The new 12AT7 tubes arrived (I ordered two just to have a backup) and with great anticipation I plugged one in… and found that the output waveform hadn’t really changed that much. Hmmm….
That had me staring at the circuit diagram and doing some more careful analysis, which basically showed that the device is working as designed. Then I entered the circuit into LTspice and ran a simulation – and sure enough the output waveform looked very close to what I’ve been seeing. Turns out that folks on the internet also mention the non-sinusoidal waveform that this thing puts out. So it has actually been working as designed all along. The design is basically a compromise as to what can be achieved with just two common tubes.
Once again here is the output waveform with nothing connected to the output, and the level turned all the way up. The frequency is set to approximately 600KHz in the AM band.
So what happens to this output if you actually connect it to a radio? I connected the output to the antenna input of my old Hallicrafters S-40 with about a 3-ft length of 300 ohm twin lead. Here’s what the waveform looks like at the output of the KG-650.
Sorry for the blurry image, but you can get the idea – there is definitely some 600KHz in there but an awful lot of harmonics as well. At the radio antenna inputs it looks like this.
Once again apologies for the poor photo. This is basically a slightly attenuated signal from the one at the output of the KG-650, which is to be expected.
The radio is actually able to tune this signal at 600KHz on the dial, and if I turn on the modulation in the KG-650 you do get a tone in the radio. Not a very pure tone, but a tone nonetheless. So it could be used as a rough alignment tool, which is why I bought it in the first place.
Now I need to decide if I’m going to leave it the way it is and just use it that way, or if I’m going to try to improve it. Decisions, decisions.
It looks like a new 12AT7 tube may be in order for this unit. Here’s what I found as I was tracing through it. You can refer to the schematic, there’s a link to that in the last post.
First I removed the 6C4 audio oscillator tube so I knew the audio modulation circuit was not going to affect the measurements.
Then I started looking at the RF modulator and what I found was that it was oscillating quite nicely, but the amplitude of the oscillation was so high that it was overdriving the second triode stage, thus causing the distortion. Here are the signals on the gate and plate of the right-hand triode in the circuit diagram (pins 7 and 6 respectively).
Both signals are at 50V/division and ground level is one line down from the center of the display.The gate signal is on the bottom and the voltage on the plate is the top waveform. The frequency is set to about 200 KHz.
Looking at the data sheet for the 12AT7, it’s clear that by the time the gate voltage gets to about -4 volts, the tube should be shut off completely and the plate voltage should be at maximum. But what we see here is that the gate voltage is reaching about -70 volts before the plate voltage reaches maximum. So it appears that the gain of this tube is way down from the specification. That’s not too surprising since the tube appears to be original – it has a “made for Knight” stamp on it.
I did then replace the 6C4 audio oscillator tube and checked that circuit, which seems to be working fine and provides a nice sine wave at around 600Hz to modulate the RF signal.
So now it’s time to order a 12AT7 tube and see if this diagnosis is correct.
Sorry it’s been such a long time since the last post. I wanted to show you the latest piece of equipment that I picked up today at a hamfest – a Knight KG-650 RF generator. This should be really handy for aligning and testing radio gear, assuming I can get it to work properly. Here’s what the front panel looks like:
Here’s what the insides look like – appears to be in decent shape:
So I powered it up slowly with a variac and watched the power supply voltage carefully – the supply looks very clean with very little ripple. So I went straight to measuring the RF output. Unfortunately the output is not a clean sine wave, instead the lower part of the waveform is distorted as you can see below. This happens across the entire frequency range. The good news is that other than the distortion, the unit seems to work. Next step is to trace the signals through. Luckily it is a fairly simple circuit – I’ll attach the circuit diagram as well.