Source for 2-4 Uf Capacitor 3000V for Modulator Projector

[K5UJ]

It depends on whether the cap is wired in such a way as to place the +HV DC across the capacitor. The only case where substantial audio voltage would appear across the cap would be towards the low frequency end of the range, and the capacitive  reactance would be a lot higher than the modulating impedance.  I would say it it is safe to use the same voltage rating for the blocking capacitor as that of the HV filter caps in the power supply furnishing the +HV.

This is what is amazing to me about AM.  There is a paradox in that supposedly AM is technically simple compared to SSB but the waveform is much more complex and then you get into this sort of engineering--I am not sure this sort of thing is covered much in any books I have but I need to keep reading.  The key factor I missed was that this is audio voltage and is not to be considered in the same way DC is.   Anyway thanks Don for explaining.

[k4kyv]

Since the audio voltage is superimposed on the DC voltage, you have to consider both. The peak voltage that appears across the capacitor is the sum of the DC voltage plus the peak audio voltage.

The SSB waveform is extremely complex as well. Just look at the mathematics involved in calculating the component values that go into a crystal lattice filter.  Also the waveform.  For example, with SSB you can't use hard clipping to  limit the modulation peaks the same way you can with AM.  Try sending square wave modulation through an ideal SSB transmitter, and you end up with peaks of infinite amplitude.

[K5UJ]

The SSB waveform is extremely complex as well. Just look at the mathematics involved in calculating the component values that go into a crystal lattice filter.  Also the waveform.  For example, with SSB you can't use hard clipping to  limit the modulation peaks the same way you can with AM.  Try sending square wave modulation through an ideal SSB transmitter, and you end up with peaks of infinite amplitude.

To me SSB is only a very dynamic CW signal.  It varies with power and frequency offset from an "imaginary" suppressed carrier zero beat frequency.  But it is basically a rapidly shifting CW signal consisting of a combination of frequencies to represent audio frequencies either above or below the suppressed carrier with varying RF power to represent audio amplitude.  I find AM to be much more complex and difficult to understand spectrally and mathematically.

[WD5JKO]

Most modulation transformers have a gapped core to help it handle the DC current of the RF final. We know that eliminating the DC current, and using a reactor helps a bunch since we get full magnetic BH for the audio only. That said, then we need this AC coupling capacitor, the topic of this thread. But what about eliminating the cap, and just placing the reactor directly across the modulation transformer? If the winding resistances are the same of each winding, then the DC current through the secondary of the modulation transformer is cut in half. Wouldn't this be good enough, eliminate the need for the cap, and eliminate any turn on/off spike?

Jim
WD5JKO

[k4kyv]

Most modulation transformers have a gapped core to help it handle the DC current of the RF final. We know that eliminating the DC current, and using a reactor helps a bunch since we get full magnetic BH for the audio only. That said, then we need this AC coupling capacitor, the topic of this thread. But what about eliminating the cap, and just placing the reactor directly across the modulation transformer? If the winding resistances are the same of each winding, then the DC current through the secondary of the modulation transformer is cut in half. Wouldn't this be good enough, eliminate the need for the cap, and eliminate any turn on/off spike?

I recall that Dave's original question was about using an audio output transformer connected backwards to an audio amp originally designed to drive a speaker.  If the output transformer was designed for push-pull tubes, it probably doesn't even have a gap, and if single-ended, its DC current rating may be much less than the plate current of the 807, so the coupling capacitor may be a necessity.

The idea of running a reactor without a coupling cap presents some considerations.  Firstly, the inductance of the reactor and that of the mod xfmr secondary are in parallel, which lowers the total inductance.  The mod xfmr would get worthwhile relief from the DC only if the DC resistance of the reactor were equal to or less than that of the secondary of the transformer. If you have a high-inductance reactor with unusually low DC resistance, then it could be used without a capacitor with no problem.

I would suggest finding a good 2 to 4 mfd non-electrolytic capacitor with a working voltage rating about 25-30% higher than the plate voltage on the 807, and use it.  I believe all these concerns about spikes and overshoot, and the need for a DC voltage rating 300%-400% of the actual DC voltage, amounts to a lot of needless worry over a non-existent problem.