PW Series Modulated Transmitter

[WU2D]

The mosfet idea is awesome. It must be operating linear mode so you would need to balance a few things out to get the parameters optimized and a heatsink! Well basically the same with a tube I guess. Can you dredge up a valve circuit modulating a couple of 807's?

[DMOD]

The mosfet idea is awesome. It must be operating linear mode so you would need to balance a few things out to get the parameters optimized and a heatsink! Well basically the same with a tube I guess. Can you dredge up a valve circuit modulating a couple of 807's?

I assume you mean dual 807's modulated by the Mosfet circuit. We did that with the Globe Chief 90.

If you are referring to Tubes Modulating tubes then the dual KT88's modulating dual 807's is your cup of tea.

The Mosfet solid state modulator is the most efficient.


Phil

 

 

[K8DI]

I assume you mean dual 807's modulated by the Mosfet circuit. We did that with the Globe Chief 90.


Phil

Phil, can you comment on a couple things here?

1) operating/key down bias comes from where? Grid drive? The bias voltage while operating seems to be based on the 75v zener. Is the capacitor in parallel to stabilize the bias or allow audio to go around the zener?

2) you show plate current as 13mA less than cathode, yet the screen is shown providing 16mA. Is that an error, or is the remaining 3mA coming from the grids, and if so, isn't that low for a pair of 807s?

3) how scalable is this circuit, plate voltage-wise? Does the FET need to withstand the full plate voltage, or some known fraction? If I stick an 11N90 below an 813 at 2100v B+, will it reward my folly with a load of shrapnel?

Ed

[DMOD]

Phil, can you comment on a couple things here?

1) operating/key down bias comes from where? Grid drive? The bias voltage while operating seems to be based on the 75v zener. Is the capacitor in parallel to stabilize the bias or allow audio to go around the zener?

Operating control grid bias comes primarily from the exciter's RF drive via grid-leak bias.

The short explanation is this. The Mosfet has a very low RDS or Drain-to-Source resistance at high audio level peaks so the cathode of the finals are near ground at this time.

At the other end, when there are audio valleys, the RDS is high allowing the final cathodes to go to approximately 0.35XVp for dual 807's. The high positive cathode potential represents a very low grid bias voltage. In some situations, the combination grid leak bias + the cathode potential can exceed the control grids' negative voltage spec. The "PB" components influence the total control grid voltage at the "valleys" to insure that the control grid's negative voltage spec isn't exceeded.

The power supply furnishes KeyUp bias which was a replacement/holdover from the Globe Chief 90 circuit. As such, there is at least -100 volts on the key circuit at Keyup. The 6.8k 2W resistor R2 does limit current at Keydown so as not to tax the Bias supply. This bias could also be used to cutoff exciter stages with proper dropping resistors.

2) you show plate current as 13mA less than cathode, yet the screen is shown providing 16mA. Is that an error, or is the remaining 3mA coming from the grids, and if so, isn't that low for a pair of 807s?

The figure of IP originally quoted was a holdover from the Globe Chief 90 design in which the owner wanted slightly less currents. In this updated design, Ip has been corrected and is Ip = IK - 24 mA. Screen grid current = 16 mA and control grid current = 8 mA. Since the metering is at the Mosfet Modulator's Source, actual Ip is less.  

3) how scalable is this circuit, plate voltage-wise? Does the FET need to withstand the full plate voltage, or some known fraction? If I stick an 11N90 below an 813 at 2100v B+, will it reward my folly with a load of shrapnel?

This circuit is good for 6146, 807 and similar type finals where the plate voltage doesn't exceed 900 volts. See Reply #16 for a Mosfet circuit for higher voltage tubes. This is the one I will be using to modulate my new 4-400 transmitter. An updated circuit with extra voltage protection is also included below.

