Radiophile valve transmitter

[Terry]

From a Google* search, the general consensus is that it appeared much later than that, in the May 1989 issue of Electronics Australia.

Pdf is available here: http://messui.the-chronicles.org/valves/VR198905.pdf

A lower quality copy can also be found on UKVRRR, a variant with an EM84 modulation level indicator appears in the same thread!

* Other search engines are available.

I was interested to find the following disclaimer on the National Library of Australia's archive page relating to Electronics Australia: "Members of Aboriginal, Torres Strait Islander and Maori communities are advised that this catalogue contains names and images of deceased people. All users of the catalogue should also be aware that certain words, terms or descriptions may be culturally sensitive and may be considered inappropriate today, but may have reflected the author's/creator's attitude or that of the period in which they were written."

[Aerodyne]

Just for clarity, my article describing a pantry transmitter is not the one sought by Ian.

Tony Thompson

[Mike Watterson]

Almost any AM Local oscillator circuit can use Octode/Heptode OR Hexode + triode, as long as similar HT, LT, etc as long as two of the sections of a Heptode/Octode/Nonode Mixer/Osc are wired in a triode type oscillator using cathode and first grid as feedback.
Then if you replace IFT mixer out anode load with a tuned MW loop and the RF in with AF in, you have an AM modulator.
A two valve version using as the mixer almost any Pentode (Osc Anode to G2, audio to G1) , Hexode, Heptode, Octode etc and any valve as the triode Osc (tie G2 & Anode) will often work in the same circuit.

Above 30MHz it's better to use a triode or triodised "self mixer oscillator" with an AF driver and RF out buffer amp. (dual EF80, ECC85 and maybe an EF80 or EF86 driver).

The ECH81 is readily available, as is the NOS Russian equivalent. It will work down to 14V HT (use a car radio circuit) with positive bias. I'd probably use two so as to have an AF preamp and RF buffer/Driver. A single 12V or 24V PSU is viable.

[Terry]

I designed the MiniMod ...

... and a very god job you did of it too, Ian, by all accounts.

I note that the design reappeared in UKVRRR for anybody who is interested ...

[radioman]

Chas has published a number of valve-based 'pantry' transmitters over the years, but I think the one the OP is referring to was based on a circuit from long ago...
My Radiophile mags are randomly stored in assorted piles/boxes/locations so not easy to search through unfortunately.
For something else, this 6AS6 based design looks interesting :-http://www.radiomuseum.org/forum/hi_fi_a...itter.html

Andy

[ppppenguin]

The designer of that TX on Radiomusuem has done a very thorough job. The 6AS6 is a relatively unusual valve, design for dual grid control: http://www.r-type.org/exhib/aaa0065.htm so it's unlikely a commonplace EF91 or EF80 would work so well. Some frequency changer valves ought to work.

[Mike Watterson]

Joe Sousa is a very expert Electronics Design Engineer of ICs and has a website on Philbrick history. He is vintage radio collector, vintage valve collector and experimenter.

The 6AS6 is based on curiosity rather than a pragmatic approach based on common NOS valves. You'd need a heptode or at minimum a hexode. While I correspond with Joe and think he comes up with great stuff, I'd not build that one.

So build a slightly different design based on ECH81
Search 6I1P / ECH81 / 6AJ8 tubes. Lot of 10 pc. NOS!
Very many NOS East European suppliers.
The Triode section uses identical circuit to any AM Radio. Easily adapted for crystal, but a double gang tuning cap can tune triode and other half the loop on Anode of the Hexode section, the grid of which is fed with Audio instead of RF from an aerial.

If you want 12V or 24V (better) look at 12V ECH83 car radio mixer/osc. Same valve. You do need two to run the heater off 12V. So the second one can be AF amp and RF amp, or some other 300mA 6.3V valve.
Or use 2 x EF80 or 2 x EF91 as pentode mixer and triode Osc, though you'd want a normal HT I think.

Here Joe Sousa discusses advantage of ECH81 over heptoe/octode
http://www.radiomuseum.org/forum/ech81cu...ces.html#1

[Semir_DE]

Hello everyone,

I realize this thread is about three years old, but just in case someone here is interested I had built an AM transmitter/modulator using the method described by Joe Sousa with the 6AS6. In my design I used an EF80, however, as this is fairly common and easy to obtain. It can be modulated at G3 and by using the differential method with an output transformer it will produce a very nice linear modulation up to 100%:

   

Please check out my article on Radioforum.org:

https://www.radiomuseum.org/forum/linear..._ef80.html

It is in German but if there is interest here I would be happy to translate it to English.

Since I have many P- valves I designed the whole transmitter using P valves (apart from the EF80) like PCF80 and it has a PM84 as a modulation level indicator.

