Why am I making modifications to my tube amp? I think history has something to do with it. I consider the tone of an electric guitar's sound to be more important if one isn't a virtuoso rock guitarist and just as important if one is a virtuoso rock guitarist. For me it's the former. The truth is that I'm quite unhappy with the sound I'm getting out of my amps. I didn't think about it until lately when I hit my overdrive pedal to play during an instrumental break in a song my band was playing. It was a practice session and while no one complained, I was frustrated with what I heard. I traded in that amp as part of buying my Fender Champion 600.
My Vox AC30 is a great amp but I can't play it loud. I won't give up that amp but right now I'm focusing on the little five watt amp I've discussed before. The Champion 600 is sort of a reissue of the original released in 1949. Back then nobody knew what electric guitars were supposed to sound like. The electric guitar was first produced in the 1930s and Leo Fender was producing electric lap steel guitars with amps since the mid forties. Amplifiers were designed from the RCA handbooks that many amp builders received at that time. The Champion 600 was designed for students but some of the best rock and roll guitar songs ever came from the output of those amps.
This reissue is not really a reissue. The original 5B1 circuit has been replaced by a current one. Never mess with perfection and always keep it simple. Fender did not do that. The original preamp circuit had no cathode bias capacitor. A tone stack was nowhere to be found. The negative feedback circuit didn't connect to the cathode of the preamp. These are in the current amp and while I think the intention was to keep the power tube from distorting at too low a volume, those circuits affected the tone that the original had.
I took a chance buying this amp but I knew the original circuits were easy to modify. It paid off with the amount of information I found online. I've found kits that cost almost twice as much as the amp. I found mods that cost nothing to implement. I have to stay in the middle of the road. Since the last time I wrote about this I've found other mods that might be as effective and easier to make.
I've made two modifications so far that have cost me no money. The biggest difference came when I cut out the negative feedback circuit. I'm a former technician, not an engineer, but I have a hunch as to why this was there. The original amp had a tube rectifier. The rectifier is made up of two or four diodes that convert the AC electricity from the electric company to DC electricity. A tube rectifier was two diodes. The current amp has four solid state diodes wired as a bridge rectifier. The bridge rectifier is better than a tube because current can change the characteristics of the tube. In amplifiers, excess current from overdriving the tubes cause the rectifier tube to "hesitate" providing the current. The result is a compression that can be heard. This added to the tone acceptably and I believe that by using the output to the speaker and feeding back to the cathode bias circuit in the second preamp stage would achieve a similar affect. I don't know but I found that the sound I got when I cut out the circuit was better.
The second modification I made was taking out the cathode bias capacitor in the first preamp stage. The verdict is still out on this mod. I haven't noticed a difference yet.
I have decided not to break the tone stack. Instead, I will just deal with the midrange resistor. I was going to replace it with a resistor twice its value. Now instead, I want to place a three-way switch to the console that will change the resistor's value to two and three times the original value. I also want to replace the tone stack capacitors with better ones of the same value. These capacitors can be bought at The Tube Store. So I think one order is necessary to be made along with ordering the replacement tubes for the amp, my biggest mod.
I'm not convinced of replacing the speaker yet. I have a better idea: I will buy a better speaker for my isolation box and decide then what the difference is.
This is more detailed than the previous mention. I will report back soon with an updated report.
(Update 11/28/2010 6:27AM - I found out that Blackface Fender Champs had the negative feedback to the cathode of the second preamp stage along with a tube rectifier. My hunch was wrong - Gary)
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