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Revamping A Modulator Input Circuit




 
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Author Topic: Revamping A Modulator Input Circuit  (Read 6634 times)
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KA2DZT
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« Reply #50 on: April 21, 2018, 07:04:13 PM »

when you use the center tap of a full bridge loaded transformer to have a lower DC voltage, DON'T forget to put a diode in series with the center tap!!


I've built many of this type of power supply,  never needed a diode in series with the CT.  The FWB diodes are already in series with the CT.

Fred
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #51 on: April 21, 2018, 08:21:50 PM »

When you don't put a diode in series, you lift the AC winding to a DC voltage! In this case, your AC winding will be 300VDC
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KA2DZT
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« Reply #52 on: April 21, 2018, 08:48:45 PM »

When you don't put a diode in series, you lift the AC winding to a DC voltage! In this case, your AC winding will be 300VDC


Doesn't matter,  The winding is already 300 volts above ground with FWB rectifiers.  What is at the CT is a 300 volt DC pulse at 120 Hz.  You don;t need any diodes in series with the CT.  The FWB diodes are already in series with the CT.  Current is only flowing in one direction from the CT.

The xfmr winding does not have any DC voltage on it.  It's all AC voltage.  If it's a 600vac winding then there is always 300vac at the CT.

OTH you must use a choke input filter off the CT.  If you use a cap filter or even a CLC filter then the high voltage output also becomes a cap input filter.  So then you could use a diode in series with the CT.

I never use cap input filters for xmtr service.

Fred
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PA0NVD
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« Reply #53 on: April 21, 2018, 11:59:28 PM »

I am sorry Fred, you are wrong in this case.
The center tap in your opinion should be connected to a filter cap for the 300VDC. So there is NO ac at the centertap, the 300V filter.cap short-circuits the AC to ground. When the center tap is directly connected to a cap charged to 300 VDC, the upper half of the winding + THE 300VDC will charge the HV caps to 600V or deliver the current to the 600VDC user in case of an choke input filter for the HV. This charging current comes from the top value of the half sec + the 300VDC in the cap, and NOT from the whole winding (until the 300V cap is partly discharged) so the cap gets a VERY HIGH surge discharge current every time the HVcap  is charged or the HV user draws current, when the HV delivers current to a load.. That results in a high ripple at the 300V when you draw current from the HV When you don't draw HV current or the current by the 600V user is MUCH lower than the 300 V user, no problem, but that is NOT the case.
You have to put a diode in series with the center tap in this application OR the 300VDC should be a choke input filter, but than you have less voltage than 300VDC for the low voltage users..
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KA2DZT
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« Reply #54 on: April 22, 2018, 12:54:24 AM »

No that's not what I said.  I said that the CT should have a choke input filter.  If you use a cap input filter on the CT then that will cause the HV filter to also become a cap input filter, even though there may be a choke input filter on the HV.  You're saying about the same thing, only saying it differently.

If you use choke input filters you don't need a diode off the CT.  If you use a cap input filter on the CT, then yes, you should put one or two diodes in series with the CT to prevent the HV from seeing the cap on the CT.

A cap directly on the CT will charge higher than RMS voltage.  This higher DC voltage will bias the CT higher than the RMS value.  This will cause the HV to also rise to a higher voltage.

That's why I never use cap input filters for xmtr service.

Fred

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PA0NVD
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« Reply #55 on: April 22, 2018, 09:07:02 AM »

Ok You are right. Now I read a gain, we indeed tell the same at a different way.
For the CT I normally use a diode and a cap input to have more voltage, smaller and simpler and because so far I used it as a low current supply. Saves a choke
This tread seems also good to improve my English.. Wink
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KA2DZT
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« Reply #56 on: April 22, 2018, 09:59:04 AM »

Ok You are right. Now I read a gain, we indeed tell the same at a different way.
For the CT I normally use a diode and a cap input to have more voltage, smaller and simpler and because so far I used it as a low current supply. Saves a choke
This tread seems also good to improve my English.. Wink

Your English is not that bad,  I've seen worse.  But, I'll re-write your first line.

OK, you are right.  Now I read it again, we indeed say the same thing in a different way.

The rest of your post is OK FB

Fred
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PA0NVD
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« Reply #57 on: April 22, 2018, 10:04:56 AM »

Thanks for the lesson Fred, Highly appreciated
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N1BCG
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« Reply #58 on: April 22, 2018, 10:51:09 AM »

I'm simulcasting this with another thread but wanted to share the solution that's working well. The 250V point supplies the 6SN7 (15mA) while the 150V point supplies the four 6146 screens (35mA) and OA2 regulator (15mA). The power transformer doesn't seem to notice the 65mA drawn from the center tap.

