Thursday, January 1, 2009

Fender Champion 600 Presence Plus Mod

This post is one in the series of mods I made to this Champion 600. I wasn't sure what to call this one so for now it's dubbed "Presence Plus". It pretty much the same as a presence boost except that the boost has a much wider frequency range so it effects more than just the traditional "presence" frequencies. It extends from the high frequencies down to just above the resonant bump in the speaker response. The idea is to have the feedback loop maintain control over the low end looseness of the speaker but be transparent to the high and body frequencies. The effect is to give the amp a bit more gain and openness without boosting the low end flab.

Here's the modified schematic. The added components are shown in red:

There are a few other mods in progress here so don't let the altered circuit board throw you off. The presence mod consists only of the huge black cap in the upper right labeled 16 uF 350 VDC, the red switch it's connected to and the black jumper wire that completes the connection across R23 in the pc board. That's all there is to it.

The 16 uF cap is ridiculously over rated at 350 VDC. I really wanted a 16 uF cap. I have 10's and 22's in stock but nothing in between, so I pulled a 16 from the parts bin and used that. There's so little voltage across R23 that pretty much any voltage rating you find will work (and take up a whole lot less space).


Here's a view from the top:

And here's the switch on the bottom. You can see the silver Sharpie line I've drawn to position pilot holes for future switches.

be careful when placing the switch on the outside edge. Mine just barely clears the wood support for the chassis inside the cab. I stupidly forgot to check before I drilled and if it had been another 1/8" out the chassis wouldn't have fit back in.

Here's a closeup of a schematic for the mod. In the full schematic a the top of the post the feedback loop path to ground is highlighted in blue. Below you can see the added components in red.

I find that the Champion 600 has plenty of high end response (especially with the sort of lousy stock speaker). If you find otherwise, you can fiddle with the value of the 16 uF cap. I don't think think you'll find much use in going lower, but raising the value will restrict the low end response and the effect will be more of a high mid / treble boost.

If you want to order the cap for this mod, have a look at the Presence Plus Mod "kit"


Fernando said...

One more great addition.
I planned to add a traditional "presence" ctl. but I'll try this first.

I also plan to test different values for the feedback resistor. The 5F2A uses 22k, some others 57k, 100k, etc. I'd like to have finer control of the feedback to explore. So I maybe try with a pot first and later add a switch with a couple of them.
Do a pot on that position need to be of higher than usual watage?

Fernando said...

Do a pot (*) on that position need to be of higher than usual watage?
And how higher res. values will be related to the cap value in your mod?

(*) I'd strat with a 100k pot to explore from 1k to 100k, then move to 1M to see if there's something interesting near "no feedback" that I preffer to cutting the fb loop completely.

akavalve said...

The pot can be a 1/4 watt one.

In that circuit the cap value has more to do with the 47 ohm resistor it is in parallel with. So it's frequency response should stay pretty much the same.

Remember that the feedback resistor is part of a feedback circuit and it's value isn't chosen in isolation. You'll see that the earlier Champ circuits without the cathode bypass cap on the second stage connect the feedback resistor right to the cathode of that stage. This forms a voltage divider with the 1.5K cathode resistor.

Later Champs (including the Champion 600 RI) have a cathode bypass cap on the second stage which means the feedback can't be connected to the cathode any more or it will simply be shunted to ground through the bypass cap.

So another resistor is added below the cathode resistor in order to form the other half of the voltage divider in the feedback circuit.

That resistor is much smaller (just 47 ohms), in order not to effect the bias and frequency response of the second stage.

This means that what is usually considered the "feedback resistor" also has to change in value in order to have a similar ratio.

So follow the path to ground for the feedback loop when comparing the resistor values. The actual amount of feedback has to do with more than just the value of the resistor connected to the speaker tap.

All that said...if you throw a pot in there you'll probably never bother with switched fixed values. Make sure to put a resistor of some minimum value in series with your pot. Ideally this would be the stock resistor value, but if you really want to increase the negative feedback (you probably won't once you hear it), you could make it smaller.

John Purgatory said...

I just replaced the negative feedback resistor with a 10k pot, which I modified to be no-load at 10. In fact, there's hardly a noticeable difference from about 7 right up to unloaded. There's a huge difference at the bottom of the control; at 0 the volume is dropped way down and it is quite clean, even with the volume cranked, and as you turn it up a bit it comes back to life. Around 5 it seems there's enough resistance to pretty much negate the negative feedback ;) I was originally planning on using a 25k or 50k pot, but it turns out the 10k is plenty, even a 5k no-load pot would do fine.

Andy, is there a problem with not having a fixed resistor in series with the pot, as you suggest? Like I said, the volume really drops at 0, but it doesn't seem to be doing anything noticeably bad, could I be ignorant to something?

