Conference napalm::guitar

    I'll had a chance to look at a 2 X 15 cabinet since entering the
    previous note, and it was wired in series, yielding 16-Ohm impedance.
    My assumption would be that the two speaker outputs on the amp are
    probably wired in parallel, 'seeing' 16 ohms when a single cabinet
    is used and 8 ohms when an extension is connected.  Is this correct?
    

    I have a Old 50 Watt Fender Bassman Head with the "ext spkr" jack.
    I am running it into two Marshall 15"s in seperate enclosures. 
    ie.  Two cables from the head to the speakers.  
    
    Am I still driving 50 watts or something less? 
    

    
    Re: .2
    
    I have an ancient Bassman head as well as the schematic for the
    early 70s "master volume" 50 watt head.  In both cases, the two
    speaker jacks are in parallel, not series.
    
    Interestingly enough, the main speaker jack is a switching jack.
     If you don't plug into it, the output transformer is shorted (I
    guess the amp tolerates shorts better than opens???) so if you plug
    into *only* the ext spkr jack, you get no sound!!!! (plus possibly
    a smoked amp!!)

	In a previous reply, I said that the "ext spkr" output
    	on Fender heads was a series connection. Forget it. I went 
    	home last night and checked all the Fender schematics I 
    	have, Twin, Super Reverb, Quad Reverb, Bandmaster, Dual
    	Showman, and the VibroSonic, and all have parallel ext spkr
    	outputs as stated in .4 . I don't have any schematics for 
    	bass heads, but have a schematic for any Fender amp with a
     	master volume.                          
    
    	Neal
    
    

    Yea, I kinda figured, 'cause if they were in series it seems to me
    you'd have to be using both or nothing.  That suggests to me that
    perhaps he individual cabinets might have been wired for 16-ohms,
    which sort of makes sense, assuming that the amp will deliver more
    power into an 8-ohm load, and that you'd want that extra power to
    drive the extra speakers.
    					joe
     

    A notes few back....

    An amplifier that uses an output transformer (versus direct coupled),
    needs to be loaded, hence a shorting jack on the output of the
    amplifier head. Leaving it open can cause your transformer to fail
    (and they are usually quite expensive to replace these days).

    For those of you with direct coupled amplifiers, you want to avoid
    shorting the output stage, as smoke, fire and brimstone may be the result.
    
							Jens

    
    
    	I'd like to get rid of my Laney 2-12 cab if this will work.  Would
    one of our technical dudes, help me with this?  I'd like to use my
    Concert head to drive the speaker in the cab of my deluxe and select
    one amp or the other using an A/B.. but I'm worried about how one amp
    sees the load of the other amp... It would look like this... Am I
    crazy, I'm not going to do anything until I hear... heh heh
    
             +------------------+---------------+
    Mr Concert		Mr Deluxe		Mr. Speaker
            (-)----------------(-)-------------(-)
    
    Mr. Electronics!!!!   Greeeeeeee Veeeeeee King!!!!!  (make him happy
    ladies and gentlemen.....)

Steve,

	The only way you can use both amp heads is to build or buy a 
switching system which will A/B between amps,and put a speaker load
on whichever amp your not using at the same time along with redirecting
the speaker leads to the amp your using. Perhaps finding a 1 12 cab 
for the Concert head would be easier/cheaper and just accept the tonnage 
for the price you pay for your sound.

If you do this switching thing, bear in mind that tube amps don't
enjoy open loads very much. If you ever noticed that the output jacks
provide a DEAD SHORT when nothings connected to them. If you run the 
output to a switching device, that device will need to provide that
short in place of a valid load.

--rjb

    Re -.1:  Is this really true?  I don't see how having a shorted output
    jack is electrically different than laying a hunk a wire across the
    terminals for the speaker leads...a dead short, in other words.  But,
    all of my amps warn against loads lower than 8, or 4, or 2 ohms, depending 
    on the amp.  Now you're proposing that I put a load of 0.000...0001 ohms
    (a short) across the amplifier outputs.  Could you elaborate?
    
    Regards,
    John

re: .11

I think he's saying a dead short is better than no load whatsoever.
I'm not sure I buy this, as either one would have a good chance of
toasting your output transformer.  Can anyone shed light on this?

-- Sam

    re.-1 Yep....
    
re-.2
	well,Ron I disagree,running a tube amp with no load or a short will 
fry his output x-former and/or shorten tube life. The transformer is suppose to 
be seeing 4 to 8 ohms. You prefer to dissipate heat through a 4-8 ohm load as 
opposed to the x-former. So you really want a load on the xformer when it is 
disconnected from the speaker and the other amp is being used.
	
	Here's my scenario:
Switch Position #1
                 
Amp A:

	Input--------- Speaker/sound

Amp B: 

	No Input------ Load/no sound

__________________________________________
Switch Position #2

Amp A:

	No Input------ Load/no sound

Amp B:

	Input--------- Speaker/sound




	Buy a 1 12 cab for the Concert....and an A/B switch,it's easier... :^)
your Deluxe will feel better!
    	

    RE:                <<< Note 900.13 by ROYALT::BUSENBARK >>>
                            -< Fender fried...... >-
    
    
    
    	Oh, ayuh, I'd like to say that I already knew this stuff, and was
    jest checking in, but I can't, <sob> I'm a dummy when it comes to
    electronics.  Thanks much.. I get it now.  No matter what I do, unless
    I build a bunch of switches that would include a dummy load for one of
    the amps.. I'd be pushing a signal into the transformer of one or the
    other amps at infinite load.. and... hey, do I smell smoke???
    
