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Driver transformer recommendations




 
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n4joy
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« on: January 04, 2022, 10:39:44 AM »

Hello.  The Thordarson 20D76 driver transformer for my Lettine 130 has a short. The 20D76 is a 5.2:1 (single plate to PP grids).  The modulator consists of a 6N7 speech amp, 807 driver, and a pair of 807s.  The modulator transformer is a 75 watt Thordarson 21M56.

Knowing that original driver/interstage transformers are very difficulty to locate, what options do I have?  The best I can find from Hammond is the 124B, but the turns ratio is 1:3.

I appreciate the guidance!

Chris, N4JOY
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IN A TRIODE NO ONE CAN HEAR YOU SCREEN


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« Reply #1 on: January 04, 2022, 12:20:30 PM »

Hi Chris,

Hit me up, I can help with that iron. Wink

73DG
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n4joy
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« Reply #2 on: January 04, 2022, 12:56:01 PM »

Thank you, Dennis!  I just sent you a PM!

Chris, N4JOY
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n4joy
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« Reply #3 on: January 11, 2022, 11:57:44 AM »

I've looked at some Hammond options and have done research on 807s in class B.  The Lettine 130 has two 807 finals (about 140 watts input) and  the modulator section consists of an 807 driver and two 807s in class B.  The 807 driver runs approximately 740V on the plate and around 250V or so on the screen with a 750 ohm cathode resistor and 47uf bypass cap.  I see that the plate impedance would be high at around 39K ohms. The two 807s in class B at 740V should have a grid resistance of around 7K ohms.  The original driver transformer (step down) was 5.2:1 (primary ratio to 1/2 secondary).  Interestingly, Lettine only used 2.2K ohm resistors to connect the 807 modulators screens/grids for class B.  Everything I read says the 807s prefer high voltage/low current in this operation and to use 20K or 22K resistors.  This probably explains why I never could achieve 100% modulation.

I looked at reversing the Hammond 124D (1:1.49 turn ratio or 1:2.2 impedance ratio) because it has a CT on the primary and secondary, but the input and output impedances are only 7K and 15.8K ohms, respectively (I would reverse the primary and secondary to function as a driver transformer).

Perhaps another option is the 124B and reversing the primary and secondary.  I would have to add an artificial center tap on the primary (now serving as a secondary).  The primary of the 124B is 10K ohms and the secondary is 90K, but I would setup the secondary (now serving as primary) to 45K using the CT.

Hopefully, my understanding and logic makes some sense!

Chris, N4JOY
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« Reply #4 on: January 11, 2022, 12:50:03 PM »

Quote
The Lettine 130 has two 807 finals (about 140 watts input) and  the modulator section consists of an 807 driver and two 807s in class B.

I am curious about that transmitter. Do you have a schematic of the modulator and transmitter that you could post?

Thanks Terry
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« Reply #5 on: January 11, 2022, 01:29:54 PM »

Quote
The Lettine 130 has two 807 finals (about 140 watts input) and  the modulator section consists of an 807 driver and two 807s in class B.

I am curious about that transmitter. Do you have a schematic of the modulator and transmitter that you could post?

Thanks Terry

Hi, Terry.  Unfortunately, an original manual and schematic is impossible to find.  I need to make a schematic when time permits.  Unlike the more common 240 and 242, the Lettine 130 is rather rare.  It consists of a 6N7 speech amp, 807 driver, and a pair of 807s running class B.  Two 807s make up the finals and an 807 serves as the oscillator.  Power supply is 1780VCT, 3B28s, and LC.  B+ is around 740 volts under load and output is around 70 watts.  The modulation transformer is a 75 watt Thordarson.

Lettine made some interesting value choices.  The 240, for example, is not capable of 100% modulation unless the cathode resistor for the 6L6GC modulators is dropped from 750 ohms to approximately 450 and a bypass cap is added.  A screen bypass cap is also needed at the 6L6GC. 

