Fender Champion 600 mod help...

[CroakerSmoker]

Started modding the fender champion 600 reissue today and am having problems. First thing I did was change the tubes and replace the speaker with a 6" Jensen MOD. Everything was working great at this point. I did a little research on modding the circuit and thought i would give it a go...I decided to follow a mod by a guy that did the mod on his blog. He had good results and seemed to know his stuff so i basically copied his circuit to try to get this little amp to sound better. The first thing i did was try to bypass the tone stack by lifting out R21, R20, R22, and R18. I added a coupling cap with a value of .0047uF 630VDC from top of R18 to bottom of R20. I also replaced C2 with a .022uF 630VDC cap. For the first gain stage (R9) i replaced with a 220 ohm resistor. I then dropped the plate voltage at R11 by replacing with a 16k resistor. At R12 i replaced with a 33k resistor. The guy on the blog instructed to remove three of the large black caps (C7, C5, C6) to reduce some of the filtering. I have not done this yet because I don't have the parts on hand.
Here is the problem: I turned on the amp to see if there was an improvement. The tubes warmed up and i started to test it out. The amp sounded tighter on the bottom end and at first thought it was an improvement. I then realized that the amp was turned almost all the way up and was not very loud at all. The amp on 12 before the mod was much louder. I also noticed a loud low pitch hum/rumble coming from the speaker as soon as the amp heated up. I cannot figure out where i went wrong. Is this rumble and lack of volume from not replacing the three filtering caps or did i screw something else up? Im stumped and was hoping some champ guy out there could help me out.
Here is a picture of the mod...

[olddawg]

Sounds like you did several mods at one time. I would put it back stock and see if you still have a problem. If you still do then troubleshoot the stock circuit. IMHO, changing the speaker and using NOS tubes is the best mod you can do to most of these small amps. People start chopping them up for no real reason. Usually an OD pedal is all you need.

[soundguruman]

When you took out R18, you killed all the mid / low frequencies
when you took out C8, it accomplished absolutely nothing....
and so on and so forth.
(a poster child for why you "should not" follow directions on the internet, to mod an amplifier)
So, look at the schematic, and put it all back to stock.

[52 Bill]

Without a schematic for the amp, I have no idea as to what your mods have done to the circuit. Maybe if you post one, we can help you figure it out.

[soundguruman]

Fender Champion_600_schematic.pdf

[52 Bill]

Are you sure that you got the values right from the blog?

I can understand the removal of the tone stack, but I don't understand the changes to the driver and the power supply sections. A 220 ohm plate resistor? Removal of the filter caps?

[soundguruman]

The safest is to just first restore it to stock. It's harder to get that wrong.
To mod this amp, a second preamp tube is really the ticket. But, I don't know if the power to run it is even there (?)
It's the heater current that really clenches the deal.
So to really get what you were after, you need more preamp.

[olddawg]

The safest is to just first restore it to stock. It's harder to get that wrong.
To mod this amp, a second preamp tube is really the ticket. But, I don't know if the power to run it is even there (?)
It's the heater current that really clenches the deal.
So to really get what you were after, you need more preamp.

Or just plug a $40 Bad Monkey OD pedal on the input. And by all means put it back to original first.

[kleuck]

You can have way more gain from a Champion 600 without adding a tube : remove the nfb, remove the tonestack, use 220 or 470K plate loads and according cathode resistors (3K3), bypass both cathode resistors.

[J M Fahey]

You got what you bargained for, by messing with a perfectly working amp.
The "guy from the blog" must be ROFL in his padded cell.
Apply the best mod of them all: back to normal.

[JHow]

...The first thing i did was try to bypass the tone stack by lifting out R21, R20, R22, and R18. I added a coupling cap with a value of .0047uF 630VDC from top of R18 to bottom of R20.

If you go to the bottom of R20, aren't you making a voltage divider that effectively turns down the volume permanently? Or am I mis-reading this?

..For the first gain stage (R9) i replaced with a 220 ohm resistor.

Are you saying that you changed your 100K anode resistor for a 220 ohm one? 220K maybe? Am I looking at the right schematic? 200 would be way too small resistor.

...The guy on the blog instructed to remove three of the large black caps (C7, C5, C6) to reduce some of the filtering. I have not done this yet because I don't have the parts on hand....

Why on earth would you remove the filter caps?

