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ART-13 Crap Out




 
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WU2D
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CW is just a narrower version of AM


« on: November 30, 2009, 09:21:21 PM »

Well after 15 years of trouble-free service, my ART-13 decided it did not do AM. Of course I was worried that I blew the transformer (especially after sending my spare up to GZB). The complaint was that I was distorted and the modulation was very low. Of course I did not have to hear that or look at a scope to know something had popped. I could tell from the lack of "singing" in the transformer and wiggling of the current meter that something was wrong.

I have a loop pickup on a BNC connector in the RF cage and a quick connection to the scope with a generator and dummy load showed a very sick waveform indeed.

Several years ago I built up an extender cable so I could actually take the audio module completely out of the rig. This was a big help when I developed my improved module several years ago. My first job was to find it! This is not so easy in my shack.

First I pulled the 811A's and I saw that one of them was showing a lot of mercury on the midriff. I replaced them with my brand new spare set. Some improvement - now I had symmetry at least. I was modulating up and down. Still nasty looking. Then I replaced the 6V6 driver tube and things got even better with audio easily beating stock but still nowhere near normal. I have had problems with ultrasonics and this was starting up again. At this point I got serious and killed the HV and pulled the module. I began systematically investigating each stage of the module.




* ART13AudioModule_WU2D.jpg (193.53 KB, 1500x1125 - viewed 358 times.)
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WU2D
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CW is just a narrower version of AM


« Reply #1 on: November 30, 2009, 09:35:19 PM »

The original ART-13 module is very simple. It has a 12SJ7 pentode mic amp into a 6V6 which is the driver for the 811's. Now this is a pretty spartan lineup which lacks gain with a very military sound to it. Some folks have had success force feeding audio into the rig with an equalizer. I had two modules, so I decided a long time ago to modify just one to see what I could accomplish. The new circuit uses a 12J5 into a 6SN7 into the 6V6 and I added feedback and opened up the values to get better audio. I have plenty of gain but the thing has always been touchy however especially with feedback and I have had a plague of ultrasonics off and on.

So after seeing that the 12J5 was micro-phonic and getting instability in general, I decided to rebuild the module and be more careful about the grounding. The first circuit change was to switch from the glass 12J5 pre-amp to a 12SR7. This is a similar tube but it is metal. Other than simply improving the grounding, the only other major change was the elimination of voltage feedback from the 6V6 back to the cathode of stage 1 of the 6SN7 to simple shunt feedback using 700K from plate to plate (6V6 plate to the stage 2 6SN7 plate). This shunt feedback method was very stable and effective. This allowed my global feedback to work more effectively as it lowered the 6V6 impedance. The resulting module is much improved.


* ART13Stock.jpg (469.43 KB, 1696x2332 - viewed 346 times.)

* ART13Audio_New_WU2D.jpg (205.75 KB, 2274x1706 - viewed 365 times.)
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N2DTS
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« Reply #2 on: November 30, 2009, 09:37:33 PM »

Well, it looks like fun.
I never had an art13, but they look neat.

What do you mean by a lot of mercury on the side of the tube?
They dont have any inside, except for whatever the getter is, which is flashed when the tube is made.

Most 811's seem to be good unless there are holes in the plates!

Brett
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N2DTS
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« Reply #3 on: November 30, 2009, 09:43:18 PM »

I never had luck with feedback unless I bypassed it for very high audio freq's because of the phaze shift in the iron.
I would try something like a small cap across the cathode resistor where the feedback goes in, to shunt any supersonic stuff to ground.

Brett
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WU2D
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CW is just a narrower version of AM


« Reply #4 on: November 30, 2009, 10:01:12 PM »

If you gots good iron - you donts needs no feedback!!! Aggga ggagga

Unfortunately this is not the case with military Iron which is a bit restricted in its response so the feedback smooths a lot out!

You know you are probably right - I should pop the original 811's back in and see.

Mike WU2D
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KB2WIG
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« Reply #5 on: November 30, 2009, 10:06:43 PM »

I feel your pain.


klc
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N2DTS
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« Reply #6 on: November 30, 2009, 10:14:37 PM »

I remember Bruce, wa3jvj always ran tubes with holes in the plates.
He refused to pay over $.25 for an 811a for his globe king 400 (the wall of sound).
He always ran his worst tubes, I dont know why, he had loads of tubes.

I suppose they must wear out eventualy, but the art13 only runs about 1200 volts I think, hard to ruin them at that voltage. I always ran mine at 1500 volts zero bias without any sign of color on the plates.

When things get old, I tend to suspect caps, leaky old dried out caps, resistors that change value, tubes are last, as I have not had a tube of any kind fail for 25 years at least...

Brett
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W3RSW
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Rick & "Roosevelt"


« Reply #7 on: December 01, 2009, 10:53:43 AM »

Quote
...tubes are last, as I have not had a tube of any kind fail for 25 years at least...
whoa...
It's bad luck to even mention tubes.

I hope I wispered  that quietly enough.  You know what's going to happen to yours now, don't you?  Grin
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RICK  *W3RSW*
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« Reply #8 on: December 01, 2009, 01:50:46 PM »

Mike

There is a guy on eBay that is selling a bunch of ART13 mod xfmrs -- for the record

Al
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