electronics

How to Build Your Own DCR Measuring Tool

Let’s build one of my favorite DIY guitar tools that I use daily in my shop. I’ll show you two versions and then explain how to put them into action.

Welcome back to Mod Garage. After receiving numerous requests to show more DIY tools for guitarists, today we’ll explore one of my favorites. For years I’ve used this one in the shop daily and I’m sure you’ll love it. It’s cheap and easy to build, but very effective for analyzing circuits of electric guitars and basses without opening the electronic compartment or lifting the pickguard. It’s a kind of adaptor or extension to measure a pickup’s DC resistance (DCR) from outside the guitar. After building one, we’ll discuss how to interpret the measurements.

The DCR of a pickup is by far the most common parameter you can read when reading pickup descriptions and often it’s used as an indicator of the output. The reason for this is that it’s easy to measure, but, sadly, it doesn’t tell us anything about a pickup’s output nor its tone. To quote pickup designer Bill Lawrence: “DC resistance tells you as much about a pickup’s tone and output as the shoe size tells you about a person’s intelligence.”

I’ve written about DCR as a pickup parameter in detail and you can read about it in “Mod Garage: Demystifying DCR.”

DCR is not a primary parameter in pickup design. It’s simply the result of the type and gauge of the pickup’s wire, the number of turns, and other parameters like the winding pattern, etc. But it isn’t completely useless, and we can use it as a good reference point for analyzing pickups both inside and outside a guitar or bass circuit. All you need for this is a digital multimeter (DMM). You don’t need an expensive calibrated precision DMM—any entry level DMM will work. You can get a simple digital DMM for $10, but if you want to invest in a better device, it can’t harm.

The easiest way to analyze a pickup is outside a circuit. Simply set your DMM to ohm and connect the two pickup leads to your DMM. If your DMM doesn’t have an auto-range function, set it to 20k ohm. Now you’ll get the DCR reading for your pickup. You can compare it to the factory specs of your pickup and it should be close. If your DMM shows “infinite” or “overload,” you know the pickup wire is broken. Let’s say your pickup should read 7k ohm, but yours reads around 2-3k ohm. Your pickup likely has a short circuit somewhere in the winding. Used this way, the DCR is always good to quickly check if a pickup is alive or not.

To quickly analyze a guitar or bass circuit with one or more pickups, you first need to build the DIY adaptor tool this column is about. There are two different versions, and you don’t need much for this:

Version #1: This is the quick and dirty version. You need a standard 6.3 mm straight mono plug (the same type on all your guitar cables), some wire of your choice (preferably in two different colors), and two insulated alligator clips.
Version #2: A more elegant version that you can also use with a scope if you have one. You need the same parts as for version #1, but instead of two alligator clips, you need two 4 mm banana plugs, and the two wires need to be longer than what you’d use for Version #1.

So, heat up your soldering iron and let’s get to building version #1.

Solder one piece of wire to the HOT terminal of the mono plug and another one to the GROUND terminal. I prefer a red wire for the HOT and a black wire for the GROUND terminal (Photo 1).
Solder an insulated alligator-clip to each end of the two wires, preferably a black one to the black wire and a red one to the red wire. Ready!

Version #2 is built the same way, but, instead of alligator clips, you solder a 4 mm banana plug to each end of the two wires, if possible, also in black and red. The two wires should be long enough that can place your DMM and/or scope at some distance from the guitar. In Photo 2, you can see version #1 on the top and version #2 on the bottom.

The difference between the two versions is that with version #1 you put the plug into the output jack of the guitar, connecting the two probes of your DMM to the alligator clips: the black probe of the DMM goes to ground (black wire) and the red probe goes to hot (red wire) as seen in Photo 3. With version #2, you need to remove the two probes from your DMM, plugging the two banana plugs directly into your DMM or your scope, also seen in Photo 3.

Both versions work equally well. Version #2 is just easier to operate when you also want to use the adaptor for a scope.

For a quick check, you can also directly touch the hot and ground terminals with the probes of your DMM, but you need both hands or a second person for this if you want to play around with the controls or the pickup-selector switch.

