The reactance tube needs to deviate the center frequency up and down equally and linearly. Otherwise, there will be distortion. I found that the value the 6C4's cathode resistor was very critical in determining this as viewed on the spectrum analyzer. In my experiments, the cathode definitely did not want to have a bypass cap. Also, I've never seen a pentode used as a reactance modulator. I think it is the plate-to-cathode capacitance that gets varied by the grid voltage. I'm not certain but maybe the presence of screen and supressor grids might inhibit the linearity of this action.

With regard to the slug material...the presence of ferrous core material (iron, steel, ferrite) will increase the inductance (and lower the frequency) while non-ferrous metals (copper, brass, aluminum, etc.) will decrease the inductance and increase the frequency.

Dave

Interesting because most of what I saw were pentode reactance tubes. For example, the datasheets for 6AN8 and 6AZ8 specifically list reactance tube as applications for the pentode.

I'm certainly no expert but had also never seen one configured like yours and presume it works the way you say. But all the ones I've seen, triode or pentode, have a phase shift network off the tank feeding the grid. That causes an out of phase plate current injected into the tank simulating a reactance, which can be either inductive or capacitive depending on the network, like this page explains http://www.vias.org/basicradio/basic_radio_34_02.html As I mentioned in the first post, I plagiarized the basic mod/osc configuration I'm using from an RCA patent, although I've changed it up some.

Btw, the reason a 6CU8 was my initial 'dollar days' choice is because it's listed in the 1972 RCA 'Application Guide' under reactance tubes but I don't know what criteria they use to decide one is 'good' for a reactance tube while others aren't in the list.

Well, that's what I thought but the screw is steel and frequency goes up, so you can see why I was surprised. Well, let me revise that. I thought they were steel but didn't check and Occam's Razor says that's a better explanation than inventing a new theory.

Good point but I think I made a rash assumption and it's not steel.

There seem to be at least two basically different reactance tube circuits. It appears that Dave's circuit makes use of the tube's inherent interelectrode capacitance which is higher in a triode. The second type of circuit incorporates an external capacitor between plate and grid, and this can work with either a triode or pentode. In my AM stereo transmitter, the reactance tube phase modulator uses a variation of this second type with the external capacitor and pentode (or tetrode).

I also found that the tube bias is critical to get equal deviation on either side of center. In my phase modulator circuit, it turns out that phase shift is a very non-linear function of tube transconductance, and transconductance happens to be a very non-linear function of grid bias. These two non-linear functions bend in opposite directions though. So, by choosing the proper grid bias and screen voltage, it's possible to have the non-linearities cancel each other out over the operating range.

Now that I've done some internet searches and reading about reactance tube modulators, I see that most typical circuits do use pentodes. But I've noticed that AFC circuits in FM tuners usually use a triode, often a 12AT7...half of which is used as an oscillator and the other half for AFC control. So my transmitter circuit is basically similar to an oscillator/AFC section in a tuner but with the AFC control operating at audio frequencies. Wondering what advantage a pentode has over a triode when used as a reactance tube modulator?

Dave

RCA used a pair of 6AQ5s modulating a third one in their BTE-10B exciter. Also, the screw mentioned is likely not steel but nickel plated brass, and as was pointed out, brass lowers inductance.

Scott Todd

Yep. During my research I picked up the instruction manual and schematics for that one. They're also using them push pull with demodulator negative feedback.

I cut one open and they're not brass.
These things are 30 year old 'left overs' from an electronics shop and I think they're non magnetic 18-8 stainless steel so it may be a case of yes they are (steel) but no they're not (magnetic).

I've seen circuits similar to Dave's using transistors but I couldn't find anything giving tube capacitance vs grid voltage.
This is an area I'd like to learn more about and all the sources I've read always seem to leave 'something' out.

If I understand correctly you're phase modulating a crystal osc and from what I've gleaned that also varies with the modulating frequency. At least, that was in the description of the Armstrong system and why audio went through a "pre-distorter." In short, I'm not sure how things translate from PM to direct FM, like the reactance tube circuit I'm using.

