Can you run an external cabinet along with a Polytone Mini-brute?
JimKroger
1. I know the Mini-Brute IV powers a 4 ohm speaker. If you plug in a
speaker cabinet (4 or 8 ohm) to the external speaker jack is it in
parallel or series? Does it lower the load (that's dangerous right?)
or increase the resistance (that's OK right?)?
2. I have a Polytone Mini-brain (with the sonic circuit and
haromonics, about 10 yrs old) with 15 inch Polytone cabinet. I just
bought an old Mini-brute IV. It sounds MUCH warmer than the newer
Polytone which has not been played much. I want to know if it's the
old and broken in speaker, or the electronics. Is there any way, short
of opening it up and installing switch, to turn off the internal
speaker and power the external speaker? Or, to just power the internal
speaker from an external source? I guess that would take installing a
jack.
Thanks.
Jim
Bg
But before I got the RE Cab, I had a tech change on of the jacks in
back to a 1/4' (Maybe Headphone) jack that shut off the connection to
the internal amplifier, so that I could use the box as a speaker Cab.
Didn't sound anywhere near as good as the RE Cabs. though.
I don't keep that amp at home here, so I can't even check which one of
the 2 jacks he modified, it was quite a few years ago.
I think the RE website said it was OK to use and external cab with
your Mini Brute, as long as it's an 8 Ohm cab.
It'll still drop the impedance below 4 Ohms., and I wouldn't want to
play that way with the volume turned up too high.
Bg
JimKroger
Thanks. That's what I have trouble understanding. It seems like adding
another speaker adds additional resistance so 4 + 8 is more ohms. But
maybe it provides more signal path so the resistance is lower.
Jim
Guy
wrote:
We're talking impedance here which is not quite the same thing as a
raw resistance. Still the formulae for raw resistance is a good guide.
If you were to connect your external speaker in <series> with the
internal speaker then you would simply add the two resistance figures
- eg 4 + 8 = 12 Ohms since there is a single current path through both
speakers.
But an external jack is virtually certain to be wired in parallel
(otherwise if in series the external speaker would have to be
permanently connected to get any sound at all) so unless it
simultaneously cuts out the internal speaker you have the situation of
two signal paths which you mentioned. Here the formula for the
effective total resistance is :-
(R1 * R2) / ( R1 + R2)
So for a 4 + 8 ohm scenario, the effective resistance is 32 / 12 =
2.66 Ohms.
Of course because the voltage across both speakers is the same, the
power delivered into each speaker will (again this is not quite right)
differ being Voltage squared / Resistance for the speaker. But the
perceived sound output would depend on speaker efficiency also.
FWIW I have an early 1980s vintage Minibrute 3 and the manual says
'100 watts RMS into 3 ohms/15" speaker/single channel'.
Guy
Kevin Van Sant
wrote in message
<90305ca2-2886-409b...@g25g2000yqn.googlegroups.com> :
I have a Polytone mini-brute IV or V, whichever has the 15" speaker
and switchable horns. The other night I plugged in an RE Twin 8 as
an extension cab. It worked great and didn't blow up.
________________________________________
Kevin Van Sant
http://www.kevinvansant.com
info, music, videos, etc...
dave t
I always forget those amps run at 4 ohms. If they have an ext spkr jack, I
guess they must be able to handle 2 ohms. Sure, w/ a switching jack, the
ext spkr could be added in series, but that wouldn't make sense, since it
would lower the pwr output of the amp. OTOH, I've seen stupider things...
JimKroger
> On Tue, 23 Nov 2010 00:28:55 -0800 (PST),JimKroger<jkk...@gmail.com>
Hi Guy, yes, I found a post somewhere recently stating the ohms for
all the polytones and it agrees with your manual. I downloaded a copy
of the new polytone manual and it says it's OK to connect an external
4 or higher cabinet. But from what you describe this goes below the
rated ohms and isn't that the dangerous direction?
So with the formula for a 4 ohm external (polytone or my RE twin 8),
(4*4)/(4 + 4) = 2 ohms. This sounds dangerous but I would think
Polytone would expect you to stick a 4 ohm Polytone external cabinet
in that external speaker jack. Now I'm puzzled.....
JIm
JimKroger
> On Mon, 22 Nov 2010 19:18:39 -0800 (PST),JimKroger<jkk...@gmail.com>
> wrote in message
Well, that's encouraging :-).
Guy
wrote:
>On Nov 23, 3:36 am, Guy wrote:
Hi Jim,
Sorry for delay in replying - PC has been in bits all weekend but OK
again now!
I think it's difficult to be confident how "safe" it's going to be. As
you lower the resistance in a DC circuit then the amount of current
draw increases - Ohms Law says :-
I = V / R where I = current (Amps)
V = Voltage
R = Resistance (Ohms)
So you can see that if you were to stick a piece of thick wire across
the output creating a dead short, the resistance would effectively be
zero, the current would try to rise to near infinity and in the
absence of any circuit protection the output stage of the power amp
(at least) is going to blow.
Some power amps are protected and can cope with dead shorts but I
doubt the Polytones are. However they are fused - at least mine is -
which might save the day although they are likely slow blow.
You also have to consider how much Voltage you are producing. If you
are using the amp near flat out into an effective 2 ohms then I'd be
really worried. But at a moderate volume then the average voltage out
will be considerably lower than maximum so the actual current into 2
ohms will not be so high.
Of course these are AC circuits which do not exactly follow the simple
laws for DC circuits.
I'd proceed with caution. Can you get any information directly from
Polytone? In my experience it's hard. I did write to them once some
years back giving my amp's serial number and asking for some info. I
was actually looking to buy another Polytone and said so in my note so
I thought they might reply but they didn't. I landed up buying three
German AER amps instead so that was some business Polytone might have
had if they been a little more customer friendly.
Guy