Conference napalm::guitar

    
    Yes, that's exactly what the old Scholz Power Soak was, a high power
    L-pad.  
    
    

    SO, did they (Power Soak, that is) work?  If so, how come they didn't
    catch on (or did they)?
    							joe

    
    	Yeah, but it really beats on the amplifier, by making it work
    hard, cookin up the power tubes real good for just a *little* bit 
    of sound...
    
    	The dual ganged master volume at the control grids of the power
    output tubes is the "right" way to do it..."power soaks" are a way
    that most people can deal with it - at the expense of the amplifier
    and, apparently their wallet. A $150 "resistor"...
    
    	How come MESA-Boogie's master volume control isnt a giant 100W
    potentiometer connected across the speaker output?
                                                     
    	Joe Jas

    By the some token, how come the Mesa's Master Volume is just placed
    between the preamp & the power stages (at least in the one I have
    schematics for)?  From what note 246(I think) says, the MV at the
    control grids can also hammer on the tubes - I don't know enough
    hollow-state to comment, only report.  As for L-Pads, I really can't
    see why anyone would want to install one (2) in a stereo system,
    or is there some benefit beyond merely making the amp work harder?
    
    							joe

    
    Also, the Power Soak wasn't foot-switchable  8;(  which made it
    of little interest to giiging players who needed both clean and
    dirty sounds.
    
    Altair and EU Wurlitzer (the dreaded Boston Strangler box) made
    similar devices as well.
    
    One company, Jim Kelley, made an amp with such an attenuator built
    in that *was* foot switchable, but to do it, they had to install
    a *relay* into the speaker path!!!  I'm not sure if the FACS amps
    are still being built or not.  Many reviews of it said that due
    to the relay setup it had even better overdrive sounds than the
    Boogie!!!
    
    

    
    Re: .4
    
    In stereo systems, L-pads are commonly used to balance tweeter output
    vs. woofer output and to allow volume control of speakers in remote
    locations (like driving speakers on the porch from the amp in youd
    living room).

    

    I see, so they really weren't built with things like 'Marshalls on
    10' in mind.  Well, I knew it sounded too simple - sure glad I asked
    before wasting money a/o my amp.
    						joe

    The Sholtz Power Soak is designed to keep a constant load
    on the output of your amp, i.e. switchable 4, 8, 16 ohms.
    EU Wurly's "Boston Strangler" is just a high wattage wire-
    wound rheostat. They sound great until your head blows up,
    which happened quite frequently. Sholtz had a better idea.
    	
    Neal
    
    
    

    Jim Kelly amp's are out of buisness,but I have heard that some his
    ex employees are still repairing them......
    
    							Rick

    I think the main reason the Power Soak never caught on big is a
    combination of the previous mentioned things, summarized; 1) relative
    high cost, 2) not footswitchable, 3) accelerates amp wear.  I also
    believe that a lot of people never really understood what it was
    supposed to do. 
    
    I have one (which I bought for $40 from a pawn shop) that I use with my
    Hiwatt amp.  It allows me to get a really nice screaming tube
    distortion at moderate volumes.  This is a big advantage for me, for
    several reasons.  The amp is a 100 watt 1x12 combo.  Even with a big
    EVM speaker, there is no way I can crank the amp up enough to get more
    than a little distortion without having a nice blown speaker and
    bleeding ears, perhaps I could with an external cabinet(s), but I don't
    want that much volume, or a big heavy cabinet to lug around right now. 
    
    I realize that it accelerates wear on the amp componants, but the wear
    is no more severe then running the amp at full volume through regular
    speakers, just cuts the volume back.  Since I have another amp for
    personal practice, I don't consider the wear to be a major detriment.
    One big advantage of this setup is being able to get the consistant
    sound at varying volumes.  If I had a simple (smaller) tube amp, which
    I cranked all the way up to get the sound I wanted, I would then have
    absolutely NO control over the volume (without running through a PA).
    The Power Soak also eliminates impedence matching problems by
    presenting the amp with a constant load, regardless of the speakers
    being driven. 
    
    All in all, it meets my current needs VERY well.  It allowed me to get
    the sound I wanted from the amp I had without having to invest a lot of
    money.  To buy another amp, sell this one, and STILL not have the
    flexability that I currently have using the Power Soak.  It's not for
    everyone, but I works well for me. 
    
    Greg

    Another aspect of the power soak - it alters the sound of your amp.
    Maybe not enough to bother some people, but I tried power soaking
    a 50 watt Marshall head and the result was not pleasant to my ears.
    It seemed to rob the sound of high end and fullness and also reduced
    the dynamic response (i.e. how the amp tracked "touch").  The reason
    is that power soaks are (I believe) straight resistive loads; speakers
    are not, the impedance of a speaker has reactive components and
    thus changes slightly with frequency.  The power soak will let you
    run your amp at distortion levels, but don't expect it to sound
    like the same amp cranked through a real speaker.
    
