Long-delayed update -- I bought some replacement grille fabric, cut a new front piece, painted it black, put the fabric on, mounted the speaker, put it all together. Now I have a nice working Fender Champ. I haven't done anything with it (except turn it on and plug in a guitar) since December, at least.
I need to get back to it, replace those caps, maybe put in a new eyelet board. Put in a standby switch, an FX loop, etc., etc. But for now I'm happy with it. (And it doesn't smell!)
Tuesday, February 19, 2008
Sunday, November 11, 2007
One step back, one step sideways
"I collect molds, spores, and other fungus." -- Egon Spengler
I haven't posted for a while, since I haven't done anything with the amp for a while. Work has been very hectic, I haven't even picked up a guitar for several weeks, and when I have picked one up, it's been the acoustic.
The champ has been sitting there on the table next to my computer at home, staring at me, saying "why don't you play me? why don't you fix me?" and I've been ignoring it, with some guilty feelings.
I've also been smelling it. Yes, smelling it. When it arrived (as I wrote previously) "It smells like the wettest, mustiest basement you've ever been in." And to my surprise, that hasn't improved. It has continued to smell terrible. When is this thing going to dry out?
Today, while waiting for something on the computer I was idly poking at the front grille, when I realized that the grille is attached to a piece of wood that is separate from the speaker baffle. In fact, it's attached by Velcro. So I pulled at it, it came off, and lo and behold -- mold! Fluffy white mold, growing in between the grille and the speaker baffle!
I took out the electronics and speaker and took the frame down to the basement, thinking I might be able to sand it off, then wipe down the wood with Clorox to keep the mold away. I started to carefully remove the fabric from the grille, then I noticed that the mold had grown under the fabric.
"OK, I'll have to replace the fabric."
I pulled all of the staples holding the fabric to the grille, and the grille started to come apart in chunks. The wood (some sort of pressboard) had gotten soaked at some point and had rotted.
"OK, I'll have to replace the grille."
So I started in on the frame itself, and pretty quickly determined that the mold had gotten under the Toulex (the black vinyl stuff on the outside of amps). I started to peel up the Toulex (which was coming loose) and saw that the mold had gotten into the wood.
"OK, I'll just have to replace the whole thing."
I ripped off the rest of the Toulex so I could see how the thing was constructed. It's not very complicated joinery, and I think if I had to I could cut pieces of the same shape and size and assemble a replacement box. (Before that I'd probably ask a friend who has a wood shop in his basement to help out... something I wasn't looking forward to doing, but if I got stuck I'd do it.)
So I came back upstairs and noticed the not-as-old Champion 110 sitting under the table. I bought the Champion 110 six months ago, off Craigslist, for $15. It's a mid-1990s solid state amp that I thought I might be able to fix. I sent away to Fender for the schematic, resoldered the loose parts, but couldn't get it working. I kept the dead amp (and its 10" 8 ohm speaker) around in case I got involved in some other project that called for a speaker cabinet.
Well here I was, with working electronics from a 1973 silverface (tube) Champ, and with a reasonable cabinet for a 1993 (solid state) Champion 110. I thought hey, let's see how much space is left over in the Champion 110 cabinet if I put the Champ electronics in.
And lo and behold, the cases are the same size! It must be like the old legend about railroad gauges and Roman roads -- once you've got an assembly line set up to build Champs, you re-use the stuff you have, even if none of the parts are the same any more. Different electronics, different-sized speaker, different construction methods, but still the same dimensions.
All I need to do is to drill some mounting holes for the electronics and I can pop the '73 champ into the '93 cabinet, and I'll be moving forward. I've got to do something about the speaker -- it's an 8 ohm speaker, the Champ is designed to drive 4 ohms, and I'd rather use the Weber speaker anyway, but the hole is the wrong size. So that may take some work. But in comparison to building an entirely new cabinet, it's nothing.
I haven't posted for a while, since I haven't done anything with the amp for a while. Work has been very hectic, I haven't even picked up a guitar for several weeks, and when I have picked one up, it's been the acoustic.
