Fabricating and Troubleshooting the Homebrew Hybrid SSB Transceiver
Hybrid SSB transceivers were popular in the 1960's with solid state devices in the lower level stages and vacuum tubes in the RF Power Amp stages. Well you know N6QW, I did it in reverse with vacuum tubes in the lower level stages and solid state devices in the audio module and the RF Power Amplifier stages.
About two years ago I had the receiver working but my care giver duties overshadowed any further work. About a month ago as a form of diversion I undertook finishing the radio.
It was then I found a problem with hum on my transmitted signal. A bit of fault tree analysis led me to two possibilities, the microphone amplifier (12AU7) or the balanced modulator (6JH8) or shudder both. A bit more testing found it to be solely a problem with the 6JH8.
More testing led me to only one possibility, the AC voltage being applied to the filament on the 6JH8. A switch to DC voltage on all filaments resolved the hum problem on transmit. A friend who does a lot of work with vacuum tubes confirmed that AC on the filament of the 6JH8 was a well known problem to audiophiles.
There are still some refinements that need to be implemented but the rig is on the air. Initial reports were about the hum issue, but no more. I also tweaked (in software) the Carrier Oscillator input level from the Si5351. Right now the carrier suppression level is 40 dB -- I think with a bit work work on the circuit I can get 45 dB or better. The power output is 7 watts on 40M. I heard myself on the Half Moon Bay Web SDR (350 miles) and made contacts into Oregon, Nevada, Arizona and New Mexico. Reports now on the signal quality are all positive.
So could you replicate this rig? The only tricky part is the Transformers. The IF transformers are single winding at 9MHz in a shielded can. That part is a piece of cake! The Balanced Modulator transformer is a modified transformer from a Drake TR-4 (T14) and moved up to 9MHz. That might be harder to scratch build.
The Trick and Trouble Maker is the compact size of the tube chassis. With a bad case of FFS (Fat Finger Syndrome) their ain't much room to install parts on the 4.5 X 6 inch aluminum plate -- that is nine tubes on that small space plus the transformers, crystal filter and relay.
The initial build two years ago had a magnificent ILI 3.2 Inch Color TFT display. Once I installed the transmitter stages I found that once you hit the PTT, the display went blank. It was an issue with transients or RFI that was blanking the display. Some quick code tweaking put a 16X2 LCD on the panel and no more problems on PTT.
The sensitivity is quite good but one refinement has to be an AGC and that can be done with just a bit of effort. I also want to get the ILI 9341 Color TFT back on the panel. The Arduino/Si5351 already is fitted with the hardware and so it is just a matter of reloading the code onto the Arduino Nano. But I will also have to track down the why it blanks on PTT. I have a smaller ILI9341 display on the Spillsbury Tindal Replica rig and at the same power level and no issue with a blanking on PTT.
So a trip back into 1960's SSB Homebrew with vacuum tubes no less.
73's
Pete N6QW