So I'm building this:
http://www.schematicheaven.com/newamps/badcat_minicat.pdf
There is a 180pF capacitor across the "Volume" knob. What does this do exactly and can I use a 150pF capacitor instead?
Also, whats the best way to discharge the capacitors after I've turned it on?
Last one, promise: When I ground the third pin from the mains power, do I tie the rest of the amp grounds to that or should I keep the grounds separated?
Thanks
http://www.schematicheaven.com/newamps/badcat_minicat.pdf
There is a 180pF capacitor across the "Volume" knob. What does this do exactly and can I use a 150pF capacitor instead?
Also, whats the best way to discharge the capacitors after I've turned it on?
Last one, promise: When I ground the third pin from the mains power, do I tie the rest of the amp grounds to that or should I keep the grounds separated?
Thanks
I'm sure 150 pF will be fine. It's probably just a minor peak at the high end of the audio range, or perhaps something needed to keep the amplifier stable.
You shouldn't discharge the capacitors after you turn it on. It should be after you switch it off. The best way, assuming you are doing it for safety, is with a resistor, maybe a few thousand ohms, couple of watts. Hold it on the capacitor for a few seconds. Check with a voltmeter.
Yeah, tie all the grounds together.
You shouldn't discharge the capacitors after you turn it on. It should be after you switch it off. The best way, assuming you are doing it for safety, is with a resistor, maybe a few thousand ohms, couple of watts. Hold it on the capacitor for a few seconds. Check with a voltmeter.
Yeah, tie all the grounds together.
Run a 330K 2 Watt resistor from your 325 V point, right off your bridge, to ground. It will run just shy of One milliamp, and dissipate 320 mW of power.
This will insure your amp will draw down its B+ when off. After ?? seconds...let me get the calculator again!
____________________________________________________________Rick...
This will insure your amp will draw down its B+ when off. After ?? seconds...let me get the calculator again!
____________________________________________________________Rick...
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Always work with the amp unplugged from the wall. DIscharge caps with a resistor. If you are not sure if things are discharged, get out your volt meter and check.
The cap you describe is what we refer to as a "brightness cap." In many many Fender amps you would find a "brightness" or "bright" switch next to the input jack. Switching just such a cap in and out across the volume control is the job of such switches.
Look at it this way, the small value cap will look like a low impedance to higher frequencies, but appear impassable to lower freqs. SO your highs will bypass the volume control, go right past it. Thinking about it, you can see that with the volume all the way up, that cap would be shorted across, and have no effect. The lower you turn the volume, the more effect it will have. It serves to brighten up the sound by sending the higher edge sounds on through without reduction by the volume control. Play with the bright switch on any Fender to see what it does.
This one is hard wired, but if you are cloning the amp, nothing prevents you from adding a switch in series with the cap to switch it in and out - your own bright switch.
Nothing critical about the value. The smaller the value, the higher the frequncies affected. The difference betwen 180 and 150 will be small. You could just as easily use 120, 100, or even smaller. All depends on just how much body you want along with that extra sizzle. Experiment some if you like. Try 100pf, try 220pf.
YOur third wire ground should have its own chassis attachment point. In fact your are specifically NOT supposed to use that point for grounding other things. Also, you are supposed to make the ground wire a bit longer inside the amp than the hot and neutral wires. Reason being that IF the cord pulls out and rips the wires loose, you want the ground wire to be longest so it is the last to break. Well, perhaps longest is the wrong word, I think it is a matter of it should have the longest slack.
The cap you describe is what we refer to as a "brightness cap." In many many Fender amps you would find a "brightness" or "bright" switch next to the input jack. Switching just such a cap in and out across the volume control is the job of such switches.
Look at it this way, the small value cap will look like a low impedance to higher frequencies, but appear impassable to lower freqs. SO your highs will bypass the volume control, go right past it. Thinking about it, you can see that with the volume all the way up, that cap would be shorted across, and have no effect. The lower you turn the volume, the more effect it will have. It serves to brighten up the sound by sending the higher edge sounds on through without reduction by the volume control. Play with the bright switch on any Fender to see what it does.
This one is hard wired, but if you are cloning the amp, nothing prevents you from adding a switch in series with the cap to switch it in and out - your own bright switch.
Nothing critical about the value. The smaller the value, the higher the frequncies affected. The difference betwen 180 and 150 will be small. You could just as easily use 120, 100, or even smaller. All depends on just how much body you want along with that extra sizzle. Experiment some if you like. Try 100pf, try 220pf.
YOur third wire ground should have its own chassis attachment point. In fact your are specifically NOT supposed to use that point for grounding other things. Also, you are supposed to make the ground wire a bit longer inside the amp than the hot and neutral wires. Reason being that IF the cord pulls out and rips the wires loose, you want the ground wire to be longest so it is the last to break. Well, perhaps longest is the wrong word, I think it is a matter of it should have the longest slack.