By the way, Nico's English is quite good. If this thread were all in Nederlandse, then I'd be in deep trouble as dat is niet gemakkelijk ;-)

Drawings, schematics? Modulation section? Speech Amplifier?

Ugh. The original schematic was drawn in pencil and the paper is wearing thin from all the erase-and-change that's been going on. Need to have things settle before sitting down to the drafting table! And by the way, I'm hesitant to show that the modulator uses a (G-A-S-P!) capacitor input supply (here come the torches, pitchforks, lectures, and pontification).


* IMG_7786.JPG (1831.92 KB, 4032x3024 - viewed 59 times.)
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PA0NVD
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« Reply #59 on: April 22, 2018, 11:44:34 AM »

No problems from my side, the current draw is low. But I should put a diode instead of a resistor in series with the CT. What is your opinion in this case Fred?
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« Reply #60 on: April 22, 2018, 12:50:42 PM »

I'm simulcasting this with another thread but wanted to share the solution that's working well. The 250V point supplies the 6SN7 (15mA) while the 150V point supplies the four 6146 screens (35mA) and OA2 regulator (15mA). The power transformer doesn't seem to notice the 65mA drawn from the center tap.

By the way, Nico's English is quite good. If this thread were all in Nederlandse, then I'd be in deep trouble as dat is niet gemakkelijk ;-)

Drawings, schematics? Modulation section? Speech Amplifier?

Ugh. The original schematic was drawn in pencil and the paper is wearing thin from all the erase-and-change that's been going on. Need to have things settle before sitting down to the drafting table! And by the way, I'm hesitant to show that the modulator uses a (G-A-S-P!) capacitor input supply (here come the torches, pitchforks, lectures, and pontification).

Haha  Grin  And I too think Nic's english is pretty good.

You must have one Honkin' transformer.  Smiley

As long as your ripple voltage at the 250V point does not result in hum on the 6SN7 plates you should be good to go.

BTW, I am not trying to hijack your thread or trying to tell you what to do, just making suggestions.    Cheesy


Phil

* Clarks LV Power Supply.pdf (37.18 KB - downloaded 28 times.)
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N1BCG
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« Reply #61 on: April 22, 2018, 01:10:56 PM »

You must have one Honkin' transformer.  Smiley

BTW, I am not trying to hijack your thread or trying to tell you what to do, just making suggestions.

The original transformer failed years ago so I put in my prized RCA color television transformer. I'm not using the 5 or 6V windings, just the HV and now with CT. Yep, it's honkin'.

No worries and no hijacking here. With all the mysteries of this transmitter, this thread could easily morph into a completely different topic. To fill in some of the blanks for the sliderule jockeys, R1 which comes off the CT is 1500 @ 5W, and R2 which drops to the regulator is 6.8k @5W.
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KA2DZT
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« Reply #62 on: April 22, 2018, 01:48:54 PM »

No problems from my side, the current draw is low. But I should put a diode instead of a resistor in series with the CT. What is your opinion in this case Fred?

The resistor is part of the filter.  Whether a diode in series with the CT is needed really depends on the resistor value.  A few hundred ohms and it would not be much of a RC filter. Almost more like a cap only filter. So I would add the series diode off the CT.

If the resistor is a few thousand ohms, probably you would not need the diode.  But,  adding the series diode in all cases really doesn't hurt anything.  Diodes are cheap enough.  It is just something that I never do because I always use LC filters.

Fred
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PA0NVD
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« Reply #63 on: April 22, 2018, 02:11:52 PM »

I consider an LC filter is absolutely the best Fred, also less stress for the rectifier and the transformer, but sometimes space and availability of chokes drove me to other solutions. I produced many 27 MHz RF generators for plastic welding, 300W, 500W, 1 kW and 2 kW with MOSFET's. The voltage for the driver and the control electronics was derived this way from the CT via a diode. For the other electronics this voltage was fed to a switching regulator to get + /- 12V and 5 Volts. To find chokes for 35V, 2 - 3 amps was not easy and a reliable source for many years was problematic, especially for series production and the size was a problem as well. In the USA these things are more easy.
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KA2DZT
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« Reply #64 on: April 22, 2018, 02:20:30 PM »

Clark,

Those RCA TV xfmrs can handle a lot of over load.  Especially if you're not using the filament windings.  I had about 20 NOS RCA TV xfmrs from the 50s.  I'm using two in one of my HB xmtrs.

