Hello All!
I'm a complete noob when it comes to amplifier design. I was attracted to the simplicity of the 5F1 (Schematic), and that it is a single ended class A amplfier. This will be my first tube amp build and of course I have a few noob questions. I did search around on the internet, but couldn't find answers.
1. Why are smoothing capacitors always electrolytic? I understand that electrolytic caps aren't necessarily the most reliable. If the value and voltage rating were the same, could a Metalized Polypropylene filter be used as a smoothing cap? (I found a 22uf 630V M-P Cap here)
2. Is it possible to modify the circuit into single-ended Triode operation?
I'm a complete noob when it comes to amplifier design. I was attracted to the simplicity of the 5F1 (Schematic), and that it is a single ended class A amplfier. This will be my first tube amp build and of course I have a few noob questions. I did search around on the internet, but couldn't find answers.
1. Why are smoothing capacitors always electrolytic? I understand that electrolytic caps aren't necessarily the most reliable. If the value and voltage rating were the same, could a Metalized Polypropylene filter be used as a smoothing cap? (I found a 22uf 630V M-P Cap here)
2. Is it possible to modify the circuit into single-ended Triode operation?
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A Champ is a fine little project, especially for a novice.
1. Smoothing capacitors are generally electrolytics because those are going to take up the least room and will not be expensive. it isn't fair to call them unreliable. Maybe they don't have the life of a mylar cap, but really, we routinely find amps with 30-40 year old caps working just fine. They may be aging, and we usually will update them, but it isn;t hard to get good life from electrolytics. And those are caps from 30-40 years ago, the technology has advanced considerably since then. If your Champ needs new filter caps in 40 years, oh well.
Your linked film cap is 1.6" x 6.5" in size, that is huge. Also from that source, they are $12.95 each. A 22uf 500v axial electrolytic cap from the same source is $3.50.
2. No reason in the world you can't use radial caps. You coul duse point to point wiring, but you can also buy ready made eyelet boards or turret boards specifically for the Champ, which will simplfy the wiring a lot. They are made for axial lead caps, but you can mount radials on them too.
You will find caps come with a temperature rating, usually either 85 degrees or 105 degrees. The few extra cents the 105 degree caps cost is worth it in terms of life and reliability.
They don't NEED higher ripple, they can HANDLE higher ripple current, that is a good thing.
3. If you haven't built anything befor, I suggest just building something stock to get the hang of it, then later get into more esoteric stuff. If you buy a Champ power transformer, the heater winding is designed for one 6V6 and one 12AX7. That is 0.45A and 0.3A, or 3/4A. A EL34 alone draws 1.5A for the heater. You would be loading the heater winding with over twice the design load.
Power is what the circuit uses, not what all the parts could conceivably draw. An EL34 might be a 25 watt tube, but if you aremaking 5 watts with it, the power supply needs only supply the needs of that circuit.
You could make it a triode, would alter the gain structure as well as the power output. I am not going to look it up, but I have no idea what that would do to the impedance, and by extension the output transformer.
The Champ is not a powerful amp, 5 watts is not that much less than its rated output. Just for reference, a 10w amp would be only 3db louder than 5w. Watts are not loudness.
You may want to build this from scratch, but there are also complete kits on the market including nice cabinet and speaker. Or consider just a chassis, already punched and drilled, plated and labelled.
1. Smoothing capacitors are generally electrolytics because those are going to take up the least room and will not be expensive. it isn't fair to call them unreliable. Maybe they don't have the life of a mylar cap, but really, we routinely find amps with 30-40 year old caps working just fine. They may be aging, and we usually will update them, but it isn;t hard to get good life from electrolytics. And those are caps from 30-40 years ago, the technology has advanced considerably since then. If your Champ needs new filter caps in 40 years, oh well.
Your linked film cap is 1.6" x 6.5" in size, that is huge. Also from that source, they are $12.95 each. A 22uf 500v axial electrolytic cap from the same source is $3.50.
2. No reason in the world you can't use radial caps. You coul duse point to point wiring, but you can also buy ready made eyelet boards or turret boards specifically for the Champ, which will simplfy the wiring a lot. They are made for axial lead caps, but you can mount radials on them too.
You will find caps come with a temperature rating, usually either 85 degrees or 105 degrees. The few extra cents the 105 degree caps cost is worth it in terms of life and reliability.
They don't NEED higher ripple, they can HANDLE higher ripple current, that is a good thing.
3. If you haven't built anything befor, I suggest just building something stock to get the hang of it, then later get into more esoteric stuff. If you buy a Champ power transformer, the heater winding is designed for one 6V6 and one 12AX7. That is 0.45A and 0.3A, or 3/4A. A EL34 alone draws 1.5A for the heater. You would be loading the heater winding with over twice the design load.
Power is what the circuit uses, not what all the parts could conceivably draw. An EL34 might be a 25 watt tube, but if you aremaking 5 watts with it, the power supply needs only supply the needs of that circuit.
