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Bridging Guitar Amp



 
 
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  #1  
Old August 16th 03, 08:28 AM
Max Holubitsky
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> Isn't paralleling the outputs dangerous?

The amplifier has output transformers, so it shouldn't be. The Dynaco ST-70
was intended to be operated in parallel, and my RCA theatre amps have
instructions to be operated in parallel. I can't really see what the problem
would be so long as they're in phase.


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  #2  
Old August 16th 03, 08:33 AM
Henry 007
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Default Bridging Guitar Amp


"Max Holubitsky" > wrote in message
...
>
>
> Peter Mulvey wrote:
>
> > Hi all,
> >
> > Anybody have any thoughts on the feasibility of bridging a 2 x 50 W tube
> > power (guitar) amp? It's a Marshall 9005.
> >
> > Schematic of one side of the amp is available he
> >
> > http://www.drtube.com/schematics/marshall/9005.gif
> >
> > I would like to use this amp to drive a large cab, and 100W would be

nice. I
> > could strap the inputs together using the link terminals, but what to do
> > with the OT outputs. Is it common practice to bridge tube amps, or is

this
> > just a plain bad idea.
> >
> > Many thanks for your thoughts.......
> >

>
> If you link the inputs together, you can parallel the outputs. This will

not be
> ideal, but it will work. the output impedance will be half of whatever

taps you
> tie together - i.e. 8 ohm taps tied together gives a 4 ohm output

impedance.
>
> If you want to bridge it, you'll have to invert one of the input signals.

Really
> though, if it's a large cab, why not just connect 2 speakers in the cab to

one
> 50W amp, and the other 2 speakers to the other amp, and then tie the

inputs on
> the amp together?
>
>
> >
> > Pete.


Isn't paralleling the outputs dangerous? Why not just run 2 or more
speakers? If you have the inputs bridged then the speakers will be in phase
and you will have effectivelly 100W. Too damn loud IMO, but then again I
like 5W champs...



  #3  
Old August 17th 03, 08:42 PM
Engineer
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Max Holubitsky wrote:
>
> > Isn't paralleling the outputs dangerous?

>
> The amplifier has output transformers, so it shouldn't be. The Dynaco ST-70
> was intended to be operated in parallel, and my RCA theatre amps have
> instructions to be operated in parallel. I can't really see what the problem
> would be so long as they're in phase.


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.

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.

Cheers,

Roger

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

"Friends don't let friends vote Liberal"
  #4  
Old August 18th 03, 04:47 PM
Max Holubitsky
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> 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.
>


I find it hard to believe that any tube amplifier designed for guitar use will
have such a low output impedance. I know what you are saying, and it is 100% true
for a solid state amplifier, especially one with a large dampting factor, 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.


>
> 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.
>


Agreed 100%


>
> Cheers,
>
> Roger
>
> --
> Roger Jones, P.Eng.
> Thornhill, Ontario,
> Canada.
>
> "Friends don't let friends vote Liberal"


  #5  
Old August 18th 03, 10:06 PM
Fred Nachbaur
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Default



Jimmy wrote:
> YEs, you can paralell them, I done it and it worked OK. However driving two
> separate speakers worked even better. Obviously you are wanting some serious
> volume. Just paralelling the amps will give you 3db more. At the levels we
> are talking about I doubt if you will here the difference between one amp
> and two. You Will hear a substanial difference if you feed each amp into
> their own speakers.


Again, however, the increase is only 3 dB. You can't get sumthin' fer
nuthin'.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

  #6  
Old August 19th 03, 06:17 AM
Jimmy
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Well you can, sort of, The two amps with two speaker systems sounds
noticably louder,fuller than either the 1 amp and 1 speaker system or the 2
amps and one speaker sytem. I chalk the difference up to efficecy of the
speakers. Guitar pluckers having been doing this forever.

"Fred Nachbaur" > wrote in message
...
>
>
> Jimmy wrote:
> > YEs, you can paralell them, I done it and it worked OK. However driving

two
> > separate speakers worked even better. Obviously you are wanting some

serious
> > volume. Just paralelling the amps will give you 3db more. At the levels

we
> > are talking about I doubt if you will here the difference between one

amp
> > and two. You Will hear a substanial difference if you feed each amp into
> > their own speakers.

>
> Again, however, the increase is only 3 dB. You can't get sumthin' fer
> nuthin'.
>
> Cheers,
> Fred
> --
> +--------------------------------------------+
> | Music: http://www3.telus.net/dogstarmusic/ |
> | Projects: http://dogstar.dantimax.dk |
> +--------------------------------------------+
>



  #7  
Old August 19th 03, 07:35 PM
Tom Schlangen
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Hi Fred,

>> You Will hear a substanial difference if you feed each
>> amp into their own speakers.

