, just make sure that the 0 Ohm terminals are grounded because some, like Audio Research, are not. I thnk the Dynace Stereo 70 manual has a description of the parallel connection.

Here's the Stereo 70 manual section on paralleling channels.

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 switch 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" if an 8 ohm or 16 ohm loudspeaker is used.

Sorry, that first reply asking about a jumper to each speaker negative was in reply to bridging. Paralleling seems straight forward and my tests indicated it was okay, but thanks just the same.