Negative peak limiter for an Elmac A-54

[ka1tdq]

That's true. But like anything in life, it all boils down to whose meter swings the farthest. Mine swings to 150%.

Jon

[Steve - K4HX]

Peak meter swings are mostly meaningless. It's the average that will determine loudness.

As Dave noted, most older plate modulated transmitters do not have enough audio reserve to modulate very far over 100% in the positive direction. I recommend using a high-level negative peak limiter only to prevent splatter and to protect the mod tranny. Select the correct audio polarity and the positive modulation will automatically be greater than the negative - no limiters needed. This is all you need with these types of transmitters. Reserve any processing to increase the average, if you want an increase in loudness.

Homebrew tube rigs with big modulators and the proper transformers, appropriately modified amateur grade transmitters, some BC TXs, low level modulated rigs and Class E rigs are different animals.

[wa3dsp]

Here is a schematic of what I did in my AF67. I believe I did a similar thing in my A54 mod. It has been quite some time ago but as I remember in testing it did seem to do the job.

[N2DTS]

In doing my screen modulation experiments, I found it very interesting that you get a lot more positive modulation because of the lack of phase shift and frequency response limitations.
My plate modulated rigs have plenty of clean audio power, at least as much as carrier output, and they have good mod transformers (best I found outside of broadcast iron), yet 4X carrier is the best I could ever do, and 3X was more likely.

Screen modulation does 5X or even more.

Trying to brute force a marginal rig with a high level NCL circuit seems the wrong way to go.

[steve_qix]

Does using the 3-diode negative peak limiter with a modified Heising setup differ from that of just a straight modulation transformer with respect to hard clips?

My single FET rig is modified Heising and I plan to just use the mod transformer with the Elmac.

Jon

Yes, no problem with modified heising and the 3 diode circuit.  You insert the 3 diode NPL between the heising reactor and the RF amplifier.  Basically, the NPL goes between the RF amplifier and whatever is supplying it.

[ka1tdq]

That's how I built my current 40 meter rig and it sounds awesome.

Back in Mass, I built a plate modulated single tube rig that was also modified Heising. At first, I just used an LED to indicate negative peaks. I would just speak softer when it lit up. That was really annoying.

I then incorporated just the Zener clipper circuit and that fixed the problem. I'm also sure that splatter wasn't a problem. Living in the northeast, if I were, someone would've told me I'm sure.

I've built two rigs with these circuits with nothing but compliments on my audio. Any future rigs I build will use them too. Modern modulation methods allow for >100% talk power. Why whisper into the microphone?

Jon

[steve_qix]

You really have to pound on the N.P.L. to generate noticeable splatter.  Splatter from overmodulation in the negative direction is worse.

[Opcom]

With special circuits, especially experimenting, Please do not rely on meter swing. Please refer to scope patterns.