2204 - taming the treble + Oscillation

Discussion in 'The Workbench' started by cruisemates, Jan 15, 2021.

  1. cruisemates

    cruisemates Active Member

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    Long post - sorry...

    I have my 2204 working now. It broke a couple of weeks ago when I sold it to a GAS-head who treated it like a Thai sex therapist. He was twisting and turning every knob non-stop - spraying his cleaner in every pot & hole and using sandpaper in the inserts (I since learned that's a bad idea).

    After 30 minutes of that, it broke. I was there. First the high gain went dead, then the low. I had sold it to him without even plugging it in and it had been sitting a couple of years. Anyway, I felt obligated to take it back because he paid me a fair amount of $$ that day (two guitars + that head).

    So, we traded some things back and my goal was to make it as unbreakable as possible to resell it. So I got it working again and it was creamy and overdriven for three days. But, I was playing it and it just suddenly died again. effin'ell.

    It took a while, but I got it going again, but now it was clean and trebly even on the high input.

    Seeing nothing wrong I decided to just reflow V1 and lift the hot shield. Sure enough, found a teeny strand going from 5 to 6. Once I had it all re-flowed it sounded better than ever - but still trebly.

    Someone had posted a trick - a 3.2n cap between the purple wire and the impedance selector. I tried this and it seems to sound a lot better. QUESTION: DOES THIS NEED TO BE A CERTAIN VOLTAGE TOLERANCE? I usually build pedals and most of my caps are 25v max.

    I also took out the 1N cap on the master volume (treble bleed). It all sounds great.

    Now, I am not sure exactly why but now everything on the amp is far more interactive. I start getting overdrive at 4-5 on the preamp (high input) and a ton of it on 10. The tone stack can really tame any hum or push it louder than ever, (the range on each tone knob feels way expanded). This is not just a 2204 amp anymore, the range of possibilities is huge.

    So, all is great except - the amp is far more versatile than ever; but it is also less idiot-proof. The tone controls are way more interactive, so the mid will boost either the treble or the bass a LOT depending on which you have louder. (it seems to me it was more one dimensional before).

    Another continuing problem: If you turn the master all the way down you still get a bad loud oscillation.

    I love the amp the way it is - but to sell it, should I strive to get it back to idiot-proof? Or should I keep it more like the cool beast it has become?

    TL/DR:

    1. I am still looking for more tricks to tame the treble in a 2204.

    2. One trick I tried seems to have a strong effect; a 3.2N cap between the purple wire and the ohm selector. QUESTION: does that need to be a CERTAIN VOLTAGE RATING? I usually build pedals and most of my caps are only 25v max.

    3. Any more surprisingly effective ways to tame treble on a 2204?

    4. I replaced my Master Volume pot - why do I still get oscillation (a very loud squeal) every time I turn it to ZERO?
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Not sure about the squeal or the excessive treble on your amp, I don’t know enough about your mods to know what they do. But my stock 2204 gets very dark sounding if I turn the treble and mids to zero. Gets even darker if I turn up the bass and gain. I don’t know why some people complain that 2203’s and 2204’s are too bright.
     
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  3. South Park

    South Park Well-Known Member

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    v1 A is the gain stage check that circuit . Shielded signal cable is grounded is the grid stopper removed . Is sounds like some gain mods where done
     
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  4. AndyD

    AndyD Active Member

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    Don’t sell! You said it yourself.....it’s a really versatile amp.
     
  5. neikeel

    neikeel Well-Known Member

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    Given the history I suggest a series of overall and close-up photographs of you board and component values.
    Until you do that I cannot help and will be hugely impressed if someone else can.
    Odd re your MV oscillating when all your signal is shunted to ground.
     
  6. cruisemates

    cruisemates Active Member

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    So far the only mods in the amp right now include adding the cap on the ohm selector. That did seem to change the gain settings a lot and made it "a beast" which I like, but it is very easy to undo. Then I clipped out the brightness cap (1N) on the master volume because I usually only have that at about 20% on.

    Aside from that all I did was lift the hot shield for V1B (pin 6), and I plan to do the same for the other side of V1 today.

    I have the stock treble bleed circuit of a 470k R parallel to a 470 pF cap going to the master volume.

    It is a surprisingly stock amp compared to a lot of other circuit boards I have looked at. I guy just sent me pics of a 2104 he wants to sell for $1900, and the PCB is covered with replaced parts and piggy-backed caps & resistors. I don't have anything like that in my amp - all of the components are to schematic spec.

