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#1
Posted to rec.audio.tubes
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Class-B amplifier, control grids grounded, input to the screens.
In figu
http://1fcr.com.ar/classB.jpg you can see a very interesting diagram of the audio modulator of an old AM Gonset transmiter. A 12AT7 as voltage amplifier. one 12AU7 with both triodes in parallel as power amplifier and two 6DQ6A grounded cathode, grounded control grid with audio input to the screen grids working push pull as pure class B. R211 and R212 are only used for measurement purpose. Why would they choose to use the screen instead of the control grid as input ? . Any suggestion ? -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: --- |
#2
Posted to rec.audio.tubes
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Class-B amplifier, control grids grounded, input to the screens.
On 16 Nov, 22:06, Alejandro Lieber wrote:
In figu http://1fcr.com.ar/classB.jpg you can see a very interesting diagram of the audio modulator of an old AM Gonset transmiter. A 12AT7 as voltage amplifier. one 12AU7 with both triodes in parallel as power amplifier and two 6DQ6A grounded cathode, grounded control grid with audio input to the screen grids working push pull as pure class B. R211 and R212 are only used for measurement purpose. Why would they choose to use the screen instead of the control grid as input ? . Any suggestion ? -- Alejandro Lieber *LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map:http://1fcr.com.ar With a tube like 6DQ6 and other TV line output tubes the Eg2 needed for many purposes is much lower than Ea than for say EL34. If you examine the Ea vs Ia Ra curves for normal tetrode operation you'd find the Eg2 used for the curves, and you should see that for where Eg1 = 0V, you don't need a very high Eg2 to give a peak anode current of maybe 0.4A at low Ea, so the screens in such tubes have relatively high gm and thus don't require a huge screen to screen drive signal if the Eg1 is fixed at 0V. Examination of the screen characteristics confirm this. Hi-fidelity isn't needed and distortion doesn't worry some users of radio transmitters. Although screens draw current when they go above 0V, the Ig2 isn't huge, and the paralleled 12AU7 looks set up so that it can make the input power needed for the 6DQ6. My guess is that the IST has a fair old step up ratio, and of course wide AF BW don't matter much either, and there is no NFB used to make thing more linear. I guess the load of the RF section of transmitter has a constant input resistance so the high Rout of the modulator does not matter. In effect the 6DQ6 are working as triodes, but because of relative electrode spacing the Ra in this mode is about the same as tetrode Ra when "normal" g1 drive is used, but the gm is very much lower than gm for g1, hence the g2 µ is low. There are some who may connect g2 to g1, and drive both, a bit wild, because its like tetrode mode positive FB, but its doable. For hi-fi, one could also drive g2, but apply NFB to g1, also crazy because not much NFB can be applied because Vg2 has then to be quite high, and its high already as it is. Most modulators I've seen are mostly 807s with UL OPTs and all done like a normal Williamson or similar, but OPT is say 1:1 and sec does the business at RF amp Other tubes are 6CD6, 6CM5 etc. One may find the screen drive remarkably linear. OK, its in screen-triode mode, and there must be some internal NFB as there is in any triode, and because Ra is so airey-fairey and high ohm value, slight internal triode NFB gives linearity, but the Ra value changes little. That may sound queer, but from the curves it must be about right. I guess g1 acts to screen off space charge between cathode and g1. Anyway, the screen drive would be acceptable for hi-fi if just the right value of Eg2 at idle is chosen when Eg1 is connected to cathodes, so that a reasonable amount of Ia flows at idle, say 50mA, to allow class AB2 screen drive. I won't try the Gonset schema any time soon, but you can, so have fun. Patrick Turner. |
#3
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Commonly in those tube modulators you would see this hookup using 807s. Often both G1 & G2 are driven. A common triode modulator for mobile applications was the 6N7 & its relatives of which there are several. A 12AX7 makes a good Class B modulator for a low power rig. Sometime a carbon mike would be used on the front end amplifier driving straight into the cathode. That way the mike got the necessary current for its operation. Cheers, John |
#4
Posted to rec.audio.tubes
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Class-B amplifier, control grids grounded, input to the screens.
