811 Modulator Speech Amp

[Carl WA1KPD]

Good afternoon,

Last week I had some questions about a modulator I had picked up and adapting it from 600 ohm input to hi impedance. I got some good help but the binary Gods got angry and wiped out the page.

In the meantime I have looked at the alternatives and am proposing the following (scehmatics attached)

1.Blasting and drilling out everything ahead of the grid of the 6L6, including the limiter circuit.
2.Inserting the attached speech amp circuit in its place.
3 Using the extra real estate to mount the 300V supply to the speech amp.
4. Installing a 0-400 MA amp in the 811 circuit.
5. I plan on removing the 811 primary CT feed from the RF deck and giving it it's own 1,200+/- volt supply.

Questions,
Are there any remaining caps that should be changed in value for better audio?
Thank
Carl
/KPD

[W3GMS]

Good afternoon,

Last week I had some questions about a modulator I had picked up and adapting it from 600 ohm input to hi impedance. I got some good help but the binary Gods got angry and wiped out the page.

In the meantime I have looked at the alternatives and am proposing the following (scehmatics attached)

1.Blasting and drilling out everything ahead of the grid of the 6L6, including the limiter circuit.
2.Inserting the attached speech amp circuit in its place.
3 Using the extra real estate to mount the 300V supply to the speech amp.
4. Installing a 0-400 MA amp in the 811 circuit.
5. I plan on removing the 811 primary CT feed from the RF deck and giving it it's own 1,200+/- volt supply.

Questions,
Are there any remaining caps that should be changed in value for better audio?
Thank
Carl
/KPD

Hi Carl,

I wonder if you got any of my comments from your previous posting about this?  Shortly after that, I could not bring your posting up.  I do see some minor issues in your current mod.  The biggest one is the lack of gain I think you will have.  The second tube you have as a cathode follower which has no voltage gain.  Short of time now, but I will try to resurrect my thoughts from my previous post that became frozen and get back to you a bit later! 

73,
Joe - GMS 

[Carl WA1KPD]

Hi Joe,
I did see your comments and thought you had recommended a 12ax7 with a cathode follower, so I went looking for a 6SL7 (to stay octal) and found this one. Very possibly I misunderstood. I am no engineer, at best a Questechician having had some elementary tube theory waaaay back in High School.
I'm open to all feedback (pun intended) and circuit ideas
This one came from  http://gabevee.tripod.com/setube.html.
It looked to me like the 811's were driven by a SE speech amp so I could at least use the matching transformer from plate to the grids if I kept the single 6L6


Thanks

Carl
/KPD

[W3GMS]

Hi Joe,
I did see your comments and thought you had recommended a 12ax7 with a cathode follower, so I went looking for a 6SL7 (to stay octal) and found this one. Very possibly I misunderstood. I am no engineer, at best a Questechician having had some elementary tube theory waaaay back in High School.
I'm open to all feedback (pun intended) and circuit ideas
This one came from  http://gabevee.tripod.com/setube.html.
It looked to me like the 811's were driven by a SE speech amp so I could at least uyse the matching transformer from plate to the grids if I kept the single 6L6


Thanks

Carl
/KPD

I was trying to keep your original driver as stock as possible so one of my suggestions mentioned going from a newly added mic circuit into the existing push pull input stage minus the 600 ohm driver line transformer.  In that case you would use a gain stage first then followed by a hot cathode phase inverter which in turn would drive the first push pull stage.  So you would end up with gain from the first added stage and the next stage was to provide the phase inversion to drive the push pull grids.  That seem ok providing you were willing to add a small chassis with a tube or tubes on it depending on what tubes you were planning to use.  I had suggested 12AX7 since you get two triode sections in one tube.  Of coarse many other tube(s) would work equally well.   

My other suggestion was to use 3 FETS which would be mounted in the base of the D-104 doing essentially what the 3 tube sections that I described above.  First one for gain, then followed by a no gain phase splitter and then with that into your first P.P input tube stage. 

Many ways to go on this one especially if you wanted to modify the existing circuit. 

73,
Joe- GMS

[Carl WA1KPD]

Hi Joe,

Part of my problem is I have no idea what the impedance of the existing input transformer is. In addition, as you can see i the attachment there was one transformer that was removed. A transformer to a network and then another transformer to the grids? See attached original schematic.
Makes you wonder if Bendix had extra iron around.....
Don't get me wrong I'm open I just don't know what I am dealing with at the start of the chain.....
Thanks for the help
Carl

Joe,

Added info

First off the modulator was from  Bendix TG-16 fixed station transmitter, A couple of items of note from the pieces of the manual I have

"100% modulation of the carrier can be obtained with an input level between -10 and +4 db (reference level being 6 mw). Line impedance can be matched by using terminals 1 and 12 at terminal board E801 at the bottom of the cabinet. For local microphone input a 150 input is provided. (This transformer is missing)
By use of Type MT-114A microphone amplifier, crystal or dynamic microphones can furnish the required audio level for 100% modulation." (160 watts carrier).