This design will also work very well with EF89 and EF85. The EF183/184 valves do not work well with this method, however.

I also have a version that has an output stage that uses a PCL200 that delivers about 1W PEP. Here is an image of the transmitter:

   

In the image you can see the PM84 in front. IN the back row from Left to right:

PCL200 linear output RF Amp, EF80 AM modulator, 90V Stabilizer

The middle row:

PABC80 amplifier and rectifier for the PM84 indicator, PCF80 AF amp. and carrier oscillator.

Looking forward to your feedback

Semir

[Murphyv310]

Hi.
I like making little pantry transmitters and have done a few with Rod Pentodes. Would it be possible to post up the translation and some info on the other ones you've done, or PM the info to me please.

[ppppenguin]

That's an unusual looking technique, with the output transformer connected between the screen and anode of the EF80.

Using google translate:

realized this project in 2016. The project Hi-Fi AM Transmitter presented here by Joe Sousa had me jumping in with the 6AS6.

Before that I did not know the possibility of modulation via the G3 of a pentode in this form. I was fascinated by the possibility to achieve a 100% modulation with good linearity. Unfortunately I had no 6AS6 at hand when I started my project. That's why I studied tubes like EF80, EF85, EF89 and EF184 and measured their behavior using G3 modulation. Before I show the results of my μTracer, I would like to show you how this modulation principle works.


With a pentode, as is well known, the G3 (brake grid) is at cathode potential. Thus, it prevents slow electrons that are "beaten" from the anode as secondary emission by the fast electrons coming from direction G2 to fly back to the grid G2 and thus reduce the anode current. Without this measure, the anode current would decrease in spite of increasing anode voltage in a certain core line range of the UA / IA characteristic. This leads to the so-called "tetrode kink" in the UA / IA characteristic of a tetrode. With a Pentode, this kink does not occur at all, thanks to the G3.

However, for the faster electrons accelerated by G2 accelerated by G2, the G3 is usually "pervious". Now, if the G3 continues to be made negative, even electrons coming from the direction of the cathode are prevented from reaching the anode. They are then absorbed by the G2 and increase the G2 current. With very negative G3 and low anode voltage, the anode current can almost completely disappear in favor of a higher G2 current. The following picture shows an EF80 at 90V anode and G2 voltage. The potential of G3 (not G1!) Is shown on the X-axis. G1 is fixed at 0V in this measurement:

As the picture shows, the anode current decreases with G3 becoming more negative while the current of G2 increases proportionally. With the same scaling of the G2 and anode currents, in the area where the two characteristic curves cross, the anode and G2 currents are the same. In other words, their difference = 0.

My measurements with the μTracer showed that the effect at about 90V at G2 and anode can be used quite well, because here are larger linear areas to the left of the crossing point to find a linear modulation allow. My experiments have shown that the region in which the G2 current is higher than the anode current usually has better linearity. At voltages lower than 90V, the usable linear part of the curves decreases. A higher voltage results in flatter curves, which requires a higher drive voltage at the G3.

In the appendix I show some of my measurements with different tubes.

The effect can be used to 100% suppress carrier oscillation in AM modulation. It only needs to be formed by means of a transformer, the difference of the anode and the G2 current. Here is the basic circuit:

In the circuit I use an EF80 is operated at about -13V G3 voltage. Here, the G2 stream outweighs as can be seen above. At the same time, the NF modulates the G3. The G3 voltage varies by +/- 2-3V by the -13V mark thereby increasing the G2 current at the positive NF peaks and the negative G2 current being equal to the anode current. A carrier which is supplied to the G1 makes these fluctuations also with. If the anode and G2 currents are the same, the carrier in the transmitter is extinguished because then the difference of the currents = 0. A further increase in the modulating NF level results in a renewed increase in the carrier amplitude when passing through the zero point, since then the anode current is higher than the G2 current in the transformer. The following images show the modulation trapezoid for near 100% modulation and 110% overmodulation. Here you can clearly see the rising carrier amplitude on the right.

100% modulation of the EF80 circuit with difference G2 / anode

about 110% modulation under the same conditions.

Practical design, the circuit diagram is attached:

Since I have many P-tubes that are waiting for a sensible use, I have designed a circuit that uses a PCF80 as NF amplifier and oscillator in addition to the EF80, which represents the actual modulator. The modulation is indicated by a magical band PM84. I have solved with a PABC80 the necessary amplification of the NF signal and its rectification. Here is a circuit description:

The triode of the PCF80 V1A amplifies the audio signal to approx. 5Vss. This level is necessary to control the EF80 at the G3 enough. The 1k5 resistor in the cathode branch serves as a negative feedback to keep the harmonic distortion low. The pentode of the PCF80 V2B runs as an oscillator. Here I use again a ceramic as a frequency-determining element