The champ has been sitting there on the table next to my computer at home, staring at me, saying "why don't you play me? why don't you fix me?" and I've been ignoring it, with some guilty feelings.
I've also been smelling it. Yes, smelling it. When it arrived (as I wrote previously) "It smells like the wettest, mustiest basement you've ever been in." And to my surprise, that hasn't improved. It has continued to smell terrible. When is this thing going to dry out?
Today, while waiting for something on the computer I was idly poking at the front grille, when I realized that the grille is attached to a piece of wood that is separate from the speaker baffle. In fact, it's attached by Velcro. So I pulled at it, it came off, and lo and behold -- mold! Fluffy white mold, growing in between the grille and the speaker baffle!
I took out the electronics and speaker and took the frame down to the basement, thinking I might be able to sand it off, then wipe down the wood with Clorox to keep the mold away. I started to carefully remove the fabric from the grille, then I noticed that the mold had grown under the fabric.
"OK, I'll have to replace the fabric."
I pulled all of the staples holding the fabric to the grille, and the grille started to come apart in chunks. The wood (some sort of pressboard) had gotten soaked at some point and had rotted.
"OK, I'll have to replace the grille."
So I started in on the frame itself, and pretty quickly determined that the mold had gotten under the Toulex (the black vinyl stuff on the outside of amps). I started to peel up the Toulex (which was coming loose) and saw that the mold had gotten into the wood.
"OK, I'll just have to replace the whole thing."
I ripped off the rest of the Toulex so I could see how the thing was constructed. It's not very complicated joinery, and I think if I had to I could cut pieces of the same shape and size and assemble a replacement box. (Before that I'd probably ask a friend who has a wood shop in his basement to help out... something I wasn't looking forward to doing, but if I got stuck I'd do it.)
So I came back upstairs and noticed the not-as-old Champion 110 sitting under the table. I bought the Champion 110 six months ago, off Craigslist, for $15. It's a mid-1990s solid state amp that I thought I might be able to fix. I sent away to Fender for the schematic, resoldered the loose parts, but couldn't get it working. I kept the dead amp (and its 10" 8 ohm speaker) around in case I got involved in some other project that called for a speaker cabinet.
Well here I was, with working electronics from a 1973 silverface (tube) Champ, and with a reasonable cabinet for a 1993 (solid state) Champion 110. I thought hey, let's see how much space is left over in the Champion 110 cabinet if I put the Champ electronics in.
And lo and behold, the cases are the same size! It must be like the old legend about railroad gauges and Roman roads -- once you've got an assembly line set up to build Champs, you re-use the stuff you have, even if none of the parts are the same any more. Different electronics, different-sized speaker, different construction methods, but still the same dimensions.
All I need to do is to drill some mounting holes for the electronics and I can pop the '73 champ into the '93 cabinet, and I'll be moving forward. I've got to do something about the speaker -- it's an 8 ohm speaker, the Champ is designed to drive 4 ohms, and I'd rather use the Weber speaker anyway, but the hole is the wrong size. So that may take some work. But in comparison to building an entirely new cabinet, it's nothing.
Saturday, October 13, 2007
It lives!
My box from Ted Weber showed up yesterday (speaker, replacement capacitors, fuses, light bulb), but my tubes from Antique Electronic Supply aren't supposed to show up until Monday. I got impatient, went to a local repair place, Audio Pro (where they seem to know well what they are doing) and got a cheap trio of tubes (two are used, one is a cheap Chinese tube) so I could see what, if anything, happened when I powered it up.
To my amazement -- it worked! I plugged in my guitar, played a chord, and sound came out! It even sounded like a guitar!
The tone controls work, the volume knob works, neither is very scratchy (they're a little scratchy), and at low volume it sounds nice. At high volume it sounds not so nice, but then I haven't started replacing any capacitors yet.
