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THE AM BULLETIN BOARD => Technical Forum => Topic started by: N1BCG on April 14, 2018, 10:12:23 AM



Title: Revamping A Modulator Input Circuit
Post by: N1BCG 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?


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD 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 ;)


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG on April 14, 2018, 01:47:44 PM
Please add the grid leak in your diagram ;)

Ja, grappig ;) 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD 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


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD 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.  :)


Phil - AC0OB


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: KA2DZT 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?


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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!


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD 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? ???


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?


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 15, 2018, 05:38:36 PM
Wow Clark you are just full of surprises   ;D

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?   ???

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

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


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


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: KA2DZT 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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?


Title: Re: Revamping A Modulator Input Circuit
Post by: W1DAN 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



Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD 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


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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?


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD 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 ;D
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






Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD 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


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD 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. :)

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


 


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG 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.


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD 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


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG on April 16, 2018, 05:15:39 PM
There's no question that this has been a fascinating project...thank you all!

I've been testing using Phil's original schematic (below) and have achieved the target 100V across a 100k load with a 300V supply @ 13mA, 5V of cathode resistor bias, and approximately 5.4VAC between the 6SN7 grids.

Is this fantastic or is there something I'm not seeing that's not right?


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD on April 17, 2018, 01:30:53 PM
Seems fantastic to me ;D


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 17, 2018, 08:03:30 PM
Well, let's see. 5V Vk/470 RK ohms = 10.5 mA total circuit current for the two triodes which infers 5.3 mA plate current for each triode, which according to the tube curves is in the ballpark.

That would mean about 205 volts should be appearing on the plates and in a linear portion of the APC curves.

Now, 13mA total circuit current through 470 ohms = 6.11 volts Vk. 6.5 mA current through each RP yields a voltage drop of 117 volts, so each Vp should see (300 - 117) = ~ 183 Volts. This is also in the tube curve ballpark, and in a linear portion of the APC curves.

What are the actual plate voltages and component values now being used?



Phil

Addendum: May I suggest one more tweak to move the operation into an even more linear region:

 






Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG on April 18, 2018, 12:36:42 PM
Back in the saddle after a few busy days...

A lot was discovered by putting the circuit in the transmitter and running some real-world audio through it. First, and the most surprising, a *lot* less gain is needed. I was thinking 100V PtoP, but placing a VOM across the 6146 grids revealed that the needle wandered around the center of a 50V scale at full modulation. And that's using -6dBM of drive!  Padding, please!  Those four 6146s only need to produce 60-80 Watts of audio.

Other than that, things sounded pretty good. What's "real-world audio"? Andy Williams' "House Of Bamboo" for example. You all are too young to appreciate that ;-)


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD on April 18, 2018, 01:23:34 PM
Seem all correct to me, First of all, you need 50Vtt at each grid to get full max output, for 4 tubes 100W+ So if you need 60 - 80 Watts, approx 80Vp-p between the grids is sufficient which is approx 50V RMS. In addition, the driver design has about 100% signal margin to stay linear, so plenty of gain to drive the 6146 quad. I like the end result, you run the 6SN7 cool, long life and more reliable.
I advice to limit the drive in an audio stage directly after the mic., compressor limiter or so, Don't attenuate after the 6SN7 or lower its gain, the signal margin gives you superior audio quality.
Congrats with the result!!


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 18, 2018, 02:45:45 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...


You should be able to get approx. 240 Watts out to the antenna with a modulating power requirement of  ~ 130 watts for 100% modulation.

Neat project.


Phil


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG on April 18, 2018, 03:06:17 PM
Congrats with the result!!

Hartelijk bedankt!

You should be able to get approx. 240 Watts out to the antenna with a modulating power requirement of  ~ 130 watts for 100% modulation.

That's where the power budget comes in as I'm now limited by the power supply. Are there any 6SN7 curves that permit a B+ of 150V? I'm running 250V now but I'd prefer to tap off the screen supply for simplicity. Remaining at 250V means adding a power divider from the 600 HVB+ or locating a second clamp tap for the 100W bleeder.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 18, 2018, 03:25:24 PM
Ho Hakiwisilaasamamo Waswasimamo

Here is another potential Power Supply using a Toroidal core transformer. I have used a number of these in various projects.

Two of the HV secondaries are paralleled to provide 1.24 A.

Phil - AC0OB


Title: Re: Revamping A Modulator Input Circuit
Post by: KA2DZT on April 18, 2018, 09:23:26 PM
Should be able to run the speech amp off the 600V supply through a simple dropping resistor and a cap.  Another 15ma load on the supply shouldn't be much of a issue.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 18, 2018, 10:48:55 PM
Should be able to run the speech amp off the 600V supply through a simple dropping resistor and a cap.  Another 15ma load on the supply shouldn't be much of a issue.

But what do you do on KeyUp when the supply goes to 725V? Do you let the 300V rail go to 425V and have the tubes heat up?

We can't neglect the effect of the higher voltage on KeyUp.

One can regulate it with 0A2's or zeners.



Phil - AC0OB



Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD on April 18, 2018, 11:38:14 PM
He runs the tubes at 5 mA each, 425 V Vd shouldn't be a problem.