    If you really want to get "soaked", Harry Kolbe in NYC makes a load
    box that emulates a speaker's reactive load; in fact, it particularly
    emulates the load of a 4 ohm Marshall 4x12 cab.  The price? a mere $200
   
    /rick

    re .10   Jimmy, the owner of Mr. C's music in Marlboro "claims"
    that Jim Kelly plays in a band with him when he is not traveling
    around the world. Of course Jimmy makes a lot of claims that are
    often hard to believe. I have to admit, though, he does have a nice
    store, and a great selection of Gibson guitars. He also seems to
    be somewhat of an expert on musical equipment history, since he
    has been in business for that last 20 years and has had many
    of the major franchises popular in the 60's and 70's.
    
    Mark
    

    Re: .13
    
    Mark:
    
    I've had a number of interesting conversations with Jimmy. His
    expertise is a figment of his overactive imagination.
    
    Danny W.

    Re: 13 I mispelled Jim Kelly it should be Jim Kelley,however there
    are probably as many Kelly's as there are Smiths. And everyone knows
    one,as I had a teacher by the name of Jim Kelly who btw was an
    excellant musician(but that was in 1976).
    
    							Rick

    Yeh, but the Jim Kelly that Jimmy at Mr. C's is referring to is
    the genuine article that had his own amplifier company, and has
    played professionally worldwide.
    
    Mark

    I think Jim Kelly is playing for the Bills after a stint in the
    USFL.
    
    

    
     I have to admit that I don't the first thing about the electronics
    involved here, but that has never stopped me before 8-).
    
    The Power Soak seems to be an extreme answer to a simple problem. The
    simplest way to get an overdriven tube sound out of an amp is to do
    just that...overdrive the preamp tubes.  
    
    You can buy about a zillion effects to do just that, but they all
    change the sound of the guitar/amp combination in some way.  I use a
    preamp built into my guitar to boost the output to such a level that
    the preamp tubes distort. The natural sound of the guitar is not
    changed at all; there is just a lot more of it.  I've tried "hot"
    pickups in lieu of the preamp, but they don't put out enough of
    a signal to really get the amps attention.
    
    The 9-volt battery in the guitar lasts close to 9 months with regular
    use.  The preamp fits in with the volume controls and you would never
    know its there unless you looked inside.   I also don't have any stomp
    boxes to lug around, which is a big win as far as I am concerned. 
    
    When I was using a Twin, I used to turn the preamp volume all the
    way up and then kick in the guitar preamp when I wanted a fatter
    sound.  I still got the Fender Twin sound, but it was a very excited
    Twin!
    
    I have tried the guitar on amps that don't have master volume controls
    and, while it's not perfect, it much better than a normal guitar
    signal.  An amp with a master volume is the way to go.
    
    Anybody else use a guitar preamp?
    
    Kevin
    
    
    
    

    Yeah, Kevin's sound kicks butt.  I heard him at Sweatjam, and the
    the things his guitar was doing to that poor Peavey!

    /rick

    Along these lines, my Seymour-Duncan Convertible 100 has a power output
    control which allows you to select from 5 watts to 100 watts of output.
    I can use this very effectively to control the amount of "clean" in my
    sound, since I really don't need the full 100W output. This is in
    addition to pre-amp overdrive controls and modules that are
    specifically designed to distort. Does anybody know what this power
    output control is actually doing? I've seen similar features on a few
    other amps, such as Dean-Markley, and there were no warnings given such
    as "clamping down on the power output will shorten tube life or
    eventually fry your amp".
    
    If anybody is really interested to check it out I have schematics they
    could look at.
    
    - Ram

    
    Amps like the Duncan and the Boogies (which have low/high power
    settings) work by either shutting off some of the output tubes or
    adjusting the voltages supplied to the tubes. 
    
    I'm not sure how this affects tube longevity.
    

    Yeah, I forgot to mention that it also has a switch to shut off some of
    the power tubes. That's not what's happening with the power output
    control. Perhaps adjusting the voltages to them. While we're on the
    subject, it also has this control referred to as speaker damping. It
    seems to "tighten up" the speaker, almost as if the range of motion of
    the coil were actually being limited. But I'm pretty sure this is done
    electronically, not a mechanical effect on the speaker. Any clues on
    this one?
    
