Reduced Gain Preamp Tubes in an EL84 amp vs "just turning it down"

Discussion in 'Glowing Bottle Tube Amp Forum' started by CIM, Jul 25, 2021.

  1. CIM

    CIM Tele-Meister

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    Playing through something like say a typical 5W EL84 amp with two stages of pre-amplification curtesy of a 12AX7, the breakup can come early in the amp's volume range.

    To make an amp cleaner/later breakup I have seen videos changing preamp tubes to lower gain versions e.g. 12AY7, 12AT7 etc.

    My question is - what real different does this approach result in when compared to simply turning the level down on the volume control to a comparable drive amount with the original 12AX7 preamp? As yet I have not found any demo anywhere that does a clear A-B of this.

    To put it another way, does a lower gain preamp tube increase the amount of clean volume available before breakup or does it just change the effective clean range of the volume knob sweep?
     
  2. MuddyWolf

    MuddyWolf Tele-Holic

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    I can only relate my experience. A lower gain preamp tube doesnt give a lively snappy high headroom late breakup sound. It just sounds kind of weak. To get some kind of repsponce happening you now need to push a hotter signal into the input. So the gain still has to come from somewhere, either before the tube or in the tube.

    Many are going to contradict this, let it begin....
     
  3. mexicanyella

    mexicanyella Friend of Leo's

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    My experience with this was, years ago, I used a Tube Works RealTube II preamp in a guitar rack. It had two 12 AX7s in it. I think it was a starved-plate hybrid circuit, one tube per channel...but I’m not sure of that. Anyway, I thought its breakup was too harsh and spitty and abrupt. I experimented with 12AU7s and 12AT7s in either or both tube slots, and I think I ended up with 12AU7s in each.

    What I remember clearly was, the preamp could get waaaaay dirtier than I’d ever need in stock form, and with the tube swap, it gave me a more appropriate-to-my-needs sweep on the gain control...and it seemed like the clean-to-dirty transition was more gradual. The dirt itself seemed a little softer-edged and pleasing to my ear too.
     
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  4. tomasz

    tomasz Tele-Meister

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    I swapped preamp tubes in my Pro Junior, as it only has a volume and tone knob. That helped with some more headroom. But in my current amp, that has a gain and volume, I wouldn't care
     
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  5. VintageSG

    VintageSG Friend of Leo's Silver Supporter

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    Mostly the latter with some of the former and mug of tea.

    To achieve the same output Watts, the EL84 requires the same input voltage. A 12AU7 has lower gain, so that gets compensated for by upping the gain and volume controls, -but- the sweep(s) are more manageable.
    Tomato is one pronunciation, another is tomato :)

    Try it. A 12AU7 won't cost a fortune. Add a 5751 and a 12AY7 into the mix too and play around. Don't expect miracles.
     
  6. DavidP

    DavidP Friend of Leo's

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    I'm a big fan of 5751s in my 6V6 amps. I get a little more volume before breakup.
     
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  7. Mpd2378

    Mpd2378 TDPRI Member

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    I've swapped out the 12ax7 for 12au7 and it gave me a quieter amp but stayed clean till about 9 on the volume as opposed to 5-6.
    I preferred it as I had more control.
     
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  8. beyer160

    beyer160 Friend of Leo's

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    I have a Ceriatone Yeti, which is a hot-rodded Plexi design. That is way too much gain for me in stock mode, so I swapped the preamp tubes for 12AT7s and it sounds absolutely killer. It’s certainly not dull sounding, and I can ride the guitar volume knob just like they did in the good old days.
     
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  9. Lynxtrap

    Lynxtrap Tele-Afflicted

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    In that kind of amp, the power tube is doing most of the overdrive duty.
    To get "X" amount of overdrive you need "Y" amount of gain from the preamp. The output tube doesn't care what tubes are used, it only cares about the voltage swing coming out of the preamp.

    If you put a lower gain tube in the preamp, the difference would be that you need to turn up the volume knob more to achieve "Y" gain and "X" overdrive. With a 12AU7 you will likely not get to that point even with the volume on 10.
     
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  10. Timbresmith1

    Timbresmith1 Tele-Afflicted

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    5751 in phase splitter spot.
     
  11. Lynxtrap

    Lynxtrap Tele-Afflicted

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    ...except there is no phase splitter in a single ended amp.
     
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  12. Timbresmith1

    Timbresmith1 Tele-Afflicted

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    V2 will have to suffice, then.
     
  13. 39martind18

    39martind18 Friend of Leo's

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    Personally, I like 6L6s in my 6V6 amps- more focused, tighter low end, and because it's necessary to bias the 6L6s a bit cold (using and to preserve the transformers that come in the 6V6 amps) not a lot more volume, but more clean headroom. YMMV
     
  14. teleforumnoob

    teleforumnoob Friend of Leo's

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    My only experience is with a little Orange Micro Terror which is a 20w solid state power amp with one preamp tube.
    It came with a JJ Ax7. I changed it to a Mullard Au7 and it gave be a good bit more clean headroom without any noticeable loss in volume. It still had plenty of gain on tap for my purposes.
     
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  15. CIM

    CIM Tele-Meister

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    That was kind of what I suspected may be the case, so the main reason to swap in any lower gain preamp tube is to increase the sweep in the clean range of the amp and reduce the overdriven range available, not to change or "improve" the character or available volume of whatever clean tone the amp has to offer.

    And so if you are happy with clean tones appearing over a reduced span of the low end of the volume control sweep, there may be no real benefit in doing this.

    The only counter to this I can think of would be if the first stage of the 12AX7 drives the second stage so that it starts to add its own clipping into the signal but I don't expect there is very much drive contribution from the preamp stages in comparison to the output stage.
     
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  16. tap4154

    tap4154 Doctor of Teleocity Ad Free Member

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    I tried several different preamp tubes in my Pro Junior, and it ended up that a JJ AY7 in V1 and a JJ ECC81 (AT7) in V2 sounded best, and had less idle noise. I just recapped the amp and really reduced the idle noise to nothing, so I may try the AX7s again, but maybe not.

    I also stopped playing the attenuator game with the Pro Junior, and got a little overdrive pedal. I play clean most of the time anyway, and like a nice thick mid-rangey clean tone.

    My red knob Super 210 really likes a 5751 in V1. Makes the gain channel sound much smoother, and tames the volume control.
     
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  17. Abu Twangy

    Abu Twangy Friend of Leo's

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    I used a 12AY7 in V1 of a '97 Hot Rod Deluxe for a while. It stretched out the clean range of the volume control and dropped a little gain in the "gain" and "more gain" settings. Otherwise the amp seemed to break up at the same sound level as the 12AX7A.
     
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