#1
if anyone here saw the old thread, you know that a friend and i are working on a heavily modded fender tube amp design. after several considerations for the base amp, we decided to keep it simple and just use a champ.

so, i got a champ schematic. AA764 (non vibro-champ though).

one problem. it lists every cap as non-polarized. no way in hell that works out. some, while using the non-polarized symbol, do have a little + next to one side and a - next to the other. are these elctrolytic? or what are they? whats the deal???

http://www.schematicheaven.com/fenderamps/champ_aa764_schem.pdf
^thats the schem i'm working with.
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Last edited by AngusJimiKeith at Feb 17, 2009,
#2
They're electro, with the + or - denoting the polarity. They just didn't use the 'standard' symbol for polarized caps.

Polarized caps have had several symbols used over the decades.
#3
figures. God knows how old these schematics are
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#7
^alot. different tonestack (changed some cap values), fat switch, american/brit switch (cathode bypass cap switch), master volume (i cant even put my VJ on 5 without pissing off my parents), pentode/triode switch...

and thats only atm. there are still a few mods which have yet to be worked out.
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#9
hell yeah. so far ive got the tonestack and master volume mods edited into the schematic, but not much else.
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#10
Hows the fat switch work?

In my opinion MV and pentode/triode switch really take away from these simple 5W amps. I would just install the MV but make it a push/pull pot to remove it when you want.
#11
^yeah, the pentode/triode may not stand, that was a holdover from the deluxe mod which we dicthed. The mv switch sounds like a good idea, i'll definately throw that in. especially cause when my parents arent home, i forget about their rules and just crank my amp

btw, 220k 3w resistor in parallel with circled cap will discharge all caps when i turn it off, right?

*note--this is not the "modded" schematic
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#13
good, i dont feel like being fried....
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#14
sorry for the double post, but i this needs a bump...for updates, of course.

so...the more i thought about it, the more I wanted both the original champ sound as well as a "modded" sound. So, I threw in a second 12AX7 and basically bastardized the AA764 preamp with a 5F1 "tonestack" (not really a tonestack, its more of just one cap) into a second channel. The only mod on this channel is a switch to change the cathode bypass cap, to help eliminate flab at high volume. I used the connections from the AA764 vibro champ to figure out where the 12AX7 should fit into the schem, so there's a possibility that i'm wrong. could someone check it to make sure it's right?

the schem thus far:


if it looks edited in with paint, it is.
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Whenever I clean my guitars, my family wonders why it smells so good; I say that I exude a fresh citrus scent from hidden orifices.
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#15
Triple post ftmfl...

bump...

i'd really like to know if this is right, cause i dont want to have to scrap the idea...
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#16
Looks about right to me. What you could do instead of adding the second 12ax7 is put in a switch to change from the TS to the .02 cap. Effectively that keeeps the complexity down, but you still get both options. The line you have connecting the cathodes looks incorect but I could be incorect on that too, I don't think it should be there. BUT, putting in only one 12ax7 would solve that.

If you are bent on putting in two 12ax7s, add a cascade switch to send the out put of one stage into the other one. I'll see if I can dig up my schematic where I did this and a couple other tweeks to a planned build. Essentially a 2 channel 5F1, where the PA and the 1st channel where 5F1 and the second channel is the 5C1 pre.
#17
^thanks, i do want the second channel, i like the flexibility of having two channels. the cathode connection probably is wrong, like i said the connections were from a vibro champ, so it could very easily be wrong.

if you could dig up that schem, that would be great! thanks in advance
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#18
The connections leading to the output section are definitely wrong.

TBH I think your begin way too ambitious on paper. Its one thing to draw in little switches and channels on paper. Its another thing when you go to build this big non stock uni that has 450V DC floating around. I would seriously simplify things, I know when I started I tried to do what you were doing but got talked out of it. Thank goodness...

I would not add a second channel. I would add another recovery/boost? stage after the tone network. Use a DPDT to bypass the stage from the beginning and behind.

Also your gain boost will do nothing the perceivable to the ear. Gerald Weber says about cathode resistors, "you need to double or halve the value to be able to hear any change". If I were you I would use 820R (typically first triode Marshall value) and a 2K resistor. The 2K resistor is a bit higher than the typically value of the first triode in a Fender (1K5 usually). Having a 2K and 820R though will make the switch a bit more noticeable then if you used 1K5. If your going to add a switch might as well make it do something.

I would make the cathode resistor and bypass cap one switch. When the 820R is switched in the 680nF and you know what the other throw does then. Should give you a nice tight gritty, middy tone and a smooth, cleaner full tone.

EDIT: What is the point of having a second channel? Its not like one is a dirt channel, they are both typical Fenderish channels. They are in phase with each other though which is good, I guess if you like bridging inputs a second channel would be good.
Last edited by kurtlives91 at Feb 21, 2009,
#19
^hmm...i think it would be best for me that i stay away from the second channel then...

thanks for pointing out the cathode resistor value too...so basically its a fender/marshall switch type-thing (obiviously i wont get a true marshall tone, but itll be noticably closer)

oh, and about your question about the fat switch: i was originally going to use the circut from the blues jr. but there was no decent schematic for the circut itself...so i cheated a little and used the duncan tonestack calculator to figure out a good value cap to switch to in the tonestack... .01 on the mid seemed to give about the boost i was looking for.


EDIT: painispower solved my entire problem of "losing the champ tone" with one switch and a cap. wow im an idiot.
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Quote by Øttər
Whenever I clean my guitars, my family wonders why it smells so good; I say that I exude a fresh citrus scent from hidden orifices.
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Last edited by AngusJimiKeith at Feb 21, 2009,