Conference napalm::guitar

    Impeadance is a measurement of reactance, and it relates to AC signals,
    and the frequencies that they are measured at. An 8 ohm speaker is 8
    ohms at a given frequency (usually 1000Hz). If measured as a DC
    resistance, the impeadance measurement won't necessarily be the same
    as the resistance shown on your meter. Actually, I'm surprized that
    your numbers are as close as they are for the 4 ohm cabinet, I would
    have expected 2 to 3 ohms.

    As for the Marshal head; you might have damaged it, as most tube
    amplifiers require a load on the output transformer at all times when
    powered up. If the output transformer goes, you'll never forget the
    smell (it stinks) that it has. For now, I'd continue to use it, as it
    doesn't appear to be damaged (listen for bubbling sounds eminating
    from the transformer - If you hear them, something is wrong, or about
    to go wrong).

							    Jens

    Amplifiers (especially tube ones) rarely if ever need a load on
    them inorder to avoid damaging them. On the contrary it is too 
    much load ie too low an impedence (close to or a dead short) at
    the output that will damage an amp. Even with tube amps the output
    is coupled to the speakers through a transformer thus isolating
    the output. 
    
    You can't measure speaker impedence with just an ohmmeter.
    
    Jim
    

    If I read .2 correctly then you're dead wrong. Open the output of
    a tube amp (ie: disconnect the speaker) and your output transformer
    is swiss cheeze. Something about reflected power etc...similarly
    putting too small of a load on it (ie: less than rated ohm-age)
    will produce similar results but perhaps not as fast (depends on
    how small the load is)
    
    dbii

    
    	What happens to the output transformer is that the energy has
    no place to go when a speaker is not connected. The energy then
    tries to leak out wherever it can, usually as a very high voltage
    on the primary windings. When this voltage gets high enough in
    potential, it can jump thru the varnish insulation on the primary
    winding, causing a carbon trail. Once the carbon trail is formed,
    the voltage doesnt need to get very high, so, even with the speaker
    connected, it will still prefer conducting across the carbon trail;
    you're output transformer is ruined.
    
    	According to the Marshall schematics I have, the output goes
    *open circuit* when the speaker is disconnected; there is no "shorting
    pin" on the output 1/4" jack connectors! This leaves the possibility
    of ruining the output transformer wide open, should the amp be operated
    at a high level with the speaker cab accidently disconnected. Thanks,
    Jim!
    
    	This situation can be corrected by replacing the std "spkr 1"
    jack with one that switches the output to ground, when the plug is 
    removed. Even a Traynor amp gives you that!
    
    	Joe Jas   

    re .2 >You can't measure speaker impedance with just an ohmmeter.<
    
    Sounds believable - how can I determine the impedance of unmarked
    speakers that I have?  Is there a measurement technique that doesn't
    require an BSEE?  Can the DC resistance of a speaker be used to
    make an educated guess at the impedance (i.e. would an 8 ohm speaker
    always show a DC resistance in a certain range)?
    
    After reading these responses, I double checked the DC resistance
    measurements I took on 'known' speaker cabs, and my first reading
    was not precise.  As it turns out, the 8 ohm cab read about 6.5,
    its individual speakers (2x16ohm 12's wired parallel) measured
    about 13.2, the 4 ohm cab read about 3.4 and its speakers (2x12
    8 ohms) also read 6.5.
    
    I was trying to use these 'known' readings to guess the impedance
    of other drivers I have which aren't labeled - I would like to
    build up another cab.  Any ideas?
    
    Thanks,  Steve 

    You are fairly safe in your assumption that you can guess the
    impeadance using an ohm-meter. Most speakers are either 8 or 16 ohms
    (a few are 4 ohms, but not many). Some devices are only thier rated
    impeadance at certain frequencies (Piezo's have been discussed in this
    conferance - I guess that above 5 - 10 Khz they are low enough to work
    as as speaker load, below those frequencies, the impeadance is 1000
    ohms or more - note: I have no documentation in front of me to verify
    this, nor do I have any referances available that discusses piezos in
    depth), and others are only active based on a cross over network.

    One thing that I'll mention is that you should attempt to make sure
    that your speakers are in phase with each other. A quick way to verify
    this is with a 1 1/2 volt battery. When you touch the speaker terminals
    to the battery, it will jump in or out. If you have speakers wired
    together, they should all jump in the same direction when you attach
    the battery. (9 volt batts work too)

    As for smoked transformers, I had a Fender BASSMAN transformer that
    went up in smoke relating to an unconnected load on it. Since Fender
    was the only source for replacement parts at the time, it cost about
    $95.00 to get replaced (back in the early 1970's). I use a Twin Reverb
    these days & I never run it unloaded.

							    Jens

    re .5 Transformer backlash is only a problem with a poorly designed
    transformer coupled TRANSISTOR amplifier. And that is only if they
    were too cheap to diode protect the output. Tube amplifiers and
    non transformer coupled transistor amplifiers are not harmes by
    a no load condition at the output. The most danger to these amplifiers
    comes when you SHORT the outputs not the converse. Maybe you are
    thinking of input shorting pins, that would be correct. The backlash
    creates by the low impedence secondary of the outout transformer
    against 300+ volts in a tube amp is minimal. This is *assuming*
    the amplifier was designed properly.
    
    Jim
    

    re .8 Perhaps you're right today, however, my old Marshall went south after
    I raised the load from 8 ohms to 50+ (back when I designed the first
    power soak say '71 or so). The output transformer fried
    bigtime...standing waves ate the primary.
                                     
    At that time Marshall or any other amp I'm aware of (primarily Fenders)
    were not protected in any way...screw up the load and they fried. Just
    running a 4 ohm load when set for 8 ohms was enough to cause them
    grief.
    
    dbii

    
    
    	Hold on a second.  .5 is right, in a transformer coupled  output
    stage of a valve (sorry tube) amp if the output is not loaded there
    is nothing to absorb the back E.M.F.  The final drive in my valve
    (tube) amp runs 400v anode to cathode  then from first principals
    its dI/dT x #turns on primary = Back E.M.F = Kvolts.  What happens
    then is arcing occurs across the transformer.  This can be
    supressed by use of a snubbing zener diode but as most users of
    yester-year tech were aware of the fact it was a problem to run
    open circuit it was not normal to fit these.
    
    	I had this problem on an old HI-WATT amp which had a dry joint
    on the speaker connection, the blue sparks were very pretty for
    the little time it lasted.
    
    	Of course running short circuit dumps all the output current
    into the secondary windings & they melt basically.  So all you guys
    working on super-conducting speakers for your boogies & marshalls,
    it's not a good idea :-)
    
    	Cheers
    
    		Pete.