Vox Kensington V1242 "G" tuner.

[redneckgeek]

I have a Vox Kensington V1242 bass amp in my shop that has no "G" tuner in it. We would like to find one or build one. "Finding" would be easier than "building". I have a message in to Vox USA but so far I've had no return. The parts list refers to L! as "inductor". This would be a tunable inductor of ? value. There is also a transistor (npn) that has a color code of "yellow-Green". Does anyone have knowledge of these circuits or these two components?

[rf7]

You need Q7 from the schematic, listed as 86-5050-2 (Yellow) on my parts list. The inductor is going to be real hard to find.

[52 Bill]

If you give me a day or two, I might have a G tuner or maybe an E tuner assembly that you can modify. I just have to try and remember where they might be stored. I'll PM you if I find anything.

Alternatively I know that I have an oscillator coil that will work as a replacement for the original one. I'll let you know what I find.

[redneckgeek]

rf7. Thanks for the input. I thought the 86-5050-2 was a Vox number?? I see it in some other Vox amps. I wonder if this amp was another of the Baldwin era offerings. I spoke with one of the engineers who designed for Vox in Arkansas back in the 70's Motorola made tons of semi conductors for them to use at that time.

52 Bill. Thank you also, I'm in no real hurry. If you can find an oscillator module or an inductor, I'd love to hear from you. Thanks again.

[R.G.]

I did a quick look at the schemo, and diddled with the circuit a little in the circuit simulator. G on a bass is about 48.75Hz. Unless they designed it for a couple of octaves up and wanted the musician to do things - a lot! - by ear, then they had to run the circuit at 48.75Hz.

I modeled the inductor as a transformer with a 0.5H primary inductor, then messed with the capacitors to get it to 48.75Hz. I had to use an 18uF cap where the schemo says "680nF". So the original inductor is a big inductance - several henries if the original resonated at the fundamental frequency.

The reason I was hitting on 0.5H is that I happen to know that the commonest 1K to 8 ohm transistor output transformer that Radio Shack used to sell and Mouser still sells from Xicon has 0.5H primary inductance and an 11:1 turns ratio, which the circuit sim said worked OK in the circuit. I was after the "here's one to build", figuring you might not find a real one.

If 52 Bill can turn up one to repair, that's the biggest hope. Finding a tunable several-henry inductor is faint hope.

I can tell you how to fake it pretty closely. If you can find a CD4060, you can make a tuner out of it pretty easily. This chip has an onboard oscillator which can be tuned, and a multistage divider. In particular, it can be rigged to divide by 1024, so the 48.57Hz you want appears on pin 15 when the oscillator is running at 1024*48.75 = 49920Hz. It is much, much easier and cheaper to find an inductor and capacitor that will resonate at 50kHz than at 50Hz. Probably a thousand times easier. Single chip, low power, cheap. Mouser Electronics has them in stock for $0.29 each. The inductor for this trick will probably be about a buck. However, I notice that Mouser doesn't carry them any more and Digikey is busy not stocking them any more, so the MBAs may be closing the inductor door, too.

[redneckgeek]

"G" Tuner frequency

Thanks RG. The man I'm doing this for is a friend who loves to collect this kind of thing. He has managed to come up with an original service manual for this amp which contains the following:

"The "G" Tuner is a built in feature of your Kensington Bass Vox Amplifier to give you a more accurate pitch standard for tuning of the bass guitar and other instruments in the band."

"The tuner itself , is a modified Hartley oscillator which has been designed for a high degree of stability. It is adjusted at the factory, with precision equipment, to a frequency of 195.998 cycles per second. This is the tune pitch of the high "G" string on the bass guitar."

Then it goes on to the , "G Tuner Alignment Procedure" which really only tells you how to tune the circuit to the guitar.

Of course, logically, we don't need this at all. It's a labor of love, allot of which involves getting to research this stuff and communicate with you and other interesting people.

On another Vox amp (a Buckingham) there is an "E" Tuner built in. That one we have.
Whether or not we can glean anything of value from it or not I don't know.

[vintagekiki]

Does anyone have schematics for Vox Kensington or Vox Essex?
Thanks in advance
vintagekiki

[redneckgeek]

I have scanned the service document I have for the Kensington Bass Amp. It contains the schematic, layout, and parts list in pdf file. Hope it helps whomsoever.