Title: Re: Revamping A Modulator Input Circuit
Post by: KA2DZT on April 19, 2018, 12:04:47 AM
I was going to suggest a couple of 150 volt gas regulators in series.  In my HB xmtr the 300 volt supply is keyed along with the 600 volt supply.  My xmtr is designed in a way that only allows the voltages to rise less than 10% on key up.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 19, 2018, 12:15:36 AM
He runs the tubes at 5 mA each, 425 V Vd shouldn't be a problem.


Quote from: N1BCG
I've been testing using Phil's original schematic (below) and have achieved the target 100V across a 100k load with a 300V supply @ 13mA, 5V of cathode resistor bias, and approximately 5.4VAC between the 6SN7 grids.

By my accounting he's running each tube at 6.5 mA. I later suggested a move to 7.5 mA for each tube, for more linearity, by decreasing the cathode resistor to 330 ohms.

But that was not the point. The point is you have to consider no load voltages when working with power supplies and branch voltage rails.


Phil


Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD on April 19, 2018, 11:18:28 AM
I agree that 7,5 mA will be a little more linear, that's about the current I suggested in the very first post. When the 600 Volts jumps to 725 with key-up, the Vd for the 6SN7 will jump from 300  to approx 380V, when you use a series dropping resistor, not to 425V, Seems no problem to me.
But off course, the solution with a stabilizer will be the cleanest and prevents plops due to charging/de-charging coupling cap's during the jump which I doubt are noticeable because the 6SN7 drives a balanced amp. I should just leave it as is, but that is my personal opinion, KISS


Title: Re: Revamping A Modulator Input Circuit
Post by: Opcom on April 20, 2018, 12:25:06 AM
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.

I have found this true in experimental breadboarding. At one point I thought a transformer didn't care about being loaded, i.e. driving no grid current, why not just consider the ratio? No load is easier on it than a full or double load, right?

But as it turned out, the frequency performance was seriously affected if the load was not reasonable to the transformer's design. Alas, one of the imperfections of a 'perfect' transformer, and I mean good UTC parts not a $2 unit.

Transformer experts please comment as to why loading is important.


Title: Re: Revamping A Modulator Input Circuit
Post by: KA2DZT on April 20, 2018, 01:13:51 AM
May have more to do to the load on the secondary reflecting a load back to the primary and the source.


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 20, 2018, 01:55:59 AM
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.

I have found this true in experimental breadboarding. At one point I thought a transformer didn't care about being loaded, i.e. driving no grid current, why not just consider the ratio? No load is easier on it than a full or double load, right?

But as it turned out, the frequency performance was seriously affected if the load was not reasonable to the transformer's design. Alas, one of the imperfections of a 'perfect' transformer, and I mean good UTC parts not a $2 unit.

Transformer experts please comment as to why loading is important.

http://www.kandkaudio.com/wp-content/uploads/PSW_WhitePaper_Download_Chapter_6.pdf

See Figures 1,2 and 3.

Effectively you are broad-banding it by loading it.

But having to put a very low resistance load across an audio transformer secondary indicates to me that the transformer quality is questionable and that it has excessive internal capacitance.

The reason I use these transformers:

https://www.edcorusa.com/wsm_series

in the 600 ohm to 600 ohm (1:1) series is that they have very low internal capacitances, center taps, and good frequency response without having to load them excessively.

Another approach is to do something like this for a transformer without a centertap;



Phil




Title: Re: Revamping A Modulator Input Circuit
Post by: PA0NVD on April 20, 2018, 08:51:54 AM
It is indeed an old problem of spread inductance and capacitance. In the years 20 Philips did wind tube coupling transformers with resistance wire to flatten the resonance!!
I always load the sec until the high resonance is hardly noticeable, not more. If you want more low, lower the DRIVE impedance to the prim. The lowsof a correct loaded transformer roll off due o the inductance of the prim which you can overcome by driving at a lower impedance.
I suppose you are not looking for highs up to 20 kHz. otherwise you need more expensive transformers.


Title: Re: Revamping A Modulator Input Circuit
Post by: N1BCG on April 20, 2018, 05:24:35 PM
When Murphy strikes, he's calculated and effective...

I had the new 6SN7 driver all wired up in the modulator chassis and was giving the transmitter a dynamic test when I noticed how thin and tinny the audio was. Hyellified, I started testing audio at each stage after the processor. It didn't take long to find out that my *prized* 600:15k input transformer now had an open secondary.

So, that's that until I can locate replacement iron. Fortunately, it's small iron...


Title: Re: Revamping A Modulator Input Circuit
Post by: DMOD on April 20, 2018, 06:13:14 PM
When Murphy strikes, he's calculated and effective...

I had the new 6SN7 driver all wired up in the modulator chassis and was giving the transmitter a dynamic test when I noticed how thin and tinny the audio was. Hyellified, I started testing audio at each stage after the processor. It didn't take long to find out that my *prized* 600:15k input transformer now had an open secondary.

So, that's that until I can locate replacement iron. Fortunately, it's small iron...

Ok now I understand why you were getting such a high gain and had to pad the secondary. I recall you mentioning two different input transformers but I thought you were using the 600:500 ohm transformer.

You had a 5:1 step-up situation. +8dbm in would have resulted in 10 volts at the grids of the 6SN7.

That 15k secondary would have a lot of distributed capacitance and inductance.


Phil

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands