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Thread: Fender Twin Preamp Question.

  1. #1
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    Fender Twin Preamp Question.

    I've got a Fender Twin RI. I happen to have 6 JJ ECC83s preamp tubes laying around, 1 being Balanced another 1 being a high gain and the other 4 are standard. I know that if I were to replace the preamp tubes, I would have to replace it with 4 ECC83s and 2 ECC81's. My question is, will it be an issue if I were to just replace the preamps with only ECC83s's? Or should I follow the directions? Will there be a drastic change in tone if I were to only use ECC83S?

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    Senior Member Enzo's Avatar
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    One of the 12AT7s is the reverb driver. The tube has lower gain than the 12AX7, but it also can handle a lot more plate dissipation. The reverb driver is in fact a tiny power amp driving the reverb pan as though it were a little speaker. It really wants to be the proper tube.

    The other 12AT7 is your phase inverter. I recommend that stay a 12AT7 as well.
    Education is what you're left with after you have forgotten what you have learned.

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    Oh I see, ill stick with the 12AT7's than. As far as replacing the 12AX7's, is it ok to replace it with ie. 2 standard 12AX7's, 1 high-gain 12AX7, & 1 balanced 12AX7? Or should it just be all standard 12AX7's?

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    Some people install a 12ax7 in the phase inverter, I do it on my twin it gives it a little more grit, it is just a matter of taste

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    They don't need to be super high gain etc, but ensure that the tube vendor you use guarentees them for gain, noise and microphonics.
    Tung Sol re-issue are my preference at the moment.

  6. #6
    Supporting Member txstrat's Avatar
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    Quote Originally Posted by pdf64 View Post
    Tung Sol re-issue are my preference at the moment.
    Mine too.
    As majwild1 told: "just a matter of taste"

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    Senior Member Enzo's Avatar
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    Just keep in mind that these amps are not precision circuits or critical circuits. You can stuff most any 12AX7 in the sockets and the amp will work fine. You personally may find a preference of one tube over another. or not. And remember too that each socket does a different job, so a favorite tube in one socket may not be your fave in another socket.

    Experiment with different tubes. The journey is half the fun.
    Education is what you're left with after you have forgotten what you have learned.

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    What are your thoughts on the Gold Pin preamp tubes compared to the normal ones?

  9. #9
    Senior Member Enzo's Avatar
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    I have been working in tube circuits for some 55 years now, and in all that time, I have had excellent service from conventional tinned tube pins. I have gotten along just fine without gold. Just one man's experience.
    Education is what you're left with after you have forgotten what you have learned.

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    Senior Member ThermionicScott's Avatar
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    Plus, you'd want gold contacts in your tube sockets to prevent galvanic corrosion. I'm sure someone sells that.

    - Scott

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    twin tubes

    Use the high gain for the reverb channels first position. Use the balanced tube for the phase inverter. Use a 12at7 for the reverb and the remainder can take the other 12ax7's. Basically if the tubes in it are not dead...you shouldn't have to change them out. You should only have to change out the power tubes when they get mushy & weak sounding. If you have a microphonic pre-amp tube, that calls for changing it out obviously, but otherwise I leave them in till dead. The tube thats right next to the phase inverter doesn't carry guitar signal so its not critical as that works the trem/vibrato effect of the amp

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    when you swap the tesle/jj's in the amp will sound alot more dirty & like a tube amp....and when you bias it hotter than stock.......it will come alive

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    'Use the high gain for the reverb channels first position'

    'Use the balanced tube for the phase inverter'
    Why? Neither has any benefit that I can see.

    As Enzo says
    'keep in mind that these amps are not precision circuits or critical circuits. You can stuff most any 12AX7 in the sockets and the amp will work fine'

  14. #14
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    "'Use the high gain for the reverb channels first position'

    'Use the balanced tube for the phase inverter'
    Why? Neither has any benefit that I can see."

    V2 (reverb channel preamp) is perhaps the most tone sensitive preamp tube in a AB763 type amp, a tube with good gain and, more importantly, low noise is often desirable for best tone & dynamics. V4 is the next most sensitive. Any weak tubes can go in V1 (assuming that you don't use the Normal channel), or in V5 which can tolerate some noisy/microphonic tubes. So try subbing them around but if you don't perceive a great deal of difference don't sweat it...your tubes might just be comparable on tone & noise?

    Balanced PI tubes? Well, if you have a balanced 12AT7, then there is no harm in trying it in the PI (V6)...but you are not likely to hear a significant difference to a non-balanced tube of the same type. Typically with a BF/SF LTPI you need to start altering circuit component values before you start getting a noticably mismatched tone from the amp. You are much more likely to hear mismatches in power tube plate current before hearing the effects of an unbalanced PI. Certainly do not pay through the nose for balanced PI tubes, though a nominal increase in price is fair enough especially if the amp's owner will get better peace of mind (if he's not bothered, you shouldn't be either).

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    blue printing

    Well dual triodes are not usually matched. You might find one that has a stronger side but thats only if you swap tubes out & perhaps if you have an inverted pre-amp design in a 2 channel amp.....all points being equal.

    I think it was torres designs back a number of years ago that put out in their news letter about matching the phase inverter for better tone & performance....basically the juncture of the 82K & 100K portion of the PI, where you would swap either 2 100k's or 2 82K's in. But anyway, the overall aspect was to go thru & blueprint the resistor values to the schematic being that the resistors were 10% tolerance types. Then further going to blueprint the caps to be tight & matched...coupling caps. In stock form the variance wouldn't really make much of a drastic difference to have to use a specific balanced PI tube.......But the guy has one on hand anyway & why not put it to use.

    BTW, I'm sure you wouldn't "see" a dfference, but perhaps might hear some difference.
    Unless you refer to watching a scope.

    The tonal performance of the tesla/JJ's make for a nice dirty tube.

    Personally I blue print my circuits when I build & use specific components at specific positions, but thats just me & how I do things.

    I have probably like 8 fender 100 watt chassis. SO I can set yours up for a side by side comparison. Then you can hear a difference & tell me what you want to hear from yours. And thats generally a kid coming in with a silver face amp & wanting the conversion & tubes & gone over. So when it comes to the pay day, I walk them thru on pre-amp tubes to tune into their ears which takes a long time to swap out & play...back & forth till they decide. Alot of players can't determine the difference between pre-amp distortion & output tube distortion....they have their heads in the fuzz pedals simulators.
    So they get a bit mystified when you take the pedals away & present them with a tube amp that finally plays like a tube amp.......refering to a silverface disaster(loud & clean).

    The dirt in a twin reverb is not as drastic as that in a tweed, its much more subtle. The tesla/JJ brings it out slightly more.

    But then if you can't hear it, you may be like my GF as she has lost some of her hearing as she gets to her 60th birthday...she can't hear the phone ringer unless she is right on it......and tells me I mumble all the time when I talk.

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