How to connect 2 amp heads to one speaker?

[tbryanh]

I have a couple of Fender PA 100s that are 100w and 4 ohms.

I want to combine them together into a 200w 8 ohm bass amp.

It appears there are 2 ways to do this.

METHOD 1 (Series Transformers)
1. Connect the inputs of the first channels of Amp-A and Amp-B together.
1.A. Plug one end of a 1/4” phone cable into the input of the first channel of Amp-A.
1.B. Plug the other end of the 1/4” phone cable into the input of the first channel of Amp-B.
1.C. Eliminate any ground loops that result.
1.D. Use the remaining unused jack in the first channel of either amp as the input to the combined amps.

2. Connect the secondaries of the output transformers of Amp-A and Amp-B in series with each other.
2.A. Remove the terminal from ground on the secondary of the output transformer of Amp-A and label it T1.
2.B. Label the other terminal on the secondary of the output transformer of Amp-A as T2.
2.C. Label the terminals on the output transformer of Amp-B the same way as as they are labeled in Amp-A.
2.D. Remove The NFB wire in Amp-A from T2 and connect it to T2 of Amp-B.
2.E. Connect T1 of Amp-A to T2 of Amp-B.

3. Connect an 8 ohm speaker cabinet to the combined amps.
3.A. Connect one wire of an 8 ohm speaker cabinet to T2 of Amp-A.
3.B. Connect the other wire of the speaker cabinet to T1 of Amp-B.

METHOD 2 (Bridging)
1. Use a modified version of Step 1 of Method 1.
1.A. Instead of using a simple phone cable to connect the inputs of the two amps together as was used in Step 1 of Method 1, use an inverting stomp-box pedal to connect the inputs of the two amps together.
1.A.i. Plug one end of a 1/4” phone cable into the input of the first channel of Amp-A.
1.A.ii. Plug the other end of the 1/4” phone cable into the input of the inverting stomp-box pedal.
1.A.iii. Plug one end of another 1/4” phone cable into the the output of the inverting stomp-box pedal.
1.A.iv. Plug the other end of this 1/4” phone cable into the input of the first channel of Amp-B.
1.A.v. An alternative to using an inverting stomp-box pedal is to connect the inputs of the two amps together the way it is done in Step 1 in Method 1. Then modify an unused channel in Amp-B and use it to invert the signal in the first channel of Amp-B.

2. Label the terminals of the transformers the same way as was done in Step 2 of Method 1, but do not do any of the modifications that were done in Step 2 of Method 1.

3. Connect an 8 ohm speaker cabinet to the combined amps.
3.A. Connect one wire of an 8 ohm speaker cabinet to T2 of Amp-A.
3.B. Connect the other wire of the speaker cabinet to T2 of Amp-B.


FUNCTIONALITY?
Q1.A. Will both methods work?
Q1.B. Are there any other methods that will work?
Q1.C. Which method is best? Why?

NFB ISSUES?
Q2.A. For Method 1, will the nonlinearity correction be acceptable for Amp-A even though Amp-A is using Amp-B's NFB signal.
Q2.B. For Method 2, will the NFB in either amp be affected?

OSCILLATION ISSUES?
Q3.A. Is Method 1 prone to spurious oscillations? If so, how can they be prevented?
Q3.B. Is Method 2 prone to spurious oscillations? If so, how can they be prevented?

NOISE ISSUES?
Q4.A. Will Method 1 cause the amps to be noisy?
Q4.B. Will Method 2 cause the amps to be noisy?

OTHER ISSUES?
Q5. Are there any other issues to discuss?

[Enzo]

One major issue comes to mind. WHat benefit will be expected from this arrangement? Other than 3 decibels?


And I have no idea what that NFB plan is. The point of NFB is to help the amp correct its own distortion. WHy would you want to connect one amps output to another amps NFB line? And then why would you want to leave the second amp without its NFB? Furthermore, with two OTs in series, the output voltage will be doubled, so if you are using that output as a NFB source and feeding it back to one amp, that amp will then get twice the NFB signal it normally expects.

[tbryanh]

One major issue comes to mind. WHat benefit will be expected from this arrangement? Other than 3 decibels?

