Ultimate Ranger Audio Mods - Keeping the tubes, too (with pics)!

[steve_qix]

I have a COMPLETELY different driver circuit - much simpler     I will definitely document it here as soon as I stop making changes 

[kb3ouk]

I have a COMPLETELY different driver circuit - much simpler     I will definitely document it here as soon as I stop making changes 

good, that one was giving me headaches just looking at it. I'm thinking of maybe using it (or more likely, the improved version once you document it)  to drive something like a pair of GU-50s, they seem to be a pretty tough tube, and are really cheaper and plentiful.

[steve_qix]

I attached the schematic of the as-built, integrated in to the Ranger, audio driver to the first post of this thread.  It is also here.  It works very, very well !

The circuit is somewhat Ranger centric, however it can be adapted for other uses.  Since the Ranger has many power supply voltages available, it was quite easy to design the driver using 3 high voltage supplies.  There are other implementations that could only use 2 HV supplies, but they are a little more complex.

Because the 6DQ5 has such a high trans-conductance, and is also reasonably well screened (the screen has a lot of control over the plate current), it was most practical to simply run the control grid at 0VDC and drive only the screen with audio.  This requires about +150V peak at the screen to achieve near-saturation of the tube.  The driver will do this, no problem.

There are 2 negative feedback loops - one in each leg of the driver, and a main loop from the output of the mod transformer back to the input.  The 2nd loop is adjustable.

The 6DQ5 works very well with -15V bias on the screen (with 600VDC on the plates), and results in about 40mA of idle current as indicated on the Ranger modulator current meter.

The push to talk is implemented using a 3 pole relay.  In addition to enabling the modulator, the PTT also interrupts the screen voltage to the RF amplifier (by disconnecting the screen and the plate of the 6AQ5 "clamp" tube from the screen dropping resistor), and also breaks one of the keying lines, putting a high negative voltage on all of the driver circuits and shutting them off when in receive.

The FQA11N90 in the cathode of the modulator tubes is provided to disable the modulator when the transmitter is not in the "phone" position.  The line which originally supplied the grids of the Ranger modulator tubes is enabled only when the transmitter is in the "phone" position, so I used this voltage to enable the modulator (the PTT relay also has to be energized).

The thing sounds quite exceptional, and should make a very good driver for a linear amplifier (or low power transmitter by itself).
Here is a PDF: http://www.classeradio.com/ranger_audio_driver.pdf

[WA1GFZ]

It really sounds good but using op amps makes it so complicated. That is the reason I just used FETs in the driver Tom and I did for his rig.

[steve_qix]

It really sounds good but using op amps makes it so complicated. That is the reason I just used FETs in the driver Tom and I did for his rig.

Hi Frank,

I generally get a lower parts count using op-amps.... and of course, the differential amp action is required in many cases.  I wanted to keep the phase shift extremely low throughout the circuit.  There is really only one R/C time constant that is problematical and I think I mitigated the effects in the design.  The class A amplifier driving the output MOSFET has a 10k looking into the gate of the device - there is a potential - but with 10k into, say, 3000pF (the gate capacitance of the output MOSFET), I wasn't overly concerned    Everything else is quite phase neutral throughout the circuit (except for the mod transformer, of course!!!).

I have a variant of this circuit using only 2 power supplies - and an op-amp is used as a differential amplifier to get from DC ground at the input, to -300 V.  This works very well.  Another way is to use a DC blocking cap, however a regulated power supply would be required on the -300V DC rail, else small variations in the DC voltage will appear in the output (along with any hum, noise, etc.) unless the low frequency roll-off is moved up to the audio range and that causes low frequency phase shift.

The small amount of high frequency phase shift allows a significant amount of negative feedback to be applied to the circuit, and the non-existent low frequency phase shift allows the modulator to faithfully produce the so-called "Class E pattern" with a frequency response down to single digits.

The circuit is very small - could probably fit it on a 2 inch by 4 inch PC board.

[KC2ZFA]

Steve, thanks for the circuit !

Any guidance to anyone who wants to use this to drive
811A's ? I'm sure it's not as easy as simply substituting
tubes...TANSTAAFL 

Peter

[w1vtp]

Great!  Thanks.  I think I can build a dual voltage doubler using a reverse connected small filament transformer.  Would it have to be voltage regulated?  Don't think so.  Very interesting using the screens on the 6DQ5(s).  I bought a bunch of those tubes thinking they were a neat tube.  Probably going to use a pair of 'em on my Bendix TA12. Now I have another excuse.

al

[steve_qix]

I have a variation of this circuit that is much more useful for 811s, 833s, etc.  It only uses 2 power supplies.  I'll come up with an as-built schematic for that one in the near future   

Regards,

Steve

[steve_qix]

Al said he couldn't see the entire circuit as a .JPG, so here is a PDF version which is of very high resolution.

http://www.classeradio.com/ranger_audio_driver.pdf

[W3GMS]

Great circuit Steve and I am sure many will build it.  The Ranger is an extremely popular transmitter.

