Tweed Champ (5F1) build (JTM45ing it)

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Eugene Martone
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Tweed Champ (5F1) build (JTM45ing it)

Post by Eugene Martone » Sun Jan 06, 2013 2:30 pm

I know it sounds a little stupid, a champ being SE low power and all that, but I can think of less similar amps :wink: . Seriously tough, I know they aren't going to sound the same, but I want to get the champ a little closer to that JTM45 drive. I've searched the web for champ mods etc. and it seams most people are trying to take it the other way (like removing the feedback loop, witch makes it even less marshally).
So let's suppose Jim had started with a champ instead of a Bassman (also supposing his changes were deliberate), what would he have done?
I'll purchase most of the stuff from TAD and thinking of going with a larger cab with a 12" in it. I'll throw a g12m in there to begin with, as I have a few of those lying around, and will also purchase their "grand stack" OT witch I think is 15w with 4-8-16 taps (not shure if I'll make it switchable or just use the 16 ohm tap).
On to the circuit:
The 5F1 schematic has no cathode bypass cap on the first gain stage and I'll add either a 47 or a 25 micro cap here, keeping the 1,5k resistor. Next i thought about changing the cathode resistor of the second stage to 820 ohm to run it a bit harder. Then I thought of using sozo coupling caps making the first .022 and the second .01???(seeing as there's no tone stack to turn down the bass). I've also thought about doing some changes to the NF loop, perhaps (going against the grain and) increasing it (lowering the value of the nf resistor)? Taking the NF from the 16 ohm tap (like they did on the ? I'm really out of my depth here, I don't want a stiff amp, yet I want a smoother overdrive...
Naturally, as I haven't ordered the parts yet, I can change any plans if you think my ideas are counterproductive or just plain dumb, but it's a start. All and any ideas will be welcome :help:
Far sweeter than the lyre in tone,
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julkke
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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by julkke » Sun Jan 06, 2013 2:38 pm

Do Marshall style shared cathode and see how it works. I think it will be hard to get marshall outta champ though. But, I'm interested to see where you will end up!

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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by Eugene Martone » Sun Jan 06, 2013 3:05 pm

Yeah, it's gonna be fun to see how close I can get :rock: Seriously though, it'll still be a champ (I already have a JTM45) with a 1 tube pre and SE output.
I thought about shared cathode, but in a JTM that's for the two channels not 1st and 2nd stage (also doesn't this equate to about 1,6k?) so i thought id get further by copying the 820 of the second stage of the JTM (and 5f6a i think) and leaving that one alone? Am I wrong in thinking the shared cathode would not impart as big a difference in sound?
Far sweeter than the lyre in tone,
A voice more golden than gold...

julkke
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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by julkke » Sun Jan 06, 2013 4:48 pm

Yeah, I guess you could simulate it like that. Very cool would be to add a preamp tube but I don't think champ chassis would have room for it.

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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by Eugene Martone » Mon Jan 07, 2013 3:16 am

Yeah, that would be cool, but like you say, it's a little cramped in there (Those champs are small)... Any ideas about the NF loop thing? Am I rightinking this will smooth out the distortion, or will it just choke the amp? (I know I could build it and test, but I would like to have a schem finished to what I hope will be perfect and tweak if necessary)
Far sweeter than the lyre in tone,
A voice more golden than gold...

julkke
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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by julkke » Mon Jan 07, 2013 4:49 am

Well the JTM has lots of negative feedback... Maybe put a pot to adjust nfb?

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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by Eugene Martone » Mon Jan 07, 2013 7:25 am

Like I mentioned in the first post I'm contemplating using the 16 ohm tap for NF, more or less what Marshall did with the JTM versus the Bassman. Naturally, I could use a pot in series regardless, but has anyone tried this? Would it have a similar effect?
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julkke
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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by julkke » Mon Jan 07, 2013 8:12 am

It would be adjustable nfb control. Many have tried it and liked it, gives some more versatility.

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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by Eugene Martone » Fri Jan 11, 2013 9:08 am

So after airing this here and in another forum I've changed my plan (still not quite ready to order the parts though :roll: )

The idea at this point is (still using the chunkier OT and a greenback): 2k cathode resistor with 1uF bypass on V1a and 1k (or 820) cathode resistor on V1b. Coupling caps sozo 022. Negative feedback switchable between OT taps (4-8-16).
Any ideas about this preliminary sketch?

Also, just for fun (and maybe some inspiration), I'd like to hear some thoughts on this question from my first post:
So let's suppose Jim had started with a champ instead of a Bassman (also supposing his changes were deliberate), what would he have done?
How do you think a "JTM5" :lol: would have been built, and how would it have sounded?
Far sweeter than the lyre in tone,
A voice more golden than gold...

julkke
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Re: Tweed Champ (5F1) build (JTM45ing it)

Post by julkke » Fri Jan 11, 2013 9:23 am

Jim would have made a carbon copy clone of it :lol:

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