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Author Topic: T-368 Exciter  (Read 9373 times)
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w5hro
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« on: January 10, 2009, 10:23:34 PM »

Hey, does anyone remember the replacement tube number for the 6000 tube? I know I bought a couple, but I canít remember the number. Have boxes of tubes here, 2000+ tubes and cant find the 6000 replacements.

After I clean, rebuild and modify the exciter itís going in my HB transmitter to excite the 4-400CG via its inductive link coupling to the grid. (50-ohm to inductive link coupling) Was using the exciter externally before.


* T368_Exciter_01.jpg (652.11 KB, 1769x1155 - viewed 328 times.)

* T368_Exciter_02.jpg (1139.82 KB, 1800x1535 - viewed 348 times.)
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W3NP
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« Reply #1 on: January 10, 2009, 10:29:47 PM »

I believe it is a 25AV5.
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w5hro
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« Reply #2 on: January 10, 2009, 10:43:04 PM »

I believe it is a 25AV5.

Thanks, but that doesnít sound quite right unless there were two. I think its a 6A or 6U something? This is what happens when you walk away from a hobby for 10 years and then try to start off where you left off with many unfinished projects, yikes... I feel really old now.

Wait maybe 25 is the mil version  Huh
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W3SLK
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« Reply #3 on: January 11, 2009, 08:00:56 AM »

It is the 25AV5. However you can put a few twists on this if you are just using it as a VFO. My Elmer Eric, WB4VVI (SK), informed me that the filament wiring for the 6000 tube was independent. So, if you have a 6AV5 handy, you could use it along with a standard 6.3 vac filament supply. He also told me that the 6000 tube was nothing more than Raytheon's glorified version of the venerable 6L6. So, to be really classic, you could change the pin-out on the socket and use the 6L6 instead. Having said all that, if you are going to use it in the T-368, just go with the 25AV5.
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Mike(y)/W3SLK
Invisble airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
w3jn
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« Reply #4 on: January 11, 2009, 08:30:04 AM »

Yep, 25AV5.  Only real difference is the 25AV5's screen voltage max rating is less than the 6000.  But they're 50 cents at any hamfest, so who cares if they don't last as long as a 6000.
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WA1GFZ
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« Reply #5 on: January 11, 2009, 09:50:29 AM »

Why not just add a Zener on the screen to pull it down a bit.
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Jim, W5JO
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« Reply #6 on: January 11, 2009, 10:15:40 AM »

Speaking of these osc/mulitplier units, does anyone have the schematic for just this part?  If you have it in digital format, I wonder if you could email it to me?

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w5hro
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« Reply #7 on: January 11, 2009, 11:53:59 AM »

Speaking of these osc/mulitplier units, does anyone have the schematic for just this part?  If you have it in digital format, I wonder if you could email it to me?

I found it, the 6AU5GT will replace the 6000 and I have two. I crossed referenced over the 25AV5 and the 6000, thanksÖ

Also, I do have the schematic you need. Let me go thru it today and scan the right section(s) and I'll post or just send it to you. I have the entire T-368 manual.
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Jim, W5JO
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« Reply #8 on: January 11, 2009, 12:23:39 PM »

The schematic is all I need.  w5jpw@yahoo.com  TNX.
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k4kyv
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Don
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« Reply #9 on: January 11, 2009, 03:49:21 PM »

I have used one of those T-368 master oscillators as my VFO for many years.  I replaced the 6000 tube with a 6AG7.  It runs at but a fraction of the power, but the capacitances are very close to those of the 6000, so it doesn't try to self oscillate, and it puts out a fraction of a watt of rf, which is all I need from my VFO.

The unit doesn't have a link coupled output.  It simply uses a parallel tuned circuit to resonate the plate, which is capacitively coupled to the BNC output receptacle.  In the T-368 the piece of coax running from the M.O. unit to the 4-400 (or 4-250?) grid is so short that this causes no problem.

But mine is as  much as 10' away from some of my transmitters, and that long a coax lead to the transmitter completely deresonates the tuned circuit and the slug tuning in the coils won't bring it back into resonance.  So I added link coupling to mine.  I had an extra parts unit,  so I took it apart and cannibalised one of the ceramic bandswitch wafers.  I added the new wafer to the bandswitch in my VFO unit, and wound a few turns small gauge enamelled wire at the cold end of each of the 6000 plate coils for the coupling link, and the extra switch wafer selects the proper link which connects to a BNC socket I mounted on the back of the unit.  It took a  little trial-and-error to determine the number of turns to put on each coil, but I finally got it working on all the bands.

After the modification, the output stage wouldn't track as perfectly as it did before, but I found I could make the alignment adjustment right at the middle portion of each ham band, and the tracking would still be essentially perfect across the entire ham band, so the only thing I lost was output at frequencies I never use, anyway.

