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Author Topic: Distortion in Plate Modulated Transmitters  (Read 23967 times)
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KM1H
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« Reply #25 on: November 12, 2009, 10:40:47 AM »

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A fun thread, indeed!

Fun maybe but certainly not very informative. Subjective comments such as "my modulator sounds best because Im using xxx tubes and someone told me it sounds great" means squat without the testing data to back it up.

I really dont give a damn if a 2A3/6A3 has 2% distortion,and a $2 12B4 or low mu triode connected 807 has 4% if it cant be heard. Its all meaningless on the air and moreso especially if the modulators themselves, the iron, and the Class C stage arent textbook perfect.

Carl
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N3DRB The Derb
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« Reply #26 on: November 12, 2009, 12:03:21 PM »

Do the best you can with what you can run.

but remember, you're not talking with a static collection of lifeless metal. It's only a tool, like a hammer. The only thing that makes AM special is that other AM'ers are on the other end. I dont love doing this because the guys on the other end are running low distortion transmitters. It's the shared experiences and the lasting friendships that count.

Having said that, everyone likes nice tools in good shape. they make the work easier.

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w1vtp
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« Reply #27 on: November 12, 2009, 12:35:11 PM »

One more thought - Bruce. 

You may be hearing something that the others are not hearing -- distortion in the low frequency part of your audio bandpass.  When I did my IMD tests I noted that things were pretty good until I got to the bottom end and the distortion products went up pretty fast.  You might hear subtle artifacts where your listeners (some of whom might have limited low frequency response) might not.  Yes, bone conduction might even exacerbate these products while you are listening to your own voice over the transmitter / monitor system.  That's why I recommend a recording.

Plate modulated transmitters do have that low frequency limiting factor because of the use of transformers whereas a well designed, built and adjusted "E" transmitter would not be dealing with these limitations.  This point might explain why you are hearing these artifacts while others are not.

I offer my ability to record your signal if you wish.

Regards, Alan


* THD TEST.jpg (103.27 KB, 968x527 - viewed 751 times.)
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W2XR
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« Reply #28 on: November 13, 2009, 12:43:46 AM »

One more thought - Bruce.  

You may be hearing something that the others are not hearing -- distortion in the low frequency part of your audio bandpass.  When I did my IMD tests I noted that things were pretty good until I got to the bottom end and the distortion products went up pretty fast.  You might hear subtle artifacts where your listeners (some of whom might have limited low frequency response) might not.  Yes, bone conduction might even exacerbate these products while you are listening to your own voice over the transmitter / monitor system.  That's why I recommend a recording.

Plate modulated transmitters do have that low frequency limiting factor because of the use of transformers whereas a well designed, built and adjusted "E" transmitter would not be dealing with these limitations.  This point might explain why you are hearing these artifacts while others are not.

I offer my ability to record your signal if you wish.

Regards, Alan

Hi Al,

Thanks for the offer to make a recording of the rig the next time we are both on the air! I'll certainly take you up on it, although I don't expect to hear anything new, quite honestly. Perhaps it would be more effective for you to visit my station and listen for yourself through the pair of headphones driven by the modulation monitor audio output!

The plot you had provided of the relationship of distortion vs. frequency was interesting and not too surprising. I have seen similar results in conventional plate modulated AM broadcast transmitter proof-of-performance data, with the exception that the distortion tends to increase rapidly above say 4 or 5 Khz as well. Was this data taken at the output of your modulator, or at the output of your modulation monitor or some other quality broadband detector circuit, etc.? If the data was taken on-air, at what modulation percentage was it taken, approximately how much DC plate input to the HPA for that modulation percentage, and what was the nature of the test set-up?

Could you also advise what your transmitter audio line-up and HPA consists of? I'd be curious.

Thanks again!

73,

Bruce
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Real transmitters are homebrewed with a ratchet wrench, and you have to stand up to tune them!

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« Reply #29 on: November 13, 2009, 08:50:28 AM »

Harmonic distortion can be quite musical compared to intermodulation distortion.  Kind of like having a host of sopranos singing one, two and up octaves higher than your melody.

Now transient intermod is very unharmonious.

TIM = TMI  Grin
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RICK  *W3RSW*
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« Reply #30 on: November 13, 2009, 10:27:27 AM »

Harmonic distortion can be quite musical compared to intermodulation distortion.  Kind of like having a host of sopranos singing one, two and up octaves higher than your melody.

Now transient intermod is very unharmonious.

TIM = TMI  Grin

Hi Rick,

The only really consonant harmonic distortion to the human ear are the even-order products. Deliberately increasing the magnitude of the 2nd harmonic of the fundamental can add warmth to the sound, and make it sound "wetter", to use a term some tube guitar amp guys favor. Many older console radios were designed so the audio output stage would produce an excessive amount of 2nd harmonic distortion for that very reason; it added fullness and warmth to the sound, enhancing the saleability of the product.

On the other hand, odd order distortion products, particularly the higher-order ones, are extremely dissonant sounding and very irritating to listen to. I know this subject has been covered on this board in detail at least once fairly recently, so I won't belabor it.

I think IMD is probably the most dissonant sounding type of distortion to the human ear.

73,

Bruce
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Real transmitters are homebrewed with a ratchet wrench, and you have to stand up to tune them!

Arthur C. Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic".
w1vtp
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« Reply #31 on: November 13, 2009, 10:32:00 AM »

Bruce

"Could you also advise what your transmitter audio line-up and HPA consists of? I'd be curious."


That graph was taken from a series of tests done on an Eico 720 using the reverse AF xfmr in a modified Heising hookup (see attachment).

The problem of the low end probably was a deficiency due to source impedance of the Bogen 60 watt amp that I used.

Al


* 720 MODULATOR TEST SETUP.pdf (61.2 KB - downloaded 404 times.)
* W1VTP EICO 720 PROJECT.pdf (535.38 KB - downloaded 748 times.)
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KM1H
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« Reply #32 on: November 13, 2009, 12:22:39 PM »

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Well, I my case someone gave me an old HB rig with a pair of 6A3's and a box with several more in it. I've since thru the years collected a few more 2A3's and 6B4G's so I have plenty.

If you want to toss a pair of same brand NIB 6B4's this way I can setup some base line tests. Hopefully they will be reasonably matched but I'll also curve trace them.

Carl
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k4kyv
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« Reply #33 on: November 14, 2009, 09:12:37 AM »

I once came upon a vendor at Dayton.  In the box of miscellaneous loose tubes under the table I found about a half dozen 2A3H's.  They apparently were 2A3s with indirectly heated cathodes instead of bare filaments, with the cathode strapped to one side of the filament to make them directly interchangeable.  I asked the guy what he wanted for them and he said "one and a half each".  I thought that was a fair price and handed him $9. Then he made it clear that he meant $150 each.  I told him he could keep them. 

Maybe some audiophool and his money would soon part.
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Don, K4KYV                                       AMI#5
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Patrick J. / KD5OEI
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« Reply #34 on: November 14, 2009, 01:55:42 PM »

In the box of miscellaneous loose tubes under the table I found about a half dozen 2A3H's.

If he keeps $150 tubes rattling around in a box of mixed pulls under the table, what was on the actual table?? ridiculous price.
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Radio Candelstein
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« Reply #35 on: November 14, 2009, 03:16:01 PM »

Im using a 58 as the AGC controlled tube in the modulator deck of my 30's style rig. It was patterned after a James Millen article in a 30's QST when he wouldnt use anything but 2.5V tubes because of hum level. I used triode connected 2A5's as the drivers which have lower distortion than 2A3's.

Carl
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