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Revamping A Modulator Input Circuit




 
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N1BCG
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« on: April 14, 2018, 10:12:23 AM »

Here's the current scenario:

Four 6146s in push-pull are driven at about +8dBm by a 600:5000CT transformer. The transformer is a military item with a reported 400-5000 c.p.s. response, and it sounds it. A significant improvement is realized when the secondary is loaded with a 5k resistor, but the voltage swing drops to an unusable low level.

I'm thinking that another stage of amplification is needed, such as from a dual triode. The task is to pick one out. I have a high quality 600CT:15k line level input transformer that shows -1 to +0dB from 20-30k.

The four 6146 grids to be driven are biased at -47 Volts and each pair of cathodes have 5 Ohm resistors with bypass caps to ground, primarily for current monitoring.

I've attached a schematic of my proposed approach. Thoughts for a tube?


* Img_7754.jpg (3895.98 KB, 4027x1985 - viewed 140 times.)
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PA0NVD
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« Reply #1 on: April 14, 2018, 12:35:34 PM »

Because the anode load is a little high, 6SN7 at 6 - 8 mA per section That will give you 33 k, 2 - 3 Watt anode resistor and B+ of 400 - 500 V or  22 K at B+ of 250 VDC Plenty swing to drive the grids at low distorsion. Please add the grid leak in your diagram Wink
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PA0NVD
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« Reply #2 on: April 14, 2018, 12:40:06 PM »

do not bypass the 5 Ohm cathode resistors of the 6146 tubes. Bypassing with a cap that has less than 5 Ohms Xc is difficult. And the tube has some neg. feedback with a non-bypassed resistor which improves linearity at cost of a little extra drive. With the 6SN7 there is plenty drive available.
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N1BCG
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« Reply #3 on: April 14, 2018, 01:47:44 PM »

Please add the grid leak in your diagram Wink

Ja, grappig Wink Let's use a 47k resistor from each grid to ground since the transformer lacks a center tap on the secondary.

The Sylvania Tube Manual suggests a plate impedance of 6700 Ohms. If I used that value for plate resistors, there would be a 67 Volt drop @ 10mA, thus requiring a 164 Volt supply (7 more Volts are lost in the cathode resistor for grid bias) which could come from the OA2 (150V) regulated supply for the 6146 screens.
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DMOD
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« Reply #4 on: April 14, 2018, 03:47:25 PM »

Here is my suggestion using a 6SN7 or other low Mu (20) triode, with a stage gain of 14.

It would be helpful if you supplied more info such as a block diagram of the complete system.


Phil - AC0OB

* Speech Amplifier 6SN7.pdf (61.69 KB - downloaded 56 times.)
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« Reply #5 on: April 14, 2018, 09:21:33 PM »

Here is my suggestion using a 12AU7A triode, with a stage gain of 17, unbypassed.

Bypassing the 168 ohm cathode resistor with a 100 uF@16V electrolytic will get you more gain at the expense of increased distortion.

What you need is a decent amount of voltage drive into the grids of the 6146, so you want the load resistance of the speech amplifier to be about 5 to 10X the source resistance, which is what you see here.

+8dbm = 1.95V rms into 600 ohms.

1.95VX1.414X17 = 47V Peak per plate.

It would be helpful if you supplied more info such as a block diagram of the complete system.

And I trust this is for personal use.  Smiley


Phil - AC0OB

* Speech Amplifier 12AU7A.pdf (36.43 KB - downloaded 42 times.)
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N1BCG
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« Reply #6 on: April 15, 2018, 10:03:30 AM »

I'm leaning toward the 6SN7 solution for a variety of reasons including that an octal socket will fit perfectly in the hole used for the previous input transformer (I lack Greenlees).

For B+, my choices are the 150V regulated supply that the 6146 screens use, the 600V used on the plates via a voltage divider, or locating another tap strap for the 50k/100W bleeder.

Interestingly, this circuit can be "breadboarded" for testing using an octal relay socket with screw terminals. That might be a first, lol. Thank goodness for my EICO 1030 power supply. Just love that...

I posted a block diagram although it may not be what was asked for.