Now we can easily check four things with this tool, assuming everything is connected the way it should be and your DMM is set to ohm and auto-range or the 20k ohm scale if your DMM doesn’t have an auto-range mode:

Do you receive a reading on your connected DMM? If not, check if the volume pot is fully opened. Do you receive a reading now? If so, close the volume pot completely and see if you still receive a reading. No? Perfect, you just proved that the volume pot is alive and well.
With a fully opened volume pot and a reading on your DMM, slowly turn down the volume and watch the reading on the DMM. If you receive some crazy reading, chances are good there is a treble bleed network on your volume pot. If the reading slowly goes down to zero, you know that there is no treble bleed network on the volume pot, and you can check if it’s an audio or linear volume pot (plus the taper it has, if it’s an audio pot). Let’s say we have a 500k volume pot. When you close the volume pot halfway and receive a reading around 250k, you know it’s a linear pot. An audio pot, depending on its taper, will result in a much higher reading on the first 50 percent of the volume pot. If you read 500k until the volume pot is almost fully closed, this means the pot has a 90:10 audio taper—exactly the kind of volume pot you don’t want to have. If you read something around 300k in the middle of the volume pot, you know it’s a 60:40 audio taper.
If the volume pot is fully opened and you don’t receive a reading on your DMM, chances are good that your output jack is broken, not connected, or connected incorrectly. Please make sure there is no activated kill-switch in the circuit that can also cause this “problem.”
Turning the tone knob(s) will make no difference in the reading you receive. If you receive a slightly higher reading with a tone pot fully opened compared to when it’s closed, you know it’s a no-load tone pot.

There is a lot to discover from just the outside of any guitar or bass. So, now let’s see what we can measure from outside the instrument starting with a Telecaster with a 4-way switch. The readings in all examples are the readings I received with guitars I had in the shop, but they can be different in your instruments:

Bridge pickup only: 5.85k ohm
Neck pickup only: 6.76k ohm
Both pickups together: 3.18k ohm
Pickup selector switch in position #4: 12.30k ohm

The readings for both pickups are within the factory specs and are in a typical range for a vintage-flavored Telecaster pickup set. With a reading of 3.18k ohm for both pickups together, you know that both pickups are in parallel. With the reading of 12.30k ohm, you know that both pickups are in series with each other.

Here is the simplified math behind these readings:

Series connection: DCR pickup #1 + DCR pickup #2
- In our example, it’s 5.85k + 6.76k = 12.61k ohm, which is very close to the reading of 12.30k we received. The missing 0.31k ohm are eaten up by the resistance of the pots and the tolerance of your DMM. For this test, I chose the cheapest DMM I could find in the shop. A calibrated high-quality DMM will have much less tolerance.
Parallel connection: (DCR pickup #1 + DCR pickup #2) divided by four
- In our example, it’s 5.85k + 6.76k = 12.61k ohm divided by four = 3.15k ohm, which is very close to the reading of 3.18k ohm we received.

Now let’s repeat this with a standard Stratocaster:

Bridge pickup only: 7.07k ohm
Middle pickup only: 5.88k ohm
Neck pickup only: 5.70k ohm
Bridge + Middle pickups together: 3.26k ohm
Neck + Middle pickups together: 2.94k ohm

All three pickups are within the factory specs of this Strat. We have a slightly hotter bridge and two vintage-flavored pickups. The two in-between positions are in parallel.

Lastly, let’s try a vintage PAF-loaded Les Paul:

Bridge pickup only: 7.77k ohm
Neck pickup only: 7.09k ohm
Both pickups together: 3.74k ohm

Both PAFs have the typical vintage DCR and are in parallel in the middle position.

That’s it. Next month we’ll take a deeper look at changing wires on pickups, which is something I’ve been asked about a lot, so stay tuned!Until then … keep on modding!

by Dick Wacker – PREMIER GUITAR

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Standby Switch Myth

by patrick scheidegger

tube amplifier standby switch information

It’s a long-standing debate about what the standby switches on Fender amps are used for. That’s why Sweetwater‘s own tube amp expert, Greg Bowers, decided to clear things up and end the debate once and for all. This is his story más o menos a few Fomedits:

The Standby Switch

The myth about the lowly standby switch on guitar amplifiers has gone on since they first came on the scene in the 1950s, so no wonder it is still misunderstood. You would think that by now with the internet around everyone would be up to speed, but the myth is too enduring! I have even read articles from educated people that I respect who have not quite gotten the whole story correct because of reasonable sounding, but incorrectly applied details about vacuum tubes. Then the myth gets distorted even more, because everyone thinks these people should know what they are talking about.

These switches are notorious for causing weird problems and numerous questions from my customers like “Why does their amp pop when using it?” (they pop because they are switching anywhere from 300 to 800 volts. WOW!)I have merrily gone on repairing amplifiers over 20 years and decided to break down the mythology of standby switches based on what I know as a technician and amp builder to separate what is folklore and what is fact. At the very least, I would like to explain what standby switches are NOT used for. Here is what I learned repairing amps, doing research and reading history from much smarter people than myself.