Bass players might be able to answer this question.

Not sure. But I have heard that bass dissipates into the surroundings sooner than midrange and highs do. I don't know if this is true or not.

The concept of 3 db only being slightly louder might apply mostly to midrange and treble. For bass, maybe 3 db doesn't sound louder at all. Maybe 3 db just makes the bass travel further before it is absorbed by its surroundings.

[J M Fahey]

*SHORT* answer: you can't.
If anything, connect both power amp inputs in parallel, and let each one drive its own speaker.
Sadly, you'll need two of them.

[tbryanh]

Unlike mids and highs, bass is felt as well as heard.

Maybe 3 db makes you feel the bass twice as much even though it only sounds slightly louder.

[pdf64]

Didn't Marshall use 2 OTs in their first 100 watt amps? Similar concept, though having the 2 amps as discrete seperate units could be a significant difference and make it difficult to use Marshall's approach. But may be worth looking into how they were configured. Pete.

[Mark Black]

Unlike mids and highs, bass is felt as well as heard.

Maybe 3 db makes you feel the bass twice as much even though it only sounds slightly louder.

That's an intriguing point - I have not heard of any studies relating relative levels of body vibratory sensation to dB levels.

[Raybob]

I would think it's possible with AB/Y pedal but not using existing OTs, use a common 200w OT, with proper impedance match for number of tubes. Negative feedback might be an issue.

[J M Fahey]

Slightly longer answer.
a) still the answer is you can't. If you want it in a little more detail, read on.
b) the question is "using two Fender PA100 amplifiers in some combination or another"; it's been answered above.
NOW if the problem (which is NOT what was asked by the OP) is "can I make a 200W amp with 2 PA100 PARTS?": yes you can.
In this case what pdf64 hinted becomes practical:
Cannibalize both amps.
Kludge a monster by
c) putting 2 power transformers in parallel. Or even the full power supplies. You'll have same voltage, twice the current -> twice the DC power available.
d) build a new power stage where a *single* PI drives *eight* 6L6, in quadruple parallel
e) use both output transformers with primaries in parallel.
Secondaries in parallel will happily drive 2 ohms, or in series 8 ohms.
f) you will have a *single* NFB loop.
It will work properly either taken with the original values from one of the 4 ohm secondaries or from the new 8 ohm tap , doubling the feedback resistor.
g) It can be done, but it's clear it's not ""using two Fender PA100 amplifiers in some combination or another" .
Although I never saw that mythical schematic, maybe Marshall did something similar.
It avoids mixing NFB loops which is akin to playing with matches and gasoline.

[g-one]

As Enzo mentioned, the increase will be 3db. However, with both amps running into separate speakers the increase will be MORE than 3db due to the increase in cone area.
Why go to all that work just to short-change yourself in the loudness department?

[TransLucid]

use dual voice coil speakers....

[tbryanh]

...with both amps running into separate speakers the increase will be MORE than 3db due to the increase in cone area....

I think I have heard something like this before. Glad you mentioned this.

Maybe I will change the output transformer to 8 ohms to match the 1x18 speaker cabinet impedance.

What is a good transformer for this? Is there any difference between the silverface twin reverb OTs and the silverface bassman 100 OTs?

[pdf64]

How about configuring the two amps as a bridge? May need to invert the signal to one of them. An 8ohm load should be correct for two 4 ohm amps in a bridge arrangement? Pete.

[tbryanh]

How about configuring the two amps as a bridge? May need to invert the signal to one of them. An 8ohm load should be correct for two 4 ohm amps in a bridge arrangement? Pete.

Yeah. Maybe I will bridge the two amps.

Isn't Method 2 bridging? Shouldn't Method 2 work?

[tbryanh]

Here is a simple explanation on how to bridge a stereo amp:

You will require a phase inverter for one channel.

You then send the signal to both channels and hook up the speaker to the + terminals of the amplifier.

If you need to drive an 8 Ohm speaker the 4 Ohm taps of the OPT should be used.

bridge mono tube amp - diyAudio

Here is another link on how to bridge a stereo amp:
http://www.diyaudio.com/forums/tubes...ereo-amps.html

[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!