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Real tech info on 5751 needed

Discussion in 'Glowing Bottle Tube Amp Forum' started by ballynally2, Nov 29, 2017.

  1. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
    Hi,

    Despite countless posts in threads by amateurs swapping their 12ax7 for 5751 in V1 and V2, i'd like to hear from people with an electronic background and knowledge of Fender amps why the 5751 could be an improvement if the amp is set up for a 12ax7.
    Apart from the gain factor, some of the other variables ie voltages differ between the 2 valves.Different voltages different parameters.
    Also, when it is considered that the 5751 IS an improvement, should one change the circuit voltages (caps/resistors) to suit the valve?
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Well, if this is still unanswered I'll print out some datasheets tonight and draw up the operating points. It could well be that the "standard" 12AX7 values mesh very well with the 5751 characteristics, or even alter the harmonic structure in a pleasing way depending on what the load lines end up being.

    The other factor could be the cascaded gain in amps, with a lower gain 5751 helping present a less distorted signal to the later stages and PI.
     
    Archtop Bill likes this.

  3. corliss1

    corliss1 Tele-Afflicted Gold Supporter

    Sep 13, 2008
    Lansing, MI
    I feel like we need to define our terms. "Improvement" could mean something entirely different to two different people.

    No matter the science or measurements or whatnots behind it, the swap is performed to drop some gain in that stage. If you think that's an improvement and suits what you're doing then it is.

    I think this is one of the cases of "if it sounds good, it is good"

    Look at all the people that do the opposite - swap out a 12AY7 in a tweed for a 12AX7, and no one thinks about swapping out parts to suit that specific tube. We know what those results are, and I'm sure you could do some math behind it if you wanted, but the sonics let you hear the end result.

    Now.....if you were designing something around the 5751 input specifically that could be a bit different as you may want to hit some certain operating points.
     

  4. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Compare their respective data sheets...the info is in there!
     
    Last edited: Nov 30, 2017
    ecosse likes this.

  5. warrent

    warrent Tele-Meister Silver Supporter

    277
    Sep 15, 2009
    toronto

  6. Dacious

    Dacious Friend of Leo's

    Mar 16, 2003
    Godzone
    Why did Leo set up some amps to use a 12AY7? The reason is, he pulled his first circuits from common tube manuals - they weren't guitar-specific. So after building them, there may have been room for improvement. Circuit changes can take a while and involve ensuring they will 'live', popping a new preamp tube of lower gain is easy.

    And they may have specified 12AX7s for say phono cartridge/wax cylinder inputs. When you plug a high degree impedance guitar pickup in, it possibly produced undesirable characteristics. Like distortion, which in the early days was not what they were looking for. A low wattage Champ probably needed all the gain it could get. But a Deluxe or Vibrolux or low power Twin, not so much.

    The reason people spec 12AY7 or 5751 tubes is - they are also audio tubes. An otherwise-similar 12AT7 is designed as a driver tube, as it has a higher grid-stopper current rating it does that well.

    In theory yes you should work out plate and grid voltage of your 5751-purposed amp and optimise for 12AX7 or vice versa. But in practice, it makes not a fantastic amount of diff provided the bias resistor and caps are OK.

    In the FMV tone stack, the first preamp triode drives your volume, treb-mid-bass, and if you get distortion at that stage, even inaudible but the input sine wave is going square, it will appear at the 2nd triode and then at every point to the speaker terminals.

    So often taming that first gain stage, and then again on the output half of the first preamp tube, will smooth it out through the amp and should give you more usable range on the subsequent controls. That's what happens if you lower the gain even of a modern Fender like the Hot Rod series.
     
    corliss1 likes this.

  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    That was an interesting bit of drawing and math. Given:
    Rl = 100k
    Rk = 1.5k, fully bypassed.

    The 12AX7 biases at around 1.2V or so. Using a grid signal of 2Vp-p, I get these numbers:

    Total output swing of 140V, symmetrical (+/- 70V).
    Gain factor = 70
    Being symmetrical, that gives a very low level of distortion, i.e. the stage is very linear at this signal level.

    The 5751 biases around 1.5V.
    Using a grid signal of 2Vp-p, I get these numbers:

    Output voltage swing is 110V
    Gain factor is 55
    But, it's distinctly assymetrical. It's 50V in one direction, and 60V in the other.
    If I ran my math right, that's about 4.5% 2nd order harmonic distortion.

    Disclaimer: these are limited on my ability to read and draw on notebook sized printer paper. I don't have an 11x17 printer at home, so the graphs I'm using are a little bit small for this kind of detail work.
     

  8. JD0x0

    JD0x0 Friend of Leo's

    Age:
    27
    Feb 22, 2009
    New York
    This is generally why, I've never been a fan of simply chucking a lower gain tube in, for more 'headroom' and/or better/louder cleans, like I see often suggested. Sure, gain was reduced, but distortion increased, the load line not symmetrical anymore, which would reduce headroom.

    Ideally, you typically need to rebias the preamp tube for the change, in which case, you could just change the 12AX7 circuit if 'taming gain' or 'more headroom' is what's desired. Adding LNFB to a preamp stage is a great way to 'tame' it. NFB will suppress distortion AND reduce gain.

    Still, usually doesn't hurt to experiment with tubes that are safe in those positions. Some may like the sound of the asymmetry of the 5751.
     

  9. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
    YES, THAT'S WHAT I HEAR

    THIS ASSYMETRY DISTORTION I CANNOT PERCEIVE W MY EARS.Q:EVEN THOUGH IT'S ASSYMETRICAL, ISN'T THE CIRCUIT DESIGNED TO LEVEL OPERATION GOING FROM ONE GAIN STAGE TO THE NEXT PRECISELY TO ELIMINATE DISTORTION?

