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  • #16
    one important note that most people will tend to overlook is that its one thing to design a full range audio amp the "right" way for HiFi sound reproduction, and its something entirely different to design a guitar amp that will be so successful in production that its destined to become one of the all time classic musical instruments. there's a huge difference in the demands of musical sound production vs. musical sound reproduction. reverse engineering a classic amp typically won't get you to the same point that you'd reach if a radio engineer designed a "proper" circuit from scratch -- the results would be wildly discrepant. it shouldn't come as a surprise that many of the greatest guitar amps of all time were made the "wrong" way.

    personally, i was quite surprised to see that Marshall power transformer schematic that was only rated at 150mA. I would have expected something quite higher, but it is what it is. Tom was my original source for the schematic. maybe he could comment on his source and whether he thinks the schematic is accurate. If I were designing a Marshall, I never would have spec'd something so under-rated, as under my continuous sine wave bench tests (that are nothing at all like a guitar signal), the amp would have been extremely saggy until the PT blew out. Maybe that explains why I don't have a job designing Marshalls.

    looking at tube spec sheets, they do list some rather demanding PSU requirements for 50W applications, but its important to consider the difference s between those kinds of applications on paper and the kind of application you're dealing with in the real world when you want to build a guitar amp:

    1. playing guitar is an intermittent duty application, not a continuous duty application.

    2. guitar is a musical signal that undergoes lots of amplitude modulation; its not a pure sine wave whose amplitude is fixed and invariant

    3. the power spectrum of the guitar signal is very limited, it is not a full range audio signal.

    4. the output signal is permitted to be heavily clipped at its power rating, having lots of harmonic and intermodulation distortion; its not required to be exceptionally clean.

    each of those considerations individually lowers the bar for the amount of power that a PSU needs to supply for the gutiar amp application. in aggregate they lower the PSU demands even more. this means that the tube data sheets that list specifications for HiFi and RF operation undoubtedly overestimate what you'll need for MI/guitar amp operation.

    part of the art in designing your amp is to determine how low you can go without compromising the integrity of the circuit. some of the best sounding guitar amps of all time were built by tweakers and experimenters who kept scaling down as a cost saving measure. as a result their amps were all built the "wrong" way. in retrospect, its a good thing that they were not built by radio engineers who would have built them the "right" way, as the sonic qualities of MI amps would have suffered. its no real surprise then that most MI amps tend to have what most radio engineers would consider to be Mickey Mouse power supplies in them.

    from a practical standpoint, this means that you can probably build your guitar amp using iron that might seem somewhat underrated. as Tom said, the ultimate test is to build your amp and let us know how it turns out. you might be pleasantly surprised, or you might be sent back to the drawing board.

    As far as Hammonds go, my choice for a 2xEL34 Marshall style amp is the 372-JX. With a 120 VAC input it puts you right in the zone with 411 VDC on the plates in a Plexi style circuit. but take this with a grain of salt - its 250 mA current rating is exactly what the tube data sheets say you need to use, but in reality its over spec'd for the application. after all, there's a reason that i don't have a job designing Marshalls.
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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    • #17
      I was/am surprised at the Marshall PT choices as well.
      It’s hard to knock success though.
      While the available EL34/6CA7 data sheets clearly differentiate between sine and “music” signal driven current requirements, I tend to agree with your entire line of reasoning.

      Interestingly, Fender PTs seem to be more “classically” selected IINM.

      However, I designed an El34 amp and I am pleased with my PT selection.
      I use the Hammond 272JX (300-0-300 @ 250ma).
      I have left the thing running at full power (sine) for several hours and the increase in temperature of the PT is trivial.
      I take satisfaction and some comfort in that fact.
      I don’t like leaving my dishwasher or washer/dryer on unattended – but I don’t worry about my tube amp ..

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      • #18
        The London Power Standard amp uses the 272JX and it has a dual-dual power amplifier: 2x EL34 and 2x 6L6 running simultainiously. Now, Kevin O'Conner only claims 70 watts max out of this. Not saying what this means, just sayin'.

        I'm building the standard amp with only the 2x EL34. I had assumed that because I was dropping half the power amp, I could use half the transformer, and was therefor going to use the 272FX. Guess I should re-think that.

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        • #19
          the 272-JX would be pretty much the same choice as the 372-JX export version. the only real differences are the international primary taps and the bias tap.
          "Stand back, I'm holding a calculator." - chinrest

          "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

          Comment


          • #20
            I always run my 50W amps at lower voltages - mostly 420-430V and they're loud enough especially when connected to a 4x12" cab. Even if you can't get 50Watts out of this transformer /which I doubt/ you'll get at least 40W. I don't think that you'll notice the 10Watt difference.