On the 130, 2.2K resistors between grids for 807s (class B seem) like a very odd choice and contrary to every class B 807 modulators I've seen, which all use 20K or 22K to reduce current.  I was never able to achieve 100% modulation before the driver transformer failed.
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« Reply #6 on: January 12, 2022, 08:37:59 PM »

Well I fell on the ice today and sprained my left wrist and it hurts and is not fun! But anyhow, the audio driver is an 807 and I wonder if that is running in class A? It's interesting that they used an 807 for that but I think the art 13 does something similar using a single 6v6 to drive a pair of 811 tubes? Too bad about no manual for it? The transmitters of that time frame are not too complicated but it would be nice to have the book for it! I think the low value resistors in the 807 is because they may be running the 807 configured as triodes for class B? If you were in AB-1 then higher value screen resistors would be indicated. It would however take some doing to get enough power out of a set of 807's to fully modulate that rig in AB-1? I think AB-1 is explained as push-pull whereas class B is Huff & Puff:)
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« Reply #7 on: January 13, 2022, 12:37:53 PM »

Well I fell on the ice today and sprained my left wrist and it hurts and is not fun! But anyhow, the audio driver is an 807 and I wonder if that is running in class A? It's interesting that they used an 807 for that but I think the art 13 does something similar using a single 6v6 to drive a pair of 811 tubes? Too bad about no manual for it? The transmitters of that time frame are not too complicated but it would be nice to have the book for it! I think the low value resistors in the 807 is because they may be running the 807 configured as triodes for class B? If you were in AB-1 then higher value screen resistors would be indicated. It would however take some doing to get enough power out of a set of 807's to fully modulate that rig in AB-1? I think AB-1 is explained as push-pull whereas class B is Huff & Puff:)

Ouch!  I hope you recuperate quickly.  Yes, the 807 driver is class A.  Every article I've read about 807s in class B show 20K to 22K resistors tying the grids for triode operation.  Apparently the 807s like higher voltage and low current conditions in class B.  I installed the Hammond 124D last night and replaced the 2.2K resistors with 20K.  I don't know why Lettine would use a step down driver transformer when the load impedance of the 807 in class A at my voltages is around 6K ohms and the plate-to-plate for 807s in class B is not too far off.  I'll do some testing in the next few days and report back.

Thanks,

Chris, N4JOY


* 807 Modulator.jpg (47.77 KB, 576x284 - viewed 80 times.)
* RCA-Ham-Tips-47-05-06.pdf (1457.32 KB - downloaded 20 times.)

* 807_special_triode.png (760.13 KB, 587x441 - viewed 76 times.)
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« Reply #8 on: January 13, 2022, 12:47:44 PM »

Here's another article (75 watt class B 807) showing an arrangement somewhat similar to the Lettine 130: 807 driver, driver transformer, and a pair of 807s in class B.  I have 740V on the driver and modulator tube plates under load.  The 130 uses a 6N7 as a speech amp going right to the 807 driver.


* Amateur-Radio-AU-1962253.pdf (47 KB - downloaded 30 times.)
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« Reply #9 on: January 13, 2022, 07:21:44 PM »

Thanks, the left wrist is a small balloon right now but the pain is 90% gone! I see those resistors are there to protect the number one control grid? I thought we were talking screen grid and my brain was thinking of a GG RF amp, not audio? Maybe I bumped my head too?? I really apricated the examples you sent and will put them in my computers memory system:)

Good Luck Chris on that project!

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« Reply #10 on: January 16, 2022, 12:11:58 PM »

Okay, I am reporting back.  I made the adjustments: Hammond 124D driver wired step up (1:1.5), 22K resistors tying the grids on the 807 modulators, and dropped the cathode resistor to 270 ohms.

On the scope, I see plenty of pinching and can easily drive to distortion; however, I cannot achieve 100% positive peaks.  At best, I can only reach 70% to 75% modulation.

I do notice that my plate current is swinging negative a lot--dropping about 30ma to 40ma or so on peaks from approximately 190ma.  The LC power supply originally had three 20uf 450V caps in series; I had replaced them with 22uf. 

It seems I am running out of steam and need to up the 22uf caps to 47uf?

Chris, N4JOY
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« Reply #11 on: January 16, 2022, 01:47:03 PM »

You may be out of phase? Switch the plate caps and see if things improve?

 
Increasing the capacitance may help  I would use as much capacitance as I could reasonably fit in there but not to a point where you would have to add a surge protector circuit. (unless you have room)

Cathode resistor on the pair of 807's? What is the negative voltage on the grids at rest? I would think that for good regulation that the bias voltage here should be from a fixed source that is heavily bled for good regulation or from a regulated fixed bias PS. Perhaps Zener diodes in the cathode would be a better match? Anyone else have any ideas?