Thanks for the time spent in bringing it to the AM community.

73,
Joe, W3GMS

[steve_qix]

Great circuit Steve and I am sure many will build it.  The Ranger is an extremely popular transmitter.

Thanks for the time spent in bringing it to the AM community.

73,
Joe, W3GMS

Thanks Joe - we'll see if anyone else builds it    I think the more popular circuit will be the "general" direct coupled audio driver, which is very similar to this one, but uses only 2 high voltage power supplies (the +350V supply is not required).

There are very, very few transformer coupled modulators which will faithfully reproduce my particular voice waveform and give the so-called "shark tooth" pattern with the high asymmetry that folks get with the class E rigs, but this Ranger will actually do it!

The modifications I show here for the Ranger require a lot of work - essentially, you completely remove the modulator - all of it - and start over!  I believe I could have built an entire 400 watt transmitter, from scratch, in the time it took me to do these Ranger modifications     Of course, there were some stumbling blocks along the way - like having the repair the VFO, and the screen dropping resistor burned out during a test !!???  What's with that??  The HV bleeder was gone and there were some other problems as well.

A *much* easier modification keeps the audio driver and associated power supplies external to the transmitter.  I made everything internal to the Ranger because the transmitter is going to be used by someone else.

Quite the project!!

Regards,

Steve

[WD5JKO]

Steve,

   You might recall a long QSO on 10m we had last fall over these SS audio drivers. A fascinating subject to a lot of us. Some might hesitate at the op-amp circuitry, but with all four in a single 14 pin DIP, that is not too bad at all. The support R's and C's mostly could go into DIP socket headers to be neat and tidy.

   Is that 1 meg 1/2 watt GNFB resistor up to the task? Maybe specify 2-3 resistors in series adding up to the same value would be a safer alternative?

   I wonder if someone might whip up a PCB artwork, and/or possibly have a run of boards made?

   The G2 only drive of the 6DQ5 goes back at least 25 years ago when Pete W1VZR made a modulator this way in his Valient. As I recall, Pete used something like a 6AQ5 audio driver. Also Gonset did this in several rigs. The G-76 does it with a pair of 6DQ6 modulators.

   Looking at your circuit I wonder what might happen if a modulator tube had an internal short from the plate to G2. I recall what happened to my 16 transistor audio driver (driving 811's)  when there was a short within one of tubes...All transistors dead! I went back to a pair of 6B4's as the driver. 

Jim
WD5JKO

[steve_qix]

Steve,

   You might recall a long QSO on 10m we had last fall over these SS audio drivers. A fascinating subject to a lot of us. Some might hesitate at the op-amp circuitry, but with all four in a single 14 pin DIP, that is not too bad at all. The support R's and C's mostly could go into DIP socket headers to be neat and tidy.

That's right - it's very small and was pretty easy to build dead-bug style.  A board would make it easier, or one could use perf board (I don't have any around here, else I would have used it!).

  Is that 1 meg 1/2 watt GNFB resistor up to the task? Maybe specify 2-3 resistors in series adding up to the same value would be a safer alternative?

Good point!  I went back and looked - and it's a 1 watt resistor.  It should be 1 watt, or 2 470K 1/2 watt resistor in series or 3 330K in series, etc...

  I wonder if someone might whip up a PCB artwork, and/or possibly have a run of boards made?

The other circuit I have would be the one to lay out - it's more universal.

  The G2 only drive of the 6DQ5 goes back at least 25 years ago when Pete W1VZR made a modulator this way in his Valient. As I recall, Pete used something like a 6AQ5 audio driver. Also Gonset did this in several rigs. The G-76 does it with a pair of 6DQ6 modulators.

It certainly makes sense - the Ranger generates almost as much heat in stand-by as it does when transmitting!  Turning off the RF amp screen voltage, and killing the modulator in stand-by really makes a difference!

  Looking at your circuit I wonder what might happen if a modulator tube had an internal short from the plate to G2. I recall what happened to my 16 transistor audio driver (driving 811's)  when there was a short within one of tubes...All transistors dead! I went back to a pair of 6B4's as the driver.  

I don't think much would happen, but really - there should be a 1 amp or 1/2 amp or some small fuse between the grid and the driver.  An arc-back in a modulator tube would attempt to put high voltage back through the driver.  Once the voltage reached the driver positive power supply voltage, the internal diode within the output MOSFET would conduct, preventing any further voltage increase across the device.   An intelligent addition would be a transzorb across each output and a fuse between the grid and the output and I think I will do this both in the implementation and the schematic.