BTW, the level of voltage to which a particular cathode voltage circuit may rise is dependent on the tube gain, the number of tubes paralleled, and final loading and may vary between 0.25XVp and 0.4XVp, depending on the circuit


Phil - AC0OB

[K8DI]

Operating control grid bias comes primarily from the exciter's RF drive via grid-leak bias.

The short explanation is this. The Mosfet has a very low RDS or Drain-to-Source resistance at high audio level peaks so the cathode of the finals are near ground at this time.

At the other end, when there are audio valleys, the RDS is high allowing the final cathodes to go to approximately 0.35XVp for dual 807's. The high positive cathode potential represents a very low grid bias voltage. In some situations, the combination grid leak bias + the cathode potential can exceed the control grids' negative voltage spec. The "PB" components influence the total control grid voltage at the "valleys" to insure that the control grid's negative voltage spec isn't exceeded.

OK, that clarifies things -- mostly...  I wasn't thinking of controlling grid to cathode voltage limits, I was thinking about how bias was working.  But if voltage between elements is concerning, the specs say heater to cathode is limited to 135 volts...and the cathode will be 200+ volts and the heater is grounded. Do we need to use a separate filament supply with the center tap connected to the cathode here?

Ed

[DMOD]

OK, that clarifies things -- mostly...  I wasn't thinking of controlling grid to cathode voltage limits, I was thinking about how bias was working.  But if voltage between elements is concerning, the specs say heater to cathode is limited to 135 volts...and the cathode will be 200+ volts and the heater is grounded. Do we need to use a separate filament supply with the center tap connected to the cathode here?

Ed

Yes, if the cathode to filament potential may be greater than the spec I always use a separate center tapped filament transformer. For example as shown here, there is really no cathode in the 4-400 but this is a method to connect the modulator's Drain current to the filament:

Phil - AC0OB

[ka1tdq]

I think I found it. After trying everything with the shotgun approach, I gave in and pulled out the scope.

There's flat topping on the 2nd stage of the 12AX7 (mic preamp tube). I took this reading directly at pin 6. I took another reading after C52, the coupling 500pf capacitor (I used a .022uF) and it's always a perfect sine wave. The amplitude does decrease as I go way up in frequency due to C53 but the wave never distorts.

Jon

[WD5JKO]

Jon,

   Just because EFJ did it, does not make it right.

Looking at that circuit, as I recall you wanted to test it with the NFB disconnected. A good choice.

I just looked at the R-C Amplifier section in the 1973 RCA Receiving tube manual for a 12AX7 with 300v B+.

One concern is that RCA recommends the following stage grid resistor (R27) be 2-5X in value of the plate load resistor (R23). In your case it is 1/3X (150K/470K)which puts a nasty load on that stage. This not only reduces the gain, but also limits the undistorted P-P V output.

Suggestions:

Change Rp (R23) from 470K to 220K

Change Rg (R27) from 150K to 470K

Change Rk (R22) from 4.7k to 2.8k (2.7k close enough)

These values are from RCA, Page 645.

Jim
Wd5JKO

[ka1tdq]

Sounds good, thanks!

I’ll report back in a little bit...

Jon

[DMOD]

EFJ's speech amp is very nonlinear and their attempting to drive a 150k load with a second stage using a 470k plate resistor was ridiculous.

If you want to keep the feedback winding and still drive the driver stage with plenty of gain this is my preferred speech amp circuit:

Phil AC0OB  

[ka1tdq]

I had enough resistors on hand to make it happen. I made R27 440k instead, but that's what I had available. It all seemed to help but it's not the complete answer yet. I'm still getting clipping.

Something strange is happening too. When I inject the 1000Hz tone I can hear it coming from something in the transmitter, but I haven't been able to locate the component(s) that are generating it. The level changes as I change the audio gain control so it's something after the first stage. It happens during transmit and receive.

I'm not opposed to scrapping EF Johnson's plan and going with something else. The only part that's concrete right now is plate modulation. Everything before that is up for grabs. It wouldn't take too much effort to switch to Phil's schematic.

