Tube amps.

ecktronic said:
I have a Marshall JCM2000 DSL50 tube amp with 1960 4x12 cab. I was wondering if anyone knew exactly what the presence knob does. I know it changes the sound and makes it sound better, but does turning up the presence knob drive the tubes more for a smoother warmer sound or is it something else?

http://www.marshallamps.com/images/products/dsl/dsl50.html
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The presence control adds "presence". The higher you turn it, the more "in you face" the guitar sound is. The lower you turn this control the more the guitar tone softens and blends in with the other instruments.
 
I have a Fender Bandmaster with a presence control. It seems to be an eq boost that accentuates the mid frequencies (when I listen), and is in the feedback loop of the last amplifier stage, including the output tubes and the output transformer, so it may have some effect on the tube drive at those frequencies.

http://www.kbapps.com/audio/schematics/tubeamps/fender/bandmaster5e7.html

I don't know squat about tube circuits - maybe somebody can explain its action.
 
It is a feedback loop. It takes the signal after the power tubes, filters out the low end, and sends it back through the power tubes again.
 
Farview said:
It is a feedback loop. It takes the signal after the power tubes, filters out the low end, and sends it back through the power tubes again.
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This is probably just splitting hairs, but it looks like it variably shunts the high frequencies to ground. Assuming this is a negative feedback loop, this would have the effect of increasing the gain at those frequencies. I don't understand why it is done at the output stage of the circuit, rather than in the preamp section. Does it produce a substantialy different sound or effect if done this way, or is it just a matter of convenience or circuit simplicity?
 
In Fenders, the loop feeds back into the preamp, from after the output trannie. The loop is there to keep the output from running away or oscillating in a poorly designed amp, so that is why it comes from the output stage. In a good one, it keeps the gain in the design limits. You are right, it is a negative loop, in Fenders anyway, and selectively dumps the highs from the feedback loop to ground.
It comes after the trannie because.....I don't know. :)

I would imagine the presence control is jumped on that loop like you said, for convenience.
 
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i'm pretty sure that in my marshall (JCM 800) the presence is just a high end filter, that is, higher than the treble. am i wrong?
 
boingoman said:
In Fenders, the loop feeds back into the preamp, from after the output trannie. The loop is there to keep the output from running away or oscillating, so that is why it comes from the output stage. You are right, it is a negative loop and selectively dumps the highs from the feedback loop to ground.
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I understand why the neg feedback is there - I don't understand why the presence control is part of it. What is the sonic difference in this, or simply having a high frequency boost in the tone controls of the preceding stage?
 
mcolling said:
i'm pretty sure that in my marshall (JCM 800) the presence is just a high end filter, that is, higher than the treble. am i wrong?
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That is the result, but it isn't part of the tone stack.
 
crazydoc said:
I understand why the neg feedback is there - I don't understand why the presence control is part of it. What is the sonic difference in this, or simply having a high frequency boost in the tone controls of the preceding stage?
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Since you are feeding a distorted signal back to the beginning, it adds another layer of fuzziness to those highs that are fed back through. Kind of like what an aural exiter has been reported to do.
 
Cheers all. I suppose it doesnt mater too much if i totally understand it. I know it makes it sound better when i put it at half way. In mine it sounds like it is boosting the mids making the overall sound thicker and creamier and maybe even more distinguishable with the notes.
Ive had my amp well over a year and i dont really know why i havent bothered playing about with it till now. The thing cost me almost £1000 so you would think i would know the ins and outs of it. Suppose thats just my laziness.
 
the presence control is part of a negative feedback loop. (If it were positive feedback the amp would turn into an oscillator). No , the amp won't become unstable without it, at least not that amp. It (the precense control) tweaks the amps bandwidth, that is its range of audible and non-audible frequencies. But it does it at a cost; gain. More high end means a bit less gain and vice versa. One can compare its effect on bandwith by measuring it where the amplifier gain equals 1 (unity-gain bandwidth).
 
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