NPC for AM Transformer radios is HERE... Tubed, Solid State, etc. ANY high level TX

[unit248]

"With careful tuning of the radios output section, and a 1969, THIS is the way to get 75 PEP out of a 29."

Can you really hit 75 watts on a positive modulation peak with a single 2SC1969? (Disclaimer: I don't know the actual specs for a 1969 so I could easily be talking out of my butt, but I was under the impression that since it was originally chosen for standard CB radios designed for SSB operation with 12 watts PEP, the upper limit wasn't that high.)

Obviously, negative peak compression is meant to deal with the case where you have more headroom available for positive peaks than for negative ones (by rounding out the negative peaks instead of letting them turn square when they hit the 0 volt line), but there does come a point where you run out of positive headroom too (presumably when you drive the positive peaks beyond the supply voltage). I thought for sure that a single 2SC1969 would end up clipping its positive peaks well before you hit 75 watts.

I guess a better question would be: even if you can do that, will you still be able to maintain linear operation? Granted, you're introducing some distortion by doing NPC in the first place (you're avoiding the IMD effects of having the waveform flatten out, but still: the audio waveform leaving the radio won't look like the one that went in -- technically that means distortion), but I would think that if the RF amplifier stages don't retain linear operation across the whole output range, you're going to induce some additional distortion near the tops of the positive peaks.

-Bill

 

[Toll_Free]

The distortion introduced by the NPC circuit will be MUCH less than from baselining the audio.

Another issue is this: If you DO manage to drop the carrier to insane levels, the RECEIVING detector is 9 out of 10 times a simple diode.... And THEY can't handle the distortion created when going much over about 150 percent positive mod.... Hence the FCC mandated 125 percent MAX on broadcash equip... During the "loudness wars" of the broadcash days, it was common to run 200+++ percent positive modulation, and they'd use a circuit similiar to the one I posted to limit baselining the audio.

The way the 29 (and most other transformer modulated / plate modulated ) / 25 / 21 / etc hit 75 watts PEP is this: The transformer in them is connected as an AUTOTRANSFORMER... The MODULATED voltage is MUCH more than 13.8 volts! 13 volts is actually the CARRIER voltage... They need to impress ANOTHER 13 volts on TOP of it to get 100 percent modulation... Which will quadruple the output, giving the textbook 4 to 1 carrier to PEP level we look for. It is possible (if I remember correctly, it's been about 6 to 10 years since I tried doing one, and EVERY chassis was INDIVIDUALLY tuned on the output pi network to hit > 45 watts!!!!) to get near 40 - 50 volts PEAK on the output of the transformer.

A LOT of the AVG increase is due to the non-baselining. If you limit the negative excursions of the negative peaks so that they NEVER cause a FLATLINE in the modulated RF envelope, you cause a LARGE increase in output power on an AVG reading meter... And you will also notice a lot of increase in range, all other things being equal... Although our ear responds to peak power, the AVG power is what actually gets the signal through.... Just like in cars, it's 'torque under the curve' that actually gets the job done, not just a 1000 horsepower engine... One that produces torque from 1800 rpm to 6000 at 800 peak horsepower will EAT a 1000 horsepower engine that ONLY produces torque in a couple thousand RPM powerband. If you can increase the AVERAGE power output of your radio (which is ALL NPC does), you've won half the battle..... LIMITING the negative peaks and ensuring you have asymetrical mod (which you'd need a good avg meter or scope to see) so your positive peaks are GREATER than your negative is also beneficial, as it increases the PEAK power on the modulator.

--Toll_Free

 

[unit248]

Hm. Ok. So the audio voltage is summed with the supply voltage at the collector, thanks to the transformer, which boosts the output beyond just what you'd expect from the 13.8v supply voltage alone.

But -- and please forgive me if I'm being dense here -- that still doesn't answer my original question: can you actually push the output of a single 2SC1969 up to 75 watts on a positive peak?

With 100% modulation, the power at positive peaks reaches the four times the carrier power: with a 4 watt carrier, you'd get 16 watts at positive modulation peaks. With 8 watts of carrier, you'd get 32 watts at peaks, and so on. NPC would change this a bit since it would give you greater than 100% modulation.

But 75 watts sounds like you'd be pushing the 2SC1969 past its design limits. I decided to download the datasheet for it, and based on the curves that it shows, it looks like power output starts to level off at around 25 watts.

Now, maybe you can push it higher, but then I have to ask again: if you do that, will it stay linear? My guess is that it wouldn't. If you were to test with a sine wave then, I would expect the positive peaks to be a little attenuated compared to the original audio input.

-Bill

 

[unit_399]

http://www.amwindow.org/tech/htm/3diodeka.htm

 

[Toll_Free]

http://www.amwindow.org/tech/htm/3diodeka.htm

And your point is? Not like I took credit for it........

I'm still waiting for your explanation as to how the carrier is still their when their is no collector voltage....?

Rather than try to steer the thunder...


--Toll_Free

 

[Shockwave]

Hm. Ok. So the audio voltage is summed with the supply voltage at the collector, thanks to the transformer, which boosts the output beyond just what you'd expect from the 13.8v supply voltage alone.

