Fender Champ 600 questions

[Electron Tornado]

I have the chassis, PCB, etc., from one of these that I would like to mod. After reading about others' mods, and these two threads, I have a couple of questions:

http://www.diystompboxes.com/smfforum/index.php?topic=68174.msg550743#msg550743

http://www.diystompboxes.com/smfforum/index.php?topic=109345.0

Schematic:  http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Champion-600-Schematic.pdf


1. Regarding the negative feedback resistor, R7, if I want to replace it with a pot, what would be a good value to use? (5k or 10k, or something more?)

2. I would like to replace the output transformer with one that will allow the amp to be used with 4, 8, or 16 ohm cabs. Will the value for R7 need to be changed (or adjusted with a pot) when using cabs with different impedances?

[Mark Hammer]

The negative feedback tames high end.  The lower the resistance the less restraint is imposed on top end.  Least restraint is, of course, provided by NO feedback.  Given that the contrast between open and most feedback (at 2k2) covers a LOT of ground.  It might be more useful to use a rotary switch instead of a pot, and have two of the settings be 2k2 and open circuit, with others covering terrain in between.

On the other hand, it may be worth your while to temporarily install a pot (safely) and see just what feedback resistance values make an audible difference.

[fraser]

yeah- when i played with the nfb on mine i found i didnt like it without the stock value.
it was just too ragged sounding.
maybe different speaker or tubes or something wouldve helped- maybe its my ears or taste,
or just my amp,
but i didnt like it.
same with bypassing the tone stack- was just too nasty.
and i like hairy tweedy sounds lol.
i did like raising the mids resistor though- r19 i think? been a while.
mind you, i run that amp wide open all the time...

[thermionix]

2. I would like to replace the output transformer with one that will allow the amp to be used with 4, 8, or 16 ohm cabs. Will the value for R7 need to be changed (or adjusted with a pot) when using cabs with different impedances?

No, not necessary.  You can have the pot if you want, but you will draw the NFB from only one of the secondary taps, and it will stay that way regardless of which one you are actually using.  Look at a Marshall schematic for an example.  Keep in mind that all else being equal, you will get more NFB voltage from the higher-ohm taps.  But if you set it where you like it from, say, the 8-ohm tap, it will stay the same if you use the 4, 8 or 16.

[Electron Tornado]

The negative feedback tames high end.

The lower the resistance the less restraint is imposed on top end.

Least restraint is, of course, provided by NO feedback. 

Mark, wouldn't no feedback be greater resistance (actually infinite resistance)?

[thermionix]

^Yes, you are correct.  Looks like Mark mistyped.

BTW, if you are going to try a pot for NFB adjustment, you might want a small resistor in series, to prevent a zero-ohm situation.  I don't know what that much NFB would sound like, but I bet it would not be good.

[Mark Hammer]

Correct.  Or alternatively, the lower the resistance, the more restraint is imposed on high end.

I always like to remind folks that negative feedback was traditionally considered a way of making an amp behave better.  On my 59 Bassman (a real one, not a reissue), the Presence control (which varies negative feedback) came wired up such that clockwise reduced treble content.

[thermionix]

Funny what "factory errors" you'll find in old Fenders sometimes.  I still see the occasional bf/sf amp that has the Intensity pot wired up like a master volume.  "How can this have gone unfixed for 50 years?!"

I'm sure you know this, but to be a bit clearer, the Presence control varies the frequencies that are being fed back, not the amount of NFB in total, like the pot option mentioned above.  It is basically a tone knob on your NFB loop.

[PRR]

*) "Champ 600" is an AA-Champ with fixed tone-stack. Replacing R19-R22 with pots is an obvious mod for more varied tones.

*) The NFB from speaker also tames speaker bass resonance. This and the treble effects are very dependent on speaker system (also style and genre). What worked for Leo in 1963 may not work for you.

Upping R7 decreases NFB, makes the amp less tame. However it has to go from a fairly low (but not zero!) value then be upped a lot to give the range from "polite, hi-fi" to "bare-naked tube". It may work better to make R7 the "polite" value, and use a pot divider to reduce its effect to dead-none.

http://n557.xyz/img/Champ600-NFB.gif

*) >R7 need to be changed ..using cabs with different impedances?

I would expect a two-12" or four-10" cab to have a very different opinion of deep bass than the stock 6"(?) cone. That will lead to different NFB preference, more so than the impedance. Take your NFB pot permanently from any handy tap (I think 4r is stock). The effect on other taps will be 99.5% in-proportion, which is the right thing to do.

[Mark Hammer]

Funny what "factory errors" you'll find in old Fenders sometimes.  I still see the occasional bf/sf amp that has the Intensity pot wired up like a master volume.  "How can this have gone unfixed for 50 years?!"

I'm sure you know this, but to be a bit clearer, the Presence control varies the frequencies that are being fed back, not the amount of NFB in total, like the pot option mentioned above.  It is basically a tone knob on your NFB loop.

For 5W amps with an 8" speaker in a small cab, whatever its impact low end, the impact of varying NFB will be primarily audibly detectable for higher frequencies.

[Electron Tornado]

Thanks for the replies. Very helpful.

Should I keep the lower resistance limit on the negative feedback to the stock 2.2k, with a pot in series, or would it be worth trying something as low as 1k?


