How to connect 2 amp heads to one speaker?

[J M Fahey]

Applying SS solutions to Tube amps and viceversa does not go a long way.
As Anatech (moderator in the Forum linked) says:

Bridging a tube amp as if it were a solid state amp would be odd. I don't think I would do it that way.

As Dhaen (moderator in the Forum linked) says:

With SS, bridging brings 2 benefits that are absent from tubes:
1) Increased power for the same supply rail.
2) Ability to use half value smoothing caps on a common supply rail.

Besides, you do not have a *Stereo* Power Amp where in theory, at least , there is some possibility of success, where they are in the same chassis, use the power supply, have a common ground, etc.; but two separate PA heads.
Just wiring a phase inverter/bridger to power amp inputs and even worse, wiring the out of phase "hot" speaker signals (coming from physically separate heads) to the single speaker, will be klunky, to say the least.

[tbryanh]

Attached is a diagram on how to bridge two Marantz monoblack amps together. The Marantz manual is included too.

I think these might be transistor amps, but it seems it should work for tube amps too.

[Jazz P Bass]

The Marantz setup is called a "bridged output"
This only applies to solid state amplifiers, as far as I can tell.
After reading the excellant posts on why you cannot do what you want to do, I am struck by the post about "use a dual voice coil speaker".
This may be a rather elegant solution to your dilemma.

[Enzo]

Fair enough. I see dual voice coil speakers for car stereo subwoofers. Who makes a dual voice coil speaker for bass guitar?

[Jazz P Bass]

Talking through my hat again.
Did some research.
It appears that no one will advise running two separate signals into the dual coil speaker.
Unless you mono sum the signal.
But even then....

[Enzo]

Well, yeah. ANy signal not the same in both coils, one will fight the other. And any energy that does not become motion will become heat. SO the signals do need to be the same. But in the original premise, he was proposing a Y-cord at the two inputs, or something similar, so within the two amps being the same and within their settings being at least similar, that ought to approximate dual mono. I still don't think it is a good idea though.

[tbryanh]

It seems the bridged PA 100 amps could survive through the imbalance when one end of the phone cable that jumpers the inputs of the two amps together pulled out on one of the amps. Lets say it pulled out on Amp-B.

Amp-A would send current through the speaker and through the secondary of the OT of Amp-B.

Amp-A would not be loaded as well as it should be, but it should survive through this.

The current in the secondary of the OT of Amp-B would be the same as the current through the secondary of the OT of Amp-A. This means that the current through the primary of the OT of Amp-B would be the same as the current through primary of the OT of Amp-A.

It does not appear the imbalance causes any damage to the amplifiers.

[Enzo]

The current in the secondary of the OT of Amp-B would be the same as the current through the secondary of the OT of Amp-A. This means that the current through the primary of the OT of Amp-B would be the same as the current through primary of the OT of Amp-A.

I don't think so. Two secondaries in series of course would have the same current flowing through them - as long as they were terminated with a load. But that does not mean the primaries also have the same current flows. Let's say they are running, and you remove power from one of the amps. The other continues to function, and its output current flows through the now silent other secondary to the speaker. But the live amp has full power in the primary while the dead amp has none.

Leaving both amps powered, no signal through one amp, having current flowing through the secondary won't insure primary currents. Primary current involves the tubes as part of their circuit path. If the tubes are not conducting, no current flows, even if you are banging on the back door with a voltage.

I doubt it will hurt the amplifiers, but I don;t think your reasons are why.

And if you try your bridge approach, it won;t be enough to take the hot wire from each amp and connect to the speakers. The two amps must have their grounds connected together, or there will not be a complete circuit path for signal current through the load. This might happen by default if they ar plugged into the same outlet, but that isn't the best thing to rely on, adn may cause ugly ground loops.

[J M Fahey]

Attached is a diagram on how to bridge two Marantz monoblack amps together. The Marantz manual is included too.

Cool !! Now you know how to bridge two SS, identical power amps
Specially ones that straight from the designer's desk were made to be bridged.
Cool little BTL switch.
Is your Fender PA100 the SS version?

I think these might be transistor amps,

they are

but it seems it should work for tube amps too.

"seems?" ... what makes you think so?

Amp-A would send current through the speaker and through the secondary of the OT of Amp-B.

Such current will depend on the voltage delivered by amp A's secondary and the load impedance, which is made up from the sum of the speaker impedance (8 ohms) and the reflected plate impedance from amp B, which being pentodes, is *way much* higher than 8 ohms.
Current will be a small fraction of what it should; Amp A will be unloaded in practice.
You will have very high AC voltages in Amp B plates, courtesy of Amp A and the step-up effect of Amp B output transformer.

Amp-A would not be loaded as well as it should be,

you bet

but it should survive through this.

*Maybe*
Tube amps have been used unloaded and survived.
Try it, I'm eager to know .

The current in the secondary of the OT of Amp-B would be the same as the current through the secondary of the OT of Amp-A.

Yes ..... a *very* small current, if any.

This means that the current through the primary of the OT of Amp-B would be the same as the current through primary of the OT of Amp-A.

Yes ..... a *very* small current, if any.

It does not appear the imbalance causes any damage to the amplifiers.

It's not an "imbalance" but a much more dangerous situation.

CONCLUSION:
your arguments remind me of the 23575 arguments Victoria, my little daughter, exposed trying to convince us that a diet based on Twinkies and M&M's was *so much* better, healthier, cooler, you name it, than the boring regular food we put on her plate.
Fine and cool ... for a 9 year old girl.
Even more: a couple times she *got* to have her dinner that way ... after all I *am* a Father with a very soft heart.
Best of all? .... she "survived"

Conclusion 2: go and bridge them, it's your amps, speaker, time and money.
You don't need "approval" for something you will do anyway, no matter what.
.... unless you are only pulling our legs, having fun seeing how much will we stand answering over and over this "technical question" ... to no avail.

[soundmasterg]

Reading this thread initially brought to mind the Victoria Regal 2 amp, which uses two single ended sections through a common output transformer, going into a single speaker. The Victoria approach is different that what you had in mind, but I wonder if anything might be adapted from the approach to work?

On the other hand, as Enzo said....why bother for only 3db more?

Greg

[J M Fahey]

two single ended sections through a common output transformer, going into a single speaker.

Of course they must be fed exactly the same signal, but out of phase, or they would cancel.
Some call it "Class A push pull".
Or do they refer to single ended, class A tubes, into the same transformer winding, as in "Parallel single ended class A?"
What Gibson made 60 years ago in the Gibsonette? (2 x 6V6 SE Parallel)
They might even have each one its own winding (as to have something "new, patentable" in the tube world) so far as both windings were in phase, and exactly the same (bifilar winding preferred).
It would still be "Parallel single ended class A".

Any of the above do work.
Can't see it done with "two Fender PA100" though.

[kg]

answer: you can't!