Wow, it's been a while since I posted anything on here. I did get the parts for my mods. I changed my approach a little but I must say I'm satisfied with the results.
I'll start with the first preamp stage. I had taken out the cathode bypass capacitor as per a recommendation but I noticed a loss in gain. I discovered why when I studied tube amp design. The capacitor has no effect on the DC circuit so the cathode bias primarily controlled by the cathode resistor is unchanged. But for the guitar signal's AC the capacitor makes it act like the cathode is connected directly to ground so that the gain will not be limited by the cathode resistor. I took the direction that I had been recommended in one of the websites I visited and changed the plate resistor, cathode resistor and placed a new cathode bypass capacitor in the first preamp circuit. The plate resistor (R8) was 100k ohms 1/2 watt. I replaced it with a 1 watt 220k resistor. The cathode resistor (R2) was 1.5k 1/4 watt. I replaced it with a 1/2 watt 3k resistor. By the way, I did not check my bias current which could be bad. I had to stray a little bit for the recommended resistor values. This could effect the life of the tube. The original bypass capacitor was a 22uF/25V electrolytic. I read that such a high capacitance provides a flat gain throughout the guitar's entire frequency range. The replacement is a .68uF/630V Solen "Fast" Cap. The lower capacitance provides a bass rolloff that was recommended. The entire circuit change was to change the shape of the guitar signal away from the stock Fender shape. If that is an issue don't do it. But don't cut out C3 unless you want lower gain out of the first preamp stage.
The tone stack after the first preamp was the big gain killer. I had been recommended bypassing the entire circuit with a 0.22uF capacitor so I bought a switch to choose either circuit. I opted instead to keep the tone stack in place and just boost the gain by doubling the value of the midrange resistor (R19). I replaced a 15k 1/4 watt resistor with a 31k 1/2 watt resistor. Also, I replaced the treble capacitor (250pF) with a silver mica capacitor of the same value. I was recommended certain types of capacitors for improving tone. I also read though that Fenders used ceramic caps in their tone stacks. I can't recall if the original was ceramic but since I already took out the Fender "tone" it didn't matter. Since I was no longer bypassing the tone stack I took my new orange drop 0.22uF capacitor and replaced the second preamp output capacitor with it for improved tone. By doubling the resistance of R19 I got a noticeable gain boost and I like how the distortion sounds at the higher volume. Success.
This will conclude my guitar amp project for a while. I'm still up in the air about the speaker. Lots of people swear by replacing it. I don't know if I want to deal with a 6 inch speaker or just plug a 12 inch speaker in a cabinet into the speaker jack in the amp. Anyway I'm happy for now.
Note: Sharing this information was not to tell you step by step what to do to make better sounds out of a Fender Champion 600 amp. It is only telling you what I did. The risks taken by me to make these these mods are my responsibility. Doing any mods to your own amp is your responsibility. If your amp is under warranty it will not be afterwards. I was an electronics technician for seventeen years. Tube amplifiers use higher voltages than what comes out of your wall. Capacitors charge up and discharging a cap with your body can be dangerous. A proper soldering iron and some soldering experience is necessary to keep from damaging the printed circuit board. Don't look at what I did to be any better than the others who modified their amps. And so on...
Friday, December 24, 2010
Tuesday, December 7, 2010
Tube Amp Project - 12/7
It's been a while since I last updated this project. I've made two significant changes and changed my mind on an earlier change. I got compliments on my tone from a bandmate but there are still changes to be made.
I ordered new tubes from The Tube Store and while I was waiting for them I found some older tubes in my stock of parts I never seem to throw away. The one 12AX7 tube was never used but made in China. I didn't try it at first because I assumed the current tube was from China. The second tube I found was a General Electric I forgot about. I was excited about a big brand name tube so I pulled the 12AX7 out of the amp. Surprise! It was a Sovtek tube, Fender's brand of choice. I put the GE tube in and found out why I didn't use it before.
Tubes are mechanical devices. Their sound comes from physical plates. These plates can vibrate so precautions are taken during construction. When I cranked up the amp I got an awful audible rattle from the tube. The amp worked but the rattle made the tube useless. I'll keep the box.
So I decided to throw the Chinese tube in there. I noticed how quiet the amp was when I wasn't playing a note. But I also noticed lower gain. I was impressed though and will keep the tube. In the meantime I threw the Russian Sovtek back in the amp and then I noticed that the gain was lower too. I was starting to think another mod actually lowered the gain.
