Below is a link to a tutorial on how bad ceramic capacitors are regarding holding their capacitance value under voltage bias. Basically the smaller the capacitor (physically) and the larger the voltage across it, the less the actual capacitance will be. I have always know this to be true but I didn’t realize it was as bad as describe below. This was a good reality check for me to make sure and select capacitors for a design carefully!
So I like sparkly guitars and decided to buy one. I figured I should get the goofiest thing I could find and so I decided on a Danelectro Convertible re-issue from the late 90’s!
I knew it would have issues…and it did! But fear not, this story has a great ending!
Danelectros are cheaply made guitars but do have some nice features. My convertible has a fantastic neck made of quality maple, but not too thick(good for my playing). The rosewood fretboard is almost a 1/4 inch thick and is also high grade wood. The frets are on the beefy side which I like. You need to remove the neck to adjust the truss rod but, this is easily done and you can loosen the strings but leave them on to do this. The bodies are made of masonite and plywood composites. These materials are nice in that they are stable and tolerate of temperature variation but god help you if they ever get wet! Yes they look cool, but besides the necks, the pickups are the big claim to fame of Danelectros. The lipstick type pickups have a unique sound with a scooped mid range and a very bright high end…probably from having less self capacitance and a lower winding impedance. I don’t actually like the sound for most types of music I play, so one of my primary objectives with this instrument was to get a punchier mid range without making the low end suffer or make it sound muddy. Also contributing to the sound is a tone/volume setup with a dual gang pot and a circuit I did not care much for. As shown in the picture above this is operated via two concentric knobs.
So this is what I did to make this a really well playing and great sounding guitar
1. I took the neck off and did a complete fret leveling and re-crowning. I won’t go into the details of the process here but I just started doing this and have done fret leveling on three guitars – all with stellar results! I highly recommend that guitar players learn how to level and crown the frets on their guitars. There are lots of tutorials on youtube. It is surprisingly easy to do(you need the right tools).It takes about three hours and makes such an improvement if your guitar has uneven frets(likely). You can get everything you need from Amazon..etc.
2. I adjusted the truss rod such that the neck is ultra close to perfectly straight. I did this to get the action even along the neck while allowing for the bridge to be adjusted up higher; so it has more tension across it. It plays better and it sounds better.
After fret leveling and truss rod adjustment, this guitar plays unbelievably well. This picture shows the action from frets 8-12. It just a little higher than a credit card along the whole neck with no buzzing or weird anything!
3. Modified the the floating bridge so the strings won’t slide all over the place (problem with these bridges). I simply carved a little notch for each string along the fret wire saddle. Note that I made sure the notch is deeper on the front such that the strings don’t buzz by hitting the front of the saddle when vibrating. I did this with a little triangle and rat tail file.
4. I designed and installed a preamp that is tailored to improve the tone of the pedestrian lipstick pickup that came stock on the guitar. This preamp uses an LMV641 low noise, low power op amp. It draws less than 150uA which is amazing. The Danelectro Convertible has plenty of room to put all kinds of stuff in it, but I just opted to make this tiny amp board and use a A23 12v battery glued to the back of the cover plate. You could use a standard 9v or two 3V coin cells in series, etc. The jack on my convertible was already a stereo jack so I was able to use this as an on/off switch for the preamp. The ring connection on the jack can be use connect the battery to the ground through the guitar cable plug. The preamp has a gain of two which could be adjusted if desired by changing R4 or R3. R6 and C3 replicate the load resistance and capacitance of the volume pot that would normally be present and can be adjusted to alter tone as desired. C2 provides super high frequency roll off for amplifier stability and does not affect the tone. C6 and R3 roll off the high end just a little bit. R3 can be a POT thereby creating a typical guitar tone control. A 10k POT is a good value to use in this case. C6 could be .1, .22 or .47uF depending on taste.
After adding the preamp, the pickup sounds fantastic. It still has the lipstick high end sparkle but with a fatter overall sound. When you turn down the volume, the tone doesn’t change either because the drive of the preamp is not loaded down by cable capacitance.I love it.
This is the preamp with the battery shown in the background. A 9 volt will work also.
Preamp Schematic shown above
5. I Changed to a single volume only control. My preamp has a provision for a tone control but I opted not to use it because I didn’t want to drill another hole in the instrument.
6. The Guitar came with really nice Gotoh tuners(not sure they are original). So I did not have to upgrade these, but this is not always the case. If You have a Danelectro with cheap tuners, change them out!
7. I adjusted the pickup as close to strings as I could. I found this to improve the tone. While it also boosts output, that’s not why I did it.
Playing the Danelectro Live
Some one asked me to share the files for my 3D printed telegraph key. So I am going to do that.
Here is a link to the STL file for the key parts.
There are 3 files that print the base, key and upper bracket separately. All of the holes are included but they are undersized. Some you will drill larger and others are tapped. There is a horizontal hole in the key to run a wire back from the top key contact and the same is true for the base. I printed at 100% fill using ABS. HIPS would work well also. The magnetic damping instead of a spring works really well. The 4-40 contact screws need to be filed flat after being nipped to size.
