SJB Audio Preamp Mystery(John Phillips)??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Sol, Mar 17, 2006.


  1. Sol

    Sol Member

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    I Have a long discontinued amp by SJB Audio 'Ant'.

    It was a 30 Watt amp with 4 6V6 in the power section, but its the preamp
    thats causing the confusion.

    The valves are laid out parralel to one another 4 6V6 too the front
    and the peramp valves at the back.

    Preamp layout is as follows, looking from the back and going left to right.

    Ge 5751, 12AU7, ECC81, 6u8a (ECF82)

    It should be pointed out that that amp has both reverb and tremolo.

    Heres the rub : Which is the first valve a signal see's the 5751 or the 6u8a ?

    Out of ignorance, I've assumed the 5751 is the first..But what if it's actually
    the 6u8a instead ?

    I've made a few observations that I'm hoping will help anyone able to help.

    These are: exchanging the 12AU7 for 12AT7 increses gain, the same result
    can be obtained by fitting Ecc83 (12AX7) in the place of the 12AU7, it should be noted that occilation occurs at high master volume settings with this type

    I've found minimal changes by exchanging the other valves, athough
    removing the 6u8a results in no sound at all

    So.. is the 6u8a (ECF82) the first or the last and what is it doing?

    The amp has no choke either and uses what I believe to be current limiting resistors, what on earth does this mean?

    I'm sorry for such along post but am at a loss as to how to understand
    this quaint but toneful amp, and any and all advise recieved greatfully!
     
  2. John Phillips

    John Phillips Member

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    I have no idea, sorry. I've never seen one of those amps.

    The only thing I can usefully say is that the ECF82 is (I think) a combination triode/pentode so it's unlikely to be the phase inverter.

    Which tube is closest to the input jack? That's probably V1.

    Does pulling either the ECC81 or the 12AU7 stop either the reverb, or the tremolo, or both, but still allow dry signal through?
     
  3. Sol

    Sol Member

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    John, thanks for getting back so soon, I dont know for sure but I think
    the ECC81 next to the 6u8a is the reverb.

    I'll be able to check when I get back later tonight, so I'll tell you then:AOK

    I knew the 6u8a was a minature Pentode triode type, but are there any other amps that utilse a valve like this? And more to the point..What are they doing?

    I've heard that (Brian?Desighner?) Referred to early RCA circuit diagrams
    and had a keen interest in early Gibson amp's

    That's as much as I know for now, get back later
     
  4. Sol

    Sol Member

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    John, I followed your advice. and found the following : Removal of 12AU7and
    Ecc81 resulted in no sound, silence, I was surprised I was expecting something.
    Removing the 5751 resulted in sound, about3/4 of the output volume was lost, however both reverb and tremolo both worked.

    As a side note, without the 5751 the amp went into overdrive much sooner
    than it normaly would albeit at much reduced volume.

    Is it possible that the ECF82 (6u8a) is operating both reverb and tremolo ?

    Update,
    I went back and removed the ECC81, and although no sound from the speakers, I could distinctly hear the thump,thump thump of the tremolo
    with intensity maxed, in addition to this increasing the reverb to max
    I could hear the familiar increase in noise one gets when using reverb at extreme settings, interesting... All thoughts most welcome, and apprieciated.

    Sol:)
     
  5. John Phillips

    John Phillips Member

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    That's very strange!

    None of that really makes sense... :)

    One problem is that all of those tubes are 'dual' types, so actually there are eight 'tubes' there - and although it is usually arranged that the two halves do somewhat related functions, it's not necessarily so...

    It is obvious that at least one half of each of the ECF82, 12AU7 and ECC81 must be in the audio path, if pulling them stops the sound. It also sounds like the ECC81 is relatively early in the signal chain, which is maybe not what I would have expected - I wondered if it was the phase inverter, and although pulling it might still allow the tremolo to work if it's a bias-modulation type, it shouldn't allow the reverb to.

    The only explanation for the drop in volume and headroom when the 5751 is pulled I can think of is that it may be used in some sort of gain stage with heavy negative feedback connecting the plate to the grid (like the middle stage in a Fender AB165 Bassman), so some signal can get past even with the tube not there...

    Can you open it up and take a good picture of the inside of the chassis?
     
  6. Sol

    Sol Member

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    I have a digital camera and will be able to take some shots over the next few day's or so, however..I've no idea how to post images on this forum:confused: So what I could do is post them to you via email.

    I believe my email address can be accessed in my profile, I'll check though.

    It may interest those interested that this amp is wired throughout in true point to point.
    Not a component board anywhere to be found, and to look at such a complex amp as this wired in this way really is a thing of beauty!

    Thanks for the continued support of this forum, cool place
    :D
     
  7. Dave C

    Dave C Gold Supporting Member

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  8. Sol

    Sol Member

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    Dave C,
    Thankyou for that PDF file very interesting..It's slightly different to my own amp, but gives a good idea as to the inner workings of this rather confusing amp!

    I'll try get some images taken today some time, if anyone could host them for me I'd be most greatfull.
     
  9. Wakarusa

    Wakarusa Member

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    No personal experience with this amp, but have to disagree with you on this one point John. Ampeg used 7199 (pentode/triode) as phase inverters in several designs. Think I've seen a couple of other oddballs that mimic this layout as well. Typically the pentode is a final gain stage feeding the triode set up as a split-load or paraphase PI.

    Here's an example (from an amp I've owned for... well, forever)
    http://www.drtube.com/schematics/ampeg/g15-jp.gif

    All of that said, I've also seen the pentode/triode tubes used for reverb -- pentode as drive, triode as recovery.
     
  10. Sol

    Sol Member

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    Wakarusa,
    Thanks for your input.
    Are you saying that the 6u8a(ECF82) is the first valve (V1?) or the last in the chain (V4?).

    The layout, left to right is : 5751, ECC82, ECC81, 6u8a.

    Ive experimented with valves in diffrent places, and the position that makes most difference, is the spot now home to the : ECC82/12AU7

    Changing this valve /tube to ECC81/12AT7, increses gain somewhat.
    Whilst introducing a ECC83/12AX7, to this position brings on early overdrive
    that while sounding great, developes occilation at high master volume levels.

    Little change can be heard replacing the : 5751 to ECC83/12AX7.
    Perhaps the 5751 is there as it is low in microphonics, and tends to be well balanced on both sides of the triode.

    I'm not a tech, so this is my guestimation, if you know what I mean, lol
     
  11. John Phillips

    John Phillips Member

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    It still doesn't quite make sense...

    From that diagram, the 6U8A is the reverb drive and recovery (which makes it puzzling that removing it should stop the amp working...), the 12AT7 is the phase inverter, the 12AU7 is the intermediate gain stage (which would have the most effect on the gain, as you found), and the other tube (EF83) is V1. That amp doesn't have tremolo. That would make it likely that the 5751 is the tremolo oscillator/driver, and if it's a bias-modulation type, removing it might cause the amp to become quieter and overdrive earlier because the bias might be way out of the correct range. That would also explain why changing it has no tonal effect.

    But where's your V1? Either the whole circuit must be very different, since otherwise there isn't going to be much gain at all, or it must be 'hidden' - could there be another tube where you can't easily see it, like behind one of the transformers? Close to the input jack is the most likely place.
     
  12. Sol

    Sol Member

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    John, shoot me an email and I'll send you the images I took today of the gut's of the SJB, if you like.
    I have no means of hosting them at the moment,so thats the only thing I could think of, cheers,
    Sol
     

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