Ideas for an amp switchable between mono and stereo speaker output?

[Jennings]

A guy I know asked whether it was possible to build an amp with two preamp channels and a push/pull two valve output section, but with the option of somehow making the output stereo / splitable if he wanted...so effectively have the option of sending one preamp amp signal to one guitar speaker cab and the other to another, but driven by output amps rather than a line out arrangement into a PA/desk.  Insanity?  Unnecessary (given the line out option might be easier than two output sections)?  Seems silly to me...silly enough to ask the hive mind what they thought! 


I figured you could split out the two preamps at the PI (where I'd mix the two channels normally), but then I can't see a way round either having a double output section (so another pair of valves and an OT) making the chassis much larger and heavier.  and of course requiring a much beefier mains Tx, or additional filament Tx particularly.  I guess you could house the additional output section (perhaps with additional main Tx and PI of its own) in a separate plug in module.  Like a little mini power amp / slave amp fed by a switchable line out from the main amp's PI area.

[Merlin]

In theory it can be done, but it's probably not what you're hoping for, see page 59 of this PDF: https://www.worldradiohistory.com/UK/Practical-Wireless/50s/PW-1959-12.pdf
If it were me I'd probably build two circuits into one cabinet, each with its own low-power output stage driving a speaker simulator (like 5W or less, to keep transformer cost down), then add a couple of cheap class-D amplifier modules for the final output power to the actual speakers.
i.e. build a low power tube stereo, plus class-D output power buffers, in one box.

[kagliostro]

I think it is doable

Consider an amp where you have two separated preamp channels that are mixed at the PI or mixed before to go to the PI

then you have the PI that feeds two output tubes (one for each branch of the PI) the plates of those tubes are connected to the primary of a PP OT and the secondary of the OT to a Speaker

well, consider that in the past happened that missing PP OT some brand (italian for sure - See attached schematic of the Corland HiFi Guitar Amp) buid the power section using a pair of SE OT instead of a single PP OT

So build a power section with two SE OT that feed each one on a speaker (so you have two speakers but mono signal)

you have

Two input >> Two preamp channels >> MIX (resistors or MIX at the PI) >> PI >> Two Power Tubes (working in PP) >> Two SE OT >> Two Speakers

with this configuration the Preamp are mixed and via the PI feed the Two Power Tubes that acts in PP (also if the OT isn't a PP OT but are Two SE OT)

this way you miss the noise cancellation due to the PP Transformer windings but has anyway a PP Power Section

Then you can bypass the MIX circuit (if done with resistors as an example) and bypass te PI

you must bypass it in a way each Preamp Channel feeds the input of one power tube and you are done

Franco


p.s.: On the Corland the secondary of the two SE OT was connected to a single speaker but you can connect each secondary of each SE OT to a dedicated Speaker, so you can perform Stereo function if you bypass the MIX and the PI


The first approach to the amp that has two SE OT and acts in PP


The first Schematic of the amp as it was original


The redrawn schematic with the two original  SE OT

[Merlin]

If you have two SE OTs you might as well just build a stereo SE amplifier...

[kagliostro]

Oh, that is sure Mr Merlin

A guy I know asked whether it was possible to build an amp with two preamp channels and a push/pull two valve output section, but with the option of somehow making the output stereo / splitable if he wanted...

The question was if it was possible to build a PP amp with the option to a splitable stereo output

So I thinked that the easiest way will be the one I told previously

May be there are other way but I think this is the simpler one, of course may be I'm wrong and there are other way to build a mono PP amp that can be switched to Stereo Amp via a simple switch

Franco

[Merlin]

The question was if it was possible to build a PP amp

Oh yes I forgot that part

[SEL49]

If you want stereo outputs you must have two separate power amps as well as two separate preamps. I don't know of any way around that and still call it stereo. And without a stereo guitar what's the point? Probably the easiest way to have a true stereo would be to use two amps.

[kagliostro]

BTW I'm not saying that it will be a special feature amp, I was only considering the request and if I see a possible solution

Franco

[kagliostro]

And without a stereo guitar what's the point?

Few years ago I was doing a search in something like this direction

https://el34world.com/Forum/index.php?topic=31069.msg342408#msg342408

and this is a possible solution don't having a Stereo Guitar

https://el34world.com/Forum/index.php?action=dlattach;topic=31069.0;attach=110154

Franco

[Dave]

Just as food for thought, two output transformers can be run in parallel. So, although I have never done it, they could, at least to my mind, also be split at the flip of a switch. How you might use that as a "stereo" setup I would leave to your imagination.


Dave

[acheld]

I don't think there's any way to get around the fact that if you want a true stereo amp, you'd have to have two separate output stages. 

As to why to build:  Magnatone uses stereo effectively, though I've only heard these and never used one.   But -- for a simple true wet-dry setup, I think a stereo amp would have limited effectiveness due to how close the two speakers would be.   There's a reason why Mick and Dan use bi-amp setups rather than a stereo amp.

But thinking practically, if I were building a stereo amp, I'd build two tube based pre-amps and couple them to two SS power amps -- using something like ICEpower modules, or Dayton audio clones.   Would be far lighter than a stereo tube amp.  Many of our backs are not what they used to be !

[kagliostro]

Old Guitar stereo combo  amps has a pair of speakers that diverts by some grades, nothing empede you to use separated Speaker Cabs and place It at the right distance fom each other

Franco

[acheld]

Old Guitar stereo combo  amps has a pair of speakers that diverts by some grades, nothing empede you to use separated Speaker Cabs and place It at the right distance from each other

100% true. 

I love stereo rigs.  I have a bi-amp setup at home, and love what it can do for me. 

When I'm somewhere else, though, I just can't lug that heavy stuff around.  One amp + cab on a hand truck are all I can muster. 

But I do have buddies that lug very heavy amps/cab around all the time;  it's all about what works for you.

[kagliostro]

Note that now the amps are re amped in big structures, so a middle amp with the correct character can be used instead of to be obliged to have a big one

Franco

[mrjomo]

a couple of thoughts on the whole subject…

A) multi-channel as differentiated from “stereo”... In my li’l bedroom world, I run through mono dynamics >> mono gain/ rudeness >> a chain of stereo in and out space n time pedals. I transformer iso the left signal and send it to a “clean” amp channel. I send the other to a ruder, dirtier channel and balance levels for taste. The speakers and I make a roughly equilateral triangle. There is a lovely sense of space. It can get downright gaudy with tacky use of delay and spinny fx. This is not stereo, but it does have a lot of spacial enhancement for a signal that started life as dry and mono. To be clear, this is a mono signal processed with “stereo” ambience and routed to two different sonic channels; not really stereo. For more fun, put 3mS delay on on of the amp feeds. oooh…

B) See the Don and Carolyn Davis Sound System Engineering book for circa 1980’s descriptions of single point stereo speaker sources. It can be done, has been done, and any patents have expired. The method described is NOT very practical, particularly for a portable guitar rig. This stuff was the precursor to our contemporary TV "sound bar”.

I don’t mean to drift too far afield…
JoMo