I added extra shielding to mine, and by-passed all the power and control leads for rf.  I now run the oscillator tube all the time but it is completely inaudible in my receiver. This way, it doesn't cool down during receive periods and drift back to frequency each time it is activated to transmit, as the stock unit has a tendency to do.   I also had to put regulated DC on the oscillator filament, as well as the plate/screen supply, to prevent line voltage variations from pulling the oscillator.  The frequency drift wasn't noticeable on 160 and 80, but it was a problem on 40, particularly if I wanted to use it for CW.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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w5hro
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« Reply #10 on: January 11, 2009, 06:25:51 PM »

The unit doesn't have a link coupled output.  It simply uses a parallel tuned circuit to resonate the plate, which is capacitively coupled to the BNC output receptacle.  In the T-368 the piece of coax running from the M.O. unit to the 4-400 (or 4-250?) grid is so short that this causes no problem.

Yes, thatís something I was considering, whether or not to convert its output circuit. They way I used it back in Tulsa was with a fairly short BNC to PL259 jumper because my RF deck (4-400CG) has a tunable inductive link coupled input circuit to the grid. It didn't seem to load its output down greatly, but I also have a small 50-ohm current balun right at the input of that input circuit. This was the easiest way to do it at the time and I may just stick with it for now. 50-Ohm to inductive link coupling. Now that I'm mounting the exciter on a deck within the transmitter I am considering making those changes as you describe especially the oscillator being on 24/7 mod. I did the same thing with my Johnson 122 VFO on my Viking II by using a resistor from cathode to gnd to just barely turn the oscillator tube on continually.

Well, I need to scan the exciter schematic before I forget.
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w5hro
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« Reply #11 on: January 11, 2009, 07:39:17 PM »

Here is the schematic part 1


* t-368_exciter_01.jpg (272.24 KB, 1279x1096 - viewed 393 times.)
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w5hro
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« Reply #12 on: January 11, 2009, 07:40:54 PM »

Here is the schematic part 2


* t-368_exciter_02.jpg (204.95 KB, 1279x837 - viewed 345 times.)
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WA1GFZ
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« Reply #13 on: January 11, 2009, 09:56:46 PM »

Brian,
The output is high z. Why not just add a cathode follower if you want to drive coax? Loading down the tuned circuits will hurt the Q and may increase harmonics.
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w5hro
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« Reply #14 on: January 11, 2009, 10:39:43 PM »

Brian,
The output is high z. Why not just add a cathode follower if you want to drive coax? Loading down the tuned circuits will hurt the Q and may increase harmonics.

I'll have to look at that again. My RF deck was designed to be driven by a 50-ohm source. Before I acquired the T-368 exciter I used other low power transmitters to do the job. I designed it that way on purpose. When I started using this exciter I didnít have any problems and I was still able to achieve the proper 4-400 grid drive and current without doing anything to it. I think the current balun probably kept everything happy. There may have been some power loss, but it wasn't great enough to even worry about and the balun probably absorbed unwanted RF, etc. coming back.

Your right though, it would be really easy to add a buffer tube on the deck, thatís simple. It could only improve things. I quickly drew up my 4-400CG input circuit, see below.

P.S. As Don said the output connecter is a BNC. The T-368 used a coax jumper to connect directly to the 4-400 grid.


* grid_input_circuit.jpg (62.74 KB, 1279x823 - viewed 304 times.)
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k4kyv
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Don
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« Reply #15 on: January 12, 2009, 01:39:33 AM »

Brian,
The output is high z. Why not just add a cathode follower if you want to drive coax? Loading down the tuned circuits will hurt the Q and may increase harmonics.

The class-C 4-400 also loaded down the tuned circuit.  If you are driving a class-C stage, harmonics won't hurt anything.  In a class-C stage, the tube conducts only over a very small portion of the rf waveform, therefore the grid current waveform is full of harmonics even if fed with a perfect sine wave, and so is the plate output.  It is the flywheel effect at the final amplifier tank circuit that smooths out the harmonics.

Modifying the unit is a lot of work, but it greatly simplifies using it as a vfo to drive a transmitter some distance away.  On my Gates and the HF-300 rig, the first stage following the vfo is wired as a grounded grid amplifier (not sure if the rf level is high enough to drive it into class-C).  On my 8005/805 rig, I just feed the coax output directly to the grid of a 6AG7 with no loading resistor, and that still excites it adequately on all bands without squirreliness.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
Steve - WB3HUZ
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« Reply #16 on: January 12, 2009, 06:59:15 AM »

Thousands of hams have successfully driven low impedance loads for decades without resorting to a cathode follower. Adding a CF Seems to be an uneccesary complication. A parallel tuned circuit and tapping down on the coil should work just fine.



Brian,
The output is high z. Why not just add a cathode follower if you want to drive coax? Loading down the tuned circuits will hurt the Q and may increase harmonics.

I'll have to look at that again. My RF deck was designed to be driven by a 50-ohm source. Before I acquired the T-368 exciter I used other low power transmitters to do the job. I designed it that way on purpose. When I started using this exciter I didn’t have any problems and I was still able to achieve the proper 4-400 grid drive and current without doing anything to it. I think the current balun probably kept everything happy. There may have been some power loss, but it wasn't great enough to even worry about and the balun probably absorbed unwanted RF, etc. coming back.

Your right though, it would be really easy to add a buffer tube on the deck, that’s simple. It could only improve things. I quickly drew up my 4-400CG input circuit, see below.

