Building A Demodulator To Monitor AM and SSB Transmissions 

I have a little audio amplifier I have been thinking about testing and posting and I will use one of the FFT SA programs and make sure I do what I can to detail hookup, attenuation of input, etc. Thanks for the suggestions.

Hi Esmond, No doubt THD is probably around 10% but distortion actually makes a signal louder and our ears don't seem to have a problem with it and may even like it, to some extent. Think of the THD in the space shuttle communication line. Got that high-pitched penetrating voice sound to it. We want to sound a little more HiFi than that... 🙂 The biggest problem I see occasionally with ham radio transmission is where too much speech processing and/or compression is used to make the plate current meter "kick" up higher indicating more power output and that causes a muffled distortion that is hard to understand. Also, too much low frequency response from dynamic mics makes voice harder to understand unless conditions are excellent. Always good to hear from you. Any interesting projects lately? I have a Collins 75A-4 receiver that I purchased recently that works great but... on power up and after maybe a couple of minutes, goes into loud distortion and the S-meter goes to zero and after about another 1-2 minutes, just "clicks" and goes normal again. A real pain to trouble shoot. Not a tube or the mechanical filter - I have eliminated them. I am using the receiver but want to fix it but it only fails as mentioned. David

The RF carrier from the signal generator is injected and mixes at the input of the demodulator in the BNC T-connector, the same place the SSB signal is connected to the demodulator and the injected RF carrier must be at the same frequency as the SSB RF. After the RF carrier and SSB RF mix, the simple diode detector can separate the audio. It's pretty amazing to me also... 🙂 Do you ever just wonder how someone actually figured this out - 100 years ago.

I think there are 2 kinds of creators. The inventive kind who just keep trying different things until they hit on something (Edison maybe) and the intuitive type who can actually understand and extrapolate further. Like Tesla? One thing I do know, no matter how clever you may be, someone smarter will come along and kick your ass and wipe the floor with it. I agree though, the early innovators in electronics were giants. Thanks for your reply, hope you're keeping well. @@ElPasoTubeAmps

You described the audio chain correctly. The "preamp" is a microphone amplifier used to get the audio level high enouigh to drive the McIntosh 40. The 3-400Z modulators are running class-B and require grid current and power (26 watts) to drive them and that is the reason for the Mac amplifier. The Mac amplifier is not necessarily for HiFi although it does a good job at that, it is just that it can deliver the required power to drive the modulator. My hobbies are ham radio and audio also. I tend to flip-flop between them over the years to which is on my mind at the time. Lately, my efforts have been on ham radio and in particular, AM on 40M around 7293 KHz. Hope to hear you on the air. 73 WA4QGA

I like the setup and hopefully I'll hear you on the air. 73. Dave AA5HL

I was reading about the 6B4G and how the anodes were wired in parallel making approx 800 ohms. That makes it easy to line drive a McIntosh audio amp. Is that a good way to look at it?

I am not sure how to answer that about having a plate resistance of 800 ohms for the 6B4G. It is true there are two triodes in the envelope wired parallel. I find that interesting. The "preamp" is actually a small power amplifier originally designed to drive push-pull 810's in a Collins KW-1. In my situation, the output transformer is an 8 ohm output capable of driving a speaker. I have never used it to drive a speaker but originally used is as a driver for another AM transmitter. The input transformer for the other transmitter is an 8 ohm to 8K ohm driving the grids of a pair of 4-125A in class AB1 and modulating a James Millen 1947 push-pull RF amplifier running 812A's. It seems the general way of having a separate driver amplifier driving a higher power modulator is 500 ohms to 500 ohms but I did not have suitable transformers like that so I chose 8 ohms to 8 ohms. Works great. So, a bit more detail is I have a 500 ohm resistor across the 8 ohm output of the "preamp" using the 6B4G driving that 250K ohm input of the Mac amplifier. It is easy to drive a line-level amplifier input like a power amplifier from a speaker impedance amplifier. The 500 ohm resistor is simply to give the preamp amplifier some resistive load to work into besides the 250K input to the Mac amplifier. I hope this makes sense. In the case of the 4-125A modulator, its input has an 8 ohm resistor shunt across the input to the input connector. The AB1 modulator using the 4-125A only requires about two (2) watts to drive it as I have the step-up ratio of the input transformer (8:8K) and the bias on the 4-125A is only about -40 volts so it is very easy to drive. In the case of the 3-400Z modulator, they are running class-B and need a good bit of grid current and power (26 watts) so the little preamp stage cannot deliver enough power to drive them so I drive the Mac amplifier with it. I also need the preamp stage as I use a condenser mic that is low output so I need a high gain microphone stage at the input. As for plate resistance of the 6B4G, I believe the plate-to-plate load (different from the plate resistance) is still about 5K or so. Unless we are trying to get the last bit of performance in power and performance out of a tube amplifier, there is quite a bit of room for differences in plate-to-plate impedance in out output transformer for satisfactory performance. Hope all this makes sense. I see from QRZ you are in Plano, TX. I think we could QSO on 40M pretty easily. I tend to hang out on AM at 7293 KHz and there is a popular gathering - not exactly a net - around 7195 KHz. Hopefully we can hook up. You might like all the pictures on my QRZ page. 73 WA4QGA

Ok sounds good. I like your QRZ page. Great pics. Yes I'm in Plano and I work at DFW airport as a radar engineering technician on about 10 different radar systems and radio communications. I've been to El Paso a number of times and I like it out there. I stayed on Zaragosa and North Loop. I even worked at the immigration detention facility in the past. There was a great Mexican restaurant that everyone loved out there, but that's 25 years ago. Enjoyed chatting with you, thanks for the videos and explanation. I always learn more from you. I'll be in touch either here or on the air.