Friday, December 24, 2010

Tube Amp Project - 12/24

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...

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.

Thursday, November 25, 2010

Tube Amp Project

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)

Song in my head: Baby I Go For You - The Blue Rondos

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!

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.

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.

Monday, October 18, 2010

Web Browsers

I remember the first web browser I ever used - lynx. It was up there with all of the other great protocols of the Internet at the time: archie, ftp, veronica, etc. Linux was relatively new and I'd always see websites with pictures in the magazines. I didn't have a good computer during that time. If I wanted to compile the latest Linux kernel, I would start it before going to bed and hope that it didn't error out before it finished, a short time before I woke up in the morning.

Eventually I got Netscape. Internet Explorer wasn't out yet (as far as I can recall) and Mosaic was not as advanced as Netscape. I built my first website in 1995 and was excited when I wrote my first Java Applets in 1996 when Java first appeared. Using Perl scripts and the CGI interface was my preferred method in those days. But Netscape handled those very well. I didn't run to IE when it was available. I just would buy (yes) the latest Netscape version. Eventually IE won the browser wars and free web browsers were here to stay. Eventually Netscape started to suffer from the financial model they built the company around. Then the open-source Mozilla project came along, opening the flood gates of innovative competition.

When Firefox came out (2004?) I thought that it would get no better. As more and more power was built into affordable PCs and cable competed with DSL to provide the highest speeds, more demand for better browsers ensued and today there are great choices. Microsoft's unfair product placement put IE to the top of the browsers but now they cannot keep up with their competitors. Microsoft puts out upgrades as slowly as their operating systems but the competitors constantly try to add new features that Microsoft then picks up with their next release. This competition makes it hard to choose which is best.

I used Mozilla Firefox religiously until recently. On occasion I thought the browser ran slow and fat on my computers. I would get the next version hoping that it would take up less RAM and run faster. On occasion I would try other browsers. I played with Google Chrome when it first came out. The I played Safari and Opera. None of them made me switch. That is until two months ago.

I decided to go through and try the others again. This time I found a successor. The winner for me is Google Chrome. I don't see speed issues. That is what I noticed about Safari. But Chrome has great extensions and Google's own extensions for my preferred GMail, Google Reader and Google Voice complement Chrome very well. Like Firefox, the HTML5 video stuff isn't there yet and I have to put up with my Mac's shortcomings with Windows Media, but right now Chrome is my choice.

This is of course, until the next competitor does something better.