Champ 600 Point-to Point Rewire! The first One?

[Tom Phillips]

...Does the negative feedback circuit have any correlation on the grounding side with the 220K resistor (or anything else other than ground)?...

Only that both circuits are referenced to ground

...IOW, when I connect the 220K resistor to my fixed bias circuit, I think I can just run the 1.5K resistor (negative feedback circuit) to ground on its own....

Reference the schematic and note that the 1500Ω cathode resistor on the driver tube is lifted above ground by the 47Ω resistor. If you run the 1500Ω resistor to ground you will defeat the negative feedback circuit.
Cheers,
Tom

[hack]

There exists no 47ohm R in the Champ Circuit. If you are referring to the 470 ohm cathode resistor, it has no connection with the 1.5Kohm R on the schematic. Layout indicates they share the same eyelet for grounding point.

In any case, that cathode resistor and bypass cap will be removed.

A fixed ajustable bias circuit will take their place.

I think I have found the answer. According to the schematic, the neg feedback circuit's 1.5K ohm R goes to ground by itself.

I wonder if the negative feedback circuit will be needed. IF omitted, there will be more room on my tagboard for the fixed bias circuit.

[Tom Phillips]

There exists no 47ohm R in the Champ Circuit. ...

Opps. I was thinking of the later Champ circuit. The 47 Ohm resistor showed up in the AB764 Champ which used a different configuration of the feedback circuit. Sorry for the confusion.
Regards,
Tom

[Mandopicker]

Using my bias method, you will not have to mess around with the 220K resistor. If you do decide to install a real Fixed Bias Supply taped off of one of the red wires leading up to the rectifier (SS or Tube). you will probably unground the 220K and insert your bias voltage there. Of course pin 8 on the 6V6 will need to be grounded. Gerald weber does suggest a good method for this and I have tried it successfully making a Champ with switchable fixed and cathode bias.

[hack]

That's a very creative outboard mount. Well done. Have you burned up any pots yet? Weber says carbon element pots can't take DC voltage.

[Mandopicker]

No problems at all so far. In teh Fixed Bias Design, the pots are much larger I think 25K. Something you might find in a Deluxe Reverb Bias Supply

I figured that if the pots were "wirewound" and 2 watts or more of high wattage, I should be fine.

When used in my Cathode Bias design, and in series with a 5 watt resistor, the load would be shared making it even more feasable.

Having said that, I would love for someone to go through the math with me to find out just how safe or not safe my arrangement is.

[hack]

A Very Creative Design. So now that you have a Champ with two distinct bias systems, do you have two very distinct sound characteristics from the same amp?

Is there a notable difference btween the cathode bias and the fixed bias on the Champ Circuit? (Plus, you can really fine tune and optimize the bias in either system) What can you report? Is it worth doing tonewise?

G Weber sez that the fixed adj bias allows for greatly improved clean tone. Rather - clean tone where none existed before.

You seem to be more a proponent of your adj cathode bias system. (Where did you get the idea to do that?)

[woodyc]

A guy brought me one of those and said he wanted a 5C1. I think of a lower B+ as an asset so the existing transformers stayed and a filament transformer and can cap were added. It worked out good. Here's how it all fit:

[Mandopicker]

That looks like it is gonna be nice. Good idea to use the extra filament trans and the cap can, too. You should have lots of space for everything on the inside. Just be careful of tube/speaker magnet placement and locations.

Have fun.

[woodyc]

Here's how I wired the 5C1 copy. You have to add a couple diodes and resistors since the power transformer secondaries don't have center taps. I also added a grid stopper to the 6V6 and a silver mica on the plate of the 6SJ7. But its still a pretty accurate fascimile of a '48 Champ.



Personally I wouldn't recommend this kind of mod since it isn't cheap and while it may sound better, the amp is worth no more (and probably less) when you finish.

[hack]

Nice clean work, Woody. Looks good. I'm running my 100 ohm resistors on the heaters to the 6v6 cathode pin out. It may run quieter than running them to zero volts. Grave yard quiet.

A much cheaper mod (rebuild) would use four ten cent diodes in full wave rectifier mode with ground. That saves forty bucks at least.

[woodyc]

Nice clean work, Woody. Looks good. I'm running my 100 ohm resistors on the heaters to the 6v6 cathode pin out. It may run quieter than running them to zero volts. Grave yard quiet.

A much cheaper mod (rebuild) would use four ten cent diodes in full wave rectifier mode with ground. That saves forty bucks at least.

Thanks. Those are probably good ideas, but I was just trying to copy a particular circuit.

P.S. To elaborate a little about these kind of mods, the reason I think the Champion 600 is a poor candidate is because its a perfectly serviceable amp as-is. I think it would be smarter to figure out how to build a simple good sounding circuit into amps like a Danelectro Nifty 50 or a Behringer Gx108 (or dozens of others). It would actually be a service to humanity because those amps go almost directly from the factory to the landfill with only a brief stop in someones closet. Mandopicker, I would like to follow you down that path.