P.S. As Don said the output connecter is a BNC. The T-368 used a coax jumper to connect directly to the 4-400 grid.

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WA1GFZ
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« Reply #17 on: January 12, 2009, 08:25:52 AM »

True, as long as the coax run between the exciter and final is short.
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w5hro
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« Reply #18 on: January 12, 2009, 10:06:30 AM »

True, as long as the coax run between the exciter and final is short.

The coax run will be less than 24 inches excluding the extra 12 inches (the current balun) on the bottom side of the RF deck that starts from the SO239 jack on the back and connects to the grid's tuned circuit.

I just need to add an output control (drive control) to control the exciters output via the new deck's front panel. Maybe another control too for other possible controllable features including a BNC out for a freq counter, etc.
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k4kyv
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Don
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« Reply #19 on: January 12, 2009, 12:27:36 PM »

Thousands of hams have successfully driven low impedance loads for decades without resorting to a cathode follower. Adding a CF Seems to be an uneccesary complication. A parallel tuned circuit and tapping down on the coil should work just fine.

Tapping down on the  coil and adding an outboard coupling link is electrically the same thing - the difference between an autotransformer and a transformer with separate primary and secondary. 

The way the T-368 slug-tuned coil is constructed, it would be much easier to simply wind the coupling line over the cold end of the coil.  I think adding a satisfactory tap not only would be very difficult; you would run the risk of damaging or ruining the very delicate coil, which is wound with a variable pitch over a paper-thin bakelite form.

Another solution would be to leave the stock exciter intact, but build an outboard matching unit (a simple L-network should work fine) to transform the  hi-Z from the parallel tuned circuit to the lo-Z of the transmission line.  But you now have an additional tuned circuit to adjust every time you change frequency.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
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WA1GFZ
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« Reply #20 on: January 12, 2009, 12:37:09 PM »

Why not just vary the 6000 screen voltage for a drive control.
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w5hro
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« Reply #21 on: January 12, 2009, 12:39:46 PM »

But you now have an additional tuned circuit to adjust every time you change frequency.

That is exactly what I don't want, another ciucuit to tune. Also, since the exciter stops around 20Mc I still want the capability to use another external exciter (probably a 50-ohm exicter) for 10 meters in the future, but I have no plans to do that anytime soon. I have the Viking II for that purpose.

Gota run... I'm at work
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w5hro
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« Reply #22 on: January 12, 2009, 12:40:34 PM »

Why not just vary the 6000 screen voltage for a drive control.

Yep, thats one way...

Gota go...
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k4kyv
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Don
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« Reply #23 on: January 12, 2009, 01:10:40 PM »

But you now have an additional tuned circuit to adjust every time you change frequency.

That is exactly what I don't want, another ciucuit to tune. Also, since the exciter stops around 20Mc I still want the capability to use another external exciter (probably a 50-ohm exicter) for 10 meters in the future, but I have no plans to do that anytime soon. I have the Viking II for that purpose.

Gota run... I'm at work

Use a doubler stage between the VFO and final for 10m.  You probably already have one or more amplifier stages in between.  Just tune the grid of one of the stages to 20m, and the plate of the same stage to 10m.  You should be able to get on 12m as well.

As I recall before modifying mine, the highest frequency range, which theoretically should go from 12 to 24 mHz, still had enough output at 21 mHz to cover 15m.  The thing amazed me.  I could put a neon lamp at the output and tune across every range and the lamp would stay at absolutely constant brilliance, except for the high frequency range, where it would drop off slightly, but not completely go out, at the very high end.

I suspect the 20 mHz limit simply means that the output satisfies customer specs at the highest design frequency of the transmitter, 20 mHz.  They considered it immaterial how well it performed above that frequency.  It would most likely be possible to align it for constant output all the way to 24 mHz.

I never used mine for 10m. though.  I picked up a DDS vfo at Dayton one year, so I used the output of that at the 40m range to quadruple to 10m, using the Eico 720 that was given to me, which was designed to cover 10m using a 40m xtal.  The thing was rock stable even at the quadrupled frequency.  But the T-368 exciter should have been satisfactory, particularly for AM.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
w5hro
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« Reply #24 on: January 13, 2009, 10:03:23 AM »

Use a doubler stage between the VFO and final for 10m.

You probably already have one or more amplifier stages in between. Just tune the grid of one of the stages to 20m, and the plate of the same stage to 10m. You should be able to get on 12m as well.even at the quadrupled frequency. But the T-368 exciter should have been satisfactory, particularly for AM.

Thanks Don, I'll look into that. I purchased the exciter because it seemed to be about the perfect match for my transmitter and it worked without any problems before I left Tulsa, 9+ years ago. I hope it still does.

P.S. I donít think I will be able to get my tower and antennas up this winter, but I am still going to try. I lost my helper yesterday, my wife's dad. I canít lift everything up by myself unless I hire someone. They both flew to the Philippines last night and are currently in the air as I write this. My wife is coming back in two weeks but her dad will be there for the next several months. I was hoping to get back on the air at least with the Viking II this month, but without a decent antenna up its somewhat pointless I think Sad

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