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T-368 Exciter




 
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Author Topic: T-368 Exciter  (Read 10395 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.


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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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« Reply #2 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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« Reply #3 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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« Reply #4 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 #5 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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« Reply #6 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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« Reply #7 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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« Reply #8 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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k4kyv
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« Reply #9 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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Steve - WB3HUZ
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« Reply #10 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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« Reply #11 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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k4kyv
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« Reply #12 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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« Reply #13 on: January 12, 2009, 12:37:09 PM »

Why not just vary the 6000 screen voltage for a drive control.
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k4kyv
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« Reply #14 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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N2DTS
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« Reply #15 on: November 10, 2009, 03:36:49 PM »

Very nice work!

Who cares about length of coax?
Its only a db per 100 foot or something like that, unless you are doing uhf....

Brett
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« Reply #16 on: November 10, 2009, 06:09:43 PM »

It depends on the plate voltage and current, no?
say 500 volts and 100 ma as a guess, 5000 ohms.

I would try it before doing anything about it...

Brett
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« Reply #17 on: November 10, 2009, 08:51:15 PM »

I thought plate resistance, and plate impedance were two different things, with the plate resistance being dependant on the Mu of the tube and so on, like the 6b4 was low plate resistance, 800 ohms.

The plate load impedance was the voltage and current you ran it at, like a pair of 813 tubes at 2000 volts and 400 ma has a plate load impedance of 5000 ohms.

I would guess a 4/1 balun would help, if you need driving power and not just voltage...

Brett

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« Reply #18 on: November 10, 2009, 09:49:14 PM »

Well, there wont be much actual LOSS in the coax in 10 feet of it....
Its not like it was real high frequency stuff you were trying to pass.

It seems like anything you would have to tune would be a pain and a step in the wrong direction...

I think Don K4KYV uses one of those to excite his rigs, no?

I have one here I never used for anything...

Brett



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The Slab Bacon
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« Reply #19 on: November 11, 2009, 08:59:01 AM »

Sounds like a job for some type of cathode follower circuit. It could also act as a buffer and give some isolation as well.
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« Reply #20 on: November 11, 2009, 02:34:08 PM »

Is there enough 'scrote' in the 6000 tube or its cousin the (6)25AV5, to drive a triode like an 833? I'm willing to bet that it would require a buffer amp.
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« Reply #21 on: January 30, 2010, 09:31:14 AM »

Brian, Do you have a schematic for the power supply you used for the T-368 exciter? I have one here and need to build a supply. Thanks. I don't think anybody mentioned it here but most of the PTO's of that type have a fundamental frequency of 1-2 MHz so you can cover 160 by bypassing the doubler. Your mileage may vary.

Mike
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« Reply #22 on: January 30, 2010, 10:48:41 AM »

Two good points. First, I wrote that without looking at the video. When I get to the exciter I'll study the manual, or, you might have it by than.

Second, I didn't look at the T3 exciter first. Most of the Collins transmitters made for ham use used this type of PTO but don't offer 160. I met a guy 10 years ago or so that changed the top band on his 310 exciter to 160 and sacrificed 10 meters. It was cool but unneeded advice here. Have fun.

Mike
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« Reply #23 on: January 30, 2010, 11:25:35 AM »

About 12 watts available from the 6000 or 25AV5, in my recollection. The two tubes are closer kin than cousins.  Direct substitute here. I think only the plate dissipation rating was slightly different, but the pinout, interelectrode capacitance, and physical size are identical.

Is there enough 'scrote' in the 6000 tube or its cousin the (6)25AV5, to drive a triode like an 833?


On another question -- I didn't see a direct answer to the distance issue, but the problem is added capacitance, not coax loss, when running a VFO into a line that's more than a few feet in length.  This was why the Johnson 122 and others had only a very short coax stub to keep it near the transmitter.

The T-368 exciter deck delivers a more finished signal than a low-level VFO however, so the distance may prove less of a problem.
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« Reply #24 on: January 30, 2010, 11:56:46 AM »


I think Don K4KYV uses one of those to excite his rigs, no?

Well, there wont be much actual LOSS in the coax in 10 feet of it....


Yep, and his has been super modified too.

Itís not so much about the power loss at 10' away itís about the tuned circuit being 10' away from the 6000 tube. That is the issue. Without using some type of matching at the 6000 tube's output it probably wonít resonate the thing very well if its 10' away.

The only thing I can think of that may work short of using a buffer tube, pi or L network is using a voltage balun with a big step down ratio. Unless anyone else has any ideas?

You can convert the exciter unit to  link coupled output. This involves adding an additional bandswitch wafer and putting a coupling coil on each one of the output coils.

