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  • Marshall

    A Friend of mine owns an 80s Marshall 2205 50w split channel head he'd like to play at home without driving the neighbors nuts. He asked me if I could convert it to less power.
    I plan to put a 15w OT into it and use 6V6 tubes.
    Recently I read that the PCB is sort of fragile in those amps and could easily be ruined when maintenanced or worked on.
    Does anyone have experience with these kind of amps? Can the leads to the OT be reached for this kind of conversion? Would I have to take the PCB out? Are the tube sockets on the PCB? There are no voltage readings in the schematic (it's acually the 4210 schematic) - does anybody know what the voltages are? JJ 6V6S can stand up to 500V plate voltage.


  • #2
    15-20 Watt is still very loud for playing at home.

    If it's not a very early version try installing a post pi MV.
    Chris Winsemius
    Vleuten, The Netherlands


    • #3
      And consider something like Yellowjackets in the sockets instead of changing the OT and bias circuits. If he wants full power back or he wants to evr sell it, they pop right back out of the sockets like a tube.

      Education is what you're left with after you have forgotten what you have learned.


      • #4
        And for reference, 5 watts is half as loud as 50 watts. 1/2 watt is a quarter as loud as 50 watts. It takes a LOT of power reduction to make an amp quiet. So power reduction is not always the best way to tame loudness.
        Education is what you're left with after you have forgotten what you have learned.


        • #5

          I am always impressed with your posts and the breadth of your knowledge...especially with regards to the real-world, hands-on experience with a variety of equipment. I greatly agree with you that it taks A LOT of power to reduce volume substantially. We're definitely on the same page there.

          But, I'm surprised (and dissappointed?) that you use the language of "half as loud" and "quarter as loud". Linear relationships (like "half" or "quarter") have no meaning when talking about logarithmic systems like our hearing. I know that you know this. I'm preaching to the choir here. But, by using words implying any sort of linear relation, ("half") we continue the falicy that linear relationships do have meaning when talking about sound volume. I think that's wrong. I mean, what does "half as loud" really mean to someone? To me, it's as invalid as saying something like "today is half as hot as yesterday". That has no meaning that I can think of.

          I do not at all mean to attack anyone. I'm sorry if it sounds that way. What I'm attacking is our human need to take something that is somewhat complex and simplify it TOO far so that we can "understand" it. But, if you simply TOO far, you don't understand it at all. That's how I feel about talking about sound with phrases like "half as loud".

          Linear comparisons work OK for human perception of things like length, weight, and time (barely). I do NOT think that they work for human perception of sound (twice as loud? twice as high pitched?), light (twice as bright? twice as blue?), taste (twice as strong? twice as salty?), pain (twice as ouchy?), smell (twice as stinky?)...and on and on...

          I agree that all these quantities can be measured in physical units where one could make linear comparisons. But, I propose that the human perception of these quantities is not suited to linear comparisons.

          Again, I apologize if it sounds like a personal attack. It is not. This is a pet peeve of mine because i don't understand the other view point. Please tear apart my argument. Please educate me.

          Last edited by chipaudette; 08-08-2008, 11:19 AM.


          • #6
            I knew about the difference in loudness not being that much but I haven't considered the yellow jackets. Seems like the best idea, especially because you can easily change the amp back to its original state. But with 15 to 20 watts it would still be LOUD.

            If I put a post PI MV in between the caps to the power tubes
            (like from < PI--cap--power tube > to < PI--half value cap--MV--half value cap--power tube > ), I think that would mean a lot of tinkering on the PCB. Wouldn't it? And I would need a stereo pot (not a big problem, though).

            On the page I found the tone bone - I think as a triode with 8 watts altogether it could be OK, even for the neighbors.

            Thanks for the replies.



            • #7
              Actually, hearing perception is extensively studied, and it's near as dammit a fact that a 10x increase in power makes something sound subjectively "twice as loud".

              I don't think that's at all the same thing as "It was 30 degrees Centigrade yesterday, and it's 15 today, so it's twice as cold". Loudness is a scale that has a well-defined zero (complete silence) so it's quite OK to use comparisons like "twice" and "half".

              Incidentally, you can do the same with temperature, that's what the Kelvin scale with its properly-defined absolute zero is for. 30 degrees is 303K, and 15 degrees is 288K, so it's really only about 5% colder than it was yesterday. For those who prefer Fahrenheit, the Rankine scale is the absolute counterpart.