Jon

[Tom WA3KLR]

Hi Jon,
I wonder what you are using for a scope probe.  It looks like you have a regular piece of RG-58 leaving  the scope input jack.  Are you just putting a coupling cap on the end of a 50 Ohm coax?  If so, this, would be a 1 Meg load with 120pf in parallel.  This is not good, but it would be best to have a 10:1 probe and use the dc coupling rather than ac coupling you have selected so then the true plate voltage is known.

  If your scope channel is not inverted, I have two interpretations of what is going on. (referencing your Ranger schematic) One is that the plate is going to cut-off on the 12AX7 but I doubt that.  You have 50 Volts peak-to-peak swing at the plate and the 12AU7 grid should not need more than 18 V peak to peak in the normal application.  I  think your test is way overdriving the 12AU7 and you have grid current flow on the 12AU7 providing a clamp on positive plate swing of the 12AX7.  Pull out the 12AU7 and see if the clipping goes way (and more swing).

Anyhow not having followed the whole thread at all,  I don’t know what is your problem, but I think you are getting off the track with the overdrive in the present test.

My 2 cents worth,
GL OM.

[ka1tdq]

I am using a proper scope probe but I did switch it over to DC coupling, as you suggested. Plate voltage on the 12AX7 2nd stage settles out at about 213 volts.

Maybe I am overdriving the mic jack more than normal use. I'm attaching a YouTube video link to show what I'm talking about. This is after Jim's modifications and the clipping has gone down quite a bit.

When I pull the 12AU7, the remaining little bit of clipping goes away. The audible tone goes away as well, so something doesn't seem right with the next stage.

https://www.youtube.com/watch?v=cd5zrPRniv8

Jon

[DMOD]

I am using a proper scope probe but I did switch it over to DC coupling, as you suggested. Plate voltage on the 12AX7 2nd stage settles out at about 213 volts.

Maybe I am overdriving the mic jack more than normal use. I'm attaching a YouTube video link to show what I'm talking about. This is after Jim's modifications and the clipping has gone down quite a bit.

When I pull the 12AU7, the remaining little bit of clipping goes away. The audible tone goes away as well, so something doesn't seem right with the next stage.

https://www.youtube.com/watch?v=cd5zrPRniv8




Phil

Jon

What are the DC plate and cathode voltages on each 12AX7A triode and on the 12AU7A driver stage?

Put about 50 mV p-p into the mic connector and measure the p-p plate voltage on the first stage, and then while scoping the second channel, turn up the mic gain and determine at what p-p voltage it gets to just before flat topping.

Phil

[ka1tdq]

I was taking some initial voltage readings when I powered the radio on again and something started to groan. I'm not sure what it was so I powered it off.

I sat for a few minutes and stared at this thing.

I'm going to change gears and make it a hybrid. I'm going to install a solid state audio driver in the chassis to feed the driver transformer. There's 12 volts inside that I can use to power it. The KT-88's look cooler so I'll put those back in. I need to get creative on those two small tube sockets, but I'll put something there.

Onward to Phase 3

Jon

[K1JJ]

Something strange is happening too. When I inject the 1000Hz tone I can hear it coming from something in the transmitter, but I haven't been able to locate the component(s) that are generating it. The level changes as I change the audio gain control so it's something after the first stage. It happens during transmit and receive.  Jon

Hi Jon,


Especially when using plate modulation...

The tones you are hearing in the rig are often caused by the plate coupling cap, audio transformer talk back or other spots that torture parts. But to be sure, get an insulated piece of plastic pipe, maybe 3' long and 1" diameter. Put it to your ear and listen to the individual transmitter parts while the tones are causing the noise. You will find the source quickly.

Another technique is to use a long insulated fiberglass/ plastic rod to touch the components. You will often hear a pitch change or amplitude change when the noisy part is touched.    (by changing resonance mass or inducing the megaphone effect)

Sometimes it can be a very small sustained arc. Put a big mirror behind the rig and turn out the lights. Sometimes you will see it in the dark.