    SAME AS ABOVE.
    NOTE: SORRY FOR THE CAPITAL LETTERS, FOLKS.I JUST NEEDED TO MAKE MY COMMENTS DIFFERENTIATE FROM THE QUOTES.
    AND ALSO:I HAVEN'T BE ENTIRELY UPFRONT AS I DIDN'T MENTION THAT I FIND THAT WITH MY NOS BLACK PLATE RCA 5751 I'VE GREATLY REDUCED THE DISTORTION FROM THE ONSET LEAVING MORE ROOM FOR THE OTHER GAIN STAGES.THE SIGNAL IS MUCH CLEANER.LIKE I MENTIONED: I DON'T HEAR ANY ASSYMETRY DISTORTION W THE 5751.AS A GUITAR AMPLIFIER PREAMP SIGNAL IS NON LINEAR TO SOME DEGREE THIS DISTORTION MIGHT NOT BE PERCEIVED AS 'GRIT' BUT AS HARMONIC IE ACTUALLY PLEASING.HOW MUCH ASSYMETRY AFFECTS THE SIGNAL AND IN WHAT WAY MIGHT BE CALCULATED BUT HAS TO BE PERCEIVED BY LISTENING.
    WHAT I HEAR IS A LOT OF CLEAN SIGNAL WITH LESS OUTPUT THAN A 12AX7 BUT WHEN ADJUSTED FOR VOLUME MORE USEABLE HEADROOM..THAT'S CONTRARY TO THE LAST QUOTE.INTERESTING ISN'T IT? IT MIGHT BE THE DIFFERENCE BETWEEN DOING THE MATH, LOOKING AT GRAPHS AND LISTENING TO ITS RESULT.HARMONICS ARE ALSO ALWAYS AN INTERESTING SUBJECT.I KEEP MY INPUT SIGNAL RATHER LOW TO LET THE AMP DO THE WORK IE RELATIVELY CLEAN INPUT SIGNAL.THAT IS TO SAY OUTSIDE THE USE OF DISTORTION PEDALS OF COURSE.W FIXED BASS AMPS I USE THEM WHEN I NEED TO.MY LOWER WATT CATHODE BIASED AMPS PRODUCE THEIR DISTORTION ALL BY THEMSELVES.
     
    Last edited: Nov 30, 2017

  10. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    I put two of them in my Ramparte and I got more distortion than with anything else.

    It was a little less loud, but I got a lot of distortion even with the cool channel. Never been able to find another tube that does that in that channel.

    It might be the amp design, because I then put them in my Electribe and it's the cleanest sound of anything I tried in there.
     

  11. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
    Interesting that they make such a difference in your Ramparte.As the 5751 and 12ax7 are pretty close it shouldnt give much distortion.I would also stay away from using a 5751 in single ended amps like the Ramparte.not enough output/gain..

    i'm using a 5751 in the bright (reverb) channel of my brownface Fender Vibroverb ri 6G16 circuit..
     
    Last edited: Nov 30, 2017

  12. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    4.5% is not a great deal of distortion, big picture. There's a good number of amps (Fender, for example) that use 5% THD as their reference point for max "clean" power.

    It is greater than what a 12AX7 looks to produce in that circuit, though.

    I do see that the two tubes have very different anode resistances per their datasheets. After I get the kids off to school, I'll see what an AC load line does on these graphs I marked up last night. My curiosity is thoroughly piqued now, and these questions make me get out the books and learn more.
     

  13. SPUDCASTER

    SPUDCASTER Poster Extraordinaire Platinum Supporter

    Tried the Eurotubes "SRV" tube option with the 5751 V1 in my AC15C1. Left it in for a while. Went back to the 12AX7. Liked it better.

    I trust my ears. That's my tech info.:D
     
    telemnemonics likes this.

  14. ballynally2

    ballynally2 Tele-Meister

    Age:
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    376
    Jun 7, 2009
    donegal ireland
    Good idea! Thanks..awaiting your findings.
     

  15. ballynally2

    ballynally2 Tele-Meister

    Age:
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    376
    Jun 7, 2009
    donegal ireland
    Cathode bias resistor values looks to be the same for both valves..plate resistance slightly different
     
    Old Tele man likes this.

  16. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'm holding the circuit values constant to see what changes.

    The 5751 gets more harmonic distortion, looking at the working load lines vs the DC ones. The 12AX7 looks to stay pretty balanced under dynamic conditions, where the 5751 actually becomes even more lopsided. Looks to be that the bias point falls at a region where the grid voltage curves are noticeably closer together and show more uneven spacing vs the 12AX7's lines.
     

  17. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
    ok Clint, thanks for the work.Of course this leads to the further question: what does that mean? Is it significant and why? As the valve no doubt has cathode self biasing to some degree.What's the relation between those specs and the other gain stages of the amp.
    The first preamp valve in my vv takes care of stage 1(initial boost of the input signal, one half of the valve) and 2 (tone stack, other half). As the vol pot is a variable voltage devider it is also involved in the reverb return (12ax7) an PI (12at7).they all interact.
     

  18. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
    me too but...then i want to know why.just a habit i developed..
     

  19. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    5751 are lower volume and lower current, while 12AT7, 12AY7 and 12AU7 are lower volume and higher current.
     

  20. ballynally2

    ballynally2 Tele-Meister

    Age:
    53
    376
    Jun 7, 2009
    donegal ireland
     
    Last edited: Nov 30, 2017
    Paulie Walnuts likes this.

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