The tubes are all 1920s battery tubes - British Cossor brand. The oscillator and the first speech amp are Cossor 210HL Triodes, the buffer is a Cossor 215SG Tetrode and the final PA is a push-pull pair of Cossor 215P Power Triodes, neutralized.
The American equivalents to these tubes would be an '01A Triode, a '22 Tetrode and a '71A Power Triode.
I'm probably going to change the speech amp to be another 215SG Tetrode to be followed by another 215P Power triode for Heising modulation.
In the photos, one triode is only receiving filament power now, it's not wired in for speech yet - that' the one I'll probably change to be another Tetrode, it'll give me more gain and simplify the modulator.
Yes, the coils are homemade. I have about 10,000 feet of #26 wire, some of which I around a boiled-in-wax cardboard tube, sealed with wax (I melted spare unscented Ikea white candles in a pot and dipped the coils into this). I like making coils this way, it gives me more control over the circuit. I have my small L/C meter which I use to wind the coils precisely.
I used this same approach in making my mini-loop receiver where I made my own variocoupler). I used the same British tubes in this receiver as well and I've received broadcasts from Russia, Cuba, China, Romania and Taiwan on it).
I'm not on the air yet, I have no antenna set up and there's simply too much snow and ice to do anything like this now....I was thinking of making a loop antenna for transmitting purposes but I'm budgeting my time over Xmas/New Year holidays.
I used Brass strips, stapled down to the pine base, as my conductors. In a way it sort of resembles a modern PC board with it's flat conductors. The coils are all hand-made from cardboard tubing, held together and sealed with wax from molten candles. The copper tubing is suspended with some plastic chopsticks Epoxied into place.
I was having a problem with parasitics initially. Neutralization was tricky as well since the parasitics were getting in the way of doing a proper job of neutralization.
What could I do to isolate and halt the parasitics? Were they in the buffer (which is high gain)? In the oscillator? In some intercoupling between the final tank and the hand-made coils? Would I have to erect metal screening between these stages?
Well, I found that I COULD squelch the parasitics when I removed the buffer Tetrode and shorted the plate lead to B+. That told me the parasitics were confined to the final PA!
Then I tried 150 Ohms in series between the LC circuit and the grid and that really stabilized things but reduced drive. Then I put 56 Ohms in on each PA grid and found that 99.999% of parasitics were gone and drive levels were up now! Funny, I've never seen anything in any of my older publications about putting resistance in with the grid to aid stabiliztion, only in later publications from the 40s and on have I seen this, but it really works to help stabilize the final in the old transmitter too!
I had tried using a Triode as buffer but found that I needed an extra tuned circuit on the plate-side of the coupling RF transformer to keep levels nice and Sinusoidal. Switching to a Tetrode eliminated any need for a tuned primary on the plate-side of the buffer. Perhaps the very high plate resistance of the Tetrode helps the final jump into instability whereas the low plate resistance of a Triode buffer ahead of the Push-Pull PA would squelch parasitics in the grids in older designs? Hmmm!
I experimented for about a week in my home lab - while I have my boss' Spectrum Analyzer over the holidays - and found that keeping the grid-leak capacitance as low as possible in the master-oscillator contributed to stability and a very low harmonic content, about -45dB below the fundamental. So I'm using 8.2pF in the grid leak, along with 15k+1.2mH to discharge.
A higher grid-leak capacitance I found forms a nice time-constant which encourages "squegging" at some frequencies and draws grid-current from the oscillator tank, contributing to significantly higher harmonic content through asymmetrical loading, current draught and ultimately distortion on the oscillator tank.
I tried running it on higher voltages than normal....these Cossor tubes are rated to maximum 120 volt but I managed to get them up to 180 volt successfully.
With increased bias in the final (25 volts instead of around 10-ish), plate-current draw was no higher than running at 120 volts. But I didn't want to push these tubes so hard....the next step would be to build-in a power supply into the bottom, including a modulator. I want to separate the buffer and oscillator supply from the final PA supply.
Something about these old tubes, their filament resistance between them isn't consistent after these years. I'm running filament power from 5VDC, dropping through series resistances on each socket. I worked out 30 Ohms for the Triodes and 20 Ohms for the Tetrode based on dropping 3 volts at their rated filament current (0.1A for the Triodes, 0.15A for the Tetrode) and found discrepancies not connected to oxidation (the pins on these tubes must be silver-plated, they had a dark/black oxide layer which I cleaned off with 200 grit sand paper). So I ended up tailoring a few resistances to each tube in lieu of any rheostats.
Frequency stability is good, at the low frequency end (80 meters) I'm seeing drift in the order of a few hundred hertz over perhaps an hour. At the top end, it's a little more, like about 1kHz over 45 minutes or so. My power supply drifts a bit too so I know it's FM'ing my oscillator a bit.
These are all battery-operated tubes BTW, the schematic doesn't show that because I have only 12AX7 and 6AQ5 in the CAD library. I found that being the battery-operated type, they run much cooler since they have no heater/cathode mass as AC operated tubes do, so I think that helps stability!
The spectrum analyzer screen shot shows the fundamental at the far left, the second harmonic toward the middle and the third harmonic towards the right. Each vertical division represents -10dB. Not bad! And there are no Crystals in the circuit!
I find it awesome that such old technology can STILL serve a purpose and actually function decently given care in the details. No esoteric, high-tech materials are really needed!
More at:
http://99.239.146.96/Projects/Transmitter/Transmitter_V2/
