Replacing V1 in Princeton requires biasing ?

Discussion in 'Amp Central Station' started by Fullmoon07, Nov 1, 2018.

  1. Fullmoon07

    Fullmoon07 Tele-Meister

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    If I replace the stock V1 in my Silverface Princeton Reverb '68 Custom with a Tung Sol 5751 do I need to have it biased again ?

    68CustomAmpsv5_hi-1897297f58a4d53b4c8e09b25a037ec2.jpg
     
  2. Singin' Dave

    Singin' Dave Friend of Leo's

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    Absolutely not. Only rebias when replacing power tubes and/or rectifier
     
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  3. Fred Rogers

    Fred Rogers Tele-Afflicted

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    Nope
     
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  4. Fullmoon07

    Fullmoon07 Tele-Meister

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    Thanks. What tonal changes do you predict with the Tung Sol 5751 in V1? More clean clarity perhaps? And what clean tone would a 12AY7 in V1 produce, clarity?
     
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  5. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    Don't confuse "cleaner" with "headroom".

    The 5751 has 30% less gain than a 12AX7.

    What this means in a practical sense is that, where the 12AX7 yielded a particular tone and volume at "7" on the dial, with the 5751 you're going to need to turn that knob up to "10" to get the same sound.

    If this sounds like "more headroom" to you, I have a bridge in Brooklyn I'd like to talk to you about.

    To get *more* headroom, you need *more* power, not less. Scotty understood this. So should we all.

    But, to be fair, "more gain" doesn't mean "more power". It really means the water is coming out of the hose faster. To get "more power", you want to replace the garden hose with a fire hose.

    In an amp, it means you'd need to re-design the entire amp. The 12AX7 is already at 100%. Want more? It would just overload the front-end of the amp. Gee, just like the bazillion boost pedals already available.
     
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  6. Singin' Dave

    Singin' Dave Friend of Leo's

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    1300 is correct - a 5751 will just give you more play on the volume knob before breakup, and a 12AY7 even more still, perhaps even to the point where the amp doesn't fully saturate on 10.

    The bigger difference is the BRAND of tube you choose. The fun thing about swapping preamp tubes is you can just roll them in and out of V1 and hopefully land on something that sounds best to YOU.

    For me, I'm a huge fan of RCA tubes in my driver slots on my SF amps (V1 in my VibroChamp and my two Princetons, and V2 in my DR, VR and Pro Reverb). I love Amperex Bugle Boys too but they are crazy expensive these days. For new production, I do actually like the Tung Sols.

    The RCA's (or other NOS) ain't cheap these days, but will last you a long time and are worth it for that one key slot.

    That said, even two tubes from the same maker of the same type can sound a little different. So roll the ones you have through V1 and enjoy!
     
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  7. AxemanVR

    AxemanVR Tele-Afflicted Ad Free Member

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    Low gain tubes can effectively change the gain structure, so they can also shift the tone characteristics.

    This may be good or bad.

    One possible consequence is having a duller more lifeless sound or you could experience no apparent change at all.

    But, since it doesn’t hurt to try, go ahead and experiment all you want...


    `
     
    Last edited: Nov 2, 2018
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  8. Andy B

    Andy B Friend of Leo's Silver Supporter

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    The TungSol 12AX7 is my favorite new production tube for the first preamp tube in most amps.
     
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  9. Fullmoon07

    Fullmoon07 Tele-Meister

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    Any experience with Genalex Gold Lions ?
     
  10. JD0x0

    JD0x0 Poster Extraordinaire

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    So, while we're on biasing and 'headroom' I should mention, that biasing isn't 'required' when changing preamp tube type, IMO, it's recommended if you want to get the most out of the new tube type.

    For example, if you have a 12AX7 center biased (max headroom) and drop in a lower gain tube, you could be throwing the load line off center, which will REDUCE headroom. We can adjust the biasing by replacing the cathode resistor on that triode's cathode to rebias the new tube to the center of its load line, for max headroom. In most cases though, the gain reduction of the tube compensates with less signal and most people wouldn't notice the loss in headroom, though it's technically there.. The posts above are correct, that just dropping in a lower gain tube doesn't increase 'headroom' it just reduces signal, which requires you to compensate with more volume, giving the perception of more 'headroom'

    So, while rebiasing is definitely not required and totally safe to ignore in most cases, when changing preamp tube type, it's recommended, if you intend to run the tube at a specific operating point (to have max headroom, for example). Most people don't mess with preamp tube biasing, because you're basically talking about redesigning stages of the amplifier, and it's a bit complex for people that don't have experience drawing and calculating load lines.
     
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