I've also bypassed the tonestack with a .022 cap, and done the Frondelli mod on the preamp, so this amp is really crazy high gain with the negative feedback taken out, and adding it back in to around the stock value really cleans it up in a useful way. I could see just using a switch to flip between stock and no feedback loop, those would be two very useful tone options; I'm not sure the pot is really necessary, and it just kinda complicates things.

I'm going to add a switch with the 16uF cap as you suggest, Andy, I could see that being another very useful option.



John Purgatory said...

I've got a couple of ideas to further this mod. For one, I'm thinking of replacing the pot with a switch, to switch between the stock 2.2k negative feedback resistor, and the feedback loop removed. Then, I would use the pot in series with the 16uF cap as in your mod, so that I could gradually blend the cap into the circuit when the switch is in the stock position. So, it would be a gradual control of the mid/treble boost, and if I want a full out boost I could flip the switch to remove the feedback loop completely. The question is, what value of pot resistance would be necessary to effectively block the capacitor's path to ground?

My other idea is to keep the pot as I've got it now (as a variable resistor controlling the negative feedback loop) and add a switch to flip between two capacitors to ground, such as the 16uF you suggest, and a larger value to get just a treble boost as you suggest in the last paragraph. This way, I've essentially got a 3 band tone control (sort of), with a full boost control using the pot, a mid-treble boost with the 16uF cap and a treble boost with the other cap. My question here, is what value of cap would be suitable? Does it work in the usual way capacitors work, in that doubling the value equals an octave?


akavalve said...

With regard to the pot in series with the 16 uF cap:

Trick is you can't pick a pot value that will be effective over it's range and completely remove the effect of the cap.

A 50 ohm log taper should work well for a decent range of control - but good luck finding one!

Probably easier to go with the switched caps - the doubling/octave rule does apply when picking the values.

If you do install this cap and leave your pot in place of 22k resistor, put a 47 ohm resistor in series with the pot to be safe. If the pot is set to zero resistance the cap will essentially short the amps output to ground at high frequencies. The 47 ohm will protect against that.

Best, Andy

martin.distefano said...

Hey there! First off, your blog is excellent! It's fun (and educational) to read through what you're doing.

Ok, I'm currently building a Champ 600 from scratch and am ready to hook the circuit up to the transformers and speaker. One thing I haven't done yet is the connections to J19 and J20 (I think those are the right numbers). Each connection point has five spots allocated. I'm assuming that these can be connected directly without the use of the connector (or whatever this piece of hardware is called) that is shown in one of the above pictures? Any insight from you would be awesome! Thanks!

akavalve said...

I assume you're building on a tagboard or something, not on a PCB right? In either case, there's no need for the disconnects on the stock amp - they're just there for ease of assembly and disassembly.

martin.distefano said...

i figured as much, but just wanted to make sure. thanks for your help!

martin.distefano said...

actually, one more quick question. is it necessary to use a 4-ohm speaker? I have an old 8-ohm laying around, but i'm not sure if it will mess the circuit up somehow. I could connect the 8-ohm in parallel with an 8-ohm resistor to make the overall resistance 4, but do you think that would mess up the circuit? thanks again!

Anonymous said...

Hi Andy,

Thanks for a great blog.

I just completed the presence plus mod, and switched on it sounds very thin, way too bright. This is with a 12AX7 preamp. You say there's not much use using a lower value cap. Why not? Will it allow more bass? Could you suggest an alternate cap size or other component that would tone down the presence a bit?

Thanks again.

André Coelho said...

Hi! Thanks once again for all the great information!

After going for the tone stack bypass, I´ve decided it is way too much for me. Just didn´t care much for the sound. Still, I thought a mid or body control would be useful, so I have used a 50k pot at r19. At 0, it sure helps preventing feedback when using the amp for harmonica.

I want to go to my second mod now, which would be adding a presence control.

I am thinking that adding a 16uF would somehow boost some frequencies I am trying to avoid here (upper mids). Would it be too hard to add a pot for that?

Otherwise I guess I will experiment with different cap values.


Anonymous said...

I performed this mod using a 22uf cap and I must say, very nice. It's more of a bright switch with the 22uf but it sounds amazing. This was a very simple way to make enough of a change to affect the overall tone in a positive way and with the switch I can toggle between stock and bright. Thanks for the info.

Anonymous said...

I write from Italy.
Congratulations on the excellent changes. Congratulations!
I made this change with a 16UF Cap.da. + switch and light blue.
With a Fender Stratocaster, the difference is not very clear.
With a Gibson Les Paul, the difference is greater.
In the coming days I will try to be a Chapter 22 uF then I will keep you informed.