You could make it a triode, would alter the gain structure as well as the power output. I am not going to look it up, but I have no idea what that would do to the impedance, and by extension the output transformer.
The Champ is not a powerful amp, 5 watts is not that much less than its rated output. Just for reference, a 10w amp would be only 3db louder than 5w. Watts are not loudness.
You may want to build this from scratch, but there are also complete kits on the market including nice cabinet and speaker. Or consider just a chassis, already punched and drilled, plated and labelled.
Thanks for the reply Enzo! I was scared nobody would reply, so I simplified the questions- but you already answered them! 
Thanks again!
Size isn't really an issue, nor is cost. The "standard" Sprague Atoms also cost $12.95 for the 20uf 500V version, plus in my mind I get the reliability of a self-healing metalized polyprop! The low ESR and Dielectric Absorption can't hurt I imagine. The tighter tolerance of +/- 5% vs +/- 20% is really nice too. More than anything I just didn't know if electrolytics had some special property that they could be used as smoothing caps vs other type caps.
Another reason I was actually attracted to the 5F1 circuit is because it is low power. I'll be using the 5F1 with a 12" speaker that has an efficiency of 99dB 1W/1M! I'd rather not blow my hearing out or annoy neighbors, so I figured low watt output could get me into distortion faster at "useable" levels. I want to tweak this amp so it could go from clean to full on metal, haha. I'll be taking the feedback resistor out of the circuit to get to distortion sooner- and if I need even more gain I'll add a bypass cap to the cathode resistor. I have a lot of experimenting to do! I'll also try and find a variable potentiometer that is rated for 5W as an attenuator, or maybe just wire in a switch with a fixed value 5W or 10W resistor as a "quiet" switch.
I'll stick with the 6V6 for now, and it should be a lot easier to modify the circuit into Triode mode with it VS an EL34.
Thanks for the help!
Thanks again! Size isn't really an issue, nor is cost. The "standard" Sprague Atoms also cost $12.95 for the 20uf 500V version, plus in my mind I get the reliability of a self-healing metalized polyprop! The low ESR and Dielectric Absorption can't hurt I imagine
. The tighter tolerance of +/- 5% vs +/- 20% is really nice too. More than anything I just didn't know if electrolytics had some special property that they could be used as smoothing caps vs other type caps.Another reason I was actually attracted to the 5F1 circuit is because it is low power. I'll be using the 5F1 with a 12" speaker that has an efficiency of 99dB 1W/1M! I'd rather not blow my hearing out or annoy neighbors, so I figured low watt output could get me into distortion faster at "useable" levels. I want to tweak this amp so it could go from clean to full on metal, haha. I'll be taking the feedback resistor out of the circuit to get to distortion sooner- and if I need even more gain I'll add a bypass cap to the cathode resistor. I have a lot of experimenting to do! I'll also try and find a variable potentiometer that is rated for 5W as an attenuator, or maybe just wire in a switch with a fixed value 5W or 10W resistor as a "quiet" switch.
I'll stick with the 6V6 for now, and it should be a lot easier to modify the circuit into Triode mode with it VS an EL34.
Thanks for the help!
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Here is the power transformer I'll be using (Classic Tone). If I'm figuring this out right, the heater is able to supply 2.25A? I have no idea how to read these things. Thank god for guides like this for noobs like me.
http://www.classictone.net/40-18019.pdf
http://www.classictone.net/40-18019.pdf
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Atoms are overpriced too then. In fact somewhere is a picture of one cut open to reveal the smaller cap within it. They make them for nostalgia customers.
Never forget these are just guitar amps, not precision lab gear. This schematic lacks notes, but many Fender schematics of the era have notes on them that all readings are +/-20%. After all you are already changing the 8uf and 16uf up to 20uf. Like building a lawnmower engine to NASCAR specs. Balancing your crank to the microgram, etc.
You can use the fancy caps if you like of course, just in my personal view it is like demanding only eggs from the best of show prize chicken to make my omelet. Overkill.
The guitar signal is small and weak, the first stage of the amp lifts the signal level away from the noise floor. The volume where it is after that helps keep noise under control. You could move it to the grid of the 6V6 if you want, or even have one in both places, but they all are still volume controls.
Removing the fedback will make it incrementally louder. Any gain increase will make it louder.
Never forget these are just guitar amps, not precision lab gear. This schematic lacks notes, but many Fender schematics of the era have notes on them that all readings are +/-20%. After all you are already changing the 8uf and 16uf up to 20uf. Like building a lawnmower engine to NASCAR specs. Balancing your crank to the microgram, etc.
You can use the fancy caps if you like of course, just in my personal view it is like demanding only eggs from the best of show prize chicken to make my omelet. Overkill.
The guitar signal is small and weak, the first stage of the amp lifts the signal level away from the noise floor. The volume where it is after that helps keep noise under control. You could move it to the grid of the 6V6 if you want, or even have one in both places, but they all are still volume controls.