The tubes came in last Thursday and I was too busy to really evaluate them. The 12AX7 is made by JJ and they have a really good reputation. The 6V6 is from Tung-Sol and it was highly recommended. I was happy to notice that the hum levels lowered and the tone was brilliant. The Tung-Sol replaced a Chinese 6V6. I couldn't be happier with this power tube. The gain however was still down and so I did some research. Going back to the removal of the C3 capacitor, that is something I should not have done.
I spent the weekend researching modification choices and discovered a tone changing set of part replacements that I am going to do. It will replace C3 with this mod and should keep out harsh lows and provide more gain. Instead of adding a three-way switch I am doing a tone stack bypass switch instead. That should help fix the problem I'm having with single coil pickups. My Epiphone Les Paul's humbuckers are sweet sounding but the pickups on my Strat copy are weak. The tone stack bypass will boost the single coil pickup and hopefully give me a true "Champ" sound.
The greatest sound change came from removing the grill cloth from in front of the speaker! I read a forum where everyone was recommending it. I could see why after I took it off - it was a pice of microfiber cloth that looked like it should have been on upholstery. The cloth was too restricting and the treble boost was enormous after I took it off. I had some screen material so I doubled it up and placed it in front of the speaker. I'll have to take pictures.
I hope to have the complete mod finished next week. I will take some pictures. Speaker replacement is still up in the air but as of right now I doubt that I will replace it. The isolation box is up in the air since I've discovered an attenuator that might be easier to implement. My band played last Saturday and I was very pleased with my tone. This Fender Champion 600 reissue was a great choice for the project.
I ordered new tubes from The Tube Store and while I was waiting for them I found some older tubes in my stock of parts I never seem to throw away. The one 12AX7 tube was never used but made in China. I didn't try it at first because I assumed the current tube was from China. The second tube I found was a General Electric I forgot about. I was excited about a big brand name tube so I pulled the 12AX7 out of the amp. Surprise! It was a Sovtek tube, Fender's brand of choice. I put the GE tube in and found out why I didn't use it before.
Tubes are mechanical devices. Their sound comes from physical plates. These plates can vibrate so precautions are taken during construction. When I cranked up the amp I got an awful audible rattle from the tube. The amp worked but the rattle made the tube useless. I'll keep the box.
So I decided to throw the Chinese tube in there. I noticed how quiet the amp was when I wasn't playing a note. But I also noticed lower gain. I was impressed though and will keep the tube. In the meantime I threw the Russian Sovtek back in the amp and then I noticed that the gain was lower too. I was starting to think another mod actually lowered the gain.
The tubes came in last Thursday and I was too busy to really evaluate them. The 12AX7 is made by JJ and they have a really good reputation. The 6V6 is from Tung-Sol and it was highly recommended. I was happy to notice that the hum levels lowered and the tone was brilliant. The Tung-Sol replaced a Chinese 6V6. I couldn't be happier with this power tube. The gain however was still down and so I did some research. Going back to the removal of the C3 capacitor, that is something I should not have done.
I spent the weekend researching modification choices and discovered a tone changing set of part replacements that I am going to do. It will replace C3 with this mod and should keep out harsh lows and provide more gain. Instead of adding a three-way switch I am doing a tone stack bypass switch instead. That should help fix the problem I'm having with single coil pickups. My Epiphone Les Paul's humbuckers are sweet sounding but the pickups on my Strat copy are weak. The tone stack bypass will boost the single coil pickup and hopefully give me a true "Champ" sound.
The greatest sound change came from removing the grill cloth from in front of the speaker! I read a forum where everyone was recommending it. I could see why after I took it off - it was a pice of microfiber cloth that looked like it should have been on upholstery. The cloth was too restricting and the treble boost was enormous after I took it off. I had some screen material so I doubled it up and placed it in front of the speaker. I'll have to take pictures.
I hope to have the complete mod finished next week. I will take some pictures. Speaker replacement is still up in the air but as of right now I doubt that I will replace it. The isolation box is up in the air since I've discovered an attenuator that might be easier to implement. My band played last Saturday and I was very pleased with my tone. This Fender Champion 600 reissue was a great choice for the project.
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