Tube Amp History

Back in the 1940s -50’s there were no books or schools for making guitar amplifiers. Amplifying a guitar was a relatively new idea. Most great guitar amp companies were not founded by textbook electronic engineers or scientists, but smart service technicians who experimented with the recommended RCA vacuum tube circuits already published to get a better sounding or louder amplifier. This is true even to this day.

Designers often push the limits of what a tube can handle to see if it will work past its conservatively rated parameters used for AM radios and Public Address amplifiers. This is kind of like what hot rodder’s do to cars. Special effects using very odd looking devices or circuits also find their way into designs. And yes, there are actual technical mistakes made by these self-trained designers that become accepted norm for a given model. So I learned to expect any reason could be possible for just why standby switches exist!

It’s not a mute switch for breaks

Historically, I have yet to see an amp made with standby switches until Leo Fender was around. He is accredited for first inventing the idea and I have no reason to doubt this. Leo Fender adopted the standby switch design from reading vacuum tube service manuals. He was self-trained in electronics and developed his own designs. Basically, his switch disconnects the high voltage from the circuit, but the big question is why?

Leo Fender did not intend them for use during beer breaks as a mute switch (the biggest myth of all), even though this is what everyone thought he meant by the “standby” switch label and used them this way! A “mute” switch is a common switch often used on audio amplifiers but never designed the way Leo Fender’s “standby” switch is wired to the high voltage. A mute switch simply connects the audio signal to ground, stopping it from passing through the amplifier, just like turning the volume control all the way down.

One should note the term “standby” has been used occasionally in place of the word “mute” on other switches that actually are audio “mute” switches for taking breaks, further adding to the public confusion. All guitar amplifier companies are infamous for incorrectly labeling or coming up with cute names for a switch’s function. Leo Fender also is known for mislabeling what technically is a tremolo circuit control as a “vibrato”. This is probably because he did not know how to play guitar? Maybe he could have come up with a better name than “standby” that is less confusing? Too late now…

The addition of standby switches on tube amps is accredited to Leo Fender.

It’s not for protecting tubes

Leo Fender did not use the standby switch to protect the tubes, because it actually is not good to have the tubes on a very long time in standby, which is a fact from the RCA tube manuals. There are so many people who get this part wrong. Beware advice given by some internet guru who was just regurgitating someone else’s myth that sounds technical, but is just wrong!

This myth started with a misunderstanding of the old RCA tube manual recommendation for using standby switches when running very, very high voltage radio station transmitter tubes. RCA was NOT talking about the tubes used in a guitar amplifier. The tubes used in guitar amps are the same type tubes used in Grandma and Grandpa’s old tube radio receivers, TV’s and record players, etc., which you never see with standby switches, do you? Therefore, why would a guitar amplifier be different than these other devices? Because they are not! Fender’s first “Tweed” amplifiers also did not have a standby switch!

For Leo Fender, tubes were cheap back then and actually made much stronger than tubes we have today, so why would he have this supposed concern for tube life? In order to get the tone he wanted, many of his designs are actually very hard on tubes pushing the limits of their power capabilities, therefore it stands to reason that tube life was not his concern.

The standby switch on a Fender amp was put there by Leo to solve a problem he had later when building the much demanded larger power amplifiers using higher voltages to operate.

Don’t leave your tube amp on standby for too long. It’s bad.

The actual reason for standby switches

It’s all about the capacitors!

As the public asked for louder amplifiers, Leo Fender began to build amplifiers with higher power supply voltages. When first turning on the amplifier and before the tubes are warm, tubes do not conduct high voltage, so there is no “load” on the power supply. This phenomenon would allow voltage to rise above the maximum voltage rating for the large capacitors used in the circuit, putting them at risk of shorting out from the stress. This was especially true when Fender started to use solid state rectifier diodes that provided power supply voltage instantly when the mains power was turned on.

While the tubes are warming up, the standby switch removed the high voltage from the circuit until the tubes filaments were warmed up to operating temperature and the power supply voltage would be loaded down by the tubes to the nominal safe operating voltage for the capacitors.

Sure, Leo could have installed much higher voltage rated capacitors that could safely handle the voltage rise, but these were very expensive back in his day. His company’s goal was to produce high quality, but lower cost amplifiers (and guitars), so keeping the price down was important to him. Therefore, the standby switch was a cost-saving design feature much cheaper than the alternative very expensive capacitors.

The standby switch removes high voltage from the circuit while the tubes warm up.