Do you have a source of curves for the shapes mentioned? I mean, where on the gm curve are you operating that cancels what shape? All the descriptions of reactance tube direct FM I've seen imply operating on the (middle) 'linear' portion of the gm curve but, then, tank frequency is the square root of LC so I have trouble seeing how a linear reactance change produces a linear frequency change. On the other hand, one reference I have gave two equations for supposedly the same thing, one linear and the other with a square component, so all it did was confuse me more. I pretty much went "RCA did this so I will too." I.E. Engineering rule #1: if at all possible, plagiarize

Now, there is definitely AF non-linearity in the pentode because they aren't known for linearity to begin with and Rk doesn't provide enough degenerative feedback to make much of a difference, which is why I did Rk negative feedback. This has the effect, though, that the injected reactance current is no longer a function of grid voltage, per see, but is being directly modulated since sensing across Rk makes the thing essentially a variable current source. The net result, it seems to me, is that not only is AF linearized but the gm curve should be too since if gm deviates from linear then so will current through Rk, which NFB will force back to linear (we are assuming screen current closely follows plate current by an essentially fixed ratio). Or, put another way, the gm term is mostly removed from the equation since its purpose is to reflect how much current is flowing in the reactance circuit but that is now determined by NFB around Rk.

The desirability of that is based on my understanding that the reactance tube descriptions I read wanted linear operation and, as I noted in a previous post, when feeding test tones to the transmitter and monitoring the output of an FM radio (frequency response test) I do not see any distortion but, then, my eyeballs can't discern fractional percents on the scope.

The only time I detect obvious distortion is when I screw up the osc in some way, like overloading it with too much antenna coupling.

What do you think?

This is new stuff for me so I don't really know but, speculating, it could be to reduce the inter-electrode capacitance effects because the 'controlling' function is intended to be the phase network. Another generic difference I can think of is triode plate resistance is dramatically lower but whether that matters I don't know. AFC also doesn't care much about linearity but don't take that as saying yours isn't. I'm just noting the function itself doesn't care.

And, then, pentodes have more gain. If it weren't for pre-emphasis and inherent non-linearity the 6BL8 had enough sensitivity all on its own and I ran it that way, at first, using a PC for pre-emphasis. The 6KE8 is 12 dB more sensitive.

I like to give things funky or cute names so I think my BIC pen tunable coil shall be called the "PenTune."

Well, I soldered in yet another 5-turn 0.5" diameter air core coil (wound with #18 magnet wire) today on my "New Band FM Transmitter", and I have the tap at 2 turns from the coil bottom. Without the pickup coil yet in place, I could decipher the signal from the transmitter throughout the entire FM band on my Califone AM/FM/CD boom box. On a side note, it takes about 25 seconds for the oscillator to warm up after the LED comes on (upon power up). Now I'm where I left off.

I don't understand what "throughout the entire FM band" means. The transmitter splatters signal all over the RF spectrum, or the boombox front end is swamped, or you can 'tune' the coil by squeeze/pulling it?

By "decipher" you hear something? Does it sound right?

Flipperhome said: "I don't understand what "throughout the entire FM band" means. The transmitter splatters signal all over the RF spectrum, or the boombox front end is swamped, or you can 'tune' the coil by squeeze/pulling it?

By "decipher" you hear something? Does it sound right?"


It means that the carrier (from the xmitter) was picked up from 88Mhz to 108Mhz on my Califone AM/FM/CD boom box. On my transmitter, I am using a 3-9 pF variable air cap that's mounted on the transmitter front panel to adjust the frequency.

Yes, I hear something, but I'm not sure that it sounds right. It sounds right now more like a jammer than anything else. When I put my finger on the audio input jack (on the transmitter front panel), the signal is then gone.

Take note that the pickup coil isn't yet in place, and there is no antenna.

I could hear the signal from the xmitter anywhere on the FM band on my Califone AM/FM/CD boom box by turning the 3-9 pF air variable tuning cap that's mounted on the transmitter front panel.

Please pardon my density but that still confuses me. I'm going to presume you mean it comes in on the boombox at the frequency you tune the transmitter to and not across the entire dial all at once. In which case it would seem you have an oscillator.

That description sounds like mine before I put in caps across the power supply rectifiers. Past that I'd check B+ ripple and scope out the audio section to verify all that was working properly.

Flipperhome said: "I'm going to presume you mean it comes in on the boombox at the frequency you tune the transmitter to and not across the entire dial all at once. In which case it would seem you have an oscillator."