But -- and please forgive me if I'm being dense here -- that still doesn't answer my original question: can you actually push the output of a single 2SC1969 up to 75 watts on a positive peak?

With 100% modulation, the power at positive peaks reaches the four times the carrier power: with a 4 watt carrier, you'd get 16 watts at positive modulation peaks. With 8 watts of carrier, you'd get 32 watts at peaks, and so on. NPC would change this a bit since it would give you greater than 100% modulation.

But 75 watts sounds like you'd be pushing the 2SC1969 past its design limits. I decided to download the datasheet for it, and based on the curves that it shows, it looks like power output starts to level off at around 25 watts.

Now, maybe you can push it higher, but then I have to ask again: if you do that, will it stay linear? My guess is that it wouldn't. If you were to test with a sine wave then, I would expect the positive peaks to be a little attenuated compared to the original audio input.

-Bill

No you can't expect to get 75 watts PEP from a 2SC1969. You might get close to double it's rated power someplace around 40 watts PEP for intermittent use. I wouldn't count on it being very reliable either. This mod Toll Free has shared is more of an audio mod then a power mod. If you're going to do any mod to a radio it should be to improve audio, not increase power.

Even these new bolt on heat sink final replacements that claim 75 watts overlook an important problem for hi level AM use. Radios that use the modulation transformer get their audio directly from the audio chip with no buffer. Ask the 4 watt radio to key 8 watts and you need twice the modulation power from the audio chip because the current has doubled. This happens as a result of the final collector impedance dropping in half.

Then you have to consider that poor little modulation transformer having to support twice the DC current on it's secondary while it's primary overloads the audio chip to the point where it probably flat tops on positive peaks. The extra current on the mod transformer will cause more distortion in your low end due to core saturation.

The point here is if you want the best audio on AM, don't modify your radio to produce 2 to 5 times it's original carrier and still expect to have the headroom required to reach the positive peaks this type of mod can do. This mod should let you run over 100% modulation without noticeable distortion and with a significant decrease in bleedover.

 

[Shockwave]

This circuit is amazingly simple. The current path trough R2 / D2 supplies just enough energy to keep the final amp at a minimum carrier level during the negative peak cut off.

 

[Kamikaze]

Fun watching new folks trying out positive peak schemes.

O.G. Villiard has written a lot of interesting things on super/ultramodulation.

A quick google search turned this up:

http://www.arrl.org/files/file/Technology/tis/info/pdf/5103049.pdf

 

[Toll_Free]

Hm. Ok. So the audio voltage is summed with the supply voltage at the collector, thanks to the transformer, which boosts the output beyond just what you'd expect from the 13.8v supply voltage alone.

But -- and please forgive me if I'm being dense here -- that still doesn't answer my original question: can you actually push the output of a single 2SC1969 up to 75 watts on a positive peak?

With 100% modulation, the power at positive peaks reaches the four times the carrier power: with a 4 watt carrier, you'd get 16 watts at positive modulation peaks. With 8 watts of carrier, you'd get 32 watts at peaks, and so on. NPC would change this a bit since it would give you greater than 100% modulation.

But 75 watts sounds like you'd be pushing the 2SC1969 past its design limits. I decided to download the datasheet for it, and based on the curves that it shows, it looks like power output starts to level off at around 25 watts.

Now, maybe you can push it higher, but then I have to ask again: if you do that, will it stay linear? My guess is that it wouldn't. If you were to test with a sine wave then, I would expect the positive peaks to be a little attenuated compared to the original audio input.

-Bill

Yes, pushing a 1969 to 75 pep is REALLY pushing it. It's not a mod I'd push on to customers, it's a mod that I did (as well as quite a few others) back in the early part of the decade for the keydown scene. They do NOT last forever at that level... HOWEVER, it is a selling point when you can actually watch a 5Kw slug wriggle with JUST a radio The invent of the RFX-75, MOSFET conversions, etc. pretty much pushed these types of mods to the back burner, but some people still question how they where done, or if they where actually EVER done.

The 25 watt level of the 1969 is pretty much pointless for class C high level modulation.... With 'plate mod', you can pretty much figure on 3-4 times the diss level of the transistor as PEP value of output. The 1969 in a 148 or other SSB chassis is run class AB.... In the 29 and associated 'xformer modulated' radios, it's run class C.

Your mathematics is correct for 100 percent modulated radios, that modulate symmetrically. With ANY negative clipping, compression, etc. that throws it out the window.... And you need TWICE the voltage to get 100 percent positive peaks... To get the 'textbook' modulation percentages from the 29, you need 26 volts of complete swing.... Twice the B+ to negative baseline... Or, optimally, to about 1 volt above baseline (which is what the circuit I presented actually does, prevents the carrier from EVER reaching 0, or the applied voltage, actually.... You ONLY prevent the modulator from doing it's job, really.... Hitting 0 volts on negative excursions)

Hope that helps explain it a bit better. NO, we are not getting linear output, BUT, we are 'plate modulating' a final amplifier, which means it HAS to run Class C. Class C is EASY to get 3 times Cdiss or Pdiss, 4 times even... 3 x 6146s, a 30 or so watt tube, will net 400 watts easily of plate modulated AM..