Removing the fedback will make it incrementally louder. Any gain increase will make it louder.
Putting the voltage divider later might also cause earlier stages to distort if the input is large enough, regardless of the pot setting. (I don't know if that happens in this specific circuit.) In that case it would act more like a master volume control and less like a "gain" control. An amp with a master volume but no gain control would be somewhat questionable.
So its location could be a compromise between this issue and the noise issue.
So its location could be a compromise between this issue and the noise issue.
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Chaerin, I was actually thinking about this as well. With the volume pot after the first gain stage, the first gain stage can be driven quite far into distortion at a low potentiometer setting, while the rest of the circuit distorts much less.
I think other than the insertion noise problem, the theory was that the voltage swing on the second and third gain stages will be greater than the first anyway- so it was the best compromise position at potentiometer settings near the middle of the range- to allow each stage to distort more equally.
I guess I'll just have to experiment with the volume pot at each position and see what I like best. My only concern is that I have no idea how much voltage a potentiometer can safely take...
I think other than the insertion noise problem, the theory was that the voltage swing on the second and third gain stages will be greater than the first anyway- so it was the best compromise position at potentiometer settings near the middle of the range- to allow each stage to distort more equally.
I guess I'll just have to experiment with the volume pot at each position and see what I like best. My only concern is that I have no idea how much voltage a potentiometer can safely take...
ATOMs are for "looks", or to meet ancient contracts. Many modern Asian caps work better, at half the price and size.
> why is the volume control after the first gain stage?
Put it in front. Some amps do. Problem is that a guitar amp is expected to boost light 20mV strums to full power. Sensitivity that high, it hisses when full-up. If you put the volume pot in front, it hisses all the time, even when full-down.
OK, so move it after the 2nd stage. At the heavy-whanging extreme of playing style, a guitar will output 200mV maybe 500mV. Two stages at gain 25 each is Gv=625. Level at second stage output is 125 to 310 Volts. But with 250V (any reasonable) supply it can only do 50V. Heavy whanging will be grossly distorted *before* you can turn it down.
Further: power tube needs BIG swings. They are actually designed so-that they can be driven conveniently with a driver eating about the same B+ as the power tube's screen, or dropped a bit. If you put vol-pot between the last driver and the power tube, and turn-down much, the driver clips before the power tube and you do not get ALL the 5.7 Watts you paid so much for.
With free choice, put your volume control where the sensitivity is around 100mV. Any lower and you get into the hiss. Much higher and the previous stage is liable to overload before you can turn it down.
The 5F1 has sensitivity at vol pot wiper near 250mV. Maybe higher than my guideline, but this IS a cheap amp.
500mV into 5F1 first stage makes 12V at the plate. This won't overload, and due to Rk won't even make much THD. The later AA-Champ with bypassed cathodes will get into "sweetness" territory. But 5F1 is a very popular plan.
The 6V6 triode-connects very well. Triodes have not been popular as guitar amps, too "tame".
BTW, the PT sold "for Champ", and many of the original, are actually same-as DeLuxe. Not worth making both types. Two 6V6 and two 12AX7. So it will supply some more B+ and heater, but barely enough for EL34 (where did that come from?) and leave none for little tubes.
> why is the volume control after the first gain stage?
Put it in front. Some amps do. Problem is that a guitar amp is expected to boost light 20mV strums to full power. Sensitivity that high, it hisses when full-up. If you put the volume pot in front, it hisses all the time, even when full-down.
OK, so move it after the 2nd stage. At the heavy-whanging extreme of playing style, a guitar will output 200mV maybe 500mV. Two stages at gain 25 each is Gv=625. Level at second stage output is 125 to 310 Volts. But with 250V (any reasonable) supply it can only do 50V. Heavy whanging will be grossly distorted *before* you can turn it down.
Further: power tube needs BIG swings. They are actually designed so-that they can be driven conveniently with a driver eating about the same B+ as the power tube's screen, or dropped a bit. If you put vol-pot between the last driver and the power tube, and turn-down much, the driver clips before the power tube and you do not get ALL the 5.7 Watts you paid so much for.
With free choice, put your volume control where the sensitivity is around 100mV. Any lower and you get into the hiss. Much higher and the previous stage is liable to overload before you can turn it down.
The 5F1 has sensitivity at vol pot wiper near 250mV. Maybe higher than my guideline, but this IS a cheap amp.
500mV into 5F1 first stage makes 12V at the plate. This won't overload, and due to Rk won't even make much THD. The later AA-Champ with bypassed cathodes will get into "sweetness" territory. But 5F1 is a very popular plan.
The 6V6 triode-connects very well. Triodes have not been popular as guitar amps, too "tame".
BTW, the PT sold "for Champ", and many of the original, are actually same-as DeLuxe. Not worth making both types. Two 6V6 and two 12AX7. So it will supply some more B+ and heater, but barely enough for EL34 (where did that come from?) and leave none for little tubes.
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