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Screen Modulation - 6DQ6 or 6146 for best linearity?




 
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Author Topic: Screen Modulation - 6DQ6 or 6146 for best linearity?  (Read 2205 times)
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w8khk
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« Reply #25 on: September 19, 2022, 02:08:33 PM »

Somewhere in the 1952 issues of CQ is a big write up on the Rothman system.  July 1952 I think.

Also:

https://worldradiohistory.com/hd2/IDX-AUSTRALIA/IDX/Amateur-Radio/50s/Amateur-Radio-AU-1952159.pdf

https://patents.google.com/patent/US2765443A/en

Sounds interesting......

Matt, thanks for sharing.  I discovered those links when the Rothman was previously mentioned.  Will not fit in the T-60 build, but perhaps in the other exciter I am building.  I am not a great fan of the "controlled carrier" sound, but it could prove interesting to play with.
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« Reply #26 on: September 19, 2022, 04:57:24 PM »

Mike, I just noticed you mentioned you were interested in the Mini CNC.

Attached are a couple pictures, one showing the controller based upon Arduino, to which I added limit switches with optically-isolated interface.

This mill is under $400 via Amazon, and is very solid.   If you are interested in one, I suggest you opt for a build that uses machined aluminum for the structure.  Avoid plastics or 3-D printed brackets at all costs, they just do not hold up or provide accurate positioning with the stresses of the high-speed cutting bits.

That is very interesting and the cost is surprisingly low. Does this mean that I can finally ditch my nibbling tool forever? 😉
I looked on Amazon and they had a bunch of them for sale. Will check them out.

Edit: 09/20/2022 5:07 AM. Removed second part of post. K9MB


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« Reply #27 on: September 20, 2022, 01:01:49 AM »


...One of the things I did not like about the T60 is the low B+ voltages and with that the voltage doubler circuit.
One thing that a voltage double has, though- is that one end of the transformer winding goes to ground.

I have never seen a Full Wave Voltage Doubler circuit with one end of the secondary transformer going to ground.

A good FWVD design has the lower end of the secondary transformer going to the center connections of the first set of filter capacitors, and then to a set of series filtering resistors, and then on to another set of filter capacitors, loaded with a set of bleeder resistors.

In fact, when properly designed, one can get at least three voltages: HV,  MV, and LV from a FWVD.

Below is what I use for a transmitter that uses dual 6146 Finals or a sweep tube final such as a 6DQ5.

As for the T-60 final, 90 mA plate current is about nominal.

Phil - AC0OB

* Voltage Doubler Full Wave LV and HV PS.pdf (36.51 KB - downloaded 53 times.)
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K9MB
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« Reply #28 on: September 20, 2022, 06:13:15 AM »


...One of the things I did not like about the T60 is the low B+ voltages and with that the voltage doubler circuit.
One thing that a voltage double has, though- is that one end of the transformer winding goes to ground.

I have never seen a Full Wave Voltage Doubler circuit with one end of the secondary transformer going to ground.

A good FWVD design has the lower end of the secondary transformer going to the center connections of the first set of filter capacitors, and then to a set of series filtering resistors, and then on to another set of filter capacitors, loaded with a set of bleeder resistors.

In fact, when properly designed, one can get at least three voltages: HV,  MV, and LV from a FWVD.

Below is what I use for a transmitter that uses dual 6146 Finals or a sweep tube final such as a 6DQ5.

As for the T-60 final, 90 mA plate current is about nominal.

Phil - AC0OB

Hi Phil,
Thank you for pointing out the obvious. I appreciate it.  I have no explanation for being an idiot, in this instance, since I knew better if I had been fully conscious…😉🤪

That low voltage winding could theoretically be inserted in series with the bottom of the winding, but given that it is a cheap Import, it might not be able to handle the voltage to ground, so suggestion withdrawn..
73, Mike
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« Reply #29 on: September 20, 2022, 11:26:12 PM »

For what it is, the 6DQ6 is probably capable of maxing out the power supply. Look at the peak current to be had for the 6146-like heater current, which is somewhat a measure of reserve emission, not that the average rating should be exceeded. Tons of current with low screen and plate voltages.

Unless you want to put a larger transformer in there or go outboard with the power supply, would I be wrong to say that the 6DQ6 is enough? 

