Mesa F-50: ready to retube

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pulteney, May 21, 2020.

  1. pulteney

    pulteney Member

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    Hello there, I hope this finds you as well as can be. I'm finally ready to pull the trigger and retube my F-50. Yay!

    So far, with a 12AX7 in V1, I have used the clean channel to play jazz, rhythm'n'blues, rock'n'roll, and the drive channel to play rock (gain at 10 o' clock max, most times at 9 o' clock). I hardly ever need high gain.

    I happened to get my F-50 by chance some 10 years ago, in exchange for a Roland JC-120 I was selling. Since day one, I have had this love/hate relationship with the F-50 because of the excessive volume and gain. After taming the volume with an EQ pedal, things have got much better. If I can tame the gain, too, I feel this Mesa might eventually turn into a kind of 2+1-channel Fender amp that I can't find elsewhere. Deep down, I think I'm a Fender tweed/blackface kind of guy, but since I already have a good amp, why not make the most of it?

    These are the tubes I've shortlisted:

    V1 (input stage both channels / clean channel): Psvane 12AU7 or a 12AY7 if I find the 12AU7 too clean. I might even try a Mesa Boogie 12AT7 just to find out why so many people say 12AT7s suck in V1.

    V2 (2nd input stage both channels / channels 2&3 boost stage): Mesa Boogie SPAX7

    V3 (channels FX send / return): Mesa Boogie SPAX7

    V4 (Phase inverter): Mesa Boogie SPAX7 or Mesa Boogie 12AT7 or Psvane 12AU7. I'd like to experiment a bit here, see if I like a 12AU7.

    Thanks a lot for your comments and suggestions.

    Stay safe!

    M
     
    Last edited: May 22, 2020
  2. Humble Texan Fan

    Humble Texan Fan Member

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    I would avoid going really low on gain with the purpose of increasing headroom. Headroom has more to do with high output - low gain alone tends to sound sluggish and small, going against the design of the amp, really.

    I'm unsure asto why you're looking into the costy and hifi geared Psvane.

    For cleans I would give the long-plate JJ (borderlining on SS clean)and some version of the Shuguang 7025, like the PS. The GL 12AX7 is truely great for some midrange warmth.

    The PI could very well be the 12AT7, which can slam the outputs harder without compressing or clipping. It could serve a good purpose for the FX loop as well.
     
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  3. pdf64

    pdf64 Member

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    Here's a schematic
    I've no experience with this particular model, but the nature of the design means that it works the way it works, trying to alter that with preamp tube types may not have as much effect as might be expected.
    I think that's a (widely held) misapprehension, the AB1 amps we use don't work like that.
     
    Last edited: May 21, 2020
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  4. pulteney

    pulteney Member

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    Thank you both for your comments.

    That particular Psvane tube was mentioned on TDPRI by someone who seemed to be on the same wavelength as me when it comes to Mesas, i.e. great amps with consistently too-hot preamp sections for people who are not into metal or Santana.

    Anyway, he wrote he bought his F-50 from the Mesa Boogie shop on Sunset Blvd in LA. They did some tube swapping in the store and went straight to a 12AU7 in V1. Though 12AU7s are the lowest output preamp tubes, he found plenty of (more usable, IHO) gain, and thought the cleans were way better, and with more range of adjustment before gain and distortion kicked in. He tried 5751s and other mid-output tubes, but found them essentially identical to the stock 12AX7s. For a F-50, a good 12AU7 in V1 was, for him, right on the money. The 12AU7 is his regular swap and it works really well for him.

    It's a shame I never got him to tell me what kind of music he's into! That might have set a few things straight.
     
  5. pulteney

    pulteney Member

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    To minimize risk, I think I'll just stick to four Mesa SPAX7 and give a NOS 12AY7 a try. I guess/hope I can always resell it if I don't like it.

    EDIT: After doing some more research, I'll use one Mesa SPAX7, in V1. See post #9.
     
    Last edited: May 22, 2020
  6. HotBluePlates

    HotBluePlates Member

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    Don't try to tame excessive gain with a tube-swap. You can get at least a mild reduction, but really a high-gain preamp like the Mesa is about its circuit design, not what tube you used.

    If you don't like the F-50, sell it and get an amp that sounds the way you want, right now. Otherwise you'll spend a lot of money and come to the conclusion the amp "just sounds like that."

    Psvane 12AU7s retail for like $34 per tube. I didn't spend even $10 per tube on UK Mullard and German Telefunken 12AU7s. You'll be charged an awful lot of money for a Chinese 12AU7 that may not even make the amp sound like you want.

    If you're dead-set on experimenting with 12AU7s or 12AY7s, buy the cheapest thing you can find so that you know it will get the basic channel response where you want it (and I strongly suspect it won't). If that stage works out, then consider spending a lot more money on tubes marketed for a higher price point.

    There's no "risk" other than spending a bunch of money. Why buy a NOS 12AY7 when it also means spending more money?