I plan to use this amp with a single 12", 16 ohm cab. Why 16 ohm? I also have a Vox AC4TV that I repaired and decided to just leave stock (it's 16 ohm), I also have a Vox AC15C1 that can use either 8 or 16 ohm.  These are all just the amps, bought as projects, that also need to be re-housed.

[thermionix]

Should I keep the lower resistance limit on the negative feedback to the stock 2.2k, with a pot in series, or would it be worth trying something as low as 1k?

I guess it depends on whether you might think it may sometimes need more than the original amount of NFB.  If it were me, and I hadn't heard the amp yet with the intended speaker, I would probably start with a 1k fixed resistor in series with a 2.5k pot, so that I could have "stock" in about the middle, and selectively more or less.  But if I already had an opinion that the stock setup was too much, or too little, I would obviously shoot for a different range.

[Electron Tornado]

2. I would like to replace the output transformer with one that will allow the amp to be used with 4, 8, or 16 ohm cabs. Will the value for R7 need to be changed (or adjusted with a pot) when using cabs with different impedances?

No, not necessary.  You can have the pot if you want, but you will draw the NFB from only one of the secondary taps, and it will stay that way regardless of which one you are actually using.  Look at a Marshall schematic for an example.  Keep in mind that all else being equal, you will get more NFB voltage from the higher-ohm taps.  But if you set it where you like it from, say, the 8-ohm tap, it will stay the same if you use the 4, 8 or 16.

Just so I'm clear about your answer, can I use separate jacks for the different outputs while leaving the NFB wired to only one of the secondaries? Can I leave the NFB wired to the 4 ohm secondary and still be getting NFB while using either the 8 or 16 ohm outputs?

[PRR]

> Can I leave the NFB wired to the 4 ohm secondary and still be getting NFB while using either the 8 or 16 ohm outputs?

Yes.

The several taps are the SAME (99.9%) signal, just different voltage/current. Nail your NFB to whichever worked before, and switch your speaker jack among the 4 8 16 taps as appropriate.

[Electron Tornado]

Thank you!

[Electron Tornado]

While looking at some more info on negative feedback, I also found some info on the presence control found in some Fender amps. It looked like it would be interesting and an easy mod to add. So, I added a presence control like the one here, using the same pot and cap values:
http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Bassman-5E6A-Schematic.pdf

I made the negative feedback a simple toggle switch between the stock 2.2k resistor and zero negative feedback. I tested with an 8 ohm, 12 inch speaker.** Any difference when switching between stock negative feedback and zero is subtle at best, and the presence control really has no effect at all.

I'm really not sure why removing the negative feedback seems to have so little effect. I'm wondering if the issue with the presence control is because the 5k pot is essentially in parallel with the 47 ohm resistor R23: http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Champion-600-Schematic.pdf 

With R23 so small, I'm not sure that a larger pot in the presence control would work. Would isolating the presence control from R23 with a capacitor solve the problem?





** The amp has been modded with the new output transformer. The fixed resistors in the tone section are also pots now and work well.

[thermionix]

I'm looking at the ratio of R3 to R23 (47 to 1.5k) and thinking this circuit has very little NFB to begin with.  That could explain the very subtle results you're getting.

Personally, I would have rewired the circuit to 5F1* specs, ditched the tone stack altogether, but that's just me.  I never felt the need to mess with the NFB on a 5F1, but YMMV.


*V1A cathode resistor should be bypassed, 25uF.  It's omitted by error in the factory schematic and layout.  The circuit sounds quite boring without that cap.

[Electron Tornado]

I'm looking at the ratio of R3 to R23 (47 to 1.5k) and thinking this circuit has very little NFB to begin with.  That could explain the very subtle results you're getting.

Personally, I would have rewired the circuit to 5F1* specs, ditched the tone stack altogether, but that's just me.

Looking at the 5F1 schematic (and the 5E1 as well):  http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Champ-5F1-Schematic.pdf
the negative feedback goes directly to the cathode of the second half of the12AX7 ("V2-B" in the Champ 600).

I wonder if changing the NFB in the Champ 600 to the cathode of V2-B, and jumping R23 would make the NFB and presence control more effective. That kind of begs the question - why is the NFB routed differently in the Champ 600?

[PRR]

> why removing the negative feedback seems to have so little effect.

As thermionix says: there's hardly any NFB.

You "should" (may) hear a difference when working around the speaker's bass resonance. A $5 six-inch may slap madly with no NFB, less with some NFB.

This all says that the 5F6a type presence is not gonna do much. (5E6a is a bit different, and may not be "wow" even in the otiginal. 

> 12 inch speaker

You may rarely play down around a Twelve's resonance, and it may have enough air-paddle area to absorb your whole Six Watts without distress, NFB or not.

> ditched the tone stack altogether

Then you will have "too much" gain, and can re-purpose it for NFB to smooth the output tone (New for 1961!) or just waste it off between stages.

Fender was facing a hoard of KAY and Kent and other bottom-feeder copycat brands and models, "needed" the extra knobs to look "better" in the showroom row, even to the point of taking NFB to the added 47r instead of the 1.5K to eek a wee bit more gain out of an over-stretched plan. 

[Electron Tornado]

> why removing the negative feedback seems to have so little effect.

As thermionix says: there's hardly any NFB.

Is there a good source that will explain why there is so little NFB? I understand what NFB does, but apparently I don't understand it enough to fiddle with it effectively.

What would changing the NFB circuit in the 600 to that of the 5F1 do?