If you can find a junker T-368 or T-195 exciter unit, disassemble the bandswitch, clip the wires and remove one of the ceramic wafer sections.  Carefully remove the pieces of wire from the solder lugs on the salvaged wafer, avoiding damaging them.

On your good unit, remove the buffer/multiplier unit from the main chassis. Remove the slug rack and take off the output coils. Romove the phillips screws from the coils, and pull them out.  These coils are plug-in and come out easily when the screws are removed.

You want to add the salvaged bandswitch wafer to the bandswitch assembly in your good unit. Place it near the output  coils, taking care that it will be placed clear of other components.  Once you have figured out just where to put the extra wafer, mark the spot on the metal sleeve spacer that goes between wafer sections.  Carefully disassemble the bandswitch assembly, noting exactly where each little piece of hardware goes. Pull out the long screws and  sleeves.  Be careful not to  lose any of the little washers or spacers.

Cut the long metal sleeve at the point where you marked it.  File away enough metal from one of the pieces of sleeve on each side, so that the new wafer can be inserted using the same hardware as the rest of the wafers, and the total length with new wafer inserted will be the same length as it was before cutting.  This is one of those cases of measure a half dozen times and cut once. Make sure each side is spaced exactly the same as the other.

Re-assemble the bandswitch including the new wafer.  Make sure it is indexed the same as the  rest of the wafers.

Take off the shields from the output  coils.  Don't be surprised to find  that the bakelite coil forms have turned dark brown from heat. Now, modify the output coils.  On the cold end of each of the coils, the end nearest the grounded point, add a coupling link, using enamelled wire about the same gauge as the wire used to wind the coils. I used a thin sheet of insulating material between the main coil and the link.  Good quality non-acid paper will do.  If I recall correctly from 20 years ago, I used about 8 turns on the 160m coil, 4 on the 80m one, 3 on the 40m one and 2 on the 20m one. I was able to salvage any needed parts from the junker unit. You might have to do some trial-and-error with the number of turns on each link coil for best performance.

Ground the bottom end of the link coil to the ground point on the main coil.  I seem to  recall that there might be a spare prong on the plug-in  coil assembly to accommodate the hot side of the link coil.  If not, I don't recall it being too difficult to add one, along with a female receptacle on the coil socket, or I may have run a wire lead directly from the hot side of the coupling link to the bandswitch. (I don't feel like taking mine apart to see exactly how I did it).  In any case, I remember that this was not too difficult.

Wire the leads from the coupling links to the new bandswitch wafer, and run this output lead to the BNC connector.  Of course, you first have to remove the original wiring that goes from the from the hot end of the output coil to the BNC.

Now, you have to re-align the output tuned circuits. In mine, after adding the links, the output circuits would no longer track properly.  Most likely, the additional load resistance and reactance  from the link coupled load changes parameters in the tuned circuit.  Connect a dummy load and rf output indicator to the BNC.  I used my Hammy Hambone Mirage wattmeter on the lowest scale in the SWR position, to gain enough sensitivity so that it would indicate a fraction of a watt.

The circuit probably won't track across the entire range no matter what, but it will track well enough to cover the main ham band within each range, which should be perfectly satisfactory unless you plan to do something like pirate broadcasting.  Following the procedure in the T-368 manual, align each range, focussing on the middle of the ham band. It should be possible to get uniform output across each entire ham band covered by each range.  One exception, I couldn't get mine to align properly on 40m and still have output on 30m.  This might be possible using trial and error with the number of turns on the output coupling coil for this range.

On mine, I replaced the 6000 tube with a 6AG7.  The internal capacitances of the two tubes are nearly identical, I had a bunch of good 6AG7's on hand, and I didn't need the full output from the 6000.  I had to re-wire the tube socket since the pin connections are not the same. I run about 200 volts on all the tubes, except for +150 regulated on the PTO unit.  I can get about 250 milliwatts output into a 50-ohm resistive load on 160, 80, 40 and 20, but I use it only on 160, 80 and 40. The vfo works with all 3 of my transmitters, feeding with RG-59, 58, or even the two types spliced together, with lengths up to 15 feet.

In one rig, I simply feed the coax to the grid of the first tube, a 6AG7.  In the Gates broadcast transmitter and HF-300 rig, the crystal oscillator circuit is modified to grounded grid input.  I grounded the grid of the xtal oscillator, the 12BY7 in the Gates, and the 59 in the HF-300 rig, and feed the VFO output directly to the cathode through a coupling capacitor, with an rf choke to DC ground.
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Don, K4KYV                                       AMI#5
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