Revoicing Advice Needed 71 4-hole Pa 50'watt.

Discussion in 'The Workbench' started by Trapland, Apr 21, 2017.

  1. Trapland

    Trapland Well-Known Member

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    So this PA is coming along. I'm learning its character and am starting to trust it but I'm trying to get a little closer to my other 50jmp mkii that I adore.

    Essentially, I need less lower mids. It's rediculous how middle-bass it gets above 7. I expected some rounding but this is like a mid boost. There's also rediculous amounts of gain. To match my fav head on 7 this one needs to be on 2.

    Some thoughts.

    Make the 71 sound like the 76.
    The 71 has shared v1 cathode with a 25m cap ? It looks big. But that should be bassy all the time and it's only too bassy at higher Volumes. Reduce it?

    The first couple couplers are .02 rather than .022. Is this a huge thing
    The time circuit has a 33k slope. Would a 56 k push the mids lower?

    I'm running conservative NFB, 47k on the 8 ohm tap..1 cap on presence control. Could I reduce gain and lower mids with the coupling caps then get some drive back with less nfb. That could bpgive me a more scooped tone with plenty of overdrive when cranked that should clean up well with the guitar volume.

    Please you ideas are welcome.
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Your amplifier is setup for more lower/vocal frequencies.
    It does not have the BRIGHT channel circuit which includes the .68uF/2.7k cathode and probably .0022uF coupler. I would think it does not have the bright capacitor, usually 1000pF or 4700pF or maybe 5000pF. It most likely has .1uF couplers after the PI stage. All of that and maybe other things revoices the amplifier. It will be vocal range dominant all the time especially when turning up the volume.

    Fundamental frequencies for voice which applies to public address (PA) are about 80Hz to 300Hz.
     
    Last edited: Apr 21, 2017
  3. johan.b

    johan.b Well-Known Member

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    Adding a 470pF Over one off the 470k mixer resistors might be all that's needed. It'll be like a super bass
    J
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    In your bathtub.
    Use the tone stack calculator (and...)
    http://www.duncanamps.com/tsc/
    Remember that If bass does not enter the first preamp stage (cathode bypass cap), the tone stack cannot control bass that is not there to start with.
    https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

    Use the V1 cathode bypass cap to control how deep the lowest bass goes (or does not go)...and use the tone stack adjustments to balance the highs mids and lows (slope resistor).

    Too much low frequency is not a good thing, because the lows will compress too much at V2.
    This will make the amp sound muddy and overloaded in the lows.
    (so get rid of the 25uF bypass cap) Try using 0.68 instead...

    The key to the Marshall sound is to let the highs pass, and attenuate the lows.
    The higher the frequency, the less attenuation.
    The lower the frequency, the more attenuation.
    Turns out like this EQ curve: /
    (so put a 500pf across the 470K mixer resistor)

    The end result being that the highs will compress at V2, and the lows will be cleaner w/ no compression.

    "The first couple couplers are .02 rather than .022. Is this a huge thing?"
    no.

    The best advice I can give you is:
    Don't play a Marshall on 2 - play it wide open.
    Cause it will never sound like a Marshall should, and the tone will fool you.

    If you always play a Marshall on 2 --- you have the wrong amp.

    Don't try to adjust the tone stack...and mids or bass---with the amp set on "2."
    Cause it will never be right.
     
    Last edited: Apr 21, 2017
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    He's got it right.
     
  6. Trapland

    Trapland Well-Known Member

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    I misspoke. I meant to say the first couple of coupling caps are .02 rather than .002. Could this be where the big lower mid boost comes from when turning past 4-5? I don't think there are any .002s.

    It's easy enough to try a 470pF a mixer. I'm really trying to get more toward the 1987 mkii (converted to EL34) sound.

    Ampmadscientist, I get they sound best turned up. But 10 it not always the best choice for me. I prefer some bright cap effect, and not quite as much gain as most 4 holers make. This particular amp is as loud as its going to get by 2 and just makes more gain and distortion as you turn up. It's a bit sterile though, then I turn up to 6-7 and this lower mid hump just takes over, along with already too much gain. My other marshalls have a broader "warming up". That make sense?

    I don't have any flubby bass problems. Turning down the bass and middle control really have no effect on this lower mid boost. I could try to mock up a spectrum analyzer to see where the boost is centered but I'm pretty sure it's near 400-500hz.

    I can't help but feel it's effect comes late in the signal path.
     
  7. johan.b

    johan.b Well-Known Member

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    Try the 470pF. it will almost certainly do what yo I want.
    If the amp is relatively untouched, solder close to the body of the 470k resistor so you don't mess up the red paint on the turret solder joint. If you like it and decide to keep, take some mild acid, like vinegar on the solder to make it grey and dull and in a few weeks no one will know
    j
     
  8. neikeel

    neikeel Well-Known Member

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    The shared bypass cap on V1 is usually 250uF i.e. way bigger than is needs to be for guitar.
    In that circuit to get a difference in the channels I find that the 500pF bypass cap on channel mixer works well in combination with a 100pF cap on the volume pot of that channel - that is the spec of the Model 1989 Organ amps. These sound best with volume of that channel on 8 (+/- 1) with unused channel on 4 (NOT bridged) Presence around 6 to taste, Bass 2, Mids on 8-10, Treble on 6-8.
    This will give you variation on your 1987. They hold together better with 16ohm/27k NFB, less likely to flub out.
    Unless you are looking for a screamer I would leave the couplers at 0.022 early on and 0.1uF in the PI as stock. I have Lead, Bass, Organ and Bass spec amps of this era and find the above spec most flexible.
    If you really want to change the low mids emphasis then change slope resistor from 56k to 33k but I would try stock Organ spec first.
     
  9. Trapland

    Trapland Well-Known Member

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    This is fun! Such great discussion from people who know these amps. Thank you all.

    I will definitely try the cap across the mixer resistor. The slope resistor is already at 33. Maybe I should try 56k?

    NFB is 8 ohm tap/47k right now and it goes from dull and lifeless with presence off to too much gain and sizzle with presence on 10. I wouldn't mind a touch more liveliness without needing the eq peak of the presence to maybe I need a bit higher value resistor. Maybe....just talking it out.

    I think I'm describing "mids" and "lows" differently than others do. I've always been guilty of that. Once guy says treble and hear it a super mids, etc. I do better with frequency centers.
     
  10. Trapland

    Trapland Well-Known Member

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    Just have a quick report. I clipped in a 560pF across the first channel mixer. AMBROSIA! As far as the EQ, it's exactly what I wanted. You guys were all 100% right, it was exactly the right part to dial in the sound I wanted.

    Let me thank you all again.

    -Gain and crunch- Now that I have the overall tone perfect, I could use some ideas for reducing the gain without changing overall character. I wouldn't mind if it didn't break up at 1.

    Thoughts I had:
    -Preamp tubes. I'm already running darker weaker Russian 12ax7s
    -Use low gain input. Obvious, but it's too little gain then.
    -Bias cooler. I'm already running near 60%.
    -Try lower gain EL34? I'm running TADs and they are CRUNCHY. I'd love to use a pair of RFTs but the plate voltage is like 470 now. I don't feel confident that I'm not just burning up a great set of tubes that can be used in a lower voltage amp.

    Am I on the right track with these ideas or am I missing something? I DONT want to change the tone much, just less crunch. I love to get my cleans with the guitar volume.
     
  11. Sickriverlespaul

    Sickriverlespaul Active Member

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    This is really good stuff. Taking notes
     

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