All-Band Valve Transmitter
This page describes a project that is nearing completion, and
follows my decision to abandon an attempt to make a broadband valve
transmitter using untuned stages. As Colin, G3VTT had correctly predicted
at the start of that project, I was indeed unable to get enough voltage gain using untuned ferrite
cored transformers. But I had fun trying, and
I re-learned a lot about valves along the way. The short term objective was to make a 30 m (10 MHz) valve QRP CW transmitter while keeping in mind my eventual desire to produce a multi-band transmitter. I have now taken that next step to make this a 10-band transmitter, covering all bands from 160 to 10 m. A variable frequency crystal oscillator (VXO) operating at signal frequency is used to control the transmit frequency. This requires the use of modern, often tiny, quartz crystals in the EF80 valve oscillator. The key requirement was to use a VXO configuration that keeps the crystal current low to avoid damaging the crystal, and to prevent frequency instability.
Variable Crystal Oscillator (VXO) Here's a photo of the early prototype crystal oscillator, including the prototype power
supply:
Following tests with several different oscillator circuits using crystals stocked by the G-QRP Club, I took the best aspects of each circuit to produce the oscillator design shown below. From the outset, I sought to use plug-in crystal units because this would provide some of the nostalgia associated with the early 'crystal controlled' valve transmitters that used the large plug-in crystals. I figured that the large housing (or shroud) available for the connector would permit installation of several small components such as multiple crystals, a miniature switch, and a small inductor that, together, could be used to determine the VXO range within a single plug-in unit (Pins 12 and 24). As the design developed, I also determined that some of the oscillator
feedback components could, if required, be placed within the plug-in unit to
provide the correct level of feedback for the selected band (Pins 10, 22,
and 24). But it didn't end there. I also decided to make the
plug-in unit 'band aware' by using spare pins in the connector (Pins 1-5,
and 14-18). I also made the connector 'state aware' by extending
the transmit/receive status to Pin 19 which is grounded whenever the
transmitter is in the 'Spot' and 'Operate' conditions.
This electron coupled Colpitts oscillator draws about 3 mA from the HT supply, and generates
about 10 V p-p on 10 MHz at the output, dropping to 5 V p-p on 28 MHz.
It is possible to short the output to ground with no discernable change in
frequency. This is the beauty of the electron coupled oscillator!
Plug-In Crystal Units Below, I have shown some example plug-in modules that provide a useful VXO range. Some modules will rely on the internal capacitance of the EF80 plus the in-circuit 22 pF fixed capacitors to provide a suitable level of feedback to start and maintain oscillation. Other band modules may require additional components within the module to achieve reliable oscillator start-up (pins 10, 23 and 24) and the required VXO range (pins 10 and 12). In particular, note the possible use of a series coil to bring the centre frequency lower in frequency, and the trick of using parallel crystals to raise the centre frequency. There is considerable scope for experimentation with different configurations, including the use of parallel crystals having different resonant frequencies up to, say, 20 kHz apart.
Here are some photos to illustrate how I mounted a miniature toggle switch within a 25-pin connector housing. Firstly, a piece of plain matrix board is cut along the perforations to produce a small piece of board of 4 clear holes by 8 clear holes (12 x 22 mm). A 6 mm hole is then drilled in the centre of the board, and the switch mounted in the hole. This sub-assembly then drops in the plastic shell. The board is a little undersized, but you can decide how precisely you wish to cut the matrix board, or similar material, to fit the space. Then it's just a case of wiring in the crystal array and screwing together the two halves of the shell.
And here's a closer look at my 12 m plug-in module:
Note that, in this case, a coil is used in series with the 24.906 crystal to pull it low, providing a large VXO range. The switch is used to provide a second VXO range by shorting out the coil. The result is that this single quartz crystal covers nearly the entire CW segment of 12 m, with a useful overlap between the two switched ranges.
And here's how I squeezed 5 crystals, a switch, and a 10 pF capacitor into one of my 40m plug-in modules:
Grid Tuning Module To get enough drive for the PA, two driver stages are required, with a tuned circuit at the grid of the pre-driver (EF80), the driver (5763), and the power amplifier (5763). Relay-switched grid tuning modules are used to select the required parallel tuned circuit for each of the three stages. Each grid tuning module uses ten Omron G5LE-1 relays (one per band). The relays are powered from a 5 volt 3-pin regulator, and the appropriate relays are operated during all three Spot/Standby/Operate states via the Band Select pin in the selected plug-in crystal unit.
The resonant frequency calculator at: were very helpful when calculating the number of turns required on the toroid cores.
This photo shows one of the three grid tuning modules.
Pre-Driver
Driver
Power Amplifier This PA is intended to provide 5 watts RF output. In practice, the power output falls off above 18 MHz to about 4.5 watts on 21 MHz, and 4 watts on 28 MHz.
Pi Network
Because this transmitter has been designed to work into a load of about 50
ohms, a large value variable capacitor for adjusting the loading over a wide
range of output impedances was not required.
Instead, fixed-value loading capacitors are selected using switches
SW1 and SW2.
The PA LOAD switch is used to select one of two loading capacitors
appropriate to the 10 to 80 m bands. A significant amount of iterative
tests were carried out to find values of inductance and loading capacitance
that provided close to 5 watts output on all bands. (By using
optimised values, a lot more power output was possible, but I selected
values that provided the desired power output into 50 ohms.) An excellent Pi network calculator can be found at: When using the above calculator, I guessed the main variables as being: Rin = 4000; Rout as 50; and the loaded Q (working Q) as 13. The
calculator was used only to provide a starting point for winding the Pi
Network coils. The final number of turns on the toroid cores was determined
through iterative adjustment of each coil, starting with 10 m. Keying and Grid Control Circuit This keying circuit provides the interface from an electronic keyer
(or straight key) for keying the Pre-Driver, Driver, and PA stages using
grid block keying. The Keying Out line is used to key an
external sidetone oscillator. The circuit also provides a
Power control for varying the transmitter power from 0 to 5 watts RF output.
The keying circuit ensures click-free CW by controlling the waveform rise and fall times to all three keyed stages. The result is a pleasant CW note having rise and fall times of 3 to 4 ms.
Transmit-Receive Control Circuit
Production Version These photos were taken during the design of the mechanical layout.
And below, you can see the main aluminium parts: all wire-brushed;
cleaned in Brasso; rinsed in soapy water, and drying in the sunshine.
Here are the 10 prototype plug-in crystal units used to align the transmitter. More work is required to optimise the VXO range and centre-frequency of each plug-in unit.
On 19th March 2023, I made my first QSO with the partially completed TX. I had a nice chat with Fred F8FXA on 10.116 MHz. I was running 5 watts of RF to a doublet, and Fred was running 4 watts from his K2, also using a doublet antenna. On 20th March 2023, more 'firsts' were made, as follows: More photos of the transmitter are shown here. Click on the thumbnail to enlarge.
Power Supply The circuit of the prototype power supply is shown
below. Because I no longer have any HT transformers, I used two 30
volt transformers back to back to obtain the HT voltages.
Note
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