You presumed right. I have an oscillator. I checked for DC ripple before, but I'll check it again. I'm reasonably sure that the audio was wired to specs. Hmmmm......This is where I left off earlier in the week.

Thanks Flipperhome.

Sincerely sorry I can't go "ah hah" right off

You didn't mention the rectifier caps. You do have them, right? I had heard one would get 'buzz' without them but, good golly Miss Molly, it sounded like a jackhammer with audio barely able to sneak in the background.

You might also want to consider your circuit grounding because at 100MHz a wire isn't just a wire. I've got my mod/osc starred to one point (with local .1 uF filter), the audio to another, and then the two are independently brought to PS ground. Even so I'm still picking up some faint residual hum because the breadboard is open air with no shielding.

Have you got an earth on it?

Yes, the power supply is well filtered (has filter caps). Yes, I do have everything wired to earth ground inside the transmitter enclosure, and the transmitter does have a two conductor grounded plug. No, I haven't yet plugged the transmitter into a grounded receptacle (since most of my home receptacles aren't grounded), but there is one in the room. It's just that it's not located in a convenient location, but I can use it nevertheless. Yes, I'm aware that I might have to install some additional cap here or there (as per the high freq).

Thanks for the valuable info Flipperhome!

There is a very detailed analysis of the phase modulator circuit that I used, and I can send it to you, but it doesn't exactly apply to the more conventional reactance tube frequency modulator circuits that you are using.

The simplest reactance tube circuit to analyse, is a pentode with a capacitor Cr connected from plate to control grid, and a resistor Rr from grid to cathode. This RC network is a phase shift network such that if an AC voltage is applied between plate and cathode, you will get a signal at the grid that is phase shifted (leading). Now, the plate current is equal to the grid voltage times the tube's transconductance. Because the grid voltage leads the plate voltage, the resulting plate current will lead the plate voltage. Hence, if you connect the plate & cathode to two points in an external circuit, they will look like a capacitor. Now, if you change the grid bias, this will change the transconductance, and therefore the effective capacitance will also change.

The relationship between plate to cathode current Ipk, and plate to cathode voltage Epk is:
Ipk=Epk*(Gm*Rr)/(Rr-j(1/(2πfCr))
where Gm is the tube's transconductance.

This means that the plate to cathode impedance Zpk is:
Zpk=Ipk/Epk=(Gm*Rr)/(Rr-j(1/(2πfCr))

In the special case where the resistance Rr is much smaller than the reactance of Cr at the operating frequency, this simplifies to:
Zpk= -j*[(1/2πfCr)/(Rr*Gm)]
which is purely capacitive, and results in an effective plate to cathode capacitance essentially independent of frequency:
Cpk=gm*Rr*Cr

However, this results in extremely low component values at VHF. So it's probably better to use more practical component values, use the more general formula for Zpk, and deal with the frequency dependence.

For the 6KE8, there is a transconductance vs. grid bias curve on the last page of this datasheet:
http://www.mif.pg.gda.pl/homepages/fran ... 6/6KE8.pdf

I've transcribed it into this table:

You can transfer the data into a table on a spreadsheet, and then apply the various circuit equations to the data, then create a chart so that you can see the linearity as you adjust various circuit parameters. This is what I did in order to design my phase modulator. If I get a bit of time, I'll see if I can set up a spreadsheet for your circuit, or at least a simplified version of it.

That's what I was thinking based on the reading I'd done. I was just curious what non-linearity you were correcting with the gm non-linearity.

Other than some nomenclature variations that's the same derivation I read in "FM Transmission and Reception," Rider and Uslan 1950, but after being complete enough to remind the reader tank frequency is the inverse of 2PiSQR(LC) they neglect to mention how varying C produces linear frequency modulation.
There's an interesting 1942 ARRL "Course in radio Fundamentals," though. Interesting because it includes a 'lab' where the effect of changing component values is explored and they provide graphs of what you should see. And as long as R is low enough frequency deviation looks suspiciously like the gm curve.

Yeah, that's what I basically eyeballed to set bias. But that's also open loop and I'm sensing Rk so NFB should further flatten it out even if I'm slightly off 'optimum'. This was really the meat of my question, what you thought about the NFB effect on gm and overall linearity.

That would be cool.