I once used two to make a 1400v power supply at 300ma load as an experiment.  Worked great.  Both xfmr were about 765vac CT.  I used FWB rectifiers on both xfmrs.  I connected the diode output of one xfmr to the negative terminal of the FWB diodes on the second xfmr.  This added the two voltages together.  I used a choke input filter off the diode output of the second xfmr.  Loaded it down to 300ma.  Ran it for hours.  1400 VDC at 300ma.

The second xfmr was modified.  I removed all the filament windings and increased the insulation around the core.

Fred
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« Reply #65 on: April 22, 2018, 02:30:29 PM »

Something like this?   Cheesy

* Clarks LV Power Supply.pdf (44.97 KB - downloaded 34 times.)

* Slide Rules More.jpg (77.77 KB, 794x472 - viewed 57 times.)

* Slide Rule Pocket_slide_rule.jpg (62.88 KB, 558x152 - viewed 54 times.)
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« Reply #66 on: April 26, 2018, 12:39:38 PM »

If the PDF schematic below is Clarks PS then I can see why there is a weird voltage drop across Rdrop2 since some of the FWB current through ground is going up and through the gas regulator circuit.

https://www.electronics-tutorials.ws/diode/diode_6.html


Phil - AC0OB

* Clarks Power Supply.pdf (46.76 KB - downloaded 33 times.)
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N1BCG
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« Reply #67 on: April 26, 2018, 09:27:36 PM »

This circuit is working quite well, and the sufficiently high input resistance (1.5k) seems to eliminate the need for a choke. If there's any weird voltage at the regulated output it's because I drew the OA2 upside down. Additionally, I cited the screen current incorrectly as the 35mA was for the RF finals, *not* the modulator screens, which is MUCH lower.

The challenge du jour is fabricating a new negative feedback ladder to accommodate the new 6SN7 input circuit. Significantly less FB is needed thanks to the higher gain/impedance of the input circuit.
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PA0NVD
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« Reply #68 on: April 26, 2018, 10:19:20 PM »

Before you make that ladder try first to add 2 cathode resistors to the 6SN7 cathodes, eg 150 Ohm or so and see the result in distorsion improvement, I think no other neg feedback is required.
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PA0NVD
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« Reply #69 on: April 26, 2018, 10:31:04 PM »

I was too fast with sending, sorry.
The best is to add these resistors and drive the cathodes with a real current source, e.g. a FET instead of a resistor to ground. That will guarantee an exact balance to the 6146 tubes.
So select a JFET that has the approx. the required drain current at zero gate volts.(in your case approx. 15 mA) The drain impedance of a JFET is very high, so the sum current of the two triodes is always the same.  Than add the 2 resistors from the drain to the cathodes. ( this is a stable and simple negative feedback methode improving the linearity of the tube.  That will give you a good linearity and an excellent balance. But all this may be a little overdone, that's more for HI-FI amps, (but still fun Grin)
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N1BCG
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« Reply #70 on: April 26, 2018, 10:36:48 PM »

The feedback I was thinking of is from the modulation transformer back to the audio input. About 3dB is what I had in mind, but because the 6SN7 offers a much higher impedance point for the feedback than the previously used 5k input transformer secondary, the FB ladder I have needs a bit more loss added.
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« Reply #71 on: April 27, 2018, 08:50:55 AM »

As it turns out, a discussion of feedback is going to bring us right back to the input circuit design. It's now looking as though the 6SN7 should have separate cathode resistors in order to provide a return point for the FB circuit.

There's a single 300 Ohm resistor now. How about 560 on each cathode, 18k on each plate, and 260V source?
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« Reply #72 on: April 27, 2018, 12:19:19 PM »

As it turns out, a discussion of feedback is going to bring us right back to the input circuit design. It's now looking as though the 6SN7 should have separate cathode resistors in order to provide a return point for the FB circuit.

There's a single 300 Ohm resistor now. How about 560 on each cathode, 18k on each plate, and 260V source?

I agree with Nic, kill any distortion up front before complicating matters with a feedback system.

The proposed circuit was biased into the more linear portion of the operating curve so distortion should be minimal.

How's about a 300V source and separate, un-bypassed 680 ohm cathode resistors like this:


Phil - AC0OB

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KA2DZT
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« Reply #73 on: April 27, 2018, 12:43:11 PM »

Phil,

Check your schematic you just posted.  You have the two cathode resistors jumped together,  right where you have the 5V notation.

Fred
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« Reply #74 on: April 27, 2018, 02:26:32 PM »

Phil,

Check your schematic you just posted.  You have the two cathode resistors jumped together,  right where you have the 5V notation.

Fred

Thanks Fred, I corrected it.  Smiley

Sometimes I need a proofreader to check my schematics.  Grin


Phil - AC0OB
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