              I was motivated by this theory to make an experimental amp, the "Deci Belle", that powerscales between three settings; 0.3w, 3w and 30w. And yes, each increment makes it sound about twice as loud. And twice as good: electric guitar just sounds better loud, and no amount of electronics can change that.
              Last edited by Steve Conner; 08-08-2008, 11:44 AM.
              "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


              • #8
                I think Enzo is getting the point accross in the only way that's going to help most people get a grasp on the subject. I liken it to power vs speed of say a motorcycle. If you have a 30bhp motorcycle that does say 100mph, to go twice as fast (top speed) you are going to need more like 8 times the power, rather than 60bhp (assuming that you are not radically redesigning bodywork, from semi faired sit up & beg to all enclosed recumbent). Halving, or doubling that 30bhp is only going to make nominal differences to top speed (loudness in the amp's case), if not to acceleration & torque (envelope of sag & fequency response?).

                Likewise with amps, if your 50W amp is WAY too loud for bedroom practice, halving the power isn't going to halve the loudness & you have to grasp the concept that <5W is going to be more like the W to aim for, which really means a different amp. 5W can still be a handful indoors, 1W or 2W are more in order for a bedroom amp unless you live in well spaced, detached, double glazed, housing.

                Of course, voicing/fidelity/tightness of response all affect how 2 amps of even the same power will be percieved, but a I said, Enzo's point is guiding folks to the idea that halving the Watts isn't going to do anything like halve the loudness.


                • #9
                  I believe ENZOs way to put things, using examples out of the real world sometimes, makes it apprehensible for people. I like that.
                  Back to the amp: the 50w amp was not far too loud when being played on 1 or 2 on the scale.
                  If he uses the tone bones and can turn it on 4 or 5 I think that could mean a good compromise between sound and trouble with the neighbors


                  • #10

                    Enzo did give an easy to understand comment about power and volume perception.

                    Another way to state is that it takes 10 times as much power to perceive a doubling of volume.
                    So, a .5W amp may sound about 1/2 as loud as 5W.
                    A 5W amp may sound about 1/2 as loud as 50W.
                    A 50W amp may sound about 1/2 as loud as 500W.

                    Of course there are a lot of variables that skew an absolute dB reading. One speaker won't work the same for all power levels. The designs of the amps tehmselves will impose some deviations.

                    But, overall, the accepted principle is that it takes 10 times the power to perceive double the volume.

                    So, as Texstrat has learned, taking an amp from 50W to 20W will not give very much volume reduction. It will not cut it by 2/5. Enzo probably stated that because there are still people who think a 100W amp is twice as loud as a 50W amp, since it has double the power.

                    And you have stated a case against Enzo's statement, but you didn't explain your own theories.

                    I think you'll get outnumbered on this one.



                    • #11
                      Hey All, thanks for being willing to discuss the "half as loud" issue. I appreciate the discussion!

                      As I said, I agreed with Enzo's proposition that you have to move by a factor of 10 to get a sizable difference in perceived volume. What I disagree with is purely language...the words...the use of words like "half" with regards to volume. I propose that "half" is not something that a human can perceive with any consistency with regards to sound volume.

                      Steve brought up scientific measurements. As I said in my first reply, I agree that there are physical units of measure for all of these quantities. We could, in fact, measure the sound waves as variations in pressure in units of Pascals (or psi, or whatever). Cutting volume until the Pascal reading was half would cut the power required by a factor of 4 and you'd get an SPL reading that was 6 dB less. Fine. But, what did that do to the perceived sound level? Did it cut the sound level in half? I don't think that anyone here would argue that. How would a person describe how much the volume changed?

                      My point is that what does perceiving "half" even mean? I propose that "half" is a meaningless word when talking about perceiving sound. Others may feel that they know what "half" feels like. Great. But, have any of you ever tried listening to something and adjusting its volume until it was "half as loud"? To me, that test illustrates the difficulty of using words like "half".

                      Steve said that he made an amp that he could switch in x10 increments (ie, 10dB). Cool experimental piece! Steve makes some really fun stuff. With that amp, he said that he felt that each x10 step did make it sound half (or twice) as loud with each step. Who am I to argue with his perception? All I know is my perception and all I can do is ask people to consider their own perception...

                      Do you feel that you can perceive something like "half" and "twice" when it comes to sound? Not just "much louder" or "less loud", but something actually close to "half" vs "quarter" or "twice" versus "four times".

                      Thanks for you thoughts. This is fun!

                      Last edited by chipaudette; 08-08-2008, 06:20 PM.


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