[R.G.]

"The tuner itself , is a modified Hartley oscillator which has been designed for a high degree of stability. It is adjusted at the factory, with precision equipment, to a frequency of 195.998 cycles per second. This is the tune pitch of the high "G" string on the bass guitar."

Doh!, I forgot the octave!

Low E on a bass is 41.2 Hz. An octave up is 82.4Hz, second fret on the second string. The high G string tunes to the fifth fret on the second string, or 97.998859Hz - again if I punched the right buttons on the calculator.

A half-step is a factor of the twelfth root of two in equal-tempered music, or 1.059463094. A=110Hz is the current musical standard for pitch, so low A is 55Hz, and E is 55 divided by 1.059... five times, five half-steps down at 41.2034446Hz, and that's the low E string on a bass.

One octave up is 82.40688923Hz, and three half steps up gives 97.998859Hz, which is the frequency of the high G string.

One octave up from that is 195.997718Hz, which is what I think the Vox guys were talking about. The G tuner, if we believe that, is an octave up from the note on the high string on the Bass.

That's easier than getting to 49Hz. The inductor value for 98Hz is somewhere near 13Hz with a .68uF cap as in that circuit. Getting to the 196Hz region is about 3.44H of inductance. Still not trivial. You're talking pot cores with tuning slugs and hundreds of turns of wire.

The CD4060 approach lets you set up an oscillator with an LC, RC, or crystal, and then divide by 16, 32, 64, 128, 256, 512, 1024, 4096, 8192 and 16,384 times all at the same time.

Trying to get 98Hz (about) out you can take your pick of running the oscillator at
1568, 3136, 6272, 12544, 25088, 50176, 200704... on up. I'd pick 25088 or 50176. That gives (roughly) 1 1.5mH inductor and a 0.1uF capacitor to get the resonance in the right range for 25kHz or the same inductor and a 0.022uF cap to get to 50kHz.

These are much more reasonable values to hit with a bought inductor.

Let me know if you want to dig into this further. The custom, adjustable inductor is the big problem with building a replacement tuner that's an exact match.

[redneckgeek]

R. G. HOLY SHNIKEYS! Why didn't I hear a "Cosine Theta" anywhere in there? No. The exact replica is not the goal. All we need is a "G" to sound if we turn on the switch. But what if ... the other amp I mentioned , Buckingham I believe, has an "E" tuner. We can get our hands on that. It is tuned to 164.81 Hz or 329.63 Hz (I don't remember which). If the "G" they use is 196Hz could these be the same circuit just tuned differently?? I'm going to play with the "E" and see if I can tune it to "G".

[52 Bill]

The schematic for the E-Tuner has a couple of different part values from the G version. I've never tried to retune one before, if you go that route let us know what happens. The original circuits were hard wired point to point on a small terminal strip. Not very complex but a little messy looking.

I'm still looking for the g-tuner assembly, but in the mean time I've located my box of coils and threw together a G-Tuner circuit to see if they would work for you. I built the circuit as was shown in your schematic and it would only get to F#. I reduced the value of the oscillator cap to 0.66uF (2-0.33uF in parallel) and it tuned to a G on my Boss chromatic tuner. I haven't had a chance to pull out my frequency meter to see what it reads out at yet.

If you want a coil, PM me.

[vintagekiki]

The tuner is factory adljust to a frequency of 195.998 Hz, or the high "G" string on the Bass Guitar. For practical LC calculation may be used Resonant Frequency Calculator
Using Frequency Calculator for high "G" frequency 195.998 Hz and capacitor .68mF required inductance is 0.96968 Henrys.
Kensington bass amp in "G tuner" used inductor Thomas PN 10-5010-4 (550mH - 1.1H)
More practical solution is to replace Hartley oscillator with Colpitts oscillator. Colpitts oscillator used one inductor and the two capacitors, and in this case can be used inductor from wah wah (0.5 Henrys). Necessary capacitance we will get with few smaller capacitance.

Resonant Frequency Calculator
Hartley oscillator
Colpitts oscillator

[R.G.]

The tuner is factory adljust to a frequency of 195.998 Hz, or the high "G" string on the Bass Guitar.

