Help with Mesa F-30

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by calieng, May 23, 2004.


  1. calieng

    calieng Member

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    I was hoping someone out there would know if a change of pots would help tame the lead channel on a Mesa F-30.

    From the factory the volume and gain controls are way too sensitive in the lower range on the lead channel. I can barely turn up to 9:00 on both the volume and gain before the amp starts blasting.

    I think the controls are 250k and not sure if they are audio or linear taper. Would a 500k pot give more range on the lower end?

    Any help would be appreciated.
     
  2. John Phillips

    John Phillips Member

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    Unlikely. Changing the pot value won't really affect the range (although it may affect the tone slightly) if the taper is the same.

    If they were Linear, changing to Log would definitely help, but my guess is that they'll be Log already.

    If you need to reduce the gain, you could try substituting a 12AT7 for one of the 12AX7 tubes in the Lead channel - I don't know which one though, I don't have a diagram for that amp. Probably the second or third from the end where the input jack is - you could just try it, you won't hurt anything if you get the wrong one. The 12AT7 will sound quite a bit different too, as well as having less gain. A 12AU7 has even less gain and is another possibility.

    That amp does really have a lot of gain on the Lead channel, it's something I noticed when I was trying one.
     
  3. calieng

    calieng Member

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    Thanks John. I'll try some different tubes.

    V1 A = Input stage both channels
    V1 B = Clean Channel

    V2 A = 2nd input stage both channels
    V2 B = Channels 2 & 3 Boost

    V3 FX send/return

    V4 Phase splitter

    Sounds like V2 lower gain would have the most effect on lead channel and least effect on clean channel although it still uses one half of the tube.
     
  4. John Phillips

    John Phillips Member

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    That sounds a bit odd... it would indicate the Clean channel has a similar amount of gain to the Lead (which it doesn't).

    I'd definitely try replacing V2 with a lower gain tube and see what difference it makes - to either channel.
     
  5. calieng

    calieng Member

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    The clean channel has two triodes and the lead channel has three triodes but I'm sure there is some other circuits invloved that make the lead channel so high gain.
     
  6. hipfan

    hipfan Member

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    Just a random thought here: if you haven't found a solution yet, maybe try a 12DW7 in the V2 spot? If I remember correctly, it's one half 12ax7 and one half 12at7. If the halves are assigned properly in the tube socket, you could retain your input stage gain while reducing the gain on your channel 2. Maybe? Could be?

    I'm no expert on this stuff, but I've heard of Fender Prosonic owners doing this for the same reasons you have. Hope this helps, or at least provokes conversation. :D
     
  7. John Phillips

    John Phillips Member

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    From your earlier list, it sounds like both have three... clean channel V1A, V2A, V1B; lead channel V1A, V2A, V2B, but that doesn't sound right.

    ...unless you mean that V2A is for 'both lead channels'. But there are only two channels - clean and lead - there is no "channel 3", it's an EQ boost on channel 2, which is done with solid-state circuitry.

    But you're right, there is so much more gain on the lead channel that I would expect at least one extra tube stage, possibly two.

    I would try swapping V2 and see what difference it makes - that will also tell you for sure which channels it operates on.

    Ha! I did think of that, but I didn't post it because it's such an obscure tube that I didn't think it would be useful. I didn't know about the Pro-Sonic application - its main use is in some Ampegs, including the SVT. I think it's actually half a 12AX7 and half a 12AU7. Whether it would be useful here would depend on which way round the halves are - and being able to find a good one that wasn't microphonic, which I seem to remember being a problem last time I needed one...
     
  8. hipfan

    hipfan Member

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    Ah, right, half 12au7 and half 12ax7. JJ is actually making one now, apparently. I saw it on Triode Electronics' website: JJ 12dw7 at Triode.

    Yeah, if the halves are the wrong way around, it could just cause tone problems, not alleviate them!
     

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