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Author Topic: tips on feeding ECO to OSC  (Read 4759 times)
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Steve W8TOW
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« on: October 13, 2006, 02:50:28 PM »

I am playing around with a couple of ECO's. One is a Millen Vari-arm ECO and the
other is a BUD, from around 1947-8....
Does anyone out there have any experience with these? I'd like them to drive a
6V6 or 6AG7 TP osc...
73 steve w8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
WU2D
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CW is just a narrower version of AM


« Reply #1 on: October 15, 2006, 12:53:13 AM »

Geez Steve,

Many read but nobody replied. I do not know about the specific VFO's that you are talking about. I would imagine that they are based on a single pentode tube like a 6AG7 or a 6AU6 in a Hartley or Collpitts Electron Coupled Oscillator. They may be buffered by another pentode. In any case the output was usually taken untuned off the plate from a choke and cap coupled out. This would produce enough voltage to excite the grid of most all of the crystal oscillators found in the ham transmitters of the day. A deluxe version might have a triode cathode follower output.

Some transmitters were designed to accept a VFO. For instance the old Heath DX-35 design used an electron coupled 12BY7 oscillator which brought the crystal from the screen grid back to the control grid. To drive this with a VFO, heath did two things, they disconnected the grid from the crystal sockets and sent it to the rear VFO in connector (hot) and second, they bypassed the screen (which was normally unbypassed in Pierce oscillator duty). Heath used a double pole crystal switch to accomplish this.

For a simple Triode or Pentode tuned output oscillator ( where the output must be resonated for the tube to oscillate), the connection is simple. Just couple voltage into the grid right at the crystal socket. If the VFO happens to have a tuned output - beware - you may form a nifty TPTG oscillator.

For a simple Triode Pierce like a 6C4 untuned output, driving a 6L6, you can not connect to to the crystal socket unless you have a switch that disconnects the bottom of the socket which goes to the tube plate and conects it to ground. This can be accomplished with a switch. The same warning goes. The VFO should not have a high Z, high Q tuned output or it will tempt the oscillation gods.

For a more complex oscillator such as an electron coupled Colpitts Harmonic Oscillator (which is by far the most popular type found in the old handbooks) you need to be careful. The oscillator (usually a 6AG7, 5763, 6CL6 or 12BY7) will want to oscillate and raise hell unless the bottom capacitor is which goes from the cathode to ground (usually 100 - 470 pF) is bypassed with something like a 0.01 uF. Alternatively, both Colpitts capacitors could be switched out of circuit with a DPDT switch.

Dunno if this helps any.

Mike WU2D
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These are the good old days of AM
Steve W8TOW
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« Reply #2 on: October 16, 2006, 03:59:15 PM »

HI Mike, thanks for the reply...
Well, my specific TX that I'm using is a old HB tx. 6V6 osc (tuned plate) then drives a 807,
next a pair of Taylor T-40's in PP.
I added a pair of VR tubes a few years ago...for LV/screen regulation. With a xtal, sounds ok,
cathode keying the 6V6...I've tried keying both 6v6 & 807 cathodes, but
really don't hear much diff.

The T40's are bias at cutoff for CW
I will send ya a schematic of the rig if ya like sometime!

Yesterday, I had success with one ECO. It is a  BUD ECO from about 1947.
I rewired it several years ago. New coils caps, etc....

By itself, it keys beautifully ...but 4-5 years ago, while on AM, I got reports of "FMing".
So I put her on the shelf...
Last night, I put some new PS caps in it, and tried driving the 6v6 osc in the T40 rig...
The BUD ECO is a 6F6 into  a 6V6 (or you can put a 6L6 in it if ya need more RF!)
It does have a VR tube in it...and a variable C output....

Well, there are slight key clicks, something I'll resolve tonight, but it doesn't chirp, buzz or any
such anoying things....
The tuning of the 6V6 & 807 in the RF deck is text book...guess I got lucky eh?
73 steve   8TOW
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
WU2D
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CW is just a narrower version of AM


« Reply #3 on: October 16, 2006, 10:41:35 PM »

Hi Steve,

That sounds like a great HB rig. Wow P-P - I have not homebrewed anything larger than a couple of 807's or 6146's in parallel. I am currently having a great time on 40M with the little 6AG7/6L6 rig that is featured as my avatar. I also am fooling with a command set that I can not tame. It wants to chirp.

As far as clicks go, I have had great success with a single cap or better yet, a cap and resistor network in series across the key jack(to ground), and a series resistor to the cathode of the 6L6 (6V6 in your case) where there is a second cap going to ground. One network handles the make and the other handles the break timing. A scope will show all.

The values will be something like a 0.22 UF and a 220 Ohm 1 Watt series network across the jack and a series 100 Ohm 2 Watt and a 0.05 UF to Ground. The osc stage is keyed directly off the key jack. In my Collpitts stage this is actually a choke off the cathode of the osc.

Mike WU2D
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W2JBL
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« Reply #4 on: October 17, 2006, 08:43:34 PM »

   the Millen Vari-arm was meant to drive the Millen Exciter, direct to the grid of it's 6L6 oscillator. it should drive anything you have with voltage to spare. those early ECO's (that's a VFO to you newbies) have enough output that you don't even need to worry about using high impeadence cable going to the rig. even (high capacitance to ground) RG58 works fine. if driving newer rigs like DX60, Eico 720 etc. they may even be a bit too hot. some like the Meisner Signal Shifter are actually low power exciters and can be coupled direct to an antenna.

     for many years i have used the main VFO out of an ART-13 (single 1625 at 400 volts) as a utility VFO around the station. i have a tuned circuit at the desired operating frequncy rigged on it's output. it puts out about 5 watts by itself, and can drive anything with no problem. these day's i use it to excite a single 813 and it's OKfine. i dig those Vari Arms! i had one years back, and zerobeating with it was like steering a boat with a fore to aft moved tiller! pre war my Dad ran a 6L6 ECO on 40 driving a TZ40 to a pair of HY51's. i have that ECO someplace.
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