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(snip)

Roger wrote:

Do not put amplifier outputs in parallel. This has nothing
to do with O/P transformers. Any decent amplifier has a
very low Thevenin equivalent generator output impedance,
well below one ohm, likely around 0.1 ohm. BTW, this has
nothing to do with the nominal 8 ohm speaker rating which is
a measure of the ability of the amplifier to deliver
current. Thus, if the individual outputs are in any way
different the difference signal would see this load , i.e.
the other amplifier, and a huge current would flow that
could cause damage. Just being "in phase" is no guarantee
of no difference signal, e.g. the gains could be different.


Max Holubitsky wrote:

I find it hard to believe that any tube amplifier designed for guitar use will
have such a low output impedance.


Roger said:

It likely won't. It will be a rather crude PP design with
no NFB and lots of distortion by Hi-Fi standards. The
output impedance will be quite high. I was going on
speculate on the value - how does over 50 ohms sound?
Likely, you could put two in parallel with no damage - but
why?
Max:
I know what you are saying, and it is 100% true
for a solid state amplifier, especially one with a large damping factor,


Roger:
The same goes for good tube amplifiers. IIRC, damping
factors of 20 are common so the Thevenin generator is a
less than 1/2 ohm source.

Max:
however,
the amplifier in question will likely be just fine. I doubt that a tube guitar
amplifier will have a better damping factor than, say, a Dynaco ST-70, and the
Dynaco is intended to be paralleled this way.


Roger:
I can't comment on the Dynaco. Does it have any NFB?

Roger (before):
The correct way to get double the power is to bridge them
(see elsewhere) or much better, IMO, use two speakers - they
can even be in the same box.



Max (before):
Agreed 100%


Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"