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Bright cap for Tweed Champ; blackface mods.

5881

Member
Messages
1,115
I have a Champ home build that has always been on the dark side for me. I built if about 10 years ago; I forget whose kit it was. The tone control was an option, but again the amp has never been as bright, or open for that matter as open as I would like. On top of that I only play hums so... Looking at the picks and below values any ideas?
Bottom line is I want it to sound "blackface"; any mods out there?
Tha
Left to right as the pictures appear:
Power/Volume knob 716P 600v Large orange
Tone control:500 pf? Small brown
No cap on input jack
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pdf64

Member
Messages
8,151
From the photos, can't properly see how the vol pot / tone pot / caps have been arranged. Need to see what is connected where on the vol pot.
Pete
 

Leaf_Mansel

Member
Messages
37
Well it looks like a typical tweed style vol and tone arrangement... but can't see the values for the pots, think they might be 1M. Also the Orange drop cap, 716p is the series number for those caps not the actual value, which would be written next line down and on the left. My guess would be that it says 472, that is 47 and 2 zeros picofarads ie 4700pf or 4.7nF. Which would tie in nicely with the other 500pF cap. Many tweed tone ccts have a 10:1 ratio between the caps.
Although it looks a fairly typical tweed tone cct., there are a few ways to wire these up so a few more pics would be useful, and confirmation of the cap and pot values.
 

5881

Member
Messages
1,115
Thanks for the input. I added another picture that shows the wiring better, and the orange cap has 472J on the second line. The pots are "A1M" volume; and "B1M" tone. Anything I could do to "open it up" would be great. I want it to sound Blackface as opposed to tweed. May be asking alot here. Should add that I am running large transformers on this; a Hammond 270CAX PT and a Hammond 125 ESE so there is a lot of output on tap for a Champ.
 
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guitarcapo

Senior Member
Messages
2,326
First thing I would try is a simple treble bleed cap across the volume pot lugs. I honestly never "got" why people rave so much about the tweed Champ. Even with single coils it always sounded muddy to me unless the volume was on "12"

Some values I use are .1 nF to .4nF...but you can play around with lots of values. If you go too large, though...your volume pot will appear to cease to function or become more of a tone knob than a volume knob.

Here's my favorite mods to the Champ circuit all on one schematic:

 

phsyconoodler

Member
Messages
4,305
The bright cap is useless on a champ unless the volume is below 6 or 7 all the time.As most of the Champ's tone is when it's cranked,it would be better to alter the bypass caps on the cathode of the preamp tube.There is not a lot of difference in bypass caps until you get below .1uf or so. A .68uf cap switched in is really nice.
Raising the preamp voltage will get you more headroom.Take a look at the dropping string in the champ and change accordingly.

In the above schematic the adition of a choke is a cool idea but the 3.3uf cap on the power tube cathode is not.It should be no less than 50 uf there.The amp really loses drive with that low a value.No need to drop the input resistors below 68k either.It may let in radio frequencies.Oh and unless the amp is cranking out far more voltage than stock,the 680 ohm resistor on the 6v6 cathode will drop too much output power.No need for that either as the stock 470 ohm will be just fine there.A silverface Champ is where the cathode resistor may be too low a value.
 

guitarcapo

Senior Member
Messages
2,326


The above schematic in action. Guitar straight into amp no pedals. Never heard a radio station in the 3 years I've owned this one.

As for the cathode cap being "no less than 50 uF", you might want to call Fender about that since they've been using 25 uF since the 1950's. I use a lower value because of all the extra gain I'm getting from the cathode cap on the second gain stage, higher voltages, and reduced input resistors. Got rid of the negative feedback too which helped. As for output power, this amp is plenty loud and goes from Matchless cleans to enough overdrive for my tastes...which I admit are subjective.
 
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phsyconoodler

Member
Messages
4,305
I did say 'may' let in radio frequencies.It really isn't nesessary to lower the value and it really doesn't add more gain or input drive or whatever.
It may have enough drive for you,but I use at least the stock size cathode bypass cap on the 6v6's.As you increase up to even as high as 1000uf the drives gets creamier.
Your amp has a nice clean tone!But it doesn't kick it out like a stock champ circuit.

The preamp cathode bypass cap is where all the frequencies are let in.
To the OP: try using a .68uf cap on V1 and go up from there to taste.You could even switch different ones in and out to get variations.
 

guitarcapo

Senior Member
Messages
2,326
It overdrives even better with humbuckers. That last part was a telecaster with the switch set to the bridge pickup..



I usually play my guitars with the volume all the way up. When I turn down the volume I never like the tone as much. Things sort of sound sterile. (I usually adjust volume more at the amp) I know there are treble bypass mods you can do to the volume pot of the guitar but it's just not the same for me. That's where changing the input resistor helps me. With the volume all the way up it allows the amp to be pushed more from the front which is a sound I like.

I think I had to reduce the cathode cap value on the power tubes in response to other mods I made that increased gain like removing the negative feedback loop, adding that cap to the second gain stage, increasing voltages etc.

A lot of my amps that I love use no cathode capacitor at all on the power tubes. (Let alone one less than 25uF)

A few of my Magnatones for example. Lots of different amp designs out there. I try to keep an open mind.
 
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phsyconoodler

Member
Messages
4,305
I like switching out the bypass cap for a low-power mod.I do that on a few amps;makes for a really cool lo-fi sound with less output.

I was referring to the bright cap mod on the amp's volume though;as you turn the amp's volume past about 6 it doesn't have any effect anymore.

I love 12" speakers with a Champ or Tweed Princeton circuit.
 






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