Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: garyboy on August 07, 2012, 03:18:59 am
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:new1: Hello i'm new here and was looking at building a marshall style tube amp. I've been reading alot of posts on this forum and gotten alot of ideas i would like to try to see if i like them. Here is what I am looking to get out of this build. I am looking at building for a high gain JCM800 circuit that i can change my mid slope (flat, cut, & lifted, and "mid voice" i guess you can call it, normal/ high gain switch, a 2M audio pot for the pre amp volume, resonance, and maybe a serial FX loop. I also read on this forum about running a SE OT with two tubes and i was looking at trying that with 4 KT77s PP running 50w instead of 100w. I'm not quite sure how to draw that for PP operation so i kinda winged it. I drew the schematic in three parts for it be easier on me to draw and read since this is the first time i used this program. Sorry if things looked cramped thrown in there. I would appreciate any input on errors, mistakes, and the design itself. I am also looking at building a dummy load box that i can run paralleled with a speaker cab. I am planning to use this amp for studio work where i can record clean tracks and heavily distorted tracks. That and it'd be used for garage jamming too.
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I found a few mistakes after i posted everything and made a few revisions.
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RE: your power amp...
I see a couple 50K pots on the grids. Are those suppose to be for bias adjustments? If so, they will not have any effect on the negative bias voltage on the grids.
Things I'd do differently. You only need one cathode resistor for each pair of tubes. Better yet, use a separate one ohm resistor under each of the four tubes but dont connect the cathodes together. Then you'll know for sure if the tubes are matched and sharing equal currents. I'd use a separate screen resistor for each tube. You have 400uF filtering on the plate node and 150uF on the screen node. Seems very excessive to me.
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Hey thanks. I was thinking of halving the resistors and putting pots there to control both sides idle. I'll just change it back to the resistors. How would be the best way to control the negative bias voltage to each side of the output section? Yeah that does seem quite a bit excessive but I have quite a few can electrolytic caps laying around so i was just going to use them in this circuit. I play alot of metal so i'd like to keep this tight as i can, but i also play alot clean also. I was maybe thinking of putting a sag resistor with a separate filter cap bank of its own that is quite a bit smaller so i can get some sag in the mix. I'll also go back to the individual 1R resistors on the output cathodes. Thanks for the input it's much appreciated.
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Here's how I did it...
http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf (http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf)
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Now that is cool. Thank you I'll see about integrating that bias control in.
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Thanks for the schematic I added the new bias supply, changed the cathode resistors back to individual 1Rs , and separated the screen resistors. I also added in a sag resistor w/ a smaller filter bank. Now with the output tubes setup would it be feasible to have them running into a 50w OT or am i approaching this the wrong way? I really like the idea of 50w amp that sounds like its a 100w amp w/ out the as much decibels.