Invader Jim

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Tuesday, May 30, 2017

Using capacitors instead of resistors to drop AC

Some background:
In the original version of my Radio Champ build, the tubes use a total of 56v at 150mA which leaves 64v that must be dropped, assuming a 120v 60Hz line. Ohm's Law and Power Law will tell you that you need a 430R 20W resistor to take up the slack (430R is the nearest standard value and you should always try to use double the calculated wattage):

R= E / I then P= I E

R is resistance in Ohms, P is the power rating you'll need in Watts, E is the voltage you need to drop in Volts; I is the heater current in Amps.

To cut down heat dissipation you could use a choke or capacitor as ballast to drop the extra voltage. You just need to know the reactance, heater current, and line frequency. It works because coils and caps are reactive; their impedance changes with frequency. If the frequency is constant, so is the impedance and the current flow. In a series-string the heaters are rated for the same current (because only one current can flow in a series circuit) and when that current flows they drop their correct voltages and all is well.

This won't be practical in every case, as a suitable part may be hard to find or just won't exist, but it can be a good alternative to power resistors, especially where heat dissipation is a concern. If the calculated value is not a standard value, plug the nearest standard value into the original reactance equation to find out what its reactance would be then use that number to re-work the Ohm's Law equation to see if the value is acceptable. Tube heaters are usually specified with an acceptable tolerance of +/- 10% of the nominal voltage rating. If the calculated value is non-standard and a standard value would provide unacceptable results, you can add a resistor to take up the slack. This way the capacitor will take the brunt of the voltage-drop, letting the resistor dissipate much less heat.

Here's how to figure the value for a capacitor. If you don't already know what reactance/resistance you'll need, use Ohm's Law:
Xc= E / I

Xc is the capacitive reactance in Ohms (you treat reactance like resistance); E is the voltage you need to drop in Volts; I is the heater current in Amps.

Now rearrange the capacitive reactance equation to find the cap value needed.

Original equation (shown for reference):
Xc= 1 / (2 π f C)

Rearranged version:
C= 1 / (2 π f Xc)

C is the capacitance in Farads; f is the frequency in Hertz. To save a bit of time, you can replace "2 π" with "6.28" and be done with it; the rounding error is negligible here.

So we need a capacitor with a value of about 0.00000622F or 6.22uF (multiply the answer by 1,000,000 to convert Farads into microfarads). 6.3uF is the nearest standard. For safety, at least a 200v rating should be used, even though theoretically it will only have about 64v across it. The capacitor must be non-polarized. If at all possible use a ceramic type, as they have excellent stability as they age. A film type would also work. Avoid electrolytics. They have a relatively short service life and age like milk.

If you want to use a choke the process is the same but the formulas are:

Original equation (shown for reference):
Xl= 2 π f L

Rearranged version:
L= Xl / (2 π f)

In this case a 1H choke would do. Remember to use a choke that can handle the heater current!
7:00 am - 0 comments - 0 Kudos
Friday, September 23, 2016

1970's Coleco Telstar Pong game schematic

Here's the complete schematic of the Coleco Telstar pong variant. It uses the General Instruments AY-3-8500-1 "pong-on-a-chip".

The 3-transistor circuit at the bottom is a variable voltage regulator that doesn't seem to serve much of a purpose here. Maybe it was intended to dial down 9v from the wall-wart to 5v for 7400-series TTL chips (though this one uses 4000-series CMOS chips so it was set for 9v anyway). I don't know if there was a version of this board that used TTL chips though so I'm just guessing here.

This thing can be vastly improved by ditching the RF section entirely (including the 4011) and replacing it with the 1-transistor circuit in red. Adjust the 1k trimpot until it looks good (the trimpot may not even be needed--YMMV). This will give a composite video signal that can be used with a modern TV. The RF section is unstable and basically useless because modern TVs can't be fine-tuned and honestly it isn't worth the hassle to try and make it work (if it works on one TV, it probably won't work on another, and it probably won't even work on the same TV twice in a row). The IC's audio output pin can go to another RCA jack to replace the in-cabinet speaker, which is rather loud and has no volume control or mute switch (you may be able to ditch the 4k7 resistor and speaker driver transistor in this case; I haven't tried it though).

It wouldn't hurt to also replace the 2MHz LC oscillator with a crystal-controlled oscillator for greater picture stability (not really a problem in my particular unit, but it's still something to think about).

