Thanks OPCOM.
Let me be sure I understand you comments. The modulator tubes are a pair of 811A’s. The tube lineup is a 6SJ7 R/C coupled to a 6C5 which is R/C coupled to a 6C5 which is R/C coupled to a 6L6 transformer coupled to the 811A’s
I understand you to say take the feedback from the 811A plate and feed it back to an 811A grid which is after the next transformer back.
This leaves out all the R/C coupled stages before the 811A’s. If I make sure the R/C coupling is designed to pass a wide frequency range with (hopefully) minimum phase shift I would think it would be better to include the first audio tubes in the loop.
But at this point I’m a dum dum and looking for guidance.
Thanks,
Jerry, W8UJX
No, in that case, you may have to have the one transformer on the loop.
It would not make much sense to try to apply NFB only to the 811 stage, and difficult because it is drawing grid current too. To make it simple, the stage you apply feedback of this sort to, should not be drawing grid current. So, the 6L6 and 6C5's are good candidates.
One thing to be careful of is that whatever you do, the polarity or phase (as mentioned above) is real important. One 811 plate cap or the other, in your case.
One, the wrong one, will cause unwanted positive feedback and a howl in the modulator (bad for the mod transformer - try it with a 5K resistor on the secondary or something for safety of the transformer), the other 811 plate cap, the right one, will produce the desired effect of negative feedback, which should only reduce the gain. This is determined usually be experiment.
Try going back from one of the 811 plates to the second 6C5 cathode. The things that concern me are the coupling modules between the 6C5's and between the 6C5 and 6L6. I have no idea what amount of phase shift would be in those, if they are used for limiting the audio response, it could vay greatly over a wide range of frequencies and cause instability. To reduce the issue, i chose to go to only the second 6C5, so only one of those things is involved.
In the schematic, I try to show a general way to approach it first. Others may disagree. Before trying it, ask someone who has worked on that set to help, maybe, because i have no experience on that unit, only generally with some high powered audio amplifiers and modulators, but all are different.
some notes for the drawing:
1. The purple + and - symbols indicate the polarity of the signal.
2. The red dots indicate 'like' polarity of the signals at the transformers. These could easily be reverse (see above about feedback). The manual does not say. You may have to scope this out. If it is 'wrong', then swapping the 811 plate caps should sort it out. OR, swapping the 811 grid leads. (not both).
3. The added network at the top is a very general way to try to put 1/3,100th of the 811 signal voltage (0.032% of it) to the cathode of the 6C5.
- That is, 4,700,000 divided by 1500 is 3,100. more or less.
- The network is part of a voltage divider from the 811 plate to GND, with a tap at the 6C5 cathode, 1500 Ohms up from GND.
- (I ASSUME the value of that resistor is 1500 Ohms because the GK500B has a better shcematic and it is 1500 on that. On the bad schematic for the "A", the value can't be read.)
So if the peak to peak voltage at the 811 plate is 1500V, then 0.48V peak to peak will be pushed onto the 6C5 cathode to 'fight' the signal which exists at the 6C5 grid. That is the negative feedback. It should be less than 6dB with this circuit but i can't guarantee that. Start small; it might not need a lot anyway.
a.) The question I would ask, before doing this, is "how much voltage is on the 6C5 grid at 100% modulation?" Whatever it is, the voltage applied to the cathode determines the dB of feedback, more or less.
b.)if the 6C5 grid voltage is 4V peak to peak, and there is 1V applied to the cathode from the feedback, then it is 25%. If the 6C5 runs 2V p-p at 100% mod., then the same feedback is 50%. and so on. Hope there is enough extra gain to make it up. It has to be made up or the modulation % won't hit 100.
4. The network has a 4.7M resistor. A 2.2M will make the feedback stronger. 2,200,000 divided by 1500 is 1466. A 2.2M resistor would put (using this logic, 1500V/1400= about 1V) 1 volt of feedback on that cathode. Note small resistors are not rated for 2-3KV so use a string of 1/2W resistors. Most 1/2W resistors are rated about 500V.
5. The network has a 22pF capacitor. This increases the feedback at the highest audio frequencies and usually helps stability, but might not be necessary. The 2KV voltage rating is because that resistor previously mentioned will have the full peak to peak 811 plate swing on it.
6. the network has a 0.05uF 3KV cap. This is to block the 1000V DC voltage of the 811's plate supply from the cathode of the 6C5. Otherwise it could raise the cathode voltage and cause distortion.
7. the 25uF cap across the 6C5 cathode resistor has to go, because it would absorb the little 1V signal we might try to feed in there with this scheme. If RF is an issue, put a 0.001uF or 500pF cap there instead.
8. Again I have to say I don't know this transmitter, and am not responsible for any results from this general discussion of what I might try to do, myself, as a starting point. If you can find more knowledge or an example of it being done, do so. And don't get shocked by the 1000V or whatever else is in there.
Others on this topic ought to comment on this.
PJ