Terry
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« Reply #12 on: January 16, 2022, 02:24:26 PM »

Thanks, Terry.  I guess it is just you and me here!  Grin  Good advice.  The power supply is full wave.  1780VCT, 3B28s, and a choke.  I also thought it might be out of phase and switched the primaries on the driver transformer--no change.  I'm barely hitting 70% on peaks.  The 40ma+ downward swing on the plate current is a big clue.  Based on the design and component values, Lettine never designed this transmitter for 100% modulation, which is very odd.  The filter caps must be woefully undersized.  I may have room for a soft-start circuit.

At least I've made some progress.  Replacing the 2.2K resistors with the 20K resistors and changing out the old driver transformer with the Hammond 124D interstage has made a difference.  It appears I'm driving the tubes better now.

I'll order some F&T 47uf caps and replace the 22uf caps as a start.  That will get me from about 7uf to 16uf after the choke.
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« Reply #13 on: January 17, 2022, 04:41:43 PM »

So Chris, what are your long term plans for this transmitter? If you were going to use it as an rf exciter then not too much would have to be done but if you wanted to drive a GG linear, I would want to get at least 125% modulation.

On my BC-610 I see the plate ma droop but the RF seems steady plus the 300 watts+ of audio on the carrier pump things back up again. From the factory I think the total capacitance is about 8uf. I hung one of those energy storage caps on the HV line and it's rated at 25uf 4KV.( I don't think they are oil filled?) Before I put that cap on there the droop was even larger but I don't remember what it was?

Going around the web I learned that your transmitter is an orchard of 807 tubes Grin If there is room, maybe another socket for a hot spare is in order?  Terry
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« Reply #14 on: January 18, 2022, 02:05:24 PM »

So Chris, what are your long term plans for this transmitter? If you were going to use it as an rf exciter then not too much would have to be done but if you wanted to drive a GG linear, I would want to get at least 125% modulation.

On my BC-610 I see the plate ma droop but the RF seems steady plus the 300 watts+ of audio on the carrier pump things back up again. From the factory I think the total capacitance is about 8uf. I hung one of those energy storage caps on the HV line and it's rated at 25uf 4KV.( I don't think they are oil filled?) Before I put that cap on there the droop was even larger but I don't remember what it was?

Going around the web I learned that your transmitter is an orchard of 807 tubes Grin If there is room, maybe another socket for a hot spare is in order?  Terry

Hi, Terry.  My normal setup is a Lettine 240 (modified for 100% modulation) and a home brew 2x813 GG.  The 130 will likely be operated stand alone, so I definitely need to get the positive peaks up!  I ordered three F&G 100uf 450V caps, which will go in series for the 130's choke input PS.  Once I get this worked out, I will test and see if the speech amp, driver, or modulation section needs further adjustments.  My measurements suggest the 807 is providing enough drive--a lot more now with the new interstage transformer. The modulator is working harder and 40ma sag is not helping matters right now!
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« Reply #15 on: January 25, 2022, 01:24:24 PM »

Just another update... the weak link appears to be my 6N7 speech amp and 2nd audio stage. The 6N7 (cascaded) uses grid leak bias and the output is anemic.  I'm not supplying enough drive to the 807 driver.  I'm going to change over to a 6SN7 in cathode bias and cascade both sections.  It will take a little work but this should stimulate the 807, so it can better drive the 807s in class B.  

I located a design for the 6SN7 "Cascade Resistance Capacity Coupled Amplifier", which should deliver an output of approximately 55 volts peak, an overall voltage gain of approximately 270.



* 6SN7GT_pic3.jpg (30.42 KB, 388x370 - viewed 37 times.)

* Capture.JPG (21.33 KB, 531x173 - viewed 20 times.)
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« Reply #16 on: January 26, 2022, 02:52:47 PM »

Just thinking out loud that if there is too much gain perhaps a gain control could be put in place of R3 with the wiper going to grid 2 ?
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« Reply #17 on: Yesterday at 05:53:44 AM »

Just thinking out loud that if there is too much gain perhaps a gain control could be put in place of R3 with the wiper going to grid 2 ?

I was debating whether the pot should go at R3 or replace the 470K grid leak resistor before the 3rd stage.  I guess I will need to test.  I like the 6N7 but it seems like an odd choice for a 1st and 2nd stage in cascade to drive an 807.

I appreciate your responses.  Perhaps next time I should add Viking Ranger" or "Valiant" to my topic title?  😆
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