>
> Again, however, the increase is only 3 dB.


No it ain't :-) and the explanation for us tech oriented
types is quite simple and has nothing to do with the electrical
power, but with speaker efficiency: When you stack speakers
(notably woofers) close enough together, fed with the same
signal, power, and in-phase, they act (because of overlapping
wave compression effects) as a single cone with a somewhat larger
cone area and efficiency than two cones separated far enough.
The drawback is higher directivity, but this might even add
to the audible effect when you are in the "beam".

You can ask any old fashioned PA systems guy why in those olde
days of under-kilowatt-PAs the speaker cabinets were stacked
like that the cones in the cabinets are located as near as
possible to each other.

Or, try to find some info about how "Schallzeilen" work;
many comparatively little diameter speakers positioned nearly
together in a vertical line - used e.g in churches, parliament
halls, and so on.

And D'Appolito design speakers get added efficiency this way,
although the primary intention was another goal (single point
wave source).

> You can't get sumthin' fer nuthin'.


That's right. In the case under discussion the resulting
higher speaker efficiency is gained by more distortion and
more directivity.

But the priciple works just fine for geetah and bass-ment
stacks, and for stacking PA cabinets, and for "Schallzeilen".

Back to the topic of this thread, I wholeheartily agree
that two amps with two cabinets (or one multispeaker cabinet,
if you can split the speakers in it to be fed by
two amps of the same type) are _the_ way to go.

An added plus is that no modding whatsoever of the gear itself
is required.

If you still don't believe, just take a classic geetah
stack (consisting of bottom cabinet, upper cabinet and
piggy-back amp) separate the cabinets at least 1 metre
horizontally apart, plug in your favourite axe and
compare the result to the verticaly oriented setup
with cabinets stacked vertically. If you don't believe
your ears, just use a sound preasure meter. You won't
be disappointed about the added dBs of sound pressure :-)

Tom

--
Anyone who considers protocol unimportant has
never dealt with a cat. - R. Heinlein
  #8  
Old August 19th 03, 08:01 PM
Fred Nachbaur
external usenet poster
 
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Default



Tom Schlangen wrote:
> Hi Fred,
>
>
>>>You Will hear a substanial difference if you feed each
>>>amp into their own speakers.

>>
>>Again, however, the increase is only 3 dB.

>
>
> No it ain't :-) and the explanation for us tech oriented
> types is quite simple and has nothing to do with the electrical
> power, but with speaker efficiency: When you stack speakers
> (notably woofers) close enough together, fed with the same
> signal, power, and in-phase, they act (because of overlapping
> wave compression effects) as a single cone with a somewhat larger
> cone area and efficiency than two cones separated far enough.
> The drawback is higher directivity, but this might even add
> to the audible effect when you are in the "beam".
> [...snip for brevity]


Good info here! Thanks, Tom!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

  #9  
Old August 20th 03, 04:49 AM
Engineer
external usenet poster
 
Posts: n/a
Default



(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"
  #10  
Old August 21st 03, 08:33 AM
Jimmy
external usenet poster
 
Posts: n/a
Default

I forgot to add. I you paralell the out puts you must put a 1 ohm several
watt resistor in series with each amp output or risk frying the amp. You
also need some way of closely matching the gain setting s of each amp. In
general it is a PITA and not suitable for any practical use.

"Jimmy" > wrote in message
...
> YEs, you can paralell them, I done it and it worked OK. However driving

two
> separate speakers worked even better. Obviously you are wanting some

serious
> volume. Just paralelling the amps will give you 3db more. At the levels we
> are talking about I doubt if you will here the difference between one amp
> and two. You Will hear a substanial difference if you feed each amp into
> their own speakers.
>
> "Peter Mulvey" > wrote in message
> ...
> > Hi all,
> >
> > Anybody have any thoughts on the feasibility of bridging a 2 x 50 W tube
> > power (guitar) amp? It's a Marshall 9005.
> >
> > Schematic of one side of the amp is available he
> >
> > http://www.drtube.com/schematics/marshall/9005.gif
> >
> >
> > I would like to use this amp to drive a large cab, and 100W would be

nice.
> I
> > could strap the inputs together using the link terminals, but what to do
> > with the OT outputs. Is it common practice to bridge tube amps, or is

this
> > just a plain bad idea.
> >
> > Many thanks for your thoughts.......
> >
> > Pete.
> >
> >

>
>



 




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