* BlockDiagram.jpg (43.55 KB, 870x418 - viewed 69 times.)
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N1BCG
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« Reply #7 on: April 15, 2018, 02:39:52 PM »

Well, well, well.  Some pineboard style testing produced interesting results using Phil's 6SN7 input circuit (attached).

B+ had to be limited to 150V in order to keep the plate voltage from exceeding 90V (per the manual). Only 2.25V was developed across the cathode resistor (suggesting a combined plate current draw of approximately 5mA) which is half of what the manual suggested yet the tubes tested "good" (I have two).

The grid resistors were changed to 25k each which loaded the input transformer properly and resulted in a -1 to +0 dB from 20Hz to 10kHz with -1dB @15kHz!

This arrangement produced a 70V PtoP output across a 200k load with 0dBm input level.

The next step will be to add a -4dB H-pad on the input for isolation and S/N improvement.

* Speech Amplifier 6SN7.pdf (59.04 KB - downloaded 48 times.)
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KA2DZT
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« Reply #8 on: April 15, 2018, 02:55:36 PM »

Does the input xfmr have a CT on the secondary??  What input xfmr are you using?  no name?? or brand name?
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N1BCG
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« Reply #9 on: April 15, 2018, 03:05:01 PM »

Does the input xfmr have a CT on the secondary??  What input xfmr are you using?  no name?? or brand name?

No, unfortunately. It came out of an LPB 25C AM transmitter. The 600Z primary has a CT, which could be used for remote switching by putting DC between the balanced line and ground to operate a relay. I believe it has a 15k secondary based on voltage ratio measurements. Pretty good specs, though!
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DMOD
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« Reply #10 on: April 15, 2018, 03:59:37 PM »

Well, well, well.  Some pineboard style testing produced interesting results using Phil's 6SN7 input circuit (attached).

B+ had to be limited to 150V in order to keep the plate voltage from exceeding 90V (per the manual). Only 2.25V was developed across the cathode resistor (suggesting a combined plate current draw of approximately 5mA) which is half of what the manual suggested yet the tubes tested "good" (I have two).

The grid resistors were changed to 25k each which loaded the input transformer properly and resulted in a -1 to +0 dB from 20Hz to 10kHz with -1dB @15kHz!

This arrangement produced a 70V PtoP output across a 200k load with 0dBm input level.

The next step will be to add a -4dB H-pad on the input for isolation and S/N improvement.

My circuit was based on the 6SN7 Characteristic Curves for plate voltages of 150V@8 mA and a cathode bias of 3.5 V, and a 600 ohm 1:1 isolation transformer. The voltage Gain of the stage was 14 (23 dBV) with the values given.

The primary of the transformer was already loaded.

The cathode resistor should be 220 ohms for a cathode voltage of 3.5V@16 mA.

Where did this 90V Vp requirement come from? Huh


Phil


BTW, have you looked at your total current/power budget? If this is a 4X4 6146 transmitter the estimated powerbudget is 700V@1.25A > 1kW if you include filament power?

* Speech Amplifier 6SN7.pdf (62.76 KB - downloaded 42 times.)
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N1BCG
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« Reply #11 on: April 15, 2018, 04:26:09 PM »

Where did this 90V Vp requirement come from?

I can't seem to avoid it. In addition to the datasheets I've found on line, these books concur for Class A service:

"Characteristics And Typical Operation": 90V @ 10mA per section
  ~ Sylvania Technical Manual

"Characteristics And Ratings": 90V @ 10mA per section
 ~ Essential Characteristics for Receiving Tubes - General Electric

*However*, the 1976 ARRL Handbook shows the 6SN7 as: "250V @ 9mA" on each plate which seems much closer to what I was seeing but didn't want to push. Of course, how could such a stout tube have to run at 90V on the anode. Weird.