    Broken Record: I strongly suspect you'll get through the tube swap finding it didn't have as much impact as you'd hoped, or that the gain was reduced but now the amp just sound "blah."

    I'm with @pdf64 on this: this is a false notion popular on some forums, but isn't how push-pull power sections or phase inverters work.

    When I sold off some of my tube stash, people bought 12AX7s all day, every day. The 12AT7s and 12AU7s sat unsold. Groove Tubes was the first to push the "change your phase inverter whenever you change output tubes" probably because they too were sitting on a bunch of unsold 12AT7s.
     
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  7. pulteney

    pulteney Member

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    Last edited: May 21, 2020
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  8. 70 Mach 1

    70 Mach 1 Supporting Member

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    One of my favorite amps.
    Wish i bought one back in the day.
     
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  9. pulteney

    pulteney Member

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    I edited post #5 after the amazing guys at Mesa wrote to me there is no need to use the SPAX7 in all positions, as it has less and less benefit the further down the signal path it is. They then added that although not necessary (sic), they would recommend it used in the V1 position.

    I hope nobody loses their job over this...
     
    Last edited: May 22, 2020
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  10. oneblackened

    oneblackened Member

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    I should mention: the SPAX7 is just a noise graded JJ ECC83S. Make of that what you will.

    Anyway... if you want to back off gain, the first thing I'd consider would be a 5751 in V2 (which happens to be the input stage). They've got a spec mu of 70 vs a 12AX7's 100, and in circuit it's a solid ~25% lower gain (~44 vs ~60).
     
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  11. pdf64

    pdf64 Member

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    It’s worth noting that’s a reduction in gain of less than 3dB ;)
     
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  12. pulteney

    pulteney Member

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    The F-50 is a really loud 50 watter, much louder than, say, a 1967 Selmer Treble'n'Bass 50 MK II.
    Is that 3dB an estimate or the actual figure? May I ask you how to calculate it? Thanks!
     
    Last edited: May 22, 2020
  13. pdf64

    pdf64 Member

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    Voltage gain dB =
    20 x log10(gain a/gain b)
    So 44/60 = 0.733
    log10 of 0.733 = -0.135
    -0.135 x 20 = -2.69

    The T&B MkII has A MUCH saggier HT than the Mesa, eg GZ34 compared to solid state, CLC filtered plate supply.
    So its ratio of overdriven power to clean power will be much lower than that of the Mesa. eg so T&B 50W clean 60W overdriven, Mesa 50W clean 80W overdriven.
    And further to that, when overdriven, EL34 tend to squish out, whereas 6L6 can seem to bite through.
     
    Last edited: May 22, 2020
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  14. oneblackened

    oneblackened Member

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    That is true, but 3dB is not an insignificant amount when you consider that 3dB is a doubling of power.
     
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  15. pulteney

    pulteney Member

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    What is 44? And 60? Thanks

    What does HT stand for? Thanks

    I got lost when you shifted from voltage gain dB to the ratio of overdriven power to clean power. Are they interconnected? I'm totally ignorant of this subject, yet I find the science behind it fascinating. Thank you very much.
     
  16. oneblackened

    oneblackened Member

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    44 and 60 are the in-circuit gain values for a 5751 and 12AX7, respectively.

    HT stands for High Tension, which is another way of saying High Voltage. In this case it means the plate and screen voltages.
     
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  17. pulteney

    pulteney Member

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    Last edited: May 22, 2020
  18. HotBluePlates

    HotBluePlates Member

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    And when you calculate dB relating to power levels or SPL, you use 10 x log (power/reference). There is a reason buried in the math of deriving the formula, but you could just commit it to memory.

    For example, in another thread we discussed re-rigging an amp to "ideal values" so that it produced 54 watts RMS rather than 45 watts RMS. Is this a "big difference"?

    10 log (54w / 45w) = 10 * log (1.2) = 10 * 0.0792 = ~0.8 dB.​

    You'd be hard-pressed to hear a half-decibel change of volume. 10 decibels is commonly accepted as "twice as loud." 1-2 decibels often seems like "audible, but small change."

    No, @oneblackened is talking about 12AX7 (Mu of 100, but in-circuit gain of around 60) and 5751 (Mu of 70, but in-circuit gain around 44).

    Guys who don't do amp design but stumble on those Amplification Factor Charts don't appreciate that actual voltage amplification in a circuit is lower than the tube's amplification factor in most cases, and in some circuits is actually less than 1 (or a signal reduction from input-to-output).
     
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  19. HotBluePlates

    HotBluePlates Member

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    Come to think of it, I have a bag of like 100-200 old production 12AU7s... more than I'll use in a lifetime (even though I now have some amps that actually use 12AU7s). If you need a few, PM me and I'll pop some on the tube tester & sell them inexpensively. You'll get 3 for the price of one of those Psvane 12AU7s, or less than the cost of a Russian 12AU7.
     
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  20. pulteney

    pulteney Member

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    Really kind of you, thanks! PM coming your way
     

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