Information about Lettine model 240X

[pa3esy]

Since two weeks I am the proud owner of a Lettine model 240X transmitter.
I examened the transmitter and saw that the modulator is not the same as in the model 240.
It has a balance input transformer instead of the 6N7 driving triodes.
Strange is the connection of the two leads to the power-tubes. They are direct connected to pin 4 and via a seriesresistor to pin 5 of the final tubes. I have never seen that before...
Who has some more information abpout the 240X model??

With kind regards from Jan Poortman  PA3ESY

[n4joy]

Hi.  I collect all things Lettine (I own 242s and 240s) and have never come across a Lettine 240X.  This is news to me!  Can you post some pics?  The 130 and 262 are elusive, but I'd love to learn more about the 240X as well!

Chris, N4JOY

[KD1SH]

   I'd like to learn more too.  I picked up a really nice looking 242 at Sussex this summer; often hear a nice sounding 240 on a regular net. There's an entry for Lettine in the "Tube Type Transmitters" book, but otherwise than that I can't seem to find out much about Lettine and other rigs they've made.  I found the address and did a Google "virtual drive" past the location - there's a small house at that location that looks to be old enough to have been the site of the original production facility, very likely a small "garage factory".

[n4joy]

  I'd like to learn more too.  I picked up a really nice looking 242 at Sussex this summer; often hear a nice sounding 240 on a regular net. There's an entry for Lettine in the "Tube Type Transmitters" book, but otherwise than that I can't seem to find out much about Lettine and other rigs they've made.  I found the address and did a Google "virtual drive" past the location - there's a small house at that location that looks to be old enough to have been the site of the original production facility, very likely a small "garage factory".

It was indeed a garage operation--father (Anthony) and son (Howard), primarily, with some hired help.  I recently spoke with a family member and was informed all remaining documentation owned by the family was destroyed by Hurricane Sandy.  I love my little Lettines and always receive stellar audio reports.

[KD1SH]

  My 242 hasn't reached the workbench yet - it's in a long line of projects - but I'm looking forward to getting it on the air.  It's very nice looking; painted finish a little dirty but not rusted or scratched, and no mouse nests or their corrosive leavings inside.
  Took me a while to come up with a manual and schematic, but a friend who is very good at internet sleuthing found it for me after I'd all but given up.  I admire the simplicity of the design, the old-school workmanship, and the rather unprofessional hand-drawn schematic is actually part of the charm.
   I'd love to find a 262 - the 6 and 2 meter version - sometime.
   Talking about the Lettine is making me rethink my project priorities a bit; maybe my DX-100, Gonset G-76, or Heath Seneca won't take much offense at the little Lettine cutting in front of them in line.

[pa3esy]

Hi Chris, Thanks for your reply and the sad message that all documentation has gone.
I will give you the address of the website-page I just finished this afternoon about the Lettine Model 240X.
The big difference between the 240 and 240X is the modulator. In the 240 the 6L6 powertubes are controlled by the 6N7 phase-inverter and in the 240X with a single plate to push-pull-tansformer. The two output leads are connected to the screengrids of the 6L6s and then with  2200 Ohms resistors to the signalgrids of the tubes. I never saw that before and wonder how that works.


So in advance a picture of my Lettine:

 

And here is the link to my Lettine page: https://www.pa3esy.nl/zenders/lettine%20240/html/model-240X_set.html

Hope you can give me an answer.
Whis you a merry X-mas and a very happy New Year,

Jan, PA3ESY

[pa3esy]

  My 242 hasn't reached the workbench yet - it's in a long line of projects - but I'm looking forward to getting it on the air.  It's very nice looking; painted finish a little dirty but not rusted or scratched, and no mouse nests or their corrosive leavings inside.
  Took me a while to come up with a manual and schematic, but a friend who is very good at internet sleuthing found it for me after I'd all but given up.  I admire the simplicity of the design, the old-school workmanship, and the rather unprofessional hand-drawn schematic is actually part of the charm.
   I'd love to find a 262 - the 6 and 2 meter version - sometime.
   Talking about the Lettine is making me rethink my project priorities a bit; maybe my DX-100, Gonset G-76, or Heath Seneca won't take much offense at the little Lettine cutting in front of them in line.