No worries on the power budget. B+ is 600V, screens are regulated at 150V, and idle current per pair is 70mA. The RF deck only produces 120W of carrier, so these four 6146s are living in Cushville.
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N1BCG
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« Reply #12 on: April 15, 2018, 04:46:23 PM »

Hmmm. So thereís gain to spare. I might change the 6SN7 grid resistors to 330 Ohms and use D.C. blocking caps on the input. This will never be driven by anything other than a bícast processor.
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DMOD
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« Reply #13 on: April 15, 2018, 05:38:36 PM »

Wow Clark you are just full of surprises   Grin

So is this circuit

1) two 6146s in ClassAB1 Push-Pull driving a single 6146 or

2) two 6146s in ClassAB1 Push-Pull driving a set of paralleled 6146's?   Huh

3) four 6146s in ClassAB1 Push-Pull paralleled driving a set of paralleled 6146's?   Huh

4) four 6146s in ClassAB1 Push-Pull paralled driving a set of four paralleled 6146's?   Huh


I don't know how they came up with those 6SN7 numbers, must be for some low noise, low gain audiophile circuit.

What was their plate load RL and supply voltage???

Both the GE and RCA data for ClassA show running the plates at 250V@9mA, with a -8V bias, but that was in the less linear region than running them 150V@8mA with a Vk of 3.5V.

My goal was to run the 6SN7 with the least plate diss. and in the most linear region.

If you want 90 V on the plate then the cathode needs to be at ground since according to the tube curves, the cathode bias should be = 0V for 90 volts@10mA.

I still do not see or understand your rationale for loading the heck out of the secondary of the 1:1 input stage transformer, if that's what it is.

Most processors I have ever delt with have an output level adjustment to lower output voltage levels.


Phil
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N1BCG
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« Reply #14 on: April 15, 2018, 06:08:24 PM »

Ah, okay. A few clarifications...

This project is to effectively drive four 6146s where V1 and V2 are in parallel, V3 and V4 are in parallel, and those sets are in push pull. Thatís the modulator. The RF deck is four 6146s all in parallel driven by a Gates BC1G Oscillator/Buffer circuit.

The secondary of the input transformer needs to loaded to a point in order to flatten out the response otherwise the lows roll off at 100Hz and the highs show an upward curve past 16kHz.
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KA2DZT
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« Reply #15 on: April 15, 2018, 06:13:48 PM »

Pretty sure you can put 450 volts on a 6SN7.  I run one in my HB xmtr at 300 volts on the plate.
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N1BCG
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« Reply #16 on: April 15, 2018, 06:31:10 PM »

Another ďAh-HahĒ moment.

Yes, 90V @10mA and -7 for bias. My error was trying to derive that bias from the cathode resistor. Canít be done with an Ep of 90, trust me. So, the next step is to see at what plate voltage I get 20mA combined and a 7 Volt drop across the cathode resistor. In theory, wouldnít it be 350 Ohms?
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W1DAN
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« Reply #17 on: April 15, 2018, 06:50:41 PM »

Clark:

Fun project. I wonder:

1. If you can eliminate the driver transformer and replace with a tube phase inverter? This will allow more system negative feedback.

2. While a left-turn, consider a derivation of the WA1GFZ MOSFET driver. This will allow lower source impedance to the 6146 grids and thus less grid current distortion:

http://amfone.net/Amforum/index.php?topic=23632.0

73,
Dan

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« Reply #18 on: April 15, 2018, 06:55:55 PM »

To my opinion do NOT use 90 Volts, too low. No Idea where that comes from, but it is not correct. I should go for a minimum of 250VDC B+ in order to have a loaded output swing with sufficient headroom to stay away from the distortion zone with max drive required for the 6146   Low distortion = headroom in signal capability. That also gives you the possibility to add some negative feedback and linearize the design by adding non-decoupled cathode resistors to the 6146 tubes Attached the datasheet of the 6SN7


* 6SN7.PNG (53.06 KB, 531x422 - viewed 54 times.)
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N1BCG
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« Reply #19 on: April 15, 2018, 07:36:21 PM »

I was going to let this go for the evening but curiosity got the best of me and I had to fire up the EICO power supply for some more tests. Having broken through the glass plate voltage ceiling, I was able to get 7.5mA per plate at 225 V at the anode.

Iíve attached a schematic showing the circuit powered from the 600V supply. The Volts shown next to the resistors represent the drop.

Perhaps the last step will be to determine what current draw is needed to fully drive the four 6146s and settle on a cathode resistor that yields 7 bias Volts and appropriate dropping resistance.