That's one of the points I made. High G on the bass guitar is not 196 Hz. It's half that. High G on a bass guitar is an octave and three half steps up from low E at 41Hz, about 98Hz. (I'm rounding to the nearest Hz instead of typing all the decimals; you'd have to tweak the inductor in anyway.) To be plainer: either they set it to 195.998...Hz -OR- they set it to the frequency of a high G string on a bass guitar. Those are mutually exclusive things, because the fundamental of the G string is 98Hz, about.

Let's recap:
A=110 is the frequency standard for modern equal tempered scales. The low E on a standard tuning guitar is five semi-tones down at 82.406889 Hz. The low E on a bass is an octave down from that at 41.2034446Hz. G string on a bass is an octave up from that plus three semitones. That's back to 82.406889, plus three semitones, which means multiplying by the twelfth root of two (1.059463094...) three times to get to 97.998859...Hz. (I'm doing this on my pocket arithmetic calculator, not on whatever "calculate your note frequency" calculator which may exist on the internet, so I can do it any time )

From here, Thomas could have gone with a real low-G oscillator. Either they did not do this and made it 195.997718... Hz and lied ... er, 'scuse me, made an error about the low G string frequency, or they got confused on the way. Or they counted on bass players not knowing the actual frequencies and tuning by listening and zero beating, which is what they told people to do.

For practical LC calculation may be used Resonant Frequency Calculator. Using Frequency Calculator for high "G" frequency 195.998 Hz and capacitor .68mF required inductance is 0.96968 Henrys.

I'd have to go get out my textbooks to be sure that Mssrs. Hartley and Colpitts don't do anything to the basic 1/2*p*SQRT(LC). But yes, the resonant calculator got it right if you consider only the base resonance of an LC - I checked it on my arithmetic calculator.

At this point I have to insert my philosophical disagreement with using online calculators. Using these things hides the math underlying what is calculated, and used by beginners, they almost always cripple the minds using them so that the underlying math is NEVER learned. I'm all for standing on the shoulders of giants, but there is no excuse for not learning the math that matters to you. End of rant.

What Thomas did was put in an oscillator an octave up from the real G string on a bass. You can get that with 0.68uF and a 1Hy inductor, all right.

But since the frequency of an LC varies with the square root of the inductance, you have to *quadruple* the inductance if you're going to halve the frequency with the same capacitor. That puts the L for a 98Hz LC at 4* 0.96Hy, or about 3.9Hy.

Even doing the octave up, you're left with finding a variable 1Hy inductor. Notice that if you go with Colpitts to use a non-tapped inductor, you're left with the problem of tuning the LC, since it is not reasonable to expect that whatever Ls and Cs you get will land you right on the target frequency. Variable capacitors are almost as rare as variable inductors. Maybe more so. Variable BIG capacitors, in the range of 68nF of variation, are probably much rarer than variable inductors. So to tune the thing in, you have your choice of rare and rarer parts.

My suggestion to go with a high frequency and divide down was based on it being easier to get mH-range variable inductors than either 1Hy (or 1/2Hy) variable inductors or 0.68uF variable capacitors. Last I checked, Digikey has variable inductors in the 1mH range. But not in the 1Hy range.

[vintagekiki]

At the scanned instructions scan0005.pdf will see that the stimer is factory designed and tuned for frequency 195.998Hz or second octave G string on the bass guitar.This is No.12 fret on G string (bass), or open G string (guitar).
Because the problem is to make LC oscillator with non-standard values of L and C, the solution is simply to make RC oscillator, similar as oscillator for the tremolo or vibrato effects (4 – 7 Hz) but with several times higher frequency (98 Hz). RC components is not critical, and oscillator can be adjusted precisely.
sorry for my language

Harmony Central - Pitch vs. Frequency - Useful pitches and frequencies
http://music-electronics-forum.com/a...8-scan0005.pdf

@R.G. About philosophical disagreement
On-line calculator is the starting point for most beginners. When design the LC oscillator it is logical to use Thompson formula for the oscillation frequency calculation. A final (precise) tuning is possible with frequency counter, or with a more accurate guitar tuner.
I've talked about several different values of capacitors connected in parallel. I have not talked about variable capacitors (0.68uF).

[R.G.]

At the scanned instructions scan0005.pdf will see that the stimer is factory designed and tuned for frequency 195.998Hz or second octave G string on the bass guitar.This is No.12 fret on G string (bass), or open G string (guitar).

