Fender 600 champion reissue complete rebuild

[Mikante]

Can i ask, why should i use two more filter caps before and after the choke?

[thetragichero]

the one before the choke is so you have a capacitor input power supply instead of a choke input power supply, which will both give you the ht voltage the amp is designed for and actually work (choke input supply generally requires a bigger choke). the one after the choke becomes your new reservoir cap

[Mikante]

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This is just for me to better understand.
the princetone 5e1 first model didn t have a choke, it was then revised and equipped with one.
In the revised 5e2 schematic you can see Fender added a choke and just replaced the first 16uf cap with 8uf, but the number of filter caps are still 3.
Since this is about the fender champion 600

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There is already a cap there i marked in red…

[johnnyreece]

The 5E2 didn't have a separate power supply node for the screens, which your amp does have (at C5 in the schematic). I've attached a crudely modified bit of schematic that should help.

[martin manning]

Can i ask, why should i use two more filter caps before and after the choke?

You're only adding one more filter capacitor, along with a choke (inductor) or a resistor. The additional LC or RC stage will reduce the ripple riding on the voltage going to the output transformer from around 10V to a few hundred millivolts. In single ended amps like the Champ, that ripple voltage will produce 120 Hz hum in the speaker, so adding an extra filter stage is a very good idea. The choke also has significant DC resistance, which will help reduce the B+.

The CLC filter simulation below (22u-4H-22u) shows the voltage at the first capacitor (green, B0), and at the second capacitor (red, B1) where most of the ripple has been eliminated. In the blue trace, the 4H choke has been replaced with a 680 ohm resistor (making it a CRC), which results in nearly the same ripple reduction, plus an additional voltage drop. In both cases there is a resistive load drawing ~12W of power to simulate the amp's current draw.

In the earlier Champ and Princeton amps the screen voltage is the same as the anode, and there is no resistor to protect the screen. They reflect an earlier and less sophisticated circuit design, and the early Princeton was basically a Champ with a tone control. In later versions, the screen voltage is lowered, and there is no choke, but additional capacitance was added to the reservoir (16+16 = 32u) to keep the ripple and hum down. Adding an inexpensive resistor between those two caps would have helped tremendously. I'd bet that Fender was well aware of the hum issue, but since these amps were marketed to students, they wanted to avoid the cost of another filter.

[Mikante]

The 5E2 didn't have a separate power supply node for the screens, which your amp does have (at C5 in the schematic). I've attached a crudely modified bit of schematic that should help.

So that electro 22uf 450v cap is the only one that you would add right?

174F1E3E-950D-4F69-8D66-28DBB5AAA642.jpeg

You're only adding one more filter capacitor, along with a choke (inductor) or a resistor. The additional LC or RC stage will reduce the ripple riding on the voltage going to the output transformer from around 10V to a few hundred millivolts. In single ended amps like the Champ, that ripple voltage will produce 120 Hz hum in the speaker, so adding an extra filter stage is a very good idea. The choke also has significant DC resistance, which will help reduce the B+.

The CLC filter simulation below (22u-4H-22u) shows the voltage at the first capacitor (green, B0), and at the second capacitor (red, B1) where most of the ripple has been eliminated. In the blue trace, the 4H choke has been replaced with a 680 ohm resistor (making it a CRC), which results in nearly the same ripple reduction, plus an additional voltage drop. In both cases there is a resistive load drawing ~12W of power to simulate the amp's current draw.

In the earlier Champ and Princeton amps the screen voltage is the same as the anode, and there is no resistor to protect the screen. They reflect an earlier and less sophisticated circuit design, and the early Princeton was basically a Champ with a tone control. In later versions, the screen voltage is lowered, and there is no choke, but additional capacitance was added to the reservoir (16+16 = 32u) to keep the ripple and hum down. Adding an inexpensive resistor between those two caps would have helped tremendously. I'd bet that Fender was well aware of the hum issue, but since these amps were marketed to students, they wanted to avoid the cost of another filter.

Thank you this is very informative.

[johnnyreece]

So that electro 22uf 450v cap is the only one that you would add right?

Yes.

[Mikante]

So that electro 22uf 450v cap is the only one that you would add right?

Yes.

OK great, the revised schematic from my previous post should be about it.
I will only bypass the tone stack and put a switch on the negative feedback to choose between two values.
Nothing else until the amp is up and running.

[Mikante]

image.jpg

New plate to cover the mess
Still needs some polish but i ended up without pads for the job.
I like it with this matt finish too

[Mark]

The Champ 600 also works well if you turn it into a 5F1. The power supply is a big part of the sound too.

Do you really miss the 5Y3 rectifier?

If not then you can increase the filter caps.

[Mikante]

I like ss rectifier better.
Anyway, i m building the amp as it was, it is easier for me to follow the original pcb in case i have doubts.
To be honest, this amp sounds incredibly good for the money.

[Stevem]

The lesser the amount of coupling caps a amp has the better the sound!

So yes simple circuits sound the best .

[Mark]

I like ss rectifier better.
Anyway, i m building the amp as it was.

I must admit I would like to hear the Champ with the octal preamp valve. The amp in the video sounds good, but P90’s usually sound good.

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[Mikante]

Nice, problem is that the 600 champ reissue has noting in common with the first model. The new one sounds very much like a silver face champ with the fixed eq.

[ChopSauce]

I would like to hear the Champ with the octal preamp valve...

... & so do I...

I've been wishing I could find one reissue model at a fair price for a long time now...

Thx for the gutshots. It's amazing to see how these little amps seem to be pis*** off the star grounding theory...