Lots of neat things can be done with this chip. It has seven games on board (two are rifle games which require a light-gun and a simple interface circuit) but the Telstar only utilizes three games (four if you catch the game selector switch between detents). If you're the DIY type, go to the website below and find (Ctrl+F) "General Instruments Catalog" for the complete datasheet, application notes, and add-on circuits for the AY-3-8500-series chips, including a crystal oscillator to replace the LC oscillator, the rifle game interface circuit, and a simple multiplexer to extend the games to four players.
7:00 am - 0 comments - 0 Kudos
Wednesday, August 24, 2016

Vox Pathfinder 10 schematic

EDIT 4-16-2017: Updated the schematic. Big thanks to j_flanders for spotting and fixing a couple of errors in my original drawing. I went over my Pathfinder again to confirm the corrections and they check out. I believe this is the final, error-free version but as always it's possible there could still be an error or two.

Several years ago I got a Vox Pathfinder 10 practice amp. Surprised by the sound, I looked for a schematic to see what (if anything) was different about it but, sure enough, couldn't find one. I figured the amp wouldn't be too hard to trace so I just did it myself. I'm posting the schematic I made here to make it available to anyone who may need or want it. Enjoy.

7:01 pm - 0 comments - 0 Kudos
Wednesday, August 24, 2016

Fender Mini '57 Twin-Amp schematic

A long time ago I went looking for a schematic for a Mini-Twin I had bought. Nobody had one, so I decided to just trace it myself. I'm making it freely available to anyone who wants it. Enjoy.

7:00 pm - 0 comments - 0 Kudos
Sunday, January 19, 2014

Tone capacitor myths

Many people like to think that the type of capacitor used in their guitar's tone control makes some sort of tonal difference, so they will toss the ceramic or poly film caps that came stock and replace them with expensive audiophile caps like Orange Drops, Black Beauties, or any of several other types.

Obligatory Disclaimer: This is an attempt to debunk the "mojo" surrounding tone capacitor types *as used in guitars* (that is NOT to say that they don't have such properties in other circuits). If you are someone who fervently believes that your expensive new-old-stock audiophile tone caps sound better than your OEM tone caps, and that anyone who says differently speaks heresy, this is your opportunity to stop reading this and leave. Again, *I am not* implying that audiophile caps have no "mojo" properties, I am suggesting that these properties can't possibly be perceived in any shunt-to-ground application, such as a guitar's tone control circuit.

First we need to look at how a guitar's tone control works. Here is a schematic of a typical guitar (for simplicity, only one pickup and its associated volume and tone controls is shown). Also keep in mind that a guitar signal is not a single frequency, but is made up of many frequencies all superimposed on top of each other.

Notice that the tone control circuit is parallel to the signal path (it is strapped across the signal and ground). When the tone pot is rolled all the way up (the wiper rotated all the way to the unconnected terminal), there is a 500k resistor in series with a 47nF cap to ground present across the signal. A cap's reactance varies with the applied frequency (reactance is basically the AC equivalent of resistance) with it being the lowest at higher frequencies, causing them to pass through the cap very easily. The pot's resistance does not change with frequency. So the total impedance (resistance+reactance) of the tone circuit will be 500,000+Xc where Xc is the reactance of the cap *at a specified frequency*. Put simply, this puts a very light load on the signal (ignoring the parallel loads of the volume pot and whatever the guitar is plugged into) and only the highest frequencies in the signal are bled off because the load gets heavier as the frequencies get higher.

When the tone pot is rolled all the way down, it is out of the circuit (being unconnected at one end) and only the reactance of the capacitor is present across the signal. This loads the signal quite heavily--but again, the load varies with frequency. All the highs are dumped from the signal, along with a good chunk of the mids depending on the cap value. The cap's reactance is highest at lower frequencies so the lows stay in the signal. With a big enough cap the tone circuit could let all the frequencies in the signal pass and the tone control would then become a terrible volume control. Conversely, very small tone caps will subtly shave off very high frequencies to smooth out the sound without it sounding like a useless, muddled, treble-dump.

It should now be obvious why the tone cap type is insignificant in this circuit: the output signal doesn't even pass through it. Whatever bits of the signal *do* pass through the cap are shunted to ground and out of the signal path, never to be heard ("ground" can usually be thought of as "oblivion"). Since this is a passive tone control (it uses no amplifying devices) it can only cut frequencies out of the signal. The circuit is not boosting lows, but is just throwing away highs.

Having said that, there are most certainly tonal differences among the various capacitor types. Any application where the main signal, or at least part of it, passes through the capacitor will let the distinct tonal properties of that capacitor to become apparent. The best way, by far, to exploit the "mojo" of a capacitor is to use it as a coupling cap in an amp or effect.

