Re: Rola speaker/extension cabinet wiring questions

From: "dlightfoot" <dlightfoot@xxxxxxx>

> I recently acquired a very clean Silvertone 1432 cabinet which had no
> chassis, but still had what I believe may be the original Rola speaker
> (codes 285949 and 15526 on the basket).  I think it would therefore date
> from the 49th week of 1959...I have no idea what the 15526 code
> speaker doesn't have a big heavy magnet on the end of the basket, either.
> Just a little cylinder about two inches in diameter, kinda copper-colored
> with a U-shaped metal guard over the end... Anyone have experience with
> these, and are they suitable for harmonica?

Not specifically, but the magnet you describe is alnico, highly prized for
musical instrument speakers.  The best way to find out if it's a good harp
speaker is to try it out.  It probably is.

Don't assume the impedance is 8 ohms.  It probably is, but we can't be
absolutely certain without testing.  It could have been reconed to a
different impedance.

Fortunately, this is very simple to check.  Use an ohmmeter, cheap,
expensive, doesn't matter.  Make sure the speaker isn't connected to
anything else.  8 ohms impedance will usualy show up as somewhere around 6
ohms DC resistance.  Voice coil resistance will never be more than the
speaker impedance.

I'll explain below, but I think your two best options are 1. change the
output transformer to one with speaker impedance taps (and install a
switch), or 2. get a PA or power amp to run the extension cab.

> I would like to use the gutted one as an extension cabinet for my other
> in order to better deal with some overenthusiastic members of my local
> community (how is that for diplomacy?) and am wondering how best to
> accomplish this.  Is it as simple as clipping the Rola speaker leads onto
> the Jensen terminals?  I assume that the Jensen is an 8ohm speaker, and so
> if I wire the extension cabinet properly (now would that be series or
> parallel?) I can maintain the overall resistance at 8ohms and thus not
> out the output transformer...

You can't wire two 8 ohm speakers to be 8 ohms.  Either you wire them in
parallel ( + to +, - to - ) for 4 ohms, or in series ( - of spkr 1 to amp, +
of spkr 1 to - of spkr 2, and + of spkr 2 to amp - you'll need a special
jack to do this, one that will open the speaker circuit when you plug in the
2nd cab) for 16 ohms.

If your amp doesn't have a switch to change speaker impedance, I wouldn't
recommend running the 2nd cabinet, especially not with a Silvertone amp.
They're VERY cheaply made - the output transformer of a 100 watt Silvertone
is about 1/5th the physical size of a 100 watt Marshall.  The transformers
(output and power) and other components are generally running dangerously
close to overload.  Danelectro made 'em cheap, and actually figured in the
"safety factor" engineered into components when designing their amps.
Manufacturers know there are variances in product, and, for example, will
design a "40 watt transformer" to handle something more like 50 watts as a
"fudge factor".

The Silvertones are great sounding amps, though.  Just don't tax them like
you would a Marshall.

"Hey Mike - I run a mismatched extension speaker (or similar) on mine and
it's worked fine for <xxx> years".

Yes, this can happen.  Some Silvertones lucked out and got the beefier
components.  Most didn't.  If it's yours, your prized vintage amp could be
the source of the smoke in "smokey places".  It's your amp, and you increase
the risk of failure if you do this.

If it were my amp, I wouldn't run the extension cab, or I'd recone the
speakers to 16 ohms and run 'em parallel for 8 ohms (BUT you'll have to
ALWAYS use the extension speaker), or (better) replace the output
transformer with a beefier one with speaker taps for 4 and 8 ohms, and
install a switch on the amp.

There's another option you might want to look into, concerting your cab into
a "powered speaker".  Look for a power amp, tube or solid state, for the
extension cab.  If you take the signal from the amps speaker thru an
attenuator/isolation transformer (2 resistors and a $4 Radio Shack 600
ohms-600 ohms telephone transformer - you need the transformer for
isolation, because your amp is probably poorly grounded), or mic it if the
amp has a mic level input (easier, but a little more prone to feedback, plus
you need an extra mic), you'll get great sound, even from a solid state
power amp.

How many watts is the Silvertone?  I'd go with a solid state power amp about
double (or more) this power, because we don't want this to overdrive (we get
that from the tube amp).  Tube distortion sounds great.  Solid state
distortion is "fingernails on the blackboard".  In fact, the harmonic
content is remarkably similar.)  You want the solid state extension amp to
run *absolutely* clean.  That's what they're best at.  Make absolutely
certain that the volume of each speaker is as close to identical as you can
get it (testing at medium "clean" volume outdoors would be good).  If in
doubt, run the extension just a tad lower.  This will protect your speaker.
Once you have this level set, you should never have to adjust it again.

Hope this helps.

BTW some LIVE tunes by The IronMan Curtis BonTemps Blues Band:
(and - hint hint - we're looking to tour)

Health care for blues musicians?  Sign the petition!

- -IronMan Mike Curtis *Southland Blues Magazine TU 8pm jam Starboard Attitude, Redondo pier
Every Sun, 2pm Stagger Inn, 9018 Alondra, Bellflower, except:
Last SUN, LARHA HarpJam, Tia Juanas, Irvine
6/11 10pm BB Kings, Universal Citywalk
6/18 7-11pm Mission Tobacco, Riverside CA

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