    Thanks again.. I ordered this cool EV 12 inch guitar speaker.. I like
    it tight... and if it sounds OK in my new deluxe, I'll probably take
    the Fender speaker out and build a little 1-12 for the concert as you
    suggest.. makes sense... Greee Veee King... saved from disaster once
    again by guitar notin' dudes!!
                                                 

Manufacturers put the shorting jack on the output probably
because they assume no one will drive the amp's input without
a speaker connected. The scenerio is that the amp gets set up
and turned on without the speaker connected well before the 
musician is ready to play. So there is typically no signal 
present before the mistake is discovered. The output stage is 
in a quiescent state. The xformer is not dissipating any power.

I'm trying to think of why the amp makers short the output xformer 
secondary rather than just leave it open. I'm sure they didn't put 
a more expensive jack in the unit just for laffs. If you assume that
they know what they're doing, then obviously (in a 
vacume tube amp) a dead short is preferrable to an open load or 
Fender et al wouldn't have built their units this way for the last 
30 or 40 years. If you are actually driving the amp, though...

Hey Jay! Got any comments on this???

    
    
    
    	OK, how about another stoopid question???  I've got this great cab
    filled with two celestion speakers (70 watters) and it's 8 ohms... 
    
    	I'm going strictly single 12 with my sound soooo... I'm sellin my
    beloved celestion cab...  But wait... if the cab is 8 ohms, that means
    they've hooked two 16 ohm speakers in parralel or 2 4 ohm speaker in
    series what keeps me from hooking a little 8 ohm resisitor across the
    leads of a 16 ohm speaker to make it an 8 ohm speaker... or from
    hooking a teeny weeny 4 ohm (is there such a thing?) to one of the
    leads of a 4 ohm speaker to make it 8 ohms...  Don't make fun of me,
    I've got a fragile ego...  would this stuff work??
    
    
    Gree Vee

No. And that's all I'm gonna say. I've gotten into
too much trouble in this note already.

--rjb

    ah come on Ron it's all in fun!!!!!,hey steve! Ron's right,and thats
    all I'm gonna say too.... :^)      The answer is no.....
    

    
    
    
    	Heh heh, thanks, dudes... Ron.. helpful advice dude.. I'll be
    saying thank-yew... 
    
    Steve

Steve,

	I have not been following this note....but from what I understand you 
have asked re: about 3 notes back......

You want to rip out (or bypass electrically) one of you speakers in the two 
speaker cab??

Yes you can do it, but you will lose power in the single speaker.

If it is a four ohm version you need a four ohm resistor in series to make 
eight ohms. The four ohm resistor must be capable of disipating the same 
wattage as the speaker. Because power is proportional to the square of voltage 
or current, you will only get one quarter of the sound power out of the 
cabinet.

If the speakers are two sixteen ohms in parallel, you will need to provide a 
sixteen ohm resistor across the remaining speaker. Sound and resistor power 
comments apply as above. 

The reason why you get less sound power is that the amplifier is providing its 
signal into the "dummy" resistor and your "lost" sound is being converted into 
heat.

Hope this helps and that I have not jumped in with mouth blazing and missing 
the point !

Dave

    	There is a good reason for shorting the output jack when there's no
    speaker connected. The transformer is an impedence matching device. The
    output impedence is relatively low (2-16 ohms). Ask yourself what is
    closer to that load, a dead short (0 ohms) or an open (infinite ohms).
    Another thing about transformers is that they reflect the impedence of
    the output on the input. This means the AC design of the amp is
    affected with sometimes catastrophic results. Jay can probably explain
    this better. 
    
    	THIS IS NOT TRUE WITH SOLID STATE AMPS!!!!!!
    
    	If you short the outputs of one of these she's a goner.
    
    FJE

    
    
    This brings up another question:
    
    	In transistor amps it's OK to put a higher load that the
    amp is rated at, with a related drop in output power.
    
    	What are the rules for tube amps? Can I put more load
    than it wants? Less load? 
    
    -T
    

    
    
        ....I've got this old super reverb...the 4-10's are wired for
    	2-ohms....and the amp has an extermal speaker jack as well....
    	...the back panel doesn't specify maximum load.....
    	...now what do think would happen to the output transformer
    	if I plug in another 8-ohm on top of the 2 ??
    
    	...what if I rewire the 4-10's to 8-ohms...
    
    	...will I take a chance of blowing something if run this thing at
    	8-ohms ???
    
                Ray.
    	

    I've heard 'never less than 4 ohms' for what that's worth. If you rewire to
8 ohms your power goes down.


  - Bob

    I'd leave your 4 10s wired the way the factory wired them. A super
    has a 2 ohm secondary. If you change the load, you'll never have
    an optimum load without your exention spkr (assuming you've
    created a combination that will give you a 2 ohm load). Admittedly,
    a 2 ohm speaker output doesn't give you much leaway for adding more
    speakers. It's really your call, but I would leave a super just
    the way the factory built it. After all, it was one of the most
    successful amps Fender ever made.

    
    
    	OK...I just called the factory...here's what they had to say..
        ....The output transformer wants to see a 2-ohm load...
    	if one chooses to change speaker configuration it is recomended 
    	that the output transformer be changed to match the load...
    	keep in mind that going to a different output transformer will 
    	effect the tonal qualities of the amp......
    
    
    	.R.