So you are 100% correct, some additional amplification is needed. As before I would like to use octals just in keeping with the era of the rig.

[KA2DZT]

The input impedance of T-202 is 600 ohms. 

[Carl WA1KPD]

KA2DTZ
OK, What would T201 have been there for?
Thanks
Carl

[KA2DZT]

KA2DTZ
OK, What would T201 have been there for?
Thanks
Carl

Good question,  I think to isolate the mike from those networks that are on the input circuit.  T-201 must be mike impedance (600 ohms?) to 600 ohms.

First network looks like a low pass filter, second one a high pass filter.

Fred

[Carl WA1KPD]

Makes sense..... thanks!

[KA2DZT]

Someone may have more to add to this.

[Steve - K4HX]

Not likely that it was for use with a mic. That looks like a line level input. I think I used to have one of these modulators. The transmitter was probably remote from the operator. So a line level connection or even a phone line would have been used.

[Carl WA1KPD]

Hi Steve,
You are correct. It was for line level input. So I am just looking foor tube based ideas on how to get enough signal into it at the grids of the 6SG7s
Carl

[KA2DZT]

I think on the schematic it has "mic" on one terminal.  Hard to see it clearly.  Could also be line level, usually 600 ohms.

Fred

[Steve - K4HX]

Tons of options. You'll need something like 40-60 dB of gain. That's a voltage gain of at least 100. I'd go for the higher end of the range to ensure any mic could be used. So a voltage gain of about 1000 would be indicated. Two sections of a 12AX7 would get you pretty close. You would need a plate to line transformer on the second section. If you added another tube, maybe a 12AU7, you could get a little more gain in the first section and run the second section as a cathode follower at 600 Ohms. This way, no transformer would be needed.

[WD8BIL]

The 12AX7/6C4 mod for the Viking 1/2 on the AMWindow.org page would do nicely. Just ditch the driver transformer and replace it with a resistor. Com off the cathode of the 6C4 with  a .05 or so coupling cap for the 600 Ohm line and you'll have plent of audio drive.

Hey... it can happen!

http://www.amwindow.org/tech/htm/viking2.htm

[w4bfs]

OK, What would T201 have been there for?
Thanks
Carl

I believe t201 is a 600 to 600 Ohm transformer to block dc and 20hz ring signals and to provide trans-hybrid balance back to the telephone company .... it is followed by the equalization network, constant impedance audio drive control (l-pad) and then grid transformer ....all of this circuitry is designed to work at standard level (around 0 dbm) which is 1 milliwatt into 600 Ohms or 2.2 v pp

[W3GMS]

Hi Joe,
I did see your comments and thought you had recommended a 12ax7 with a cathode follower, so I went looking for a 6SL7 (to stay octal) and found this one. Very possibly I misunderstood. I am no engineer, at best a Questechician having had some elementary tube theory waaaay back in High School.
I'm open to all feedback (pun intended) and circuit ideas
This one came from  http://gabevee.tripod.com/setube.html.
It looked to me like the 811's were driven by a SE speech amp so I could at least uyse the matching transformer from plate to the grids if I kept the single 6L6


Thanks

Carl
/KPD

I was trying to keep your original driver as stock as possible so one of my suggestions mentioned going from a newly added mic circuit into the existing push pull input stage minus the 600 ohm driver line transformer.  In that case you would use a gain stage first then followed by a hot cathode phase inverter which in turn would drive the first push pull stage.  So you would end up with gain from the first added stage and the next stage was to provide the phase inversion to drive the push pull grids.  That seem ok providing you were willing to add a small chassis with a tube or tubes on it depending on what tubes you were planning to use.  I had suggested 12AX7 since you get two triode sections in one tube.  Of coarse many other tube(s) would work equally well.   

My other suggestion was to use 3 FETS which would be mounted in the base of the D-104 doing essentially what the 3 tube sections that I described above.  First one for gain, then followed by a no gain phase splitter and then with that into your first P.P input tube stage. 

Many ways to go on this one especially if you wanted to modify the existing circuit. 

73,
Joe- GMS

Hi Carl,

Many good ideas here and here attached is what I was trying to explain in words!  You could easily sub another tube if you did not want to use 12AX7.  The only thing I was trying do was to keep the design somewhat similar.  If you willing to gut it, then all kind of options put out here would work well.   

73,
Joe-GMS

[WU2D]

That 6L6 driver circuit in the original modulator is interesting. You seldom see a feedback system in a single stage and when you do, it typically is a shunt type like used in the ART-13 6V6 driver. The idea is to lower the impedance of the driver and flatten its response through negative feedback. Here they actually use a center tapped transformer on the grid side and use voltage feedback.

[N2DTS]

I thought that was some sort of low pass filter for the clipper stuff.
The circuit has enough transformers to be an aircraft radio...