Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mpayne on April 08, 2011, 10:03:42 am
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I’m putting together a schematic for another amp build and was wondering if anyone would be willing to look over it and give some feedback. It’s a slightly modified JCM800 with an effects loop added. I’ve been reading Merlin’s book on preamps so the effects loop is taken right out if it. The schematic is spread out over several pages to make it easier to read.
I also have a few questions:
In Merlin’s book he talks about using anode bypass capacitors to eliminate high order harmonics in the preamp. Given that, should I add some ~100 pF caps across R1 and R2 in the preamp for insurance against noise?
Should cathode bypass caps be added to V2A in the preamp?
I was wondering if I could add a 9 volt DC output to the back of the amp to power effects? I’ve seen some small modules that take 85~240 VAC and convert it to 9VDC. Could one of those be placed in the power section without compromising the sound quality of the amp? I realize that the 9 volts would only be able to source about 300 to 500 mA.
I used the same output schematic for my last build and the presence control had little to no effect on the amps output. Could I just leave it out or is it important to the amps overall tone?
Thanks for your time,
Mike
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I wouldn't add any caps unless the amp sounds funny.A well built JCM800 is pretty damn nice as is.If you add a bypass cap across the cathode resistor on pin 3 of V2 it will bump the mids up and add a considerable amount of gain.Some like it and some don't.I would use a .68uf and see if you like the before and after sounds.
The presence on the JCM800 works pretty well stock.
Unless you have lots of room in your chassis,I would leave the 9v power supply out and on the floor where it belongs.Just my opinion.
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This is only a small adjustment to your bias circuit. Move the 47k resistor to the ground side of the 22k trimpot,this will help protect your valves should there be a problem with the trimpot.I can't say that too many marshalls have FX loops so i was glad they added one to the jubilee. This is how they did it, hope this helps :icon_biggrin:
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I wouldn't add any caps unless the amp sounds funny.A well built JCM800 is pretty damn nice as is.If you add a bypass cap across the cathode resistor on pin 3 of V2 it will bump the mids up and add a considerable amount of gain.Some like it and some don't.I would use a .68uf and see if you like the before and after sounds.
The presence on the JCM800 works pretty well stock.
Unless you have lots of room in your chassis,I would leave the 9v power supply out and on the floor where it belongs.Just my opinion.
I like the .68 to 1uf also. Even a .47uf bypassing the other 10k cathode is a cool tone too.
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KiCad?
Everything looks right to me.
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I would take the bias voltage from the junction of R42 and R43 like on the original.
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Good Catch! I'll move the bias between R42/R43. Thanks!