Bleeder resistors should have been incorporated into the design in the first place purely from a safety standpoint. In the grander scheme of things the circuit won't notice the extra load...There are a few other improvements that can be had, one right off the bat I saw was no snubber network for the diodes. It would clean-up the diode switching.....keeping noise off your B+.
______________________________________________Rick.....
______________________________________________Rick.....
So the grounds in my schematic should or should not be connected electrically to the ground from the wall?
I also had a question about biasing... What should I do to bias those tubes? I didn't think about that till just now. Its the 1.5K resistor on the cathode correct? This value may need to change? Should I throw a pot in there and a DMM to check the current? Any insight would be appreciated!
Snubber Network? If you can show me how I could do this while maintaining those voltages, that would be helpful!
I'm open to ideas, this is my first build but I also don't want to modify the schematic too too much, I already have every part on there on order.
I also had a question about biasing... What should I do to bias those tubes? I didn't think about that till just now. Its the 1.5K resistor on the cathode correct? This value may need to change? Should I throw a pot in there and a DMM to check the current? Any insight would be appreciated!
Snubber Network? If you can show me how I could do this while maintaining those voltages, that would be helpful!
I'm open to ideas, this is my first build but I also don't want to modify the schematic too too much, I already have every part on there on order.
A snubber is an extra; run without it and see if you decide to put it in. Biasing, well if you are going to run the higher B+ you can increase the cathode resistor a bit, say 1.8k just to keep the current from increasing too much. Try it and decide if it still sounds good.
The grounds, well the jury is out on that and has been for at least 75 years. As long as this is the only device you are using, you can connect grounds together. If you are working with other gear, try it and see how it goes.
The power cord has three wires. The green one goes to chassis.
The grounds, well the jury is out on that and has been for at least 75 years. As long as this is the only device you are using, you can connect grounds together. If you are working with other gear, try it and see how it goes.
The power cord has three wires. The green one goes to chassis.
I said nothing about separating the grounds electrically, but do physically. Yes, you will be grounding your amp circuits to the chassis ultimately, whether you stick grounds to chassis here and there, or if you use a star ground with a single point. But you should have a separate point to ground the mains to the chassis, separate from the point where your circuits ground to it. That is not me, that is the electrical code.
The issue is not ground or no ground, we are only taking about within the amp WHERE the grounds are connected. The chassis will be ground to everything, but the power cord ground wire should have its own green screw to chassis, and it should not be shared by the circuit grounding point(s). yes, electrically they will all be wired together.
Bias. What bias in particular are you concerned with? 1.5k is the worlds most common cathode resistor value in guitar amp 12AX7 circuits. You can certainly change it, but you ought to see how the amp performs as stock first.
I m concerned, when you refer to "the" 1.5k and metering current, that you are referring to the power tube. The 1.5k resistor below the tube is not biasing it, it is more of a grid stopper. Note it feeds the control grid, not the cathode. The EL84 is cathode biased by the 220 ohm resistor to the left of the tube. it is oddly drawn since it is shown wired to the suppressor grid, but the suppressor grid and cathode are wired together inside the tube, so connecting to one is connecting to the other. You can certainly change the 220 ohms to something else, but again, there is nothing wrong with 220 ohms there, and you ought to try the amp with stock values before massaging it.
Remember this little single ended amp is running class A, so the currents will be larger that what you might be used to in a typical class AB push pull amp. By their numbers it is running about 47ma with about 302v across the tube for an idle dissipation of about 14 watts.
The issue is not ground or no ground, we are only taking about within the amp WHERE the grounds are connected. The chassis will be ground to everything, but the power cord ground wire should have its own green screw to chassis, and it should not be shared by the circuit grounding point(s). yes, electrically they will all be wired together.
Bias. What bias in particular are you concerned with? 1.5k is the worlds most common cathode resistor value in guitar amp 12AX7 circuits. You can certainly change it, but you ought to see how the amp performs as stock first.
I m concerned, when you refer to "the" 1.5k and metering current, that you are referring to the power tube. The 1.5k resistor below the tube is not biasing it, it is more of a grid stopper. Note it feeds the control grid, not the cathode. The EL84 is cathode biased by the 220 ohm resistor to the left of the tube. it is oddly drawn since it is shown wired to the suppressor grid, but the suppressor grid and cathode are wired together inside the tube, so connecting to one is connecting to the other. You can certainly change the 220 ohms to something else, but again, there is nothing wrong with 220 ohms there, and you ought to try the amp with stock values before massaging it.
Remember this little single ended amp is running class A, so the currents will be larger that what you might be used to in a typical class AB push pull amp. By their numbers it is running about 47ma with about 302v across the tube for an idle dissipation of about 14 watts.
I think the bad cat minicat is supposed to only be a 4-5watt amp. I hope it is not a 14-watt as the mercury magnetics output transformer I got is low wattage.