Thoughts?


* B623B7B2-0D83-4E6A-8A2A-82672EFCF133.jpeg (516.15 KB, 1535x2048 - viewed 66 times.)
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DMOD
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« Reply #20 on: April 15, 2018, 09:29:52 PM »

Clark, I am not sure where your numbers are coming from but here is a tabulation derived from the 6SN7 Tube curves for various parameters, keeping in the most linear portions possible, and which is the only reliable way I have found of getting into the ballpark:

Vs = 300V
Vp = 90V
Ipeach = 10 mA
RPeach = 21k
Vk = 0V
Rkt = 0

Vs = 300V
Vp = 150V
Ipeach = 8 mA
RPeach = 18.75k
Vk = 3.5V
Rkt = 219 ohms

Vs = 350V
Vp = 200V
Ipeach = 10.5 mA
RPeach = 14.3k
Vk = 5.5V
Rkt = 262 ohms

Vs = 400V
Vp = 250V    
Ipeach = 12.5 mA   My Pick of the Litter Grin
RPeach = 12k
Vk = 7V
Rkt = 280 ohms

Vs = 400V
Vp = 250V    
Ipeach = 11 mA
RPeach = 13.6k
Vk = 8V
Rkt = 364 ohms

Vs = 450V
Vp = 300V
Ipeach = 11 mA
RPeach = 13.6k
Vk = 10V
Rkt = 455 ohms

I hope this helps

Phil




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« Reply #21 on: April 15, 2018, 10:36:33 PM »

I am sorry Phil don't completely agree that this is the best way to arrive where you want to be. You have to calculate back from the requirements of the tube drive. The 6146 need 100Vpp grid to grid at 600 V anode to be driven. That means 50Vpp per grid. In order to stay away from the distortion zone, the voltage drop over each anode resistor of the 6SN7 should be MINIMUM. twice that = 100 Volts. Due to the load of the grid resistors which is recommended to be less than 200 kOhm, say 100 kOhm for 1 tube, 47 kOhm for two tubes in parallel, take 50% more (for anode resistors in the order of 22 kOhm), so the voltage drop should be 150 V over each anode resistor. It is best to stay away from bottoming the tube, so better operate in the higher anode volt ranges. Say 250 V at the anode = 250 + 150 = 400 V VD
The recommended anode dissipation is 2,5 Watts, 3,5 Watt MAX, so you can run the tube at 10 mA per section at 15 kOhm anode resistors or e.g. 7 mA  with 22 kOhm resistors if you want to run a little cooler. It is a driver, and if you take sufficient margin at the the 6SN7 output voltage like in this example, you don't have todrive the 6SN7 more than 50% of the max output and you will stay nicely linear. No need to run the tube up to 12,5 mA
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« Reply #22 on: April 16, 2018, 12:21:07 AM »

What I gave him was a set of possible circuit values from the curves.

Each to his own philosophy of approaching circuit design. Smiley

I gave three possibilities with Vsources (Vs) of 400V to 450V.

Each one had at least a 150V margin to the top of the PS, at least a 50 volt margin for signal, and low output impedance if he decides to go with ClassAB2.

I have given a number of possible schematics so may I suggest you post a schematic
of how you think his driver should appear.
 Of course the final decision as to how he will implement it will be up to the OP.

Phil - AC0OB


 
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N1BCG
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« Reply #23 on: April 16, 2018, 06:23:52 AM »

Of course the final decision as to how he will implement it will be up to the OP.

Thatís what clip leads are for! The last components to be soldered will be the plate dropping and cathode resistors.

I had a way to do this before posting here, but itís always beneficial to broaden the options and build a discussion that could inspire others.
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« Reply #24 on: April 16, 2018, 10:16:45 AM »

Ok Phil, misunderstood, I was misled by the first  3 examples. In my opinion, the anode voltage of the first 3 is too lo to have ample signal margin. My pick is the 400 V 11 mA, though I should run a little cooler, a little less current, but that is entirely a personal opinion. Interesting discussions.
I made a simple distortion meter and learned a lot from using that in all kind of amps. From those experiences I ended up with the above philosophy. That was for hi-fi amps driving 2 x KT77 in UL
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