I was a little worried about the tubes, because last night (pre-tube) I plugged it in and measured the voltage coming off of the power transformer. According to the specs it should be putting out 660V, but I was reading 780V, and the 6.3V was reading 6.8V. I was concerned that I'd burn up the tubes. Both the proprietor at Audio Pro and my friend who knows about this stuff tell me that being 15-20% high on the voltage isn't much (or necessarily anything) to worry about. I might still replace the power transformer with one from Weber (for $35), but I'm in no immediate rush (especially now that I have a spare set of tubes).
Sunday, October 7, 2007
Volume dealer
So I'm thinking of adding a switch to cut out the tone circuit, and a switch to cut out the feedback loop. I should also remember to add a "master volume" control.
What's a master volume control? I'm glad you asked.
Part of the appeal of tube amps is the euphonic distortion they produce when they are turned up. The tubes get pushed a little too far, they begin to distort, and that creates the classic loud rock-n-roll guitar sound. But to get this distortion, you need to crank up the volume. High. And even a five watt amp (like the Champ) is very, very loud when turned up.
(Playing in a noisy bar, with a loud drummer, it's not too loud, so there's a reason you might want more power.)
But, you might say, five watts? My car stereo is more than that! My clock radio is probably more than that!
Maybe. But (human perceived) volume is logarithmic -- a 50 watt amp only sounds twice as loud as a 5 watt amp; a 500 watt amp only twice as loud as a 50 watt amp. So if a 50 watt amp is enough to overpower a noisy bar (and drummer) (and it is), a Champ's 5 watts can put out 1/2 that volume when turned up. So enough to drown out a quiet bar, or a restrained drummer. Either way, it's too loud for practicing at home. So we can't really crank it all the way up and get the distortion we want.
The trick is to put a second volume control in. You turn up the first volume control (which is usually called a gain control in this configuration) to get the tubes to distort, and turn down the second volume control (usually called the master volume) to cut back on the overall sound level. Voila! Distortion without too much sound.
But where to put the master volume in the circuit?
There are two kinds of tube distortion that people look for from this kind of amp. You can get distortion from the preamp (small volume gain) stages, or from the power amp (large volume gain) stages.
(There is a third kind of tube distortion, "sag", when the short-term demand on the power supply is too great, as happens when hitting the strings really hard, but that is more under the control of the player. And apparently it doesn't come into play with low-wattage amps, since the amps aren't drawing that much power anyway.)
(And there is the distortion you get when you push the speakers too hard, "breakup". But you don't get that until you're making a lot of noise.)
To get preamp distortion without too much volume you can put a master volume control (a "pot", a potentiometer, a variable resistor) between the preamp stage and the power amp stage. The signal there is small enough that a simple pot can do the job.
If you want to get power amp distortion at low volume you need to use some sort of a "power soak", a device that goes between the amplifier output and the speaker, to cut back on the signal sent to the speaker. An example of such a device is the Weber MASS; it's basically a fake speaker that soaks up some of the power without making noise. Doing this is more complicated (you're soaking up big power-amp power instead of small preamp power) and more expensive. But a power soak is easy to add on between the amp and the speaker, and it lets you really push all of the stages of the amp to their limit, so it's a popular add-on to an existing amplifier.
But if you're hacking on the guts of an amp (like I am), it's easy to add a master volume between pre and power amp.
In the case of the Champ the factory-supplied volume (a.k.a. gain) pot is between the first (12ax7) preamp stage and the second (12ax7) preamp stage. (The 12ax7 tube is really two amps in one tube, hence the schematic being labeled "12ax7 (1/2)".) The place to add a master volume pot is between the 2nd preamp stage and the power amp (6v6gt). Easy enough to do, I'll add it to my list.
What's a master volume control? I'm glad you asked.
Part of the appeal of tube amps is the euphonic distortion they produce when they are turned up. The tubes get pushed a little too far, they begin to distort, and that creates the classic loud rock-n-roll guitar sound. But to get this distortion, you need to crank up the volume. High. And even a five watt amp (like the Champ) is very, very loud when turned up.
(Playing in a noisy bar, with a loud drummer, it's not too loud, so there's a reason you might want more power.)
But, you might say, five watts? My car stereo is more than that! My clock radio is probably more than that!
Maybe. But (human perceived) volume is logarithmic -- a 50 watt amp only sounds twice as loud as a 5 watt amp; a 500 watt amp only twice as loud as a 50 watt amp. So if a 50 watt amp is enough to overpower a noisy bar (and drummer) (and it is), a Champ's 5 watts can put out 1/2 that volume when turned up. So enough to drown out a quiet bar, or a restrained drummer. Either way, it's too loud for practicing at home. So we can't really crank it all the way up and get the distortion we want.
The trick is to put a second volume control in. You turn up the first volume control (which is usually called a gain control in this configuration) to get the tubes to distort, and turn down the second volume control (usually called the master volume) to cut back on the overall sound level. Voila! Distortion without too much sound.
But where to put the master volume in the circuit?
There are two kinds of tube distortion that people look for from this kind of amp. You can get distortion from the preamp (small volume gain) stages, or from the power amp (large volume gain) stages.
(There is a third kind of tube distortion, "sag", when the short-term demand on the power supply is too great, as happens when hitting the strings really hard, but that is more under the control of the player. And apparently it doesn't come into play with low-wattage amps, since the amps aren't drawing that much power anyway.)
(And there is the distortion you get when you push the speakers too hard, "breakup". But you don't get that until you're making a lot of noise.)
To get preamp distortion without too much volume you can put a master volume control (a "pot", a potentiometer, a variable resistor) between the preamp stage and the power amp stage. The signal there is small enough that a simple pot can do the job.
If you want to get power amp distortion at low volume you need to use some sort of a "power soak", a device that goes between the amplifier output and the speaker, to cut back on the signal sent to the speaker. An example of such a device is the Weber MASS; it's basically a fake speaker that soaks up some of the power without making noise. Doing this is more complicated (you're soaking up big power-amp power instead of small preamp power) and more expensive. But a power soak is easy to add on between the amp and the speaker, and it lets you really push all of the stages of the amp to their limit, so it's a popular add-on to an existing amplifier.
But if you're hacking on the guts of an amp (like I am), it's easy to add a master volume between pre and power amp.
In the case of the Champ the factory-supplied volume (a.k.a. gain) pot is between the first (12ax7) preamp stage and the second (12ax7) preamp stage. (The 12ax7 tube is really two amps in one tube, hence the schematic being labeled "12ax7 (1/2)".) The place to add a master volume pot is between the 2nd preamp stage and the power amp (6v6gt). Easy enough to do, I'll add it to my list.
Saturday, October 6, 2007
Being more incremental
I was very close to ordering a pre-populated board (see below) from Ceriatone or Turretboards for $80 or so delivered, but then I got to thinking that I'd still need to buy tubes ($30-35), and a speaker ($20-100), etc... and I would have trouble thinking of this as a $150 amp after I had put another $150 into it -- not to mention the possibility of needing two new transformers ($50-$100).
So I bit a different bullet, bought a set of tubes from Antique Electronic Supply ($30) and a speaker ($20) and a dozen appropriate capacitors ($10) from Weber (and a fuse and a lightbulb too). I'll first try plugging in the tubes and speaker and see if it even starts up. Then I can replace the capacitors as the mood strikes.
This Champ is from 1973, and follows the AA764 schematic -- or at least I thought it did. The schematic says "all capacitors at least 400 volt rating if not specified", but a number of the capacitors on the board were rated for 600v. I thought that the "as-built"[*] might be a better guide than the schematic in this case, so I got the bigger capacitors.
There are other versions of the Champ, most of which do not have tone controls. I am considering putting in a switch to cut the tone controls out of the circuit, to see how it affects the tone. The other differences between this circuit and older ones (the best know are the 5C1, 5E1, and 5F1) are minor differences in component values. There is also a negative feedback loop, introduced after the 5C1, to reduce distortion -- it runs from the output transformer back into the 2nd 12ax7 preamp stage. It's easy to disable (cut the wire), and some purists prefer a Champ without it. Since I don't know what any Champ sounds like, I can't pick between different versions. Maybe I'll put a bunch of switches in so I can turn all of these features on and off at will -- tone controls, feedback, whatever.
