help me with my amp project..... Marshall 1987

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gusfinley

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Hi, I am planning on building my own custom amp based on the Marshall JMP 1987. For now I will just be building the power amp. It will have lots of custom options like pentode/triode switching, fixed/cathode bias switching, feedback switching and possible variable feedback, post phase inverter master volume.... its gonna be fun!!

I'm going to build it out of terminal strips becasue I want to be able to modify it easily, as I will be using this for experimentation to see what sounds different configurations and tube compliments give the amp...

Eventually I will build a preamp also, but I have not designed this yet - I will be using my current amp preamp for this purpose. Will there be any problem with building and running only the power amp section for now? The voltages should all be the same, but it would be able to supply more current without the loading of the preamp circuitry. I'm hoping that the components will only draw as much current as they need... Is this true or do I need to redesign the amp (Chage resistor values) if I am just using the poweramp section?

Reguarding the power supply - I would like to have this amp handle up to KT-88's, but originally it will be used with EL34's, but later I hope to try out 6V6, 6L6, 5881, 6550, KT66, KT77,KT88. I have estimates that in order to handle KT-88's (the "hungriest" tubes) I would need to supply about 275ma. I have read that power transformers such as Hammonds can be run at about 50% over thier spec current. Using this calculation, I would need a powersupply that would supply about 185ma, but only 175ma or200ma is available so I'd get the 200ma.... Is this correct work?

Also, reguarding the value of the power transformer - I am having trouble deciding which one to get in terms of voltage. Using a factor of 1.4 to get output voltage for a solid state power amp- a 300-0-300 would give me 420V after rect, and a 350-0-350 would give me 490V. which ones of these is going to work better? I would assume 490V would be better for EL34's and KT88's, while 420V would be better for 6V6, 6L6? So I am leaning toward the 350-0-350 one....

Finally, I have been unable to find any information reguarding the value/specs of the choke. What value should I use? What should is DC resistance be? What should its voltage rating be? How do these different value, effect is performance, "tone," etc.

I greatly appreciate any help!!! Hope to hear from you soon...
 
As far as the plate voltage, I personally think 420v is almost perfect for a Marshall 1987. I used to have a 69' jmp-50 that was a cross between a 1987 and jtm45 (one of marshall's early ptp experiments, i guess ;solid state rectifier)and it came with ~380 volts stock. I replaced that with a Mercury Magnetics PT that had 420 volts. The original 380 was nice for clean, but was incredibly dark and muddy when cranked. The 420 opened it up, but not to the point of harshness; it was still buttery smooth. However, once voltage goes beyond 450 (as many metal panel Marshalls do), i think there is too much treble and bass and things get harsh quick.


gusfinley said:
Hi, I am planning on building my own custom amp based on the Marshall JMP 1987. For now I will just be building the power amp. It will have lots of custom options like pentode/triode switching, fixed/cathode bias switching, feedback switching and possible variable feedback, post phase inverter master volume.... its gonna be fun!!

I'm going to build it out of terminal strips becasue I want to be able to modify it easily, as I will be using this for experimentation to see what sounds different configurations and tube compliments give the amp...

Eventually I will build a preamp also, but I have not designed this yet - I will be using my current amp preamp for this purpose. Will there be any problem with building and running only the power amp section for now? The voltages should all be the same, but it would be able to supply more current without the loading of the preamp circuitry. I'm hoping that the components will only draw as much current as they need... Is this true or do I need to redesign the amp (Chage resistor values) if I am just using the poweramp section?

Reguarding the power supply - I would like to have this amp handle up to KT-88's, but originally it will be used with EL34's, but later I hope to try out 6V6, 6L6, 5881, 6550, KT66, KT77,KT88. I have estimates that in order to handle KT-88's (the "hungriest" tubes) I would need to supply about 275ma. I have read that power transformers such as Hammonds can be run at about 50% over thier spec current. Using this calculation, I would need a powersupply that would supply about 185ma, but only 175ma or200ma is available so I'd get the 200ma.... Is this correct work?

Also, reguarding the value of the power transformer - I am having trouble deciding whi

ch one to get in terms of voltage. Using a factor of 1.4 to get output voltage for a solid state power amp- a 300-0-300 would give me 420V after rect, and a 350-0-350 would give me 490V. which ones of these is going to work better? I would assume 490V would be better for EL34's and KT88's, while 420V would be better for 6V6, 6L6? So I am leaning toward the 350-0-350 one....

Finally, I have been unable to find any information reguarding the value/specs of the choke. What value should I use? What should is DC resistance be? What should its voltage rating be? How do these different value, effect is performance, "tone," etc.

I greatly appreciate any help!!! Hope to hear from you soon...
Click to expand...
 
sweet!!

spoonie, thanks for the reply...

I was checking things out yesterday and realized that the hammond transformers I was looking into had 115V primaries, but sometimes my AC feed runs up to almost 124V!! That mean that my B+ for a 350-0-350 is going to be 511V at 120 and even more when the AC line goes up. (My are had really bad power, and there is NOTHING I can do about it!!) With the 300-0-300 at 120V I would get 438V.

The caps I am looking into are 500V, so the other 350ct secondary transformer would require more, bigger caps. I was glad to hear that plexis have the same kind of voltage as the 300ct transformer will put out!! That is the sound I want to hear out this thing!!

I'm getting pretty stoked about my project. Its time to finalize the schematic and start ordering parts, but first I need to find some place that will punch the chassis for me!!

for anyone who cares - I'm thinking that for the preamp, I will use "bright" and "boost" switches on each channel, and also a switch that will switch the tone stack between Vox and Marshall Specs. Channel switching may also be an option. I'm gonna have to pick up Tonnes of Tone, though so I can design the preamp the way I want it.....
 
For the chassis holes, you can use drill bits plus a holesaw set, or if you want nicer holes, try either a chassis punch set or a stepped drill bit which goes up to 7/8" (which will do all holes except the power tube sockets).
 
hmmmm...........

Being that I am only building one amp for now, I'm gonna see how much it will be to have someone else punch the chassis - a set of punches would run me like $100 , if I could have it punched for $20 or so that would be great...
 
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