Takeaways

In my experience, if you want your tubes and the other parts of the amplifier to last longer, put a small fan on the amplifier to get the heat out of it. Excess heat is the greatest problem, so only have the amp on when you need it. Let’s review the takeaways.

There are occasionally a few modern amplifier designs that are taking the problems with conventional plate voltage standby switches into consideration and have put in safer systems for tube warm up purposes. To be fair, these systems do not cause the same potential problems as the old fashioned standby switches. If you have one of these amps, the use of the standby switch may not be causing any harm. You will simply have to inquire about your amps features to know what is used.

However, I still refer to other much smarter engineers than I, including the RCA tube manual which do not list any standby switches in the recommended design of receiving tube power supplies. Don’t expect your tubes to last longer using them.

Don’t use it as a “beer break” switch. For short breaks, simply turn down the volume control (or mute switch if you have one) and don’t use the standby switch, so there is not that nasty pop in the house sound system that could damage speaker drivers. If the time between sound check and performing is longer than 20 minutes, turn the amplifier completely off. You only need 5 minutes at the most to completely warm up a tube amplifier.

It’s as simple as that. Why else would you use something that often pops loudly in the audio when used (remember I mentioned it cuts off the high voltage)? By the way, other brands did not use standby switches until Marshall copied Fender’s Bassman amplifier design and after the two biggest makers used these standby switches, everyone assumed you always had one on a guitar amp. Often, designers put these on amplifiers only because the public asks for them, not that they are needed. This is due to the power of the myth! These days we have other devices available to protect the capacitors and in general capacitors are much cheaper now and can be made to run at higher voltages without great cost.

Don’t put one on your amp because you were told it makes the tubes last longer! Is there a way to help my tubes last longer you say? The correct understanding of vacuum tube operational specifications prove there is no evidence that a standby switch can make your tubes last longer and actually could only hurt them if you overuse the standby mode.

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Ferrite Transformer Turns

by patrick scheidegger

Ferrite core turns ratio calculation
Push pull topology ferrite core turns ratio calculation with example
Ferrite transformer primary turns calculation
Ferrite transformer secondary turns calculation

In this article you will learn how to calculate turns ratio of ferrite core transformer for high frequency switch mode power supply inverters. High ferrite core transformers are used in almost every power electronics circuits like invertersand pure sine wave inverters. They are used to boost up or step up low dc voltage of battery and other dc sources like solar panels. Ferrite core transformers are also used in isolated dc to dc converters to step up or step down dc voltage. For example in isolated buck converter it is used to step down dc voltage and in isolated boost converter, they are used to step up dc voltage. In this article, we will learn how to calculate turns ratio of high frequency ferrite core transformer with examples.

Ferrite core turns ratio calculation

For example in boost up stage we have two options to use from power electronics converters, push pull topology and full bridge. I will explain both methods one by one. Turns ratio calculation formula and concept remains same for both topologies. The only difference between push pull topology and full bridge transformer design is that push pull ferrite core transformerrequires a center tap in primary winding. In other words, push pull transformer have two times primary turn than full bridge transformer.

Push pull topology ferrite core turns ratio calculation with example

Let’s start with example. For example we want to design a 250 watt boost up dc to dc converter. We are using push pull topology for this design. We are using 12 volt battery. We want to step up dc voltage from 12 volt 310 volt. Switching frequency of design is 50KHz. We are using ETD39 ferrite core which can handle 250 watt. It is beyond the scope of this topic to tell how to select ferrite core according to power rating. I will try to write separate article on it. The output of ferrite core will be always high frequency square wave of 50 KHz. We need to use full rectifier to convert it into dc of 310 volt. You may also need to use LC filter to harmonics or AC components from output.

Ferrite transformer primary turns calculation

As you know battery voltage does not remain same all the time. As the load on battery on increases, battery voltage will be less than 12 volt. With no load with fully charged battery, battery voltage will be near to 13.5 volt. Therefore input voltage is not constant, we must consider it while calculating turns ratio of ferrite core transformer. Cut off voltage for battery is usually 10.5 volt. We can take it as smallest possible value of input voltage to boost up dc converter. So we have following parameters now:

Vinput = 10.5 volt

Vout = 310 volt

As we know that formula of turns ratio calculation in transformer is

N = Npri / Nsc = Vin / Vout

Where Npri is number of primary turns and Nsc is number of secondary turns. We have three know variables like turns ratio which can be calculated by above equation, input voltage and output voltage. But we need to calculate primary turns to find secondary turn of ferrite core transformer. Formula to calculate primary turns for ferrite core transformer is given below:

Npri = Vin * 10^8 / 4 * f * Bmax * Ac

But for push pull it will be half the primary number of turns.

