Question on negative peak limiting

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AB2EZ:
Hi

I am thinking about putting a negative peak limiter on my plate modulated GS-35b amplifier.

Putting aside the various views on whether negative peak limiters are a good idea or not...

... I have attached a diagram of a 2-diode negative peak limiter that appears to perform all of the functions performed by Steve's (WA1QIX's) 3-diode design.  However, perhaps there is something subtle that occurs at the transition to negative peak limiting (and back).... maybe having to do with the parasitic capacitances of diodes ... that makes this 2-diode approach a bad idea (versus the 3-diode approach).

Note that when negative peak limiting is not occurring, the resistor (R) across the forward biased diode is irrelevant. During negative peak limiting, both the resistor R and the output tubes are connected to the keep-alive supply through the other forward biased diode. So the subtle difference must occur at the transition.

Any insights or information based on past experience would be appreciated.

Stu

WA1HZK:
Took me a while to get the smoke out of the shack when I tried that on the 833 rig. I think it's easier on smaller rigs. If you want to do that do it in the driver. Just my two cents. Hey, I had to try it too.
Keith

kc2ifr:
Stu,
I ALWAYS had better luck when the limiting was done in earlier audio stages. Of course it helps if the limiter is made for AM transmitters.......not a symmetrical limiter.

Bill

steve_qix:
Hmmmmmmm... Well, from an electrical standpoint, I cannot see any difference.

You will need one less diode, but you will have to insulate the resistor at the full high voltage.  Probably a wash.
But, looking things over very carefully, I think it is electrically equivalent.

On negative peak limiting in general:

I have performed high level negative peak limiting (clipping) on all sorts of BIG transmitters (5kW BC rigs for instance)  and my own 1kW tube rig ("back in the days... etc.") and never had a problem with any of them.

That being said, these were well designed transmitters with sufficient modulators (and modulation transformers) to handle the significant extra power and voltage generated with high levels of positive modulation.  150% positive is 6.25 times the DC power input of the carrier.  200% positive is *9 times more* power!

I agree with Bill on processing at low level if If IF (and only if :D  ) the transmitter is capable of passing a square wave (or close to one), and properly.  Most, if not all transformer coupled modulators will not do this across too much of the audio spectrum.  Contrast that with any of the PWM broadcast or amateur transmitters out there.  These transmitters will do that all day.

The same can be said  for a properly designed DC coupled grid modulated, low level modulated, series modulated (class A, G, H, etc.), phase to amplitude (Ampliphase), SDR or other similar transmitter which does not use transformer coupling in the high level audio path.

In these cases, low level processing (neg peak clipping, etc.) works just beautifully, and is in fact superior because the transmitter's audio filtering and low pass filters are involved AFTER the processing.

w8khk:
Quote from: steve_qix on November 17, 2008, 07:40:21 PM

Hmmmmmmm... Well, from an electrical standpoint, I cannot see any difference.

You will need one less diode, but you will have to insulate the resistor at the full high voltage.  Probably a wash.
But, looking things over very carefully, I think it is electrically equivalent.



Hi Steve,

Looking at Stu's schematic, it appears he was able to eliminate the need for D1 (reference id in your 3-diode schematic) by connecting R1 to the load side of the keepalive diode D3, instead of the source side. 

I would like to try this approach in my modified Valiant, instead of the 3-diode version, because space is at a premium.  My Valiant modulator has four 1625s in push pull parallel, driving a UTC S21 115W transformer.  No driver transformer, all tubes with cathode follower driving the 1625 grids.  I do not expect to need the capacitor input supply (with increased B+ for the modulator), because the parallel tube arrangement runs lower impedance and should handle peaks with standard choke input supply.  I can add cap input if necessary, after initial testing.  Initially I will configure 6K plate to plate impedance, with 1.5K or 2K output impedance. 

I was able to fit the original Valiant modulation transformer on the chassis, with primary and all secondary windings phased in series to use as a heising modulation reactor.  A 1uf 4kv oil cap from a microwave oven will be the coupling cap, from cold end of mod xfmr secondary to gnd.

I assume the voltage requirements are still the same, D2 series diode at least = to B+ voltage or higher, and D3 keepalive 4 times B+.  I will use 2 1KV 10A diodes in series for D2 (series diode), and 6 1KV 10A diodes in series for keepalive.  Resistor will be 2Kohms at 20 watts non-inductive.  I have a variable keepalive supply adjustable up to 60VDC.

Advice/Criticism is welcome, thanks!

73,
Rick

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