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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.
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.
Sorry guys!
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