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Old 25th March 2010, 02:34 AM   #1
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Default 180 pF capacitor and other questions

So I'm building this:

http://www.schematicheaven.com/newam...at_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
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Old 25th March 2010, 03:09 AM   #2
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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.
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Old 25th March 2010, 03:47 AM   #3
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Thanks bob! I know not to discharge them after it is on, because they would just charge up again and it would not be a good idea! So I should get a hold of a 5k-ohm 2-5 watt resistor to discharge the caps, then the amp is safe to touch?
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Old 25th March 2010, 04:01 AM   #4
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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...

Last edited by Richard Ellis; 25th March 2010 at 04:06 AM. Reason: Post thoughts
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Old 25th March 2010, 04:06 AM   #5
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Yeah both methods work. The main problem with adding a resistor permanently is that you are changing the design. So you can't claim it's the same as...
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Old 25th March 2010, 04:09 AM   #6
Enzo is offline Enzo  United States
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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.
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Old 25th March 2010, 04:14 AM   #7
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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+.

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Old 25th March 2010, 05:13 AM   #8
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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.
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Old 25th March 2010, 06:29 AM   #9
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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.
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Old 25th March 2010, 08:28 AM   #10
Enzo is offline Enzo  United States
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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.
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