Pre-amp question(s) from a newbie to Amp tech...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Cheebatone, Nov 1, 2005.


  1. Cheebatone

    Cheebatone Member

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    In an amp that has a Master Volume control in the power section, but also both Gain and Volume in the pre-amp section, is the Volume control just a 'passive' (hope that's the right term!) pot or is it also governed by a tube? If so, how do two identical tubes (say, two 12AX7's) do two different jobs? Are there circuits between the pots and the tubes that determine what the tubes do?

    Also, rather than have a two channel amp, could you put two switchable pots in front of a single tube to drive it at different rates and (effectively) get a psuedo channel-switching amp? Is this what amp manufacturers do anyway?

    Many thanks for any help given.

    Regards,

    Charly
     
  2. loverocker

    loverocker Member

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    Whole bunch of questions in there! :)

    A valve amp is a series (usually 3 or more) amplifications stages one after the other. Although valves can do different things, almost all are set as straight amplification stages, with values around them to tailor the EQ.
    So, a typical Volume/Gain pot sits between two stages, and acts to let you decide how much of the amplified signal from one stage gets fed into the next. Some amps with several cascaded gain stages have several Gain pots, others make do with just one (and are far less flexible as a result).

    Channel Volume controls and most Master Volumes are also like this , too - they just happen to sit before the power section. And yes - they are used for channel switching :) in the Soldano SLO, Mesa Boogies, etc.

    In almost all cases, these pots are 'passive' - they just act to divide the voltage. A few (very few) amps use smarter circuits with pots in positions that are more 'active' in effect - e.g. they control negative feedback, variable DC drive, etc.
     
  3. Cheebatone

    Cheebatone Member

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    Cheers LR!

    Anyone-else want to chip in?
     
  4. Wakarusa

    Wakarusa Member

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    loverocker's response pretty much covers it.

    Only edit would be that potentiometers are, in the parlance, strictly passive. Even if they are used in feedback loops and other fun places. "Active" implies a gain stage, even if the gain is unity or less than 1.

    Think active vs. passive filters. A passive filter is composed of all passive components (resistors, capacitors, inductors) while an active filter includes at least one "active" component or stage (usually an op-amp).
     
  5. VintageJon

    VintageJon Member

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    What amp are we talking about??

    -Jon
     
  6. Cheebatone

    Cheebatone Member

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    The one I'm having built. ;) :D :cool: :dude
     
  7. VintageJon

    VintageJon Member

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    OK, I see it now- Thanks!

    I go with what Todd said...

    -Jon
     
  8. hasserl

    hasserl Member

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    Well, the pot would go behind (after) the tube stage, not in front of it, but yes, you could run a gain stage thru two different pots then select which pot you want to be active using a relay. You could then set the pots differently for different levels of gain and select between them using the relay.

    It might be just as easy to turn a knob though.

    Different amp mfgr's design their channel switching amps in different ways. Some amps are true two channel amps, where the signal is split off to diferent circuits with seperate gain stages and tone shaping. Other amps switch gain stages in/out of the signal path but use the same tone shaping circuit. Others might use some variation of the dual pot system you mention. You could theoretically use a hybrid of all three.
     
  9. Cheebatone

    Cheebatone Member

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    Thanks guys! All your advice has been very helpful.

    Knowing precisely what sound you want out of an amp yet knowing precisely nothing about amp construction is a bitch of a combination but thanks, in no small part, to you - at least I'm on my way now.

    I'll post some pic's when it arrives ...next May.

    Cheers,

    Charly
     
  10. loverocker

    loverocker Member

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    :eek: What's it being made with - unobtainium? ;)


    Spill the beans, you tease! :)
     
  11. VintageJon

    VintageJon Member

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    Nothing is being made using unobtainium, it's unobtainable!!

    Things WERE made of PURE UNOBTAINIUM though and you see them occassionally: Dan Electro Reverb Transducers, Most Norton
    motorcycle parts, Switch pots for Evil Twins...

    -Jon
     
  12. Cheebatone

    Cheebatone Member

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    Nice to see there are so many fans of 'The Core' in the house!

    No, there's no unobtainium involved I'm afraid fella's (although I am trying to secure some Victorian Rosewood for the cabinet! :eek: ). It arrives on May 24th 2006 because that's my 40th birthday ...and I have the most generous wife in the World! :cool:

    Regards,

    Charly
     

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