[soundguruman]

The plan should be stock. If you put it back to stock first, then make sure it works 100% correctly.
Before you punch a hole in it and mount another preamp tube socket.
Always make sure it works first, before modifying...
No, actually internet modifications should be avoided. These mods are always causing a problem.
Take it to a good tech, and have your modding done.
Or buy a soldering station, O scope, load resistors, variac, current meter, etc...
and know how to use them all. Learn how, without electrocuting yourself.

[eschertron]

@CroakerSmoker,

I have the Champion 600 amp too, and it is my first 'Frankenamp'. So I can relate to your desire to mod the amp. It makes sense to me, that if I want to learn tube amp design and engineering, starting with a $100 amp is better than one costing many times more! :wink:

Let me repeat what's been said here already - return the amp to stock FIRST, verify that it works (or if it doesn't, you can have fun troubleshooting it), and then systematically consider each mod you want to do, and do them one at a time, verifying each mod (or each step) has brought the amp closer to what you want out of it. Now please let me add that BEFORE you do any modding, search through these discussion boards to glean the nuggets of wisdom and experience found herein. The wise ones (those who've responded above, and many more) have helped others through similar amp work. Read the historical documents, and more importantly, understand the theory BEHIND each modification that you want to do before attempting. a quick example:

One mod I did was to bypass the tone stack. I used the technique of simply 'lifting' the leg of R19 (the mid tone resistor) from ground. It's not a complete bypass, but it gets me about 20-25dB extra gain between stage 1 and stage 2 of the preamp. Also, it was simple enough to take the resistor to a switch so I can take the mod in and out at will. I learned how to look at the schematic, understand the signal flow, and anticipate what the mod will do; all by spending time on this forum. From one newbie to another, welcome and have some good, safe, fun!

[CroakerSmoker]

@CroakerSmoker,

I have the Champion 600 amp too, and it is my first 'Frankenamp'. So I can relate to your desire to mod the amp. It makes sense to me, that if I want to learn tube amp design and engineering, starting with a $100 amp is better than one costing many times more! :wink:

Let me repeat what's been said here already - return the amp to stock FIRST, verify that it works (or if it doesn't, you can have fun troubleshooting it), and then systematically consider each mod you want to do, and do them one at a time, verifying each mod (or each step) has brought the amp closer to what you want out of it. Now please let me add that BEFORE you do any modding, search through these discussion boards to glean the nuggets of wisdom and experience found herein. The wise ones (those who've responded above, and many more) have helped others through similar amp work. Read the historical documents, and more importantly, understand the theory BEHIND each modification that you want to do before attempting. a quick example:

One mod I did was to bypass the tone stack. I used the technique of simply 'lifting' the leg of R19 (the mid tone resistor) from ground. It's not a complete bypass, but it gets me about 20-25dB extra gain between stage 1 and stage 2 of the preamp. Also, it was simple enough to take the resistor to a switch so I can take the mod in and out at will. I learned how to look at the schematic, understand the signal flow, and anticipate what the mod will do; all by spending time on this forum. From one newbie to another, welcome and have some good, safe, fun!

Thanks for the info...I should have known better than screwing with it and trying more that one mod at a time, but was just interested in what the output of doing this would be...Im going to bring it back to stock tomorrow, ill use the same values as the schematic. Im going to try lifting the leg of R19 like you said. This sounds like a simpler mod for me. Thanks for the info, kind of feel like an idiot...but ya gotta make mistakes to learn from them i guess...

[CroakerSmoker]

Haha, wow, i feel like an idiot. I knew better than poking around with this amp but thought it would be fun to tinker with since i picked it up for 40 bucks from a buddy. This is my first amp to really tear in to, I was doing it mainly as a learning experience...and boy did i learn. If you don't make mistakes, then you cant learn from them i guess. I think I'm going to take the advice of most and start returning the amp to stock specs. I know I got the values of R9 wrong so I wanted to try changing this value (to original spec of 100k) to see if it would fix the problem. This solved my original volume problem and I get alot more gain than I was getting. I'm still getting the hum from the amp and it gets louder as i turn up. I don't know what it is so far but I'm thinking that ill start installing some of the stock specs back to the amp. Hopefully in the process it will remove the hum. Does anyone know a good place to start troubleshooting? What would most likely be causing this hum? Does it most likely have something to do with the changes to the driver and the power supply sections like 52' Bill said or does it have to do with the tone bridge? Any help would be greatly appreciated.