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Microphone challenge with my Lettine




 
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Author Topic: Microphone challenge with my Lettine  (Read 3792 times)
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n4joy
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« on: March 25, 2017, 11:01:19 AM »

Hello.  I presently use my tried and true Shure 450 with National NCX-3 and other boat anchor SSB gear--it works well and the audio reports are excellent.  According to Shure, the microphone has a recommended load impedance range of 15K to 100K ohms.  When using the microphone with my Lettine, audio quality is decent but the mic gain must be set too almost full; I am lucky to achieve 70% to 80% modulation.  I have had crummy luck with ePay D-104s and other crystal microphones, so I am exploring my options.  My research also suggests that the replacement crystal elements are either no longer available or less than stellar in regard to performance. 

The grid leak resistor on my Lettine's 6SJS is 2 meg.  So I assume I will have similar issues with most of the other "high impedance" microphones.  Should I consider modifying the audio circuit or are there other options to consider?  Step up transformer for the Shure 450?

Below is the audio circuit for the Lettine. 

Thank you for the guidance.

Chris, N4JOY


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WD5JKO
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« Reply #1 on: March 25, 2017, 11:24:36 AM »


Chris,

   I think I would make a few changes to that first stage. With no cathode bias, and a high grid resistor, perhaps the designers wanted some sort of contact bias (?). If you shunt that input with a Low Z winding of a magnetic Mic element, the bias will drop, and the tube operating point will shift.

   You might consider lowering that grid resistor to better match the microphone (100k?), and then add a cathode resistor (with AC bypass capacitor across it).

   Since you need more gain, perhaps change that 6SJ7 to a 6AC7 which shares the same pinout. With the 6AC7, a cathode bias resistor of 10-15K could be a starting point.

Jim
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n4joy
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« Reply #2 on: March 25, 2017, 11:50:19 AM »

Great ideas!  Looking over various transmitter schematics using the 6SJ7, I will try 1 meg instead of 2 meg at the grid and will add a 1500 ohm cathode resistor with a 10uf 250v across it.  I will give this a go before the 6AC7.

Thanks!

Chris, N4JOY
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n4joy
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« Reply #3 on: March 25, 2017, 12:34:31 PM »

It looks as if the schematic I provided is incomplete.  I am 99.9% certain there were no mods made to the audio input stage--all was original factory.  The microphone is going through a .01uf cap before the 2 Meg grid resistor.  There is also a 100pf going to ground at mic input socket--I assume this is to eliminate RF.
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n4joy
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« Reply #4 on: March 26, 2017, 03:06:59 PM »

Before modifying the circuit, I double checked the 6N7 driver, which goes to the grids of the 6L6GAs.  It tests a little on the weak side according to my tube tester.  My 6L6As and the 6SJ7 test good; all components are within tolerance.  I will order a new 6N7 before moving forward.


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W2PFY
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« Reply #5 on: March 26, 2017, 03:46:46 PM »

If you have a 6SK7 hanging around there it may work a little better as it has a higher transconductance than the 6SJ7  1575  6SK7 2350 and has the same pin out.
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n4joy
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« Reply #6 on: March 26, 2017, 07:18:43 PM »

Great idea!  The 6SK7 is probably one of the most numerous tubes in my spare parts bin!  I will give it a try.

Thanks,

Chris, N4JOY
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W2PFY
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« Reply #7 on: March 27, 2017, 07:06:54 PM »

Did you get a chance to try the 6SK7?
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n4joy
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« Reply #8 on: March 30, 2017, 03:45:43 PM »

Thanks for checking.  I replaced the 6SJ7 with a known good 6SK7 and noticed only a slight difference but the transmitter was still extremely under modulated.  I also replaced the 6N7 and did not see an appreciable change.  I double checked the circuit and all resistors in the audio/modulator sections are within tolerance and wired correctly.  Only thing left at this point is the 6L6GA modulator tubes.  They test good on my tester, but it only tests for emissions.  I ordered a new set of 6L6GCs and will report back.  I am hopeful this is the issue.
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W2PFY
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« Reply #9 on: March 30, 2017, 10:36:52 PM »