Hi, thanks for your reply.
Whishing good luck with the priorities for your work-bench. It's better to have too much work then just sitting around.
I think I will have a nice job to repair my Lettine 240X.

Here is the link to my Lettine 240X pages : https://www.pa3esy.nl/zenders/lettine%20240/html/model-240X_set.html

Merry X-mas and happy New Year,



best regards from Jan PA3ESY

[Opcom]

The two output leads are connected to the screengrids of the 6L6s and then with  2200 Ohms resistors to the signalgrids of the tubes. I never saw that before and wonder how that works.

Here is the short explanation from a military manual. I'd love to see both complete schematics to compare also.
TM11-665 C-W AND A-M RADIO TRANSMITTERS AND RECEIVERS, Page 141
https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA141&lpg=PA141&dq=modulator+special+class+B+connection#v=onepage&q=modulator%20special%20class%20B%20connection&f=false


Tetrodes operated with screen and control grids connected to each other (so that the tube operates as a triode) often are used as class B modulators.

Tetrodes and beam power tubes operated in this manner have a high amplification factor because the grid potential acts on the electron stream over a considerable portion of the space between cathode and plate.

The grid structure is such that grid current is not excessive, even when the grids are driven highly positive.

Triodes having a high amplification factor or mu also are used as class B modulators.

These triodes and tetrodes (connected as described above) require little or no bias.

The high amplification factor results in low plate current, even with no bias.


1960s? ARRL handbooks have more info:
What is not said in the military book is that the screen-to-screen drive voltage for this special class B connection is hundreds of volts and that hams often use a small backwards-connected tube-type hi-fi or public address output transformer as a driver transformer.
The reason hundreds of peak volts is required is because the full screen voltage must be met as in the usual operation.
So for a pair of 807s, it may be 700-800V peak screen-to-screen. Less for 6L6s.
The resistors to the control grids just limit the current to them, since they are so close to the cathode.

[pa3esy]

The two output leads are connected to the screengrids of the 6L6s and then with  2200 Ohms resistors to the signalgrids of the tubes. I never saw that before and wonder how that works.

Here is the short explanation from a military manual. I'd love to see both complete schematics to compare also.
TM11-665 C-W AND A-M RADIO TRANSMITTERS AND RECEIVERS, Page 141
https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA141&lpg=PA141&dq=modulator+special+class+B+connection#v=onepage&q=modulator%20special%20class%20B%20connection&f=false


Tetrodes operated with screen and control grids connected to each other (so that the tube operates as a triode) often are used as class B modulators.

Tetrodes and beam power tubes operated in this manner have a high amplification factor because the grid potential acts on the electron stream over a considerable portion of the space between cathode and plate.

The grid structure is such that grid current is not excessive, even when the grids are driven highly positive.

Triodes having a high amplification factor or mu also are used as class B modulators.

These triodes and tetrodes (connected as described above) require little or no bias.

The high amplification factor results in low plate current, even with no bias.


1960s? ARRL handbooks have more info:
What is not said in the military book is that the screen-to-screen drive voltage for this special class B connection is hundreds of volts and that hams often use a small backwards-connected tube-type hi-fi or public address output transformer as a driver transformer.
The reason hundreds of peak volts is required is because the full screen voltage must be met as in the usual operation.
So for a pair of 807s, it may be 700-800V peak screen-to-screen. Less for 6L6s.
The resistors to the control grids just limit the current to them, since they are so close to the cathode.