    - Ram

    Well, I took a look at Ram's Seymour Duncan schematics this weekend;
    here's what I found:
    
    The "power output" control looks identical to Rick Busenbark's master
    volume mod described in #256.  It basically attenuates the signal
    at the input to the power tubes, allowing the preamp and phase inverter
    stages (the stage just before the actual power tubes) to be overdriven
    at decreased volumes.  There is no reduction of voltage to the power
    tubes and they themselves are not being overdriven when the power
    output control is turned down, so you don't really get much "power tube
    distortion" with this control engaged.  Note that the Duncan also has a
    "master volume" control; this attenuates the signal coming out of the
    pre-amp but before it gets to the phase inverter.  This is the
    traditional factory mv control.  The difference is, the power output
    control gives you pre-amp plus phase inverter distortion, the factory
    master volume just pre-amp distortion.

    I also checked out the speaker damping control.  Damping refers
    to the speaker's response to transients (i.e. sharp attacks) in the
    amplified signal.  Anyone who took a basic electrical circuits course
    should remember damping of RLC circuits.  If a speaker is underdamped
    then it oscillates too much after a transient and produces a poorly
    defined tone.  The effect could be described as looseness or flapping.
    On the other hand, if a speaker is overdamped, it restricts the
    transient so that the full attack is not reproduced.  The effect is
    a tight, overcompressed sound.  Optimal damping would reproduce
    transients accurately and without undue oscillations.  Transient
    response is particularly critical in guitar amps since the guitar
    signal has a lot of sharp transients.

    Speaker damping is an electromechanical phenomenon, related to the
    output circuit, speaker construction, and even the air being driven.
    Heavy, humid air will have more of a damping effect on speaker response
    than light, dry air.  This atmospheric damping may account for some of
    the moodiness people have mentioned with regard to their amplifiers.

    Anyway, I did a little research and found out that speaker damping in
    tube amplifiers is also affected by the ratio RL/Rp, where RL = load
    (speaker) resistance and Rp = plate resistance of the output tube
    circuit.  A common way to control this damping factor is to introduce
    some negative feedback into the output stage of the amplifier.
    Negative feedback involves taking a portion of the amplified signal (e.g.
    at the output transformer) and feeding it back into an earlier stage of
    the amplifier such that it "subtracts" from the signal at that stage.
    Negative feedback in the output stage tends to decrease the value of Rp
    seen by the speaker; the more negative feedback is introduced, the lower
    Rp, and thus the larger the damping factor (RL/Rp).  This in turn increases
    speaker damping.  This is exactly what the damping control on the Duncan
    does; it controls the amount of negative feedback in the output stage of
    the amp and thus controls the damping of response in the speaker itself.
    
    So, it seems that the Duncan damping control would be particularly useful
    in adjusting the damping response of the amplifier to correct for things
    like atmospheric conditions, different speakers, and even "hot" pickups.
    You should always be able to adjust speaker damping to be in the optimal
    range for tight but accurate reproduction.  A pretty neat feature.

    /rick_the_amp_nerd

    A few minor details:
    	1. A speaker cabinet's impedance is a mix of resistive, capacitive,
    and inductive. The value of 4 or 8 ohms is only a nominal one, as
    the various components of it change depending on frequency. Another
    factor is the mechanical resonance of the speaker(s) in the particular
    cabinet, this is affected by the speaker design itself and the size
    and shape of the cab. Because of these factors, the impedance curve
    of a particular cabinet can look like a mountain range with high
    peaks and valleys but being centered around the nominal impedance.
                                                   
    	This in itself means that some cab/speaker combinations will
    give accentuated responses in certain ranges.
    
    	2.Tube amplifiers use output transformers to convert the high
    voltage in the power tube section to current to drive the speakers.
    An impedance on the output of the transformer is seen as a component
    of the input impedance on the other, and therefore is able to affect
    the amplifier's performance. In fact the presence control is not
    a tone control persay, it uses the reflected impedance in a way,
    fed back to the preamp to change the frequency response of the amp.
    
    	Taking these to facts into acount, one can see why the past
    attemps at attenuators did not get the desired results. The load
    presented to the amp is very complex, and not easily duplicated.
    Putting an L-pad in the line to attenuate the volume chnges this
    load and therefore the sound of the amp. Harry Kolbe has build a
    beast the emulate a Marshall cab as part of his amps, this is now
    available seperately and should really do the job of giving the
    right sound without the ear splitting sound level (especially in
    small basements)
    	from_a_bass_player_who_cares_(about his ears)
                                            ***
    side 

    
    According to Andy Brauer's column in the December GP, the Kolbe
    speaker emulator is a (reasonable) $200.  The address is listed
    in the column.
    
    

    I've spoken to one guitar player who has actually tried the Kolbe
    emulator (not me, I'm too poor these days); he didn't think it gave
    a "natural" sound.  Your mileage may vary.

    /rick