About the grounds: I understand what you mean overall. I was unsure before so I asked again, thank you for the detailed response.
I guess all I need to know is how to do that "snubber" network for the power supply and how to build in discharging resistors so that when I turn it off all the caps drain and it is safe to work on.
About the grounds: I understand what you mean overall. I was unsure before so I asked again, thank you for the detailed response.
I guess all I need to know is how to do that "snubber" network for the power supply and how to build in discharging resistors so that when I turn it off all the caps drain and it is safe to work on.
It is at least 15 watts, which means it consumes that many watts wall plug power. The output is 5 Watts maximum, single -ended amplifiers are very inefficient. Anyway, you just need a 330kΩ or 230 KΩ resistor across one of the power supply capacitors. The rest of the circuit will discharge through it. In the AX84 I built, it took about 30 seconds to reach safe voltage levels.
What concerns me is that this design is running the EL84 too hot, (12,5 Watts is the limit) and that the Grid2 (320 V)voltage is higher than the Anode (312V) voltage. This will considerably shorten the tube's life. You'd better fit a 2K2 resistor, instead of the 100Ω shown in the schematic. I do not care if that changes the design, it is plain bad design to specify such an operating point. They just make it so to get a bit more power on the cheap.
Anyway, hope this helped. Enjoy the build process and keep us posted!
What concerns me is that this design is running the EL84 too hot, (12,5 Watts is the limit) and that the Grid2 (320 V)voltage is higher than the Anode (312V) voltage. This will considerably shorten the tube's life. You'd better fit a 2K2 resistor, instead of the 100Ω shown in the schematic. I do not care if that changes the design, it is plain bad design to specify such an operating point. They just make it so to get a bit more power on the cheap.
Anyway, hope this helped. Enjoy the build process and keep us posted!
If the output transformer has a 16 ohm output, then change the ouput resistor to 16 Ohm, 5W. It is only there to protect the tube in the event that the amp is powered up without a speaker attached. Tube amps don't like to be pwoered up without a proper load attached. Tube Amps have been known to burn up output transformers and output tubes under unloaded conditions.
325V ^ 2 / 330K = 0.32W Therefore a 33oK 1W resistor is sufficient.
325V ^ 2 / 330K = 0.32W Therefore a 33oK 1W resistor is sufficient.
Thank you, I think I can leave this 16 OHM 5W out for now as I will never have no load connected. Well, maybe it is good to have it on there for testing/building purposes? I'm unsure I guess.
What about a 15-ohm? I cant seem to find a 16-ohm one anywhere. The impedance mismatch would not be that big a deal in my opinion.
What about a 15-ohm? I cant seem to find a 16-ohm one anywhere. The impedance mismatch would not be that big a deal in my opinion.
A load is indeed important for consideration of the output transformer. Having said that, the damage gets done when large signals are handled, causing high voltage spikes in the transformer and damaging the insulation.
To obviate such damage, you don't need to load the amplifier with its normal load. A 16 Ohm output will be safe with, say, 100 Ohms. You can't get much of an arc with that amount of load, and you aren't wasting any important amount of audio power in normal operation. If a speaker fuse blows and you turn up the gain, the 100 Ohms will protect the transformer.
To obviate such damage, you don't need to load the amplifier with its normal load. A 16 Ohm output will be safe with, say, 100 Ohms. You can't get much of an arc with that amount of load, and you aren't wasting any important amount of audio power in normal operation. If a speaker fuse blows and you turn up the gain, the 100 Ohms will protect the transformer.
The resistor is only in the circuit when the speaker is not plugged in. It uses a switched 1/4" jack to switch between the input when a speaker is attached, and the resistor for safety.
I am beginning to believe that some amplifiers are destroyed by powering them up without a proper load because without the load they become unstable and oscillate. This oscillation drives the amplifier from cutoff to saturation and back, full output.
Without a proper load on the transformer the voltage swings exceed the breakdown insulation of the transformer, resulting in it shorting out and shorting out the plate to the plate supply. Then the plate goes into meltdown.
Dead transformer, dead output tube(s).
I am beginning to believe that some amplifiers are destroyed by powering them up without a proper load because without the load they become unstable and oscillate. This oscillation drives the amplifier from cutoff to saturation and back, full output.
Without a proper load on the transformer the voltage swings exceed the breakdown insulation of the transformer, resulting in it shorting out and shorting out the plate to the plate supply. Then the plate goes into meltdown.
Dead transformer, dead output tube(s).
It's always a good idea to have a speaker fuse, but in a tube circuit probably not that important. And no, they aren't built into speakers unless you see a fuseholder in the back. If as TheGimp says, the resistor is only connected when the speaker is not, then all these esoteric considerations become more academic. The load you put on while the speaker is unplugged is very uncritical.
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