Another easy addition would be a high voltage diode in series with each MOSFET output, between the rest of the circuitry and the pull-down resistor going to the negative supply.  Since there is always current flowing, the diode will always be conducting - except in the case of an arc-back.  In this case, the diode is back-biased and nothing happens!  The transzorb would still be there, and the fuse.  This combination would provide a high level of protection for the driver.

It is really is a very sensible addition and I will add this for sure!

[steve_qix]

Thanks to Jim for the excellent suggestions on circuit protection!  I have added the appropriate components and have changed the schematic(s).  At this point, the circuit is extremely well protected, and I don't see anything coming in from the modulator tubes that could inflict any damage.  Thanks again  

[WA1GFZ]

Steve,
You made the same fuse mistake as me. Think about it the fuse wants to be in the drain lead of the FET so if it opens the tube gets yanked negative.
If it is opened in series with the drive you have a floating grid.

[steve_qix]

Good thinking about the floating grid.  It's nice to keep the fuse where it is because it so neatly protects the driver from an arc-back.... but I could definitely see where a floating grid may be problematical.  So, the best solution to this is to run a resistor from the grid side of the fuse back to the -200 V supply.  Then, if the fuse ever opens up, the grid is pulled down to -V.

That is a very good suggestion - THANKS!  

I will include this in the next schematic.

[WD5JKO]

There is really only one R/C time constant that is problematical and I think I mitigated the effects in the design.  The class A amplifier driving the output MOSFET has a 10k looking into the gate of the device - there is a potential - but with 10k into, say, 3000pF (the gate capacitance of the output MOSFET), I wasn't overly concerned    Everything else is quite phase neutral throughout the circuit (except for the mod transformer, of course!!!).

Steve, After reading this I recollected an active antenna I designed years ago using a GaAs fet as an input source follower. Here the Gate to drain capacitance was very low, and the gate to source capacitance is nulled out as the gain approaches unity. I searched for the mathematical relationship and found it here:

"The input capacitance of a source follower configuration shown in Figure 1 is defined as the sum of the gate to drain capacitance, and gate to source capacitance multiplied by, 1 minus the gate to source gain, so:

Cin = CGD + CGS(1 - AGS)

Where: AGS = RS / RS + (1/gm) if the drain resistance is much greater then the source resistance as is often the case (rd >> RS)."

Reference: http://www.freeclassnotesonline.com/Source-Follower-Amplifier-Capacitance.php


  So with your power source follower with about 260pf gate-drain capacitance, and 2700pf gate-source capacitance, your effective input capacitance will be around 500 pf when the gain is about 0.9. That is about six times less than you were thinking, so phase shift here should not be a big concern.

Jim
WD5JKO

[WA1GFZ]

Most of the phase shift is due to coupling caps between stages 

[Rob K2CU]

Instead of adding R44 and R45,  move R17 to the R44 location, and move R35 to the R45 location. Also consider that transorbs have large capacitance...on the order of thousands of pF.

[steve_qix]

Instead of adding R44 and R45,  move R17 to the R44 location, and move R35 to the R45 location. Also consider that transorbs have large capacitance...on the order of thousands of pF.

The resistors could be changed.  It could definitely be done that way!

According to the documentation, the junction capacitance of the 400 volt transzorb is under 200pF.  That's pretty low with respect to the resistances involved.  The lower voltage transzorbs have a lot higher capacitance than do the higher voltage diodes.

[IN3IEX]

The whole project is very interesting. This could be also a very high quality HI-Fi amplifier.
I am using 6KM6s, class AB1, in my modulator, which is full tube.... This is because I had them available. Anyway any color TV sweep tube is very interesting for audio.
They are as linear as classical audio tubes and you can squeeze more from the anode supply voltage available.
For instance the 6KM6 http://frank.pocnet.net/sheets/049/6/6KM6.pdf can provide 100 mA of anode current with anode voltage less than 10V.
The 6L6/807 requires 30V anode for providing the same current.
Probably you can use 6KM6s in this design. Fortunately the curves with Vg1=0 and Vg2 acting as input are available for most sweep tubes. Having screen current is obviously the drawback.
Vg1=0 curves are pentode like, this might seem strange, we might expected triode like curves, but the effect is due to the fact that Vg2 voltage is positive. Se also this http://tec-sol.com/products/tubes/svetlana/el509.pdf  that provides a larger range of grid voltages. Going from negative to positive Vg2 (with VG1=0) the curves change from triode like to pentode like. Around Vg2=0 they are very linear.

With 6KM6s I also learned that NOVAR base is different respect to MAGNOVAL. The pin diameter is smaller in NOVAR and I had to squeeze the contacts in the sockets with a screwdriver. I got russian MAGNOVAL sockets...