Be careful, of course.

T

[DMOD]

I was taking some initial voltage readings when I powered the radio on again and something started to groan. I'm not sure what it was so I powered it off.

I sat for a few minutes and stared at this thing.

I'm going to change gears and make it a hybrid. I'm going to install a solid state audio driver in the chassis to feed the driver transformer. There's 12 volts inside that I can use to power it. The KT-88's look cooler so I'll put those back in. I need to get creative on those two small tube sockets, but I'll put something there.

Onward to Phase 3

Jon

Geez Jon you change circuits more often than I change socks. 

If you do use those KT88's as the P-P modulators you'll have to reset the grid bias and change the screen voltages.


Phil

[w8khk]

Jon, you embarked upon a series modulated PW rig, nice, neat self-contained package.  I understand why you dropped the series modulation and went to Class B push-pull plate modulation, probably a wise move.  You just have a few things to work out to get it running, probably much better than the Ranger audio circuits you are modeling with your design.  You are getting closer, probably almost there, so it would be a shame to require outboard audio to complete the project if it is not absolutely necessary.  

I have been a firm believer of testing a stage-at-a-time when puzzled by unexplained phenomena when first testing an new rig in its entirety.  You haven't mentioned whether you are trying to make the modulator and final work while you debug the audio circuits.....  If you are, it might be easier to pull the HV rectifier, and fine-tune the audio driver stages, then add further stages to the mix.  You posted the Ranger schematic as your reference, and I don't know whether that is representative of your power circuits for your modulator and RF final.  But assuming the HV rectifier supplies both plate and screen voltage to the final and modulators, there should be no risk in disabling the HV for the audio tests.

If the 12AU7 stage is being over-driven, or if perhaps the bias on that stage is not sufficient, then your squeal with increased audio gain may be coming from the driver transformer with non-linear input.  I am not sure whether you plan to run the modulator tubes AB1 or AB2, but assuming AB1, the tests for the audio stages should produce the same results whether the modulators are active or not.  If they are pulling grid current, then measuring with and without the modulator tubes in place, (no plate or screen voltage required) might shed some light on the issues.

As previously suggested, test the 12AX7 stages and thoroughly resolve any probems there before adding the 12AU7.  Then add the 12AU7, and if you see flat-topping at the input, the cause may be either over-driving the stage, or insufficient bias on the 12AU7, causing grid current to flow even though the peak-to-peak signal level on the grid is within the proper range.  Thus it is important to have an idea of the expected levels at each stage of the audio path.    If you find the levels are correct, and pull grid current on the 12AU7, try increasing the 12AU7 cathode bias resistor value, thus moving the operating point and allowing greater input swing before the grid goes positive with respect to the cathode.  After solving this, you should have a clean signal at the input to the 12AU7.  You might need to elevate the plate voltage for the 12AU7, using the HV supply with a larger resistor, and capacitively couple the plate to the driver transformer primary, thus taking the saturating DC current off the driver transformer primary.  Your squeal may be resolved by these steps, as they may be caused by over-driving the driver 12AU7, and the noise emanates from the driver transformer.  I think you mentioned unexplained noises and growls, which should be rather easy to identify the source.  

Now that the driver stages are working properly, move on to the modulator tube stage, still with no screen or plate voltage, and inspect the modulator grid voltage swing, with AND without the modulator tubes.  With no actual circuit to review for your build, I do not know if you are using fixed or cathode bias for the modulators.  If cathode bias, then you will need plate and screen voltage for the prior test, lest you have no grid bias and you will surely draw grid current on positive grid excursions, sans any bias, rendering this test irrelevant.  By the way, KT-88s seem overkill for this application, and 6L6s, or 1614s should be sufficient.

