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Circuit question regarding a higher gain preamp tube

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Reeek, Oct 15, 2005.

  1. Reeek

    Reeek Member

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    My Sanos call for a 12AX7 and a 12AU7 in the preamp section. Is there any circuit danger if I put a 5751 or 12AX7 in that 12AU7 socket?

    I know putting lower gain tubes than designed for may not hurt components but what about if I upped the gain when the schematic calls for a 12AU7?

    Thanks!
     
  2. Blue Strat

    Blue Strat Member

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    Shouldn't be a problem. The gain has nothing to do with whether or not it's safe to do. It's things like heater current draw and plate current draw (internal resistance) that could be issues.

    Generally, all the familiar 12A*7 type audio tubes will work fine (with no danger) in the same sockets.

    Which Sano amp is this? I worked on one of these for a customer recently (can't remember the model number) which was a very cool amp. Very Keith Richards!

    This one used 8417 power tubes.
     
  3. Reeek

    Reeek Member

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    That would likely be the 300R, Mike.

    I have two 160R's and a BA-1 which is in the flavor of a Bassman 20 but with EL84's.

    IMHO, the best Sanos are the EL84 driven amps and all three of these amps are EL84 driven but the curcuit design makes them thick and big sounding almost like a low powered 6L6GC amp. All three of these amps are amazing. I'm dropping some different 5751's in that socket to see what happens ;)

    Thanks for you reply too.

    I had a 300R but sold it and I still have the matched Bogen Sylvania 8417's you sold me for it. The original Sylvanias were ok when I didn't think they were.
     
  4. Wakarusa

    Wakarusa Member

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    Agree with Mike that it should be a safe swap, but I'm curious to see if you like the results. The 12AU7 in the 160R serves as the final gain stage and a split-load phase inverter. The 5751 may give you a bit more gain than you really want here (blocking distortion anyone?). Might try a 12AY7 if the 5751 doesn't do it for you.

    If you're feeling adventurous and want to chase down which half of the 12AU7 is phase inverter and which is gain, a 12DW7 may be just the ticket -- assuming the 'halves' line up right, it would preserve the 12AU7 characteristics in the phase inverter and make the preceding gain stage a 12AX7 ;)
     
  5. Reeek

    Reeek Member

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    Todd,

    I've used those "hybrids" before and had great results with them but I did install a USA made black plate Hewlitt Packard "selected" 5751 (looks like a Sylvania to me but what do I know!) in that socket last night and played it with my Agile AL-3000 with the alnico humbuckers and the tone is fabulaous with not too much gain so I'm going to leave it there for the time being :) I'm plugging the Suhr into it today to see how that sounds. With the 12AU7, I didn;t have enough gain to get the tonal overlfows I wanted even at 3/4. But then I had those lower output GE oval plate EL84's in ther then too.

    Thanks for the heads up though. Now that you say one of the 12AU7 stages is the split load PI, I'm going to go read the schematic so I can learn. Knowing this tidbit will give me reference to what I'm looking at ;)
     
  6. Wakarusa

    Wakarusa Member

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    Not sure I get your point. I'm well aware of the relationships between mu, Rp, etc.

    In this particular circuit I'd expect to see about twice the voltage gain from the 5751 than from the 12AU7 in the pre-PI gain stage, hence the post wondering if he'd be happy with the results.
     

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