Did you check the primary on the mod transformer. One side to center will normally read a little different from the other to the center tap. If these is a big change, it could have some shorted turns? The other thing to check is the secondary winding's while toggling the SW. If there is no difference between one position and the other, you may have a mechanically bad switch where it is staying closed and shorting out the secondary. You probably already know how to test it, but this is how I would go about it...
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n4joy
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« Reply #10 on: March 31, 2017, 08:29:59 AM »

I appreciate the suggestion.  Yes, I did thoroughly check the modulation transformer--all appears okay.  I do, however, have an update.  I decided to re-check the 6L6GAs with my Mercury tube tester.  One was okay but while checking the other, I lightly tapped on the tube and the leak/short neon lamp lighted.  I repeated the test with the same result, so this seems to confirm an intermittent short in one of my modulator tubes.  I have a pair of 6L6GCs on the way.  The plate voltage on the modulator tubes, even at 115VAC line, are on the high end for the 6L6GAs.
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n4joy
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« Reply #11 on: April 02, 2017, 11:39:29 AM »

Well, I know this Lettine is not as exciting as a Viking or Collins but does anyone else own one of these transmitters?   Unfortunately, it still appears to be undermodulated.   Undecided  Perhaps this is just normal for the Lettine?  I replaced the 6L6GCs and there was no appreciable difference.  The 25 watt Thordarson 21M54 modulation transformer checks out okay and I am seeing 450+VDC under load or so at the 6L6s' plates.  I've re-reviewed the schematic several times and all components are correct and within specs; however, the only issue I saw was a 2.2K ohm cathode resistor (pin 8 to ground) at the 6N7 that was reading close to 3K out of circuit--I will replace.  Carrier input is around 40-50 watts. 

My apologies if this is treading into the technical forum.

Chris, N4JOY
   


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WD5JKO
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« Reply #12 on: April 02, 2017, 01:31:01 PM »

Chris,

   The Lettine transmitter seems unusual to me. The RF side is overly simplistic, and the modulator seems like it got more attention to design, and then due to RF side limitations, the designers took away modulation capability. The ability to get 100% clean modulation is out the window unless you are prepared to go the distance, and one by one revisit the compromises made years ago by the designers and marketing folks at Lettine.

I will highlight just a few items that pop out at me:

* there is no way to know if the 807 has enough grid current (G1)
* multiplying a crystal to work the higher bands will likely drop the available grid drive
* The 807 has a 250 ohm cathode resistor to protect the 807 in the absence of drive. Other designs might use an overload relay, or a G2 clamp tube, or G1 protective bias. That 250 ohm resistor will limit the effectiveness of a class C amplifier, and therefore reduce the positive modulation peaks
* Is the 807 G2 screen modulation sufficient, or too much? What about phase shift at 2-5 Khz audio?


*The modulator is using contact bias for the 6SJ7 stage. Is this enough? A D-104 can produce a volt or more on peaks....we don't want to draw grid current on a low level audio stage..
* The 6L6 output uses a rather high value cathode bias resistor, and it is unbypassed.
* The 6L6 screen voltage is unbypassed to ground, and the screen voltage will cave severely under load.

In my opinion the Lettine is pretty unique. Making a performer out of it will require a fair amount of work. The question therefore, do you tweak it up, or enjoy it for what it is.

Jim
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n4joy
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« Reply #13 on: April 03, 2017, 08:30:20 PM »

Jim,

I appreciate your taking the time to respond--good information.  They were a small "garage" operation, so I am sure there were many compromises made.   Your post has me reflecting on the transmitter's limitations and possible solutions.  Some of the modifications to the audio stage and modulators can be easily done (e.g., adding a .1uf Screen Bypass Capacitor on the modulator tubes) but I will probably keep it original.  Not too many Lettines in this condition floating around--I will keep it unmodified.  This and the 6 meter version receive very good audio reports, so I can't complain.


Chris, N4JOY

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Pete, WA2CWA
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« Reply #14 on: April 04, 2017, 03:19:37 AM »

I never found that dynamic mikes work that well with old boatanchor rigs. They don't have the output that you find with a crystal mike. I'm willing to bet the rig was designed for use with a crystal mike.
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