Yep, I saw that. I was responding to your restating the factory instructions. I have copied the pertinent paragraph here:

The tuner itself is a modified Hartley oscillator which has been designed for a high degree of stability. It is adjusted at the factory, with precision equipment, to a frequency of 195.998 cycles per second. This is the tune pitch of the high "G" string on the Bass Guitar.

I've highlighted in bold type the contradiction. I believe that they DID set the tuner to 195.998... Hz. What they were either mistaken about or glossing over is that 195.998 is NOT the frequency of the high G string of the Bass Guitar. That's an octave down from 195.998Hz.

They didn't say "second octave G string on the bass guitar.This is No.12 fret on G string (bass)", they said "This is the tune pitch of the high "G" string on the Bass Guitar", no octave/12th fret involved. They were incorrect.

I apologize if I'm sounding extra pedantic, but tuners are a standard, and accuracy counts. That's why they're called tuners. It's important to be accurate about tuners.

I think that Thomas Organ did a cheaper solution that they thought was good enough. They used an octave up from bass "G" in their tuner to use a cheaper inductor instead of providing the actual bass "G" string frequency. It *was* good enough, because I bet not one bass player in 100 that bought one of these ever used it. In the first place, it's hard to tune bass by ear, and in the second, most of these bands had an organ player who could not tune his organ. So the whole band tuned to the organ, whatever that was or drifted to. I was there ...

Because the problem is to make LC oscillator with non-standard values of L and C, the solution is simply to make RC oscillator, similar as oscillator for the tremolo or vibrato effects (4 – 7 Hz) but with several times higher frequency (98 Hz). RC components is not critical, and oscillator can be adjusted precisely.

That's one way to do it, but there is a problem. Tuners simply must be stable with time, temperature, humidity, and so on. If they drift, your tuning drifts with them. In general, R-C oscillators are not stable enough to use for tuners. Period. L-C oscillators are kind of almost stable enough, but they require periodic retuning. Old L-C organs need tuned up every few years to sound their best.

Yes, you can use an RC oscillator for the tuner. However, the drift will be bad, and unpredictable.

sorry for my language

No need to apologize for your language. Your master of English is at least as good as mine, and I'm supposed to be a native speaker. I can REALLY confuse people when I try what little French, Spanish, and Japanese I've picked up. It's better I just don't try.

@R.G. About philosophical disagreement
On-line calculator is the starting point for most beginners.

This is what I disagree with. Just as it's important to know how to do arithmetic without a pocket calculator, it is critical that on-line calculators for complex things NOT be used by beginners. A beginner who uses an on-line calculator will NEVER go back and use the actual equations. In fact, they will do ANYTHING to avoid going to the real equations because it's a pain to do so. Humans are either lazy or economical in spending their personal energy, which is two ways to say the same thing. They will NOT go to the trouble to learn the underlying theory if they can just punch in some numbers and get an answer.

I know this is an anachronistic view, but it's what I think. Having learned a few facts, I can generate the notes of the entire even tempered scale with a pocket calculator, and even, given time, by hand and pencil and paper. So I don't need a working internet connection to do technical work. Notes of the musical scale, tuning frequency of an LC, rolloff of an RC, Ohm's law, electrical power, solutions to quadratic equations, and on and on and on. Yes, these things are a PITA to learn. Fortunately, most people can ignore them. Beginner techies simply can't. Not learning them leaves a hole in their knowledge that they will never fill.

When design the LC oscillator it is logical to use Thompson formula for the oscillation frequency calculation. A final (precise) tuning is possible with frequency counter, or with a more accurate guitar tuner.

I agree completely. What's important there is that the inductor or the capacitor be tunable. Or both can be tunable. But tuning is necessary to tune out the tolerances of the parts.

I've talked about several different values of capacitors connected in parallel. I have not talked about variable capacitors (0.68uF).

Yes, I know. I was trying to be clear to the readers. I know that you know this, but once a builder of this kind of tuner gets his parts and puts the thing together, the finished product will not be at 195.998Hz, or 97.998859Hz. It will be a few percent off because of the tolerances of the real parts.

If you have a fixed capacitor, you can tune the inductor. If you have a fixed inductor, you can tune the capacitor.

