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Thread: Power amp design

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    Power amp design

    I want to build a simple rack mounted mono power amp modelled after the JCM 800 power section. It will be 50watts-EL34. I have a Hammond PT 300V CT at 150mA-Is that enough current?
    For the design:
    Do I just have the input run into the Volume then into the Phase splitter then into the power tubes exactly like in the JCM800? Is there anything I should be aware of? Is it that simple?

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    Current is pretty ok I guess, maybe a little low, I mostly use overrated PTs.

    Regarding the design: just remove all "pre-amp-stuff" and maybe raise the node-resistor in the powersupply to the B+ from the PI a bit to get the same sound and feel.

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    Chris Winsemius

    www.CMWamps.com
    Vleuten, The Netherlands

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    Old Timer Tom Phillips's Avatar
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    IMHO that the transformer you have is quite a bit underrated. 300V @ 150ma is only 45 Watts of power. To run a 50W amp I would design a power supply section that could supply at least 100W. In addition, with 300VCT you will need to use a FW bridge rectifier. The FWB requires a higher secondary winding current rating than when you are using a FW center tap rectifier.
    Cheers,
    Tom

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    Be sure to leave some space to an extra 12AX7 after you builded it.

    Chris, don't get me wrong, I'm not saying the opposite of you just need to call atention for something. I've builded a power amp and I found that to be able to use it with stompboxes (100mv devices) I needed an extra triode before the PI. What I did was to add a 12AX7 an placed a gain stage at the input an after that a cathode follower to driver the volume pot, so I could use a lower value pot so it won't affect to much my tone whatever the position of the pot is.

    This might be complex and prone to error, so build the PI/Power amp first and leave space to have the option to add it in the future.

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    I think I stated it wrong. The PT is 300-0-300V and is center tapped. I wont need a bridge rectifier. Would it be better for me to use a tube rectifier and EL84's? The tube rec would be needed to get the voltage down to safe EL84 use of course. I like playing heavier music so would the tube rec help or hurt the playability?

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    Old Timer Tom Phillips's Avatar
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    Sounds like a good idea. You could build a nice 20 to 25W amp with that PT. However, think about what you really want / need. You started out wanting a 50W power amp. The build will be a significant amount of work and it may not be worth compromising your goals just to use the PT that you happen to have on hand.

    Cheers,
    Tom

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    That's what I will do then. I just finished a SLO PP 20w combo that is almost too loud, almost! I never used a tube rect. but I dont think I will have any problems. Would you recommend a 5AR4/GZ34? I just happen to have a Lenco 25w OT so the 20w project would actually be better. The OT was labeled as a Deluxe Reverb swap, so do you think it will be alright in this application?

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    I think the 300v mentioned is 300v AC, which when rectified with a diode rec will give you around 420VDC. So 420VDC @ 150 ma is 63w.

    Also, there's a hammond PT, the 272FX, which has the same specs as mentioned, 300vac CT @ 150 ma DC, and it has a VA rating of 146 watts. (Is this the PT?)
    So it seems to me that this PT should be OK for a 50 watter.

    Russ

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    That is exactly the Hammond tranny.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Russ View Post
    I think the 300v mentioned is 300v AC, which when rectified with a diode rec will give you around 420VDC. So 420VDC @ 150 ma is 63w.

    Also, there's a hammond PT, the 272FX, which has the same specs as mentioned, 300vac CT @ 150 ma DC, and it has a VA rating of 146 watts. (Is this the PT?)
    So it seems to me that this PT should be OK for a 50 watter.

    Russ
    OK But I still think you need more than a 63W B+ supply to run a 50W output capable power amp. My rule of thumb is double the output power plus some more for the other circuits. Let us know how it works out.
    Tom

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    Tom, Good advice! I already have a 20w OT from another project that I will use in this amp so I will heed your words.
    Thanks

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    Actually, I'm also working on a 50w el34 design, but I ended using the next PT up in the hammond line, the 272HX - 300vac @ 200ma. But I didn't have the PT laying around, I had the choice of buying it, but I was real tempted to use the 150ma model - saves $, weight, and space. The reason I didn't was because I heard some people say it was too small and most of the schematics I saw used the 200ma and even the 250ma model. But, there was 1 or 2 shchems that had the 150ma. I suppose if you're playing cranked at 10 all the time then the 150ma might get stressed. I to would be currious to find out how the 150ma works out. Let us know.

    Russ

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    Better Tone thru Mathematics bob p's Avatar
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    Marshall transformer schematic:


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    Last edited by bob p; 06-13-2007 at 08:33 PM. Reason: added text description

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    You could build the amp with a 150ma PT but it will likely not deliver 50W of power to the speakers.
    Look at an EL34 data sheet.
    Assuming you implement a fixed bias Class B design with about 400 volts on the plates, the plates draw 100ma each at 50-ish watts output and the screens demand about 25ma each. Add to this preamp tubes and you are inclined to use a 250 ma PT. The theoretical current demand could be more than this, but I think most will agree that the output power requirement fluctuates under normal use so your 50 watt amp will operate nicely with a 250 ma PT (particularly with that under rated Hammond iron).

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    What about a quad of EL84s?

    Voltage might be a bit high for them, and the PT may still be ever so slightly underrated for full tilt operation. Perhaps throw a fan in there to reduce tube and transformer stress.

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  16. #16
    Better Tone thru Mathematics bob p's Avatar
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    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.

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    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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    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. #19
    Better Tone thru Mathematics bob p's Avatar
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    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.

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    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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    Quote 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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    oops. I quoted Gregg and then called you Bob Sorry guys!

    Found This on the subject. Looks like a pretty handy guide.

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    Last edited by Ptron; 07-04-2007 at 06:47 PM. Reason: bad link

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    Quote 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

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    ______________________________________
    Gary Moore
    Moore Amplifiication
    mooreamps@hotmail.com

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    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

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    Old Timer Amp Kat's Avatar
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    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

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    KB

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    Quote 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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    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. #28
    Better Tone thru Mathematics bob p's Avatar
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    Quote 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.

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