Thanks a lot for this information, in the mean time I found out the circuit used in the model 240X after examening the wires and tube-connections.
I was also informed by other specialists that the control of g1 and g2 together is called "Class B Zero bias high mu PA". It seems to be a method that was often used in the years just after the 2nd worldwar.

Below the circuit of the 240X



Best regards, Jan PA3ESY

[n4joy]

Jan,

I reviewed your webpage on the 240x.  I appreciate your documentation!  I am at a loss and am wondering why this 240X was built by Lettine.  Carl, KM1H, worked part-time for them back in the day and may recall this model; he is active on QRZ.  Best on your restoration!

Chris, N4JOY

[DMOD]

Just surmising here, but it appears the space charge current impinging on the 6L6's screen, combined with the resistance of the driver transformer's secondary, would create a bias voltage at the secondary terminals. I.e., you would have a half-sine AC riding on a DC bias and the DC bias would track the half-sine's amplitude.

Interesting.

I too would contact Carl and see if he can shed more light on the theory of operation.

Phil - AC0OB

   

 

[KA0HCP]

I have seen the regular Lettine ads in QST but didn't know anything about the company and their products. Thanks for the great thread and info!  bill

[pa3esy]

Jan,

I reviewed your webpage on the 240x.  I appreciate your documentation!  I am at a loss and am wondering why this 240X was built by Lettine.  Carl, KM1H, worked part-time for them back in the day and may recall this model; he is active on QRZ.  Best on your restoration!

Chris, N4JOY

Hi Chris,
Thanks for your reply. Mabe I will return the transmitter to the model 240 when the tests with the 240X modulator do not satisfy. We will see.
Best whishes for the new year,
Jan

[n4joy]

Jan,

I look forward to reading more about your progress and perhaps we'll solve the mystery of the 240x.

Chris, N4JOY

[WA4WAX]

Here is a 240 demo.  Enjoy!

https://www.youtube.com/watch?v=dmFPepIN-a4

[n4joy]

I was revisiting your page and see that you completed the restoration of your Lettine 240x.  In addition to the 240, I also own the 242x (6 meter).  The audio/modulator section of your 240x is very similar to the 242.  The 242 uses a 12AT7 for the speech amplifier and a 6V6 that drives two 6V6s via an input transformer.  I have included a photo of the original Lettine hand drawn schematic for the 242 and a photo of the underside of my 242.  The schematic shows a 22 ohm resistor for the 6v6 cathode bypass resistor, but mine had a 250 ohm that was original to the unit.

I believe the 240x came after the 240, but documentation on the 240x is nonexistent.

Chris

[va3dxv]

and a photo of the underside of my 242.

Interesting! Yours has the same flying B+ death trap lead dangling out the back just like the one in D-lab's video posted above (timestamp: https://www.youtube.com/watch?v=UZf7_tEE9oQ&t=299)

I wonder what the deal with that is? External accessory?  

[n4joy]

and a photo of the underside of my 242.

Interesting! Yours has the same flying B+ death trap lead dangling out the back just like the one in D-lab's video posted above (timestamp: https://www.youtube.com/watch?v=UZf7_tEE9oQ&t=299)

I wonder what the deal with that is? External accessory?  

It is for mobile operation and connection to a dynamotor that supplies the b+.  The black wire is for the filaments.  The photo was taken during the middle of my cleanup so some wires are not connected.  I also had to replace addiotional rotted wiring.  The transformer was replaced as the original was toasted some time in the past.  It works beautifully now and gets stellar audio reports.

[va3dxv]

It is for mobile operation and connection to a dynamotor that supplies the b+.  The black wire is for the filaments.  The photo was taken during the middle of my cleanup so some wires are not connected.  I also had to replace addiotional rotted wiring.  The transformer was replaced as the original was toasted some time in the past.  It works beautifully now and gets stellar audio reports.

Ah I see. Crazy. Yeah the audio in the video sounded really good from what I could tell. Going to have to keep an eye open for one of these. Don't think I've ever seen one up here, they look like cool little xmtrs