In summary, taking the troubleshooting and fine-tuning stage by stage should enable you to identify issues and resolve them by focusing on a smaller section of the entire project at one time, then moving ahead with each additional stage with confidence.  I hope you do not have to shift gears and 86 all your work invested in the audio stages.  I think you will find you do not need the excess baggage of a separate audio amplifier! 

This could become a very nice larger-sized PW transmitter if you follow through as originally planned.  All this may seem long and involved, but, taken one step at a time, it should greatly simplify your tasks at hand.  GL OM!

[ka1tdq]

I am changing the plan a lot but this is getting fun now! On one of my first rigs, I used both push-pull KT-88's and 6L6's interchangeably by connecting the grid to the screen. It's a lot simpler and it sounds great! No grid bias voltage, no screen voltage.

I can appreciate the step-by-step method to get this working. I would've eventually gotten there but it was getting too tedious. This new plan is adding more excitement. And it will still be a self-contained transmitter without any external parts.

I'll keep the 12AX7 tube as the mic preamp and then drive this:

https://www.digikey.com/products/en?keywords=amp2x15

I'll use a backwards driven audio output transformer to then drive the KT-88 control/screen grids.

I also found a use for the previous 12AU7 tube socket. It'll be a glory hole with a countersunk 1/4" jack to break the KT-88 cathode ground connection for current measurement.  It also makes a convenient chimney to let the smoke out if that ever were to happen.

Jon

[DMOD]

The input impedance of the Class D amp is rather low so one would need a speech amp similar to this since the output impedance is low enough to drive a 68k load.


Phil

[K8DI]

The input impedance of the Class D amp is rather low so one would need a speech amp similar to this since the output impedance is low enough to drive a 68k load.


Phil

Phil, not a cathode follower output??

Ed

[DMOD]

The input impedance of the Class D amp is rather low so one would need a speech amp similar to this since the output impedance is low enough to drive a 68k load.


Phil

Phil, not a cathode follower output??

Ed

The output impedance of the second stage is about 40k.

Here is the problem, if your mike only produces about 50mVp-p then the p-p voltage at the plate RP1 is only 3.35V p-p.

A cathode follower stage has a voltage loss of about 30% so at its cathode is only about 2.25Vp-p, not enough to drive most modulator or driver tube grids.

With 50mV input and the mic gain all the way up, you can get a maximum of 115Vp-p at RP2 and that's more enough to drive the Modulator or driver tube grids.


Phil

[ka1tdq]

It's working and the audio is crystal clear. It's lacking a little in bass so I'll putter with that to see what I can do. Attached is the schematic I used to drive the solid state audio driver mounted on the side of the chassis. It's the Ranger's 12AX7 first stage mic preamp driving a cathode follower.

I'm using the 4 ohm tap on the driver transformer. I might try the 8 ohm.

Maximum power output is 14 watts carrier which is a nice level to drive a linear.

Jon

[ka1tdq]

I played around with RC combinations to the input of the audio amp to bring the lows back in. It worked but there's a slight bit of distortion in the audio. I'm thinking that the root of the problem might be the unbalanced DC on the 'modulation' transformer. I'm going to experiment with putting in a modified Heising circuit. This rig will probably end up with enough iron and tubes on it that I could fetch $2 grand from the audiophile guys.

I'm also going to remove the KT-88 current tap point and put a line level RCA jack there instead. Measuring modulator current really isn't necessary on a rig this small and having an option for line level input makes sense.

BUT, I literally ran out of solder this morning. I'll play some more this coming weekend when my shipments of stuff come in.

Jon

[ka1tdq]

And the finished product.

I added the modified Heising circuit and it helped the distortion a lot. I did need to cut back on my bass boost RC combination after I did because at that point I had too much bass.

After making so many circuit changes, pin 6 of the 12AX7 snapped off when I was making a change to the cathode follower. I had to solder onto the stub with a short piece of stranded wire and make a junction point for it on one of the terminal posts.

Anyway, this was a fun project and this will be my main 75m AM transmitter. I really would like to pair it with an Alpha.

Jon