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            • #21
              Originally posted by bob p View Post
              the 272-JX would be pretty much the same choice as the 372-JX export version. the only real differences are the international primary taps and the bias tap.
              Sure, but my point was that he's using the 2/372JX to supply an amp running 4 power tubes. Oh wait, I claimed I wasn't trying to make a point

              Granted it's not four EL34s, it's two EL34s and two 6l6s. And he's only claiming 70watts max, not 90watts or whatever it's possible to squeeze out of that arrangement.

              I always run my 50W amps at lower voltages - mostly 420-430V and they're loud enough especially when connected to a 4x12" cab. Even if you can't get 50Watts out of this transformer /which I doubt/ you'll get at least 40W. I don't think that you'll notice the 10Watt difference
              You're probably right there, Bob. I'll likely have the power scaling or master vol turned pretty far down anyway when I'm using the amp for guitar. I was just looking for some extra headroom for playing pedal steel through it. I don't bring an extra amp to gigs just for the steel, so currently I'm often playng it through a 20 Watter! Practically anything would be an improvement in thr headroom dept.

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              • #22
                oops. I quoted Gregg and then called you Bob Sorry guys!

                Found This on the subject. Looks like a pretty handy guide.
                Last edited by Ptron; 07-04-2007, 05:47 PM. Reason: bad link

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                • #23
                  Originally posted by Ptron View Post
                  The London Power Standard amp uses the 272JX and it has a dual-dual power amplifier: 2x EL34 and 2x 6L6 running simultainiously. Now, Kevin O'Conner only claims 70 watts max out of this. Not saying what this means, just sayin'.

                  I'm building the standard amp with only the 2x EL34. I had assumed that because I was dropping half the power amp, I could use half the transformer, and was therefor going to use the 272FX. Guess I should re-think that.
                  Really ??? wonder what made him decided to do that ??? Hmmmm........

                  Also, if Kev claims 70 watts, it's 70 watts. He is not one to run his power tubes too hard, nor either am I for that matter. I wonder if he does any voicing inside his power stages ?????

                  -g
                  ______________________________________
                  Gary Moore
                  Moore Amplifiication
                  mooreamps@hotmail.com

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                  • #24
                    I took a look at LondonPower.com. I could not find a 70 watt amp. I did see a power scaling amp for $1000 coming soon. So what, I have three power scaling amps for under a grand and my power brake is better than his is...

                    -g
                    ______________________________________
                    Gary Moore
                    Moore Amplifiication
                    mooreamps@hotmail.com

                    Comment


                    • #25
                      Some how we must have got sidetracked here. Wasn't this about getting Chunkitup's design going ? I thought Bop P. and others had some real nice info for you Chunk. For dB purposes anything ove 20 watts to 50 watts is only a 3dB difference to your ears. If you are only going to play in small venues then you dont' need more power. If you play outside gigs and big places you'll need more. You want the JCM 800 sound but only have a 150 ma power tranny. Not that big a deal as lots of EL-34 amps only run on 450 volts so your around 425 and a little less under full operation. Go for it ! Build the Marshall design and use SS rectifier and whatever tranny's you have. Fuse them and let er rip ! What's the worst that can happen. It'll blow up ? So what and I don't think it will as long as you bias the tubes right and calculate the dissipation a little over on your resistors your good. You'll be surprised how tough that tranny is and how long it will last. I think you'll get a great sound at just the right volume and will have a smoking little amp. That's all you need
                      KB

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                      • #26
                        Originally posted by mooreamps View Post
                        I took a look at LondonPower.com. I could not find a 70 watt amp. I did see a power scaling amp for $1000 coming soon. So what, I have three power scaling amps for under a grand and my power brake is better than his is...

                        -g
                        This is the one. My bad. It seems it's nominally a 50W amp. What I was remembering was this line about the amp from TUT5. "The Hammond 272JX and 1650K output transformer offer an easy 50W otuput with up to 70W available if we really need it."

                        Sorry, I'll stop hijacking now.

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                        • #27
                          This is all good reading. But here is another query. I was just looking at the new Krank Rev. Jr which is rated at 20W. He is using 2-5881's and they are rated at, well I've read some are rated at 25w and some are 30w. So how did he make a 20w amp using these big bottles?

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                          • #28
                            Originally posted by chunkitup View Post
                            This is all good reading. But here is another query. I was just looking at the new Krank Rev. Jr which is rated at 20W. He is using 2-5881's and they are rated at, well I've read some are rated at 25w and some are 30w. So how did he make a 20w amp using these big bottles?
                            Go back to the tube charts for the answer to this question. There are a number of factors that determine how much power you'll get out of a pair of tubes, and its all defined by their operating parameters. Look at the obvious things like B+ voltage, bias, OT load resistance, etc.

                            For example, if you look at Fender amps, you'll see that smaller amps like the Vibrolux Reverb and the Bassman both used a pair of 6L6 tubes, but the Vibrolux put out about 35 watts, while the Bassmen put out anywhere from 50-70 watts.
                            "Stand back, I'm holding a calculator." - chinrest

                            "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

                            Comment

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