5A6 and 3Q4 for ~10W TX

[IN3IEX]

I would like to build a tube 10W AM CW TX battery powered (6V).
I have some 5A6 and 3Q4 and I am thinking about something like:

5A6 eco oscillator VFO - two bands between 3.5 and 10.5 MHz fundamental frequency.
2 x 5A6 parallel class C amplifier, fundamental frequency.
3Q4 screen grid modulator driven by a carbon microphone with step-up transformer.

2 transistor inverter for 6V->150V

I have some preliminary questions:

Can I use a simple inductor as load for the VFO tube (no selectivity)?

All filaments of the tubes are center tapped, is it better to manage the battery as +- 3V and center tap all filaments to ground? (with resistors for each tube to reduce the voltage)

How can I key the RF output tubes for CW if I cannot disconnect the filaments from ground?

These tubes look great for tube HF QRP, but few info around, are there any old design to elaborate on?

Many thanks

Giorgio IN3IEX

[kb3ouk]

For the 5A6, the datasheet says you can run the filaments in series by putting 5 volts at 230 ma on pins 4 and 5, or in parallel by tying pins 4 and 5 together and putting 2.5 volts at 460 ma between those two pins and the center tap (pin 9). For the 3Q4, you can run the filaments in series with 2.8 volts at 50 ma on pins 1 and 7, or parallel with 1 and 7 tied together and 1.4 volts at 100 ma applied betwwen those two and pin 5, which is the center tap. I would use seperate filament batteries that are just hooked to the tube filaments only, run the filaments in series, then that way you should be able to key the finals by grounding the center tap.

[KA3EKH]

Something you may want to look at is the old WW2 Navy designed DAV or MAB radio, somewhat similar only using three tubes with one as an oscillator (1T4), second as a PA (3S4) and third as a modulator (3S4). The 3Q4 and 3S4 just about the same tube but in the DAV that I have on 3885 using a flock of 9 volt batteries in series to produce 135 volts on the plate don’t think I get much more than a quarter watt. I have used the old DAV for several years in Dayton on the WW2 radio net that takes place during the hamvention and always happy on how well it works and the design is way better and easy to work on as opposed to the BC-611 hand held radio. This is a schematic for the DAV and maybe you can find a better version or the entire manual on the web.

[KM1H]

Ive had good QRP luck in the past with 3A5's and 3S4's but have never even seen a 5A6.

Carl

[IN3IEX]

Hi all,

5A6 was used in PRC 8/9/10 VHF transceivers. Single tube FM with AFC  oscillator for 1W out.
A parallel of two 5A6 class C could provide 8 or 10 W on HF bands: very interesting !
Filament OFF during receive and low filament consumption: very interesting.
During RX this QRP TX will be completely switched off, then go trough: "filament on" - "filament on with vfo" - "filament on with full TX" with a 4 position rotary switch, that will be the "manual" sequencer.
I am still elaborating on the possible circuit.
5A6 is a tube with potentials, I think.

Giorgio

http://hilltoparmyradios.com/TM%2011-612%20PRC-10.pdf