Champ Upgrade Question

[RedDragonJack]

I've been rebuilding my champ 600 to this circuit,

I was wondering could I replace r11(screen resistor?) with a lower value?

[5992]

You can but it depends on a few things, such as the specs of your power transformer, what power tube you are using, and what the existing voltage is.

How low do you want to go? If you replace that resistor, the voltage will go up on nodes B & C. You want to be careful not to raise the voltage over the screen's limit.

Depending on the power tube you use, you may want to put a grid stopper on the power tube suppressor grid (in series with node B). This prevents the screen from burning out by drawing too much current when the amp is cranked.

[RedDragonJack]

hi, they're 12ax7 and a mil spec 6v6. The screen has 295v on it and 345v on the plate. I heard somewhere that a big difference in voltage between the plate and screen is bad for tone. The Power Transformer is the champ 600 stock one.

[5992]

Originally Posted by RedDragonJack

hi, they're 12ax7 and a mil spec 6v6. The screen has 295v on it and 345v on the plate. I heard somewhere that a big difference in voltage between the plate and screen is bad for tone. The Power Transformer is the champ 600 stock one.

I don't know what the specs for the stock PT are, but you might try putting another 10k resistor in parallel with the existing 10k to give you a value of 5k, just to see how it affects your voltage.

Originally Posted by RedDragonJack

I don't know what a grid stopper is, or what the suppressor grid is, sorry?

Pin 4 of the power tube on that diagram (if it is of a 6V6) is the suppressor grid. One frequent addition is to put a 470 ohm, 2W metal oxide resistor connection the two B node points. This doesn't affect the tone, but it makes it safer to crank the amp. The original Champ and Tweed Princeton didn't have this, but Leo never meant for guitar players to crank his amps.

[RedDragonJack]

Ok thanks, I found out the 6v6 is a tetrode and doesn't have a suppressor grid thingy but anyone that's exactly what i was thinking of with the 470 ohm thing. There is one on the original champ600 in series with a 1k resistor. IDKI'm kinda drunk atm



http://support.fender.com/schematics...0Schematic.pdf

[TonewoodSCruz]

I think some people call that the Grid2 or screen grid and the 470 Ohm is sometimes called a screen stopper, screen resistor or as I think was already mentioned, a screen grid stopper. It's a good thing to add.

[Jerry Glass]

If you decrease R11, it will increase the total tube current which will require you to increase R9 to compensate...This is a Class A amp and your screen current is constant. It has to be controlled or the tube will run away on you.

If you're gonna tweak, do it in the preamp/driver area; any tweaks in the power stage will result in instability. Keep the 470 ohm screen resistor in there.

[RedDragonJack]

Ok, thanks for the info. I have another question, what exactly does r7 do?

[guitarcapo]

negative feedback resistor.