2″ of brass 1/8″ rod (for hinge point)
2 3/8 6-32 screws (front legs)
2 1/2 6-32 screws (mounting upper bracket)
3 3/4 6-32 screws (Key adjustment and back legs)
7 6-32 nuts
2 6-32 knurled nuts
2 1″ 4-40 screws (for key contacts and holding magnets…will be trimmed)
2 4-40 nuts
misc 4-40 washers for spacing adjustment
2 .6″ diameter rare earth disc magnets with center hole
24 gauge scrap wire
View of Magnets and Contacts
This is my new hybrid guitar tube amp which utilizes a solid-state input stage, DSP reverb, and solid-state phase splitter. Only the push pull, class AB output stage utilizes tubes, namely two EL84’s run at 390 volts with cathode bias. The bias uses two 15 volt zeners which creates a bias current of about 26mA. This requires almost 30 volts of swing on the grids to drive the amp to saturation. This is accomplished with a little switch mode boost converter that generates 29 volts to drive the phase splitter opamps. All of the solid-state circuitry runs off the AC filament supply for the tubes. The solid state portion is basically my stomp amp design( also on this blog) minus the final power amp, which is replaced with the phase splitter.
A couple notes about the design: Using zeners works great, but they can fail(haven’t had a problem yet) and typically they fail by shorting(very bad for the tubes!)..so it may be prudent to parallel with 1k ohm resistors and .1 uF caps to make them more tolerate of current or voltage spikes. I use 5 watt zeners and have yet to have one blow on me with numerous amp designs.
Also the gain distribution is not ideal. This is because of the low headroom of the FV-1 reverb IC which runs at 3.3v. This requires that there be lower gain in the first two stages than is possible – degrading noise figure somewhat. Despite this, the amp is very quiet – even with noisy un-bypassed zeners in the final bias circuit.
Well I am a little behind schedule! I started a new company: Global Technology Integrators…and I am swamped! But later this summer things will settle down and I am going to get to more projects! I feel bad because I like to crank out ideas and experiments as fast as I can! I am working on some really neat stuff and I am excited to share..so eventually I will inject some fresh DNA into the site. I intend to finish my antenna analyzer, I am also working on a 20-30 watt hybrid guitar tube amp with el84’s and an LND150 fet front end. Finally, I am doing more antenna experiments that look very promising.
A resistive bridge type SWR meter doesn’t actually measure SWR – it measures impedance mismatch or impedance(however you want to look at it). SWR is calculated by the ratio of two volatges on the bridge. The SWR and impedance only relate correctly if the bridge impedance and feedline impedance are the same(50 ohms typically). A resistive bridge works fine but you can do the same thing with a simple resistive divider. When using a divider, you can use any value resistor you want(50 ohms in my design) and as long as you know the characteristic impedance of your feedline, you can correctly calculate the results with any feedline impedance (not just 50 ohm coax).By measuring three voltages across the divider(Vs, Vr and Vz), one can easily derive: total Impedance, resistive impedance, reactive impedance, SWR, and return loss. A resistive bridge is most accurate when mismatched, but the resistive divider is most accurate when match is optimal. So ask yourself would rather know more accurately what’s happening between SWRs of 1:1..to 2:1 or when it’s 3:1 or greater? The only drawback of this method is that you cannot determine the sign of reactance(capacitive or inductive) directly, but you can derive all of the magnitudes. This is because when you convert the voltages to DC values by means of diode detectors, all phase information is lost.
I decided to build a small and reasonably simple antenna analyzer for 1 -30Mhz. I wanted it to provide accurate measurement of SWR, Total Z, R and X of the ANT. Simplicity in operation and moderate power consumption were also design goals. The schematic below is what I came up with.
Preliminary Schematic(not built yet)
I have etched a circuit board and will build the analyzer in early 2016. I will update the post as I make progress. Expect some values in the schematic to change. Below is a discussion of the math required to derive all of the analyzer measurements. Vr requires a floating measurement, Vs and Vz are ground referenced.
Diagrams for Analysis
Using the Voltages Vs, Vr and Vz, one can create a triangle related to the classic power factor triangle where the hypotenuse(Vz) can be seen to be shared and the cos(ang) allows one to find the reactive and real components of the impedance. One is required to find the cos(ang) using only the lengths of the sides of the Vz, Vr, Vs triangle. These values are the measured voltages from the diode peak detectors shown in the schematic. The law of cosines provides the solution.
cos(ang) = (Vs²+Vr² – Vz²)/ (2* Vs* Vr)
Using node voltage equations, the following relationships can be derived from the simple divider shown above. Note that the triangle legs:Vs, Vr, VZ , VZr and VZx are synonymous with Zs, R, Z, Zr and Zx respectively
The total impedance seen across the signal source (Zs): Zs = R*Vs/Vr
The absolute value of the complex impedance seen across the load connections of the resistive divider(Z): Z = R*Vz/Vr
The real part of the load impedance (Zr): Zr = Zs*cos(ang) – R
The complex part of the load impedance (Zx): Zx = SQRT(Z² – Zr²)
With these equations we now have total load impedance Z, the resistive component Zr, and the magnitude(but not sign) of the reactive component Zx.
If we know Zr and Zx, we can calculate the SWR and return loss as well:
First we calculate Γ: Γ = SQRT( (Zr-R)²+Zx²)/SQRT( (Zr+R)²+Zx²)
Now SWR: SWR = 1+Γ/1-Γ and return loss: return loss = -20 log Γ
Note: R used in Γ calculation is the characteristic impedance of the feedline not the R used in the divider (in my case they will be the same: 50 ohms).
I used my new XCVR and my short portable dipole down in Ft Pierce Fl, over Thanksgiving. The rig, antenna, feedline and tools all fit in a camera bag and it was easy to setup. I made some 800- 1000 mile contacts on 1 watt and drank a few beers and smoked a stogie while doing it!
Short Dipole strung between two Palm Trees
QRP While Lounging Outside