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.

[g1]

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]

another idea

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