    Also - here are some original pics of my PCB before I made the changes described above:



    The mods are not in these pics. But as I said:

    1. Clip bright cap in master volume
    2. Add 3.3N cap to ohm selector
    3. Change out the peak boost to stock 470p and 470k in parallel
    4. Change out the hot shield so it is NOT grounded to pin 6, but grounded to chassis instead.

    Maybe it was clipping the bright cap that made it a monster, maybe I just need a lower value cap.
     
    Last edited: Jan 16, 2021
  7. mickeydg5

    mickeydg5 Well-Known Member

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    A cranked 2204 output will put out much more than a 25V capacitor can handle.
    At minimum use 100V. Go higher if possible like 160V or so.
     
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  8. myersbw

    myersbw Well-Known Member

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    Regarding that bright cap on the volume pot, did you completely remove it? (You don't want to leave it hanging in there) And, if you really want to see another difference, pop in a 120pF silver mica cap to replace the bright cap removed...notable difference that many like. Along with what mickeydg5 said regarding that 25V cap.
     
  9. cruisemates

    cruisemates Active Member

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    Thanks all. I don't have a lot of amp parts on hand, like 160V caps (I figured I would probably need a bigger cap for that purpose, tho). Yeah, I did clip the 1n completely off. I just replaced it with a 640pf which I thought was better, but I do have a 120 pf silver/mica also. I will give that a try.

    The amp is sounding pretty good right now, and it is not the beast that it was. I realized that clipping the cap (1N) was causing the most instability, so putting another cap in there did help. I still have the overdrive I like, though. And not too peaky anymore.
     
  10. wakjob

    wakjob Well-Known Member

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    I find the biggest problem with "overly bright" JCM 800 complaints...is speaker choice.

    Second, not enough volume.
     
  11. cruisemates

    cruisemates Active Member

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    Speaking of speakers - a guy was telling me that amp wattage is RMS (IIRC) and that if you are using two 25-watt greenbacks for a 2204, for example, you could easily be pushing the speakers harder than they should be and it sounds bad. Greenbacks are not a trebly speaker but what he said did make some sense.

    G12-100s and other spkrs like that can be really trebly (but they can handle the wattage). Speaker selection is a tricky thing. Marshall started out matching their speakers to the amp's wattage - but in those days some amps also came with warning tags - "turning the amp up too loud can result in distortion."

    With my 2204 I find that with the treble bleed cap it gets bassier as I give it more preamp gain, but you can compensate with more treble & presence. When you roll the gain back, you can also cut the treble & presence. It's a good balance once you figure it out.
     
  12. mickeydg5

    mickeydg5 Well-Known Member

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    You will always have to learn how to dial any amplifier in for a particular volume level.
    Every time the volume controls are adjusted, whether preamp or power amplifier, tweak all of the controls as required.

    Most musical amplifiers are set up to pass more treble at lower volume but sometimes they pass too much. As things are turned up in volume then the lower frequencies catch up. But most amplifiers are not perfectly balanced for either/or so there will be tweaking of control knobs.

    If you are going to crank a 2204 to maximum then go to 4 25-watt greenbacks. It only makes sense.

    When your friend said "easily be pushing the speakers harder than they should be and it sounds bad" it was meant that over powering two speakers of 25W RMS/50W musical with a musical amplifier which distorts to begin with and especially when pushed by larger signals from pedals/effects units will release 90 and 100W easily of DISTORTED signal.
    That means the two speakers are being powered to the maximum of musical rating but with a distorted signal. That will make the speaker react abnormal and begin to square wave. The is a crappy sound when you hear it before it melts or just blows.
     
  13. cruisemates

    cruisemates Active Member

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    Just installed shielded wire to my V1 leads - makes things quieter and tamed the oscillation from my MV pot. I changed out my PI 100k resistor because even with the amp off it kept reading 4.7k. But once I had it out I checked it and it was fine - 100k.

    Had a V1 tube blow its top on me. Not sure if it happened today or previously, but the amp was working the last I played it. - completely greyed out and the vacuum unsealed right at the top. I had done my fixes and turned the amp on for the first time today and the tube was dead. Weird.

    Amp is sounding really good now, though, I think. I only have a small room and close neighbors so I can't crank it like I would like to do. The amp has so much gain it's ridiculous. I can't get the volume past 10% on or it's like a cannon.
     

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