patrick-turner wrote:
On 16 Nov, 22:06, Alejandro Lieber wrote: In figu http://1fcr.com.ar/classB.jpg you can see a very interesting diagram of the audio modulator of an old AM Gonset transmiter. A 12AT7 as voltage amplifier. one 12AU7 with both triodes in parallel as power amplifier and two 6DQ6A grounded cathode, grounded control grid with audio input to the screen grids working push pull as pure class B. R211 and R212 are only used for measurement purpose. Why would they choose to use the screen instead of the control grid as input ? . Any suggestion ? -- Alejandro Lieber LU1FCR ......... There are some who may connect g2 to g1, and drive both, a bit wild, because its like tetrode mode positive FB, but its doable. For hi-fi, one could also drive g2, but apply NFB to g1, also crazy because not much NFB can be applied because Vg2 has then to be quite high, and its high already as it is. Most modulators I've seen are mostly 807s with UL OPTs and all done like a normal Williamson or similar, but OPT is say 1:1 and sec does the business at RF amp Zero bias 807 (1625) class B audio amplifier: http://1fcr.com.ar/807classB.jpg .... Patrick Turner. -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: --- |
#5
Posted to rec.audio.tubes
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Class-B amplifier, control grids grounded, input to the screens.
John L Stewart wrote:
'Alejandro Lieber[_3_ Wrote: ;963949']In figu http://1fcr.com.ar/classB.jpg you can see a very interesting diagram of the audio modulator of an old AM Gonset transmiter. A 12AT7 as voltage amplifier. one 12AU7 with both triodes in parallel as power amplifier and two 6DQ6A grounded cathode, grounded control grid with audio input to the screen grids working push pull as pure class B. R211 and R212 are only used for measurement purpose. Why would they choose to use the screen instead of the control grid as input ? . Any suggestion ? -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: --- The short answer is that the standing current of the Class B power amp is very low, an important consideration in this kind of application. Fidelity is not important. Voice communication would normally be something like 30-3000 Hz with 12 db/octave slopes on each side. Commonly in those tube modulators you would see this hookup using 807s. Often both G1 & G2 are driven. A common triode modulator for mobile applications was the 6N7 & its relatives of which there are several. A 12AX7 makes a good Class B modulator for a low power rig. I would say a 12AU7, not even a 12AT7. Sometime a carbon mike would be used on the front end amplifier driving straight into the cathode. That way the mike got the necessary current for its operation. Cheers, John -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: --- |
#6
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But here are more examples of Class B vacuum tube modulators. These were referred to as Zero Bias Amplifiers. Cheers, John |
#7
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In voice comm the users are interested in what they call 'talk power', the ability to communicate even in difficult envirnments. So the circuitry often includes speech clippers & audio BW limiting in order to maintain a high modualtuion index. Not hifi at all. Hifi would occupy too much RF BW. Below 300 Hz is reserved for signalling & control. One widely used sysem was CTCSS. There are about 50 standard tones used depending on the manufactuer & system. Refer to link- http://en.wikipedia.org/wiki/Continu...Squelch_System More recently comm systems are converting to the digital APCO25 which can occupy 12.5 or 25 KHz. It is backwards compatible to older sysems still in use. More advanced systems are the NXDN by Kenwood & ICOM. Another is the HPD by Motorola. Both can run in 6.25 KHz BW but are not compatible with each other. The modulation schemes are quite different. See the Aeroflex 3920 test set here- http://www.aeroflex.com/ats/products...o_Test_Set~171 System testing is getting complicated, I sold many of these. Cheers, John |
#8
Posted to rec.audio.tubes
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Class-B amplifier, control grids grounded, input to the screens.
On Friday, November 16, 2012 5:06:01 AM UTC-6, Alejandro Lieber wrote:
In figu http://1fcr.com.ar/classB.jpg you can see a very interesting diagram of the audio modulator of an old AM Gonset transmiter. Hi All, I am an amateur radio operator, and I've done a fair amount of work with the Gonset circuit. See the following post, and it starts at the bottom (reverse order): http://www.amforever.com/viewtopic.php?f=2&t=1218 Jim WD5JKO |
#9
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I once built an audio amp using PPP Class AB1 6CD6GAs running 980 volts on the plates. It made an honest 120 watts. All Hammond iron. Cheers, John |
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