Looking at various old schematics on the Free Information Society web site I see that some of the Princeton circuits look suspiciously like Champ circuits (e.g. the 5D2). I need to learn more about the differences between the Champs and the Princetons.
One thing that will cause me a little extra work in this rebuildis that the three power supply capacitors on the original amp come in a giant triple-in-a-can capacitor, a style which is not common these days (but was in the 50's and carried forward into my 1973 amp). These are hard to find and kind of expensive, so I bought three separate conventional electrolytic capacitors. The turret board doesn't have space for them, so I bought some terminal strips and I'll hot-glue them or bolt them into the chassis. One more "as-built" mystery -- the schematic says they are each 20uf, but the can capacitor is a 20uf/20uf/40uf triple. So one of the three is using the "wrong" value. Clearly more research required.
[*] This is the term architects use -- what do electrical engineers use?
So I bit a different bullet, bought a set of tubes from Antique Electronic Supply ($30) and a speaker ($20) and a dozen appropriate capacitors ($10) from Weber (and a fuse and a lightbulb too). I'll first try plugging in the tubes and speaker and see if it even starts up. Then I can replace the capacitors as the mood strikes.
This Champ is from 1973, and follows the AA764 schematic -- or at least I thought it did. The schematic says "all capacitors at least 400 volt rating if not specified", but a number of the capacitors on the board were rated for 600v. I thought that the "as-built"[*] might be a better guide than the schematic in this case, so I got the bigger capacitors.There are other versions of the Champ, most of which do not have tone controls. I am considering putting in a switch to cut the tone controls out of the circuit, to see how it affects the tone. The other differences between this circuit and older ones (the best know are the 5C1, 5E1, and 5F1) are minor differences in component values. There is also a negative feedback loop, introduced after the 5C1, to reduce distortion -- it runs from the output transformer back into the 2nd 12ax7 preamp stage. It's easy to disable (cut the wire), and some purists prefer a Champ without it. Since I don't know what any Champ sounds like, I can't pick between different versions. Maybe I'll put a bunch of switches in so I can turn all of these features on and off at will -- tone controls, feedback, whatever.
Looking at various old schematics on the Free Information Society web site I see that some of the Princeton circuits look suspiciously like Champ circuits (e.g. the 5D2). I need to learn more about the differences between the Champs and the Princetons.
One thing that will cause me a little extra work in this rebuildis that the three power supply capacitors on the original amp come in a giant triple-in-a-can capacitor, a style which is not common these days (but was in the 50's and carried forward into my 1973 amp). These are hard to find and kind of expensive, so I bought three separate conventional electrolytic capacitors. The turret board doesn't have space for them, so I bought some terminal strips and I'll hot-glue them or bolt them into the chassis. One more "as-built" mystery -- the schematic says they are each 20uf, but the can capacitor is a 20uf/20uf/40uf triple. So one of the three is using the "wrong" value. Clearly more research required.[*] This is the term architects use -- what do electrical engineers use?
Friday, October 5, 2007
Cutting corners
I have a friend who builds guitar tube amps for fun. He's built lots of tube amps for lots of musician friends of his, and he has offered to help me fix up the Champ. He's said that he probably has most of the components I need lying around on his workbench. It's tempting, but after seeing what I'm faced with what is more tempting right now is to jump ahead and buy a pre-built board with all of the components already installed.
There are several companies that sell parts and subsystems for vintange amp restorers (and people who want to make modern clones of classic amps). One of them, Ceriatone, sells a replacement board for my amp, pre-wired with all of the capacitors and resistors, for $60. Which would you prefer:
This?
Or this?
There are several companies that sell parts and subsystems for vintange amp restorers (and people who want to make modern clones of classic amps). One of them, Ceriatone, sells a replacement board for my amp, pre-wired with all of the capacitors and resistors, for $60. Which would you prefer:
This?
Or this?
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