Maybe a slight tweak to anode load resistance to take advantage of what the 6DQ6 offers over the 6146 could be in order but that's probably nit picking!
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K9MB
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« Reply #30 on: September 21, 2022, 11:47:41 AM »

For what it is, the 6DQ6 is probably capable of maxing out the power supply. Look at the peak current to be had for the 6146-like heater current, which is somewhat a measure of reserve emission, not that the average rating should be exceeded. Tons of current with low screen and plate voltages.

Unless you want to put a larger transformer in there or go outboard with the power supply, would I be wrong to say that the 6DQ6 is enough? 

Maybe a slight tweak to anode load resistance to take advantage of what the 6DQ6 offers over the 6146 could be in order but that's probably nit picking!

Well, my rocket engine example on a bicycle serves to caution on practical limits set by space and power capabilities. I suspect that a 200vA Antek toroid might be wedged in and give a lot more power, but then the tuning cap would likely arc over. It is really a wide space receiving cap.
A lot of effort would possibly yield a much more powerful transmitter but that would only add about 3dB to the signal level if it was doubled. I must agree that making a better T60 need not extend to making it more powerful. Better keying, a DDS and
Improving the modulator may make it a fun QRP AM rig and also a nice compact CW rig that can help we who ran these transmitters in our youth experience some fun and nostalgia. MB
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lu8dam
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« Reply #31 on: September 22, 2022, 08:24:35 PM »

Greetings from Argentina.
Is it convenient for you to use low level modulation on screen for audio tests?
I experienced a single 6DQ6 modulated to reactor by an EL34 (or 6L6). But for 10 watts with another 6DQ6 in class A it can work fine. I have a video of a 6DQ6 transmitter for another transformer modulated 6DQ6 where I did audiofrequency tests, with 12 watts, without linearity problems.
https://youtu.be/QElyW8HPEeM

Since I'm using a pre-emphasis, you can hear some overmodulation in the higher frequencies of the audio, but when I added a compressor that problem lessened.
Greetings to all.
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lu8dam
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« Reply #32 on: September 22, 2022, 08:29:10 PM »

And I have a question! Why do you use a modulation percentage greater than 90 or 100% in shortwave? That caught my attention, since due to a question of not obtaining deformations during fading, the modulation percentage does not reach 100%.
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K9MB
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« Reply #33 on: September 23, 2022, 10:41:36 AM »

And I have a question! Why do you use a modulation percentage greater than 90 or 100% in shortwave? That caught my attention, since due to a question of not obtaining deformations during fading, the modulation percentage does not reach 100%.

Are you talking about distortion in received audio from selective fading?
That phenomenon is endemic to short wave isn’t it?

The only way I know to counter it is by using synchronous detection. That does require sufficient s/n to work, but the distortion can be much reduced I hear.
MB
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lu8dam
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« Reply #34 on: September 23, 2022, 12:40:44 PM »

Effectively.
With a lower percentage, this phenomenon is counteracted, as it is audible (without the need for synchronous detection).
The only thing that strikes me is that I see US stations that modulate 125%, a common modulation in Medium Wave. The radio stations that are listened to in Short Wave do not reach 100%.
All this draws my attention because I have never exchanged a talk with radio constructors from other cultures and nationalities.
Sincerely.
LU8DAM
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WA4WAX
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« Reply #35 on: November 15, 2022, 06:33:03 PM »

Here is a circuit of the Rothman system with some explanations.

Get about 75% modulation, but easy to do.

https://worldradiohistory.com/Archive-DX/QST/50s/QST-1952-01.pdf
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w8khk
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« Reply #36 on: November 15, 2022, 08:17:10 PM »

Here is a circuit of the Rothman system with some explanations.

Get about 75% modulation, but easy to do.

https://worldradiohistory.com/Archive-DX/QST/50s/QST-1952-01.pdf

Matt, that is an interesting concept, but a bit over the top to fit into the Knight T-60 enclosure.  Thank you, I will file it away for future reference.  I am already pushing the limits with the addition of the DDS VFO and LCD display.  I will be back to that project shortly, but other responsibilities have pre-empted my work on it for a while.
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Rick / W8KHK  ex WB2HKX, WB4GNR
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« Reply #37 on: November 17, 2022, 10:05:49 AM »

Here is a review article from '65.

https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1965/73-magazine-09-september-1965.pdf
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