And let's not get started on the "controlled carrier' type radios... Like the DX60 and a few others.... Which is what NPC REALLY is..

--Toll_Free

 

[Toll_Free]

This circuit is amazingly simple. The current path trough R2 / D2 supplies just enough energy to keep the final amp at a minimum carrier level during the negative peak cut off.

DINGDINGDING!!!!!!

And that is EXACTLY why it can be used in ANY high level circuit, be it transformer coupled (plate or collector modulated) or series modulated (like Cobra 14x chassis, Galaxy chassis, etc).

If you do this mod, with the 'class AB conversion" of the final in the galaxy / cobra style radios one thing to keep in mind is this: You're now not attempting to keep the negative peaks on a 25 watt modulator alive, more like a 5 watt modulator.... The DRIVER is the only modulated stage now.... If you don't overdrive the final, you won't need to worry about baselining it.

--Toll_Free

 

[Toll_Free]

TOLL_FREE -

Your circuit may work (I haven't tried it yet), but I don't understand your explanation. The output from the transformer has a DC (13 volts) and an AC (Audio) component. Since D1 and D2 are reverse biased with respect to the DC, it is unaffected. But BOTH D1 and D2 are forward biased with respect to negative audio peaks and both will conduct on negative peaks. However, I think that D1 will conduct more than D2 because of the voltage drop across the Anti-spike diode. This is confusing to me. Please explain the function of D1 in this circuit. Thanks. 73s.

-399

D1 is the actual sampling diode that turns D2 on during NEGATIVE peaks. The anti-spike diode actually (in this circuit) does dual-duty, it does anti-spike as well as prevents the keepalive voltage from being presented back to the xformer.... Which could cause problems galore, depending on the mod scheme used.

D1 is SUPPOSED to conduct more than D2. D2 is their as a steering diode for the keepalive voltage. D1 turns it on.

This circuit can actually be reduced to two diodes, but becomes a BEAR to keep diodes in! LOTS of people report diodes blowing up (on tube rigs moreso than transistor, but we all know SOMEONE will try the two diode scheme with some 50 volt diodes, then complain they keep blowing it up.... The cost of 3 diodes with a 1Kv rating is what, 10 cents???)


Make more sense now?

--Toll_Free

 

[unit_399]

I'm still waiting for your explanation as to how the carrier is still their when their is no collector voltage....?
--Toll_Free

Here is a scope display of an AM carrier overmodulated by a continuous tone. If, as you claim, under these conditions the final collector voltage is reduced to zero, and the carrier is cutoff, then explain to me HOW THE UNIT CONTINUES TO TRANSMIT, AND HOW THAT TRANSMISSION CAN BE DETECTED BY A STANDARD AM RECEIVER IF NO CARRIER IS PRESENT. Common sense and logic tells me that the carrier is there.

The misunderstanding comes from the way a scope displays a modulated AM signal. There are three components to an AM signal: the carrier, the upper sideband, and the lower sideband. However, a scope cannot display these components seperately in a single trace. What the scope does is to ADD THE THREE SIGNALS TOGETHER AND DISPLAY THE VECTOR SUM. What happens is that as modultion % increases, the negative modulation peaks are subtracted making it APPEAR that the carrier is varying in amplitude. When modulation exceeds 100%, it APPEARS as if the carrier is cutoff, but this is not the case. If it was cutoff, transmission would cease.

If you look the conditions in the scope trace shown above on a spectrum analyzer, it will show that the carrier is present when overmodulated, and does not change in amplitude.

BTW, I posted the AMwindow link because I thought Steve did a good job of explaining how the circuit works, and that members might like to look at the additional info on the site.

Also thanks for the reply on the D1 question. That eliminated a lot of the confusion. 73s.

- 399

 

[Shockwave]

399, I see your point but it doesn't quite work like that. Sure when the carrier is pinched off by the 100% negative peak you would think that there would be no RF present. The problem is that when the carrier is pinched off, it is flat at the bottom. You can't gradually run into the zero baseline without causing the interference this DC component presents. It is the transition in and out of each cutoff point (the leading and trailing edge of the DC component) that causes the interference to adjacent frequencies. Not the duration of the cutoff.

 

[longhaireddwb]

I have an extra 29 in the closet that I can try this with.

What I need to know or confirmation on is...

The unlabeled diode/anti spike diode, which diode is this in a 29?
Would this be D8?
I'm still learning this stuff so be easy on me.
Thanks...

 

[Toll_Free]

I have an extra 29 in the closet that I can try this with.

What I need to know or confirmation on is...

The unlabeled diode/anti spike diode, which diode is this in a 29?
Would this be D8?
I'm still learning this stuff so be easy on me.
Thanks...

Yes, that is D8. If you want to LOWER the deadkey of the radio, and still keep the same audio in proportion, you would ADD diodes in SERIES to this one, and D1 and D2 would still be put at each end of the entire chain.

I lived in Colorado Springs for a short time, up in Cascade, too.... Used to go back a couple times a year, beautiful country!

Hope that helps.

--Toll_Free