Where Npi is primary number of turn, Vin( nom) is normal input voltage which in our example is 10.5 volt.
Bmax is maximum flux density. The unit of maximum flux density is Guass. Remember if you are using Tesla unit for maximum flux density, IT = 10^4 Guass. The value of maximum flux density is usually given in data sheet ferrite core. We usually take value of Bmax between 1300G to 2000G. This is usually a acceptable range for all ferrite core transformers. Note : High value of flux density will saturate the core and low value of flux density will lead to core under utilization. For example we will take 1500G for dc to dc converter example.
f is switching frequency converter. In our example switching frequency of dc to dc converter is 50 KHz.
Ac is effective cross sectional area of ferrite core. We have to refer data sheet for this value. In this example, we are using ETD39 core. The effective cross sectional area of ETD39 is 125mm^2 or 1.25cm^2.

We have all the values to calculate primary number of turns .i.e.

Vin = 10.5 volt, Bmax = 1500G, f = 50 KHz, Ac = 1.25 cm^2

By putting these parameters in two above formula, we can calculate turns primary number of turns.

Npri = 12 . 10^8 / 4 . 50000 . 1500 . 1.25 = 3.2

Hence Npri = 3.2 But we cannot use fractional turns. So we need to round off primary turns calculated value into nearest whole number 3. The nearest possible whole number is 3. primary number of turns for ferrite core is 3. But before that we need check either for Npri = 3 Bmax is within acceptable range or not. As I have mentioned above the acceptable range for Bmaz is 1300-2000G. But the question is why we need to check the value of Bmax again? Because we adjust the value of primary turns from 3.2 to 3. So let’s calculate value of Bmax for Npri = 3 by using above forumla.

Bmax = Vin * 10^8 / 4 * f * Npri * Ac

Bmax = 12 * 10^8 / 5 * 50000 * 3 * 1.25 = 1600G

So calculated value of Bmax is 1600G which is within acceptable range of maximum flux density. Its mean we can take Npri = 3 for further calculations. Primary number of turns for push pull ferrite center tap transformer is 3 turns + 3 turns. In any design you will need to adjust the value of Npri if it is in fraction. You can easily adjust it. But you need to check value of Bmax every time. We start with assume value of Bmax and calculated Npri. But you can also start with assume value of Npri and check the value of maximum flux density Bmax. For example suppose a value of Npri =1 and check the value of Bmax and keep repeating this process, until it is become in acceptable range.

Ferrite transformer secondary turns calculation

Now let’s move to secondary turn of ferrite core. In our design the output of dc to dc converter is 310 volt at any input voltage. Input voltage is variable from 10.5 volt to 13.5 volt. We will need to implement

Feedback to get regulated 310 output voltage. So we will take little bit higher value of output voltage so that at minimum possible input we can still get output voltage of 310 volt by changing the duty cycle of PWM. So we should design a ferrite core transformer with secondary rated at 330 volt. Feedback will adjust the value of output voltage by changing the duty cycle of PWM. You should also take care of losses and voltage drops across switching devices and you should take them into account while designing transformer.

So transformer must be able to supply 330 volt output with input of 13.5 volt to 10.5 volt. The maximum duty cycle for PWM is 98% and rest 2% is left for dead time. During minimum possible input voltage duty cycle will be maximum. At maximum duty cycle of 98%, input voltage to transformer is 0.98 * 10.5 = 10.29 volt.

By using voltage ratio formula of transformer = voltage ratio = 330 / 10.29 = 32.1. Voltage ratio and turns ratio in transformer is equal to each other. Hence N = 32.

So we know all values to calculate secondary turns of ferrite core transformer.

N = 32, Npri = 3

Nsec = N * Npri = 32 *3 = 96

So number of primary turns is equal to 3 and number of secondary turns is equal to 96. So it is all about turns ratio calculation for high frequency transformers. If you have any issue, let me know with your comments.

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DIY ‘77 Vintage Phaser Pedal

by patrick scheidegger

Fom Tooley presents:

A classic circuit from the magazine Everyday Electronics (December 1977 issue).
Build your own boutique pedal for a fraction of the cost. A separate plan of the PCB is included as the final image (6 of 6). If you build it, they will come. If you build it, please post your construction and completed photos to fomtooley.com

P=͟͟͞͞R=͟͟͞͞I=͟͟͞͞N=͟͟͞͞T=

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