So, in closing, any shunt-to-ground circuit, such as most guitar tone controls, will not display tonal differences between capacitor types. Some people will refuse to believe that, and that's just fine, though people should use their ears and experiment and not blindly buy into hype and parroted myths. Question everything! But no matter what your opinions and prejudices, remember that golden rule of guitar gear:

If it sounds good, it is good.
5:00 am - 0 comments - 0 Kudos
Tuesday, November 05, 2013

GCB-95 mods tures/gear/484555/657204

The mods are as follows:
*6n8/10n/22n/47n sweep caps on 4-way rotary switch
*Whipple inductor and stock Dunlop inductor
*Wah/Yoy mode switch
*Q1- BC109 NOS metal can
*Q2- 2N3904
*2k2 mids resistor
*68k Q resistor
*Grounded-input true bypass
*Volume/wah switch
*RGB check LED
*The switch at the heel selects Wah/Yoy and turns the LED Blue or Green, respectively.
*The switch at the toe selects Volume/Wah (or Yoy, depending on the heel switch) and turns the LED Cyan (Blue+Green).
*The rotary switch on the side selects the value of sweep cap, which changes the Wah and Yoy sounds as well as the timbre of the Volume setting.
*The toggle switch on the other side is the bypass switch. It provides grounded-input true bypass and cuts the pedal's power source so I don't have to unplug the input cable.

--Another mod you could try is replace the 220n cap on the wiper of the wah pot with a 330n. This will simulate the famous ICAR pot taper.
--Something else you could do is put a 1k pot in series with the inductor. This is the Q control on the 535Q wah, or so I've read.

The switch wiring is rather complex but it is very easy and intuitive to use.

Positioning of the Wah/Yoy switch is somewhat critical. You want it directly under the rubber pad on the treadle. To install it, you must remove the "column" on the treadle where the rubber pad is attached to. Drill it out then use a Dremel to remove it completely. You want it to be trimmed flush like it was never there. When you're done, glue on a rubber or felt pad to cushion the top of the switch. Reassemble the pedal and adjust the switch height so that it activates easily.

Since the shell is so short at the heel, you may want to try the Alpha #107-SF12020-L DPDT stomp switch (available from Smallbear, stock #SF12020-L). I used a normal-sized DPDT stomp with the terminals coming out the side and it was a bit too tall. I had to bend the bottom cover plate (and insulate it) to try to clear it.

The stock transistors are MPSA18. I changed the second transistor to a 2N3904, but in hindsight it should probably be left alone. Since it is just an emitter follower, the super-high Hfe of the MPSA18 better approaches ideal characteristics.

The BC109 tames some of the harshness of the wah because its lower Hfe dulls the resonant peak of the circuit. The stock MPSA18's Hfe is way too high and it causes a sharp resonance which can be harsh to listen to.

How it sounds:
The sweep caps are a great combo. 6n8 is a trebly sweep that's great for fast single-note riffs and tapping, or when you don't want the heel-end of the stock sweep making it sound muddy. The 22n is very throaty. The 47n is really low. It almost turns the pedal into a bass wah (a bass wah uses 68n). The location of the switch knob allows me to switch with my foot on-the-fly.

Wah mode sounds a little beefier than a stock Crybaby because of the second inductor. In Yoy mode it has a "Woy" or "Eeyoy" sound depending on the sweep cap.

The Whipple sounds a lot like the stock Dunlop inductor (which isn't bad at all), but all the reviews made me expect the Whipple was the best wah inductor made. It's a lot cheaper than a Fasel, though.

The Volume switch gives the signal a real nice warm tone and a bit of a volume and mid boost as compared to the bypassed sound. The value of the sweep cap, in volume mode, sets the range of volume (smaller values make the toe setting louder) and the amount of mids in the volume signal (smaller values for more mids). It's a nifty side-effect and can really fatten up a distortion. Staying in volume mode and hitting the heel switch to Yoy mode makes a "weh"-like sweep that has little effect on volume.
5:00 pm - 0 comments - 0 Kudos
Friday, June 26, 2009

Rules of teh GB&C.

Current mood: blank

1.  If it exists, there is an ULTIMATE thread for it.
2.  If an ULTIMATE thread for it does not exist, it will soon.
3.  Check the stickies.
4.  Google is your friend. No exceptions.
5.  Kit builds are not builds. No exceptions.
6.  Don't name your edits.
8.  Finish the break, apply Titebond Original, clamp, sand flush, refinish.
9.  Yes, it's possible.
10. New amps are not for everyone.
11. LEDs are over-rated.
12. Killswitches are over-rated.
13. It's not necessarily a bad ground.
14. Build planning threads are lame. Period.
15. Parroting is stupid.
16. Drain your caps or your head will implode.
17. Ending a sentence like this is so annoying!?
18. Parroting is indeed stupid.
19. ^I agree, parroting is so stupid.
20. Thanks in advance!
2:30 am - 9 comments - 14 Kudos