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 The original 5B1 circuit has been replaced by a current one.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)
Thursday, November 25, 2010
Happy Thanksgiving
I've been neglecting this blog. Ideas fill my head every day. I've added two modifications to my new amp. I have opinions about the TSA grop.... I mean pat-downs. My band now has a sax player. I think I'll have to make a Thanksgiving promise to update these stories this weekend.
Enjoy your turkeys!
Enjoy your turkeys!
Wednesday, November 17, 2010
More Tube Amps
On Saturday I went to Guitar Center to buy a small five-watt tube amp. I went there with one in mind but ended up falling for a Fender Champion 600. It wasn't the killer rock sound that won me over - it doesn't have a killer rock sound. What I liked was that it was designed so simply that there was room for modification. Since then I have perused the Internet and have found many mods available. The YouTube videos of the mods blew me away. I'm glad I chose this amp.
I knew that for the price, this amp was made in China. I assumed (correctly) that the stock tubes were Chinese. The American-made Groove Tubes and the Russian Tung-Sol tubes are rated far above the stock tubes. Just changing them will make an improvement. But I found out more. The only control is for volume. It goes to twelve (take that, Nigel!) and in Fender's opinion, play it at twelve and let the guitar's controls make the settings. The issue at hand is that Fender's optimum tone setting, made with a tone stack between the first and second gain stages, drop the gain too low to drive the output to the sweet spot. I plugged my Ibanez Tube Screamer in and noticed the immediate sound improvement. I will fix the tone stack issue.
Most of the mod websites suggest replacing the speaker. I will not be convinced until I play this amp for a couple of months. Speakers need to be broken in. Also, I will not replace the stock transformers until I'm convinced they are a problem. The tone stack fix is most likely the biggest fix necessary.
The first thing I'm going to do is remove the negative feedback circuit (Resistor R7). After that I will either replace the tone stack midrange resistor (R19) or add a "fat" switch to keep the stock sound available. I am weighing some advice to change the shaping of the guitar signal at the first gain stage with two resistors and a capacitor, as well as a capacitor in the tone stack. The tubes will be replaced next month.
I hope to report that I will be satisfied with these changes. I hope the speaker breaks in well and the transformers won't need replacing. The goal is to be done before Christmas.
I knew that for the price, this amp was made in China. I assumed (correctly) that the stock tubes were Chinese. The American-made Groove Tubes and the Russian Tung-Sol tubes are rated far above the stock tubes. Just changing them will make an improvement. But I found out more. The only control is for volume. It goes to twelve (take that, Nigel!) and in Fender's opinion, play it at twelve and let the guitar's controls make the settings. The issue at hand is that Fender's optimum tone setting, made with a tone stack between the first and second gain stages, drop the gain too low to drive the output to the sweet spot. I plugged my Ibanez Tube Screamer in and noticed the immediate sound improvement. I will fix the tone stack issue.
Most of the mod websites suggest replacing the speaker. I will not be convinced until I play this amp for a couple of months. Speakers need to be broken in. Also, I will not replace the stock transformers until I'm convinced they are a problem. The tone stack fix is most likely the biggest fix necessary.
The first thing I'm going to do is remove the negative feedback circuit (Resistor R7). After that I will either replace the tone stack midrange resistor (R19) or add a "fat" switch to keep the stock sound available. I am weighing some advice to change the shaping of the guitar signal at the first gain stage with two resistors and a capacitor, as well as a capacitor in the tone stack. The tubes will be replaced next month.
I hope to report that I will be satisfied with these changes. I hope the speaker breaks in well and the transformers won't need replacing. The goal is to be done before Christmas.
Wednesday, November 10, 2010
Tube Amps
In my past I was an electronics technician. In high school I studied radio and TV repair. The '70s were a different time for televisions. The transition from tube to transistor (say that fast) occurred while I was learning how to fix them. I did realize that the days of the TV repairman were numbered. I went into the Army and furthered my studies of electronics. The point I'm making is that I cherish the knowledge I gained from learning all about tubes. And I am happy that tubes are still used to amplify guitars.
Today's modern amps have digital circuits programmed to sound like the best tube amps in rock music. Why not just buy the real thing? There are reasons. Money is usually the reason. The components in a tube amp cost more. Physically, tubes require high voltages and that means stepping up the voltage from the AC source and isolating the voltage from the speakers. Those are handled by transformers. They do affect the sound so the transformers can go for $250 each. Putting any component costing that much into a product means that the product will be much more expensive. Weight is another reason. A 100 watt solid state combo amp weighs less than a 30 watt tube combo amp. Also, tubes blow, tube-generated heat can damage circuits, and so on.
When the early British rock guitarists discovered that the Vox AC15 could make a great sound, they took advantage of it. The Vox AC30, made in the sixties, helped the Beatles make great tone that everyone wanted. Eric Clapton took his chops and along with a 1959 Gibson Les Paul Standard guitar and a Marshall Combo Amp, his name briefly changed to 'God.' This tone became the standard and no solid-state amplifier could compete. Now the modern digital modeling amps are getting competitive but I don't think they are good enough yet.
In the nineties a friend of mine bought an old tube amp. He played it when he got it home and it didn't sound well. So he asked me to look at it after he found out that a pair of matching output tubes would cost him $100. I redesigned the output to replace those rarer tubes with more popular and lest costly tubes. I was thrilled with the challenge and the result and since then I wanted to do more. But things change and I went the other way.
Now I have the bug again. I own a current model of the Vox AC30. They stayed true to the early sixties Top-Boost model. My only problem is that I can't play it loud. I hope to get a five watt Bugera amp that also steps down in power to allow the user to saturate the tubes without blasting eardrums. I also want to build an amp that I design someday. With everything else I want to do, it will come down to where my passion takes me.
I hope that this last hurrah for tubes lasts a while.
Today's modern amps have digital circuits programmed to sound like the best tube amps in rock music. Why not just buy the real thing? There are reasons. Money is usually the reason. The components in a tube amp cost more. Physically, tubes require high voltages and that means stepping up the voltage from the AC source and isolating the voltage from the speakers. Those are handled by transformers. They do affect the sound so the transformers can go for $250 each. Putting any component costing that much into a product means that the product will be much more expensive. Weight is another reason. A 100 watt solid state combo amp weighs less than a 30 watt tube combo amp. Also, tubes blow, tube-generated heat can damage circuits, and so on.
When the early British rock guitarists discovered that the Vox AC15 could make a great sound, they took advantage of it. The Vox AC30, made in the sixties, helped the Beatles make great tone that everyone wanted. Eric Clapton took his chops and along with a 1959 Gibson Les Paul Standard guitar and a Marshall Combo Amp, his name briefly changed to 'God.' This tone became the standard and no solid-state amplifier could compete. Now the modern digital modeling amps are getting competitive but I don't think they are good enough yet.
In the nineties a friend of mine bought an old tube amp. He played it when he got it home and it didn't sound well. So he asked me to look at it after he found out that a pair of matching output tubes would cost him $100. I redesigned the output to replace those rarer tubes with more popular and lest costly tubes. I was thrilled with the challenge and the result and since then I wanted to do more. But things change and I went the other way.
Now I have the bug again. I own a current model of the Vox AC30. They stayed true to the early sixties Top-Boost model. My only problem is that I can't play it loud. I hope to get a five watt Bugera amp that also steps down in power to allow the user to saturate the tubes without blasting eardrums. I also want to build an amp that I design someday. With everything else I want to do, it will come down to where my passion takes me.
I hope that this last hurrah for tubes lasts a while.
Friday, November 5, 2010
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