If you select tuning the inductor, you can do this by buying an inductor which has a magnetic slug which can be turned in and out of the core for tuning. This is a standard way for buying many inductors.

If you select tuning the capacitor, you cannot get variable capacitors larger than a few hundred picofarads. So if you choose to tune by changing the capacitor, you must experimentally solder in capacitors to make the capacitance bigger by adding capacitance in parallel. Notice that do do this, you need to
(1) buy a main cap that's too small for the frequency you want, so that even the biggest capacitance it could have (i.e. +5%, +10%, whatever) is too small for the correct frequency. If your main cap is already too big, you can't make it smaller in any practical way. Yes, you can put a cap in series with it, but this is much more difficult to get right because of the unsoldering to change capacitors instead of just paralleling new capacitors.
(2) Buy an assortment of add-on caps. You could measure the frequency, buy one capacitor to tune it in, solder that in, measure, then go get another adjustment cap. That will take many days to obtain one cap at a time. So I would buy an assortment.
(3) the assortment of caps needs to be of a fineness to get you within a few cents of the final frequency needed.

Sidebar: Cent! AGH! What's a cent?

It's 1/100 of a semitone. This corresponds to a ratio of 1.0005777895, the 1/1200th root of two. Musicologists will tell you that six cents is a just noticeable difference in pitch. So that's a good target for the accuracy needed in a tuner. Notice that this is only if you intend to do it well. If one doesn't do a tuner well, one might as well use RCs for the oscillator.

Six cents is a frequency ratio of 1.0005777895 to the sixth power, or 1.003471745 if I punched the right buttons on my arithmetic calculator. Call it 0.35%. That's the maximum you can allow with drift over time if you are not going to allow periodic retuning.

So you need to tune your non-drifty LC oscillator to less than 0.35% accuracy to allow for drift.

If we assume you can buy 5% capacitors easily enough (I know this to be the case: you can get 1% and better, but they are quite expensive.) then you will need to tune out as much as 10% variation in the inductor. This is 5% variation in the inductor (if you can get 5% inductors) and another 5% in the capacitor. And you need to get the resulting error in tuning under 0.35%, probably under 0.2% if you want it to be stable over time with drift.

That 0.2% as the smallest error means your smallest parallel tuning cap you're going to put on the main tuner must itself be less than 0.2%/10% =0.02 of the main cap so it can be the smallest step. And the maximum tuning will be 10%, so you need 10%/0.2% or 50:1 for a range. If you pick a binary sequence for tuning caps, you'll need six tuning caps of values which are binary related, and starting with 5% of the main cap down to under 0.2%. This is about optimum for a tuning setup by adding a cap, then testing.

I'm familiar with tuning oscillators by adding on discrete caps. I HATED it. It's far, far better to have a tunable inductor or capacitor than to add tuning components. Since you can't get tunable capacitors, it's far better to get a tunable inductor. Since you can't get BIG tunable inductors, IMHO it's better to get a small tunable inductor and divide the frequency down. Hence the CD4060 solution.

But RNG had an even better idea:

But what if ... the other amp I mentioned , Buckingham I believe, has an "E" tuner. We can get our hands on that. It is tuned to 164.81 Hz or 329.63 Hz (I don't remember which). If the "G" they use is 196Hz could these be the same circuit just tuned differently?? I'm going to play with the "E" and see if I can tune it to "G".

Since the factory tuner is at 196Hz, if the "E" tuner version is really 164.8 or 329.6, you could mess with the main tuning cap on the "E" tuner to make it be 196. If the "E" tuner is 164.8, you decrease the main cap a little and tweak it in. If it's 329.6, you double up the main cap and then tweak it in.

My Buckingham schemo shows the main cap as 0.22uF. That argues for it being 329.6.

I bet you could stick in another 0.22 and a 0.1 or so as needed to get close, then tune the inductor to get you to 196.

However, I would be happy to finish the design of the CD4060 solution. I roughed it out last night. Something like a 4mH adjustable inductor, two 2.2nF caps and the 4060 oscillator gives you an output at 98Hz, and it's tunable with the inductor.

I did some more digging on line. It looks like tunable inductors are no longer stock items at digikey or mouser. However, you can get pot cores and tuning slugs to wind your own, so that's an option.

Even cheaper, you can use a $1 microcontroller and a $2 crystal and put out the correct frequency.