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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Mon 16, 2012 6:23 pm 
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Location: Central Pa, 17044
Does anyone have a schematic or plans for a VFO that would work with my
Xmtr? Would like to stay with tubes if possible.
Thanks, Gary KB3WYZ


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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Mon 16, 2012 6:37 pm 
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Posts: 390
Location: Madison, WI
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0501.pdf
http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2002%20No%206.pdf


Last edited by Jon the Grimm on Jul Mon 16, 2012 6:41 pm, edited 1 time in total.

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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Mon 16, 2012 6:39 pm 
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Location: Livermore, CA
Gary

A VFO is an oscillator. VFO's are fairly simple but more important than circuit is how it's built. Should be built very sturdy, regulated voltage, using cathode follower, temperature controlled. A VFO shouldn't have any noticeable drift.

Search "tube VFO circuits" on the internet. Radio Amateur Handbooks from 1950's and 1960's have circuits.

http://www.w7ekb.com/glowbugs/VFOs/vackar_vfo.html

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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Mon 16, 2012 6:42 pm 
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I suspect I am on Gary's "ignore list" since I've never received an acknowledgement
to any of my replies....

but almost every crystal controlled oscillator can be made to work as a a VFO by
adding an LC network instead of a quartz crystal.

Pete


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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Mon 16, 2012 10:03 pm 
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Location: Central Pa, 17044
Sorry Peter, Wouldn't IGNORE you..No way. Your comments are always welcome!

73's Gary


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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Tue 17, 2012 4:16 pm 
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Hi Gary

Lots of older VFO s will work fine with your transmitter. The Heath VF-1 and Johnson 122 come to mind, but many companies made their own version back in the 60s. I often see a few at swaps. Most of these will need some form of power supply if your transmitter power supply does not have excess capacity. I routinely run a homebrew 6AG7/6L6 transmitter with a Heath VF-1, and find the combination very stable after about 20 minutes warm up.

Dave

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 Post subject: Re: 807 CW Transmitter
PostPosted: Jul Tue 17, 2012 8:24 pm 
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Location: Powell River BC Canada
I didn't see any rectifier tube, so assume you are using diodes. Is so the 5 Volt winding
on the transformer might be used for the 807 until you find something different.
I will be better than the 3.7 volts you have now.

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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Sat 04, 2012 4:14 pm 
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Location: Central Pa, 17044
Hi Guys, Here is an update on my 807 CW XMTR. First of all I put two 1N4007 doides in
series for the bridge rectifier, and got another power Xfmr. 800VAC/CT 6.3VAC 3A, 5VAC
3.5A and another 6.3VAC 2amp. The XFMR is rated at 90ma.
I just hope the 400VAC from the CT and going through 3, 91.5 zenier diodes and a 8h
choke plus a 5k 10w power resistor and the 10uf 450v Cap will come down to the
required voltage for the plates of the 6AG7 tube, the max voltage is 300V.
Comments are welcome, the schematic is on page 1or2 of this thread.
Thanks, Gary KB3WYZ


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Sat 04, 2012 5:47 pm 
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Joined: Aug Tue 28, 2007 6:44 pm
Posts: 455
Location: Tustin,CA
Hi Gary,
The vintage Heathkit AT-1 xmtr ran the 6AG7's plate voltage at 400 Vdc key-down, so I wouldn't worry too much abt that 300 Vdc max (?). Besides, the RCA HB3 plate characteristic curves indicate plate voltages to 475-500 Vdc!!!
Dave - WA6VVL


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Sun 05, 2012 1:43 am 
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Posts: 6
Gary, good going !

one thing though, the xfmr secondary rating at 90 mA does not leave room for mistakes during tune-up...the 6ag7 will draw 20-30 mA so I suggest you determine the dip using a grid-dipper set at your operating frequency (with the loading cap fully meshed) before you key the rig. I'm saying this because off resonance, at 800 volts, the 807 is liable to want to draw upwards of 100 mA so you want to be as close to resonance as possible from the get-go. Also, you may want to lower the 807 screen voltage to bring the plate current draw within the limit set by your xfmr.

I based my MOPA on the same circuit as you (see below). However, I have a long lead to the grid of the 807 so 20M and up are no good...I've since replaced the 20M oscillator coil with a 160M one and now have three bands.

http://imageshack.us/photo/my-images/718/img1604vq.jpg/
http://imageshack.us/photo/my-images/820/img1600kh.jpg/
http://imageshack.us/photo/my-images/802/img1632s.jpg/

Peter


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Mon 06, 2012 5:55 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
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Location: Central Pa, 17044
Thanks Peter, That sure is nice work you do..I will take your advice on the
807 screen voltages. Hope to get the rig up and running in a week or two.

Gary KB3WYZ


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 4:13 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
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Location: Central Pa, 17044
Ok guys, need some help here. Just bench tested the new power transformer for the
807 transmitter.. The schematic is on the first page of this thread.
The new Xfmf..850Vct 6.3V, 5v. Here is the problem I am getting 850VDC for B+ 1
And 310VDC for B+ 2, BUT the 3, 91V zenier diodes in series are getting very hot.
And the 2 100uf 450V Caps are giving out a snap every now and then.
I know now, the caps are to low in voltage value, I thought them power resistors would lower the voltage to about 750VDC..WRONG!!
So is there anything I can do to keep this power supply... Bigger caps of course and
what about the zenier diodes? 4 in series instead of three?
Gosh, I'm :evil: Should have known better.
Any Ideas :oops:

Gary KB3WYZ


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 4:32 pm 
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Location: Livermore, CA
Gary

Zener diodes can't handle high current. With a 91 volt Zener @ 1 watt that would only be a little more than 10 ma.

Is your 850 volts under load. During key open on your transmitter voltage may be much higher?

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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 4:34 pm 
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Joined: Aug Tue 28, 2007 6:44 pm
Posts: 455
Location: Tustin,CA
What is the power rating for those zeners - their P/N would help?? What is the measured voltage on both sides of that 5K series resistor going to the zeners at key-up?? Is the measured 310 Vdc measured at key-up??? At key-up, all the current goes thru those zeners!! You'd almost be better off removing those zeners and that 5K and powering the 6AG7 stage from the top of that 10ufd 450 Vdc filter cap. I'm not sure those zeners are buying you much other than headaches.
Dave - WA6VVL


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 4:44 pm 
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Location: Livermore, CA
If you want regulation how about 0A2 tubes? Each one is rated 150 volts and can handle 30 ma. 0B2 can handle 105 volts @ 30 ma.

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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 4:58 pm 
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Location: Tustin,CA
Hi Norm,
The dilemma is that the 6AG7 stage can be drawing close to 30 mA key-down, so you're running those reg tubes at max current. I don't think that either approach buys you anything. At key-down, the 6AG7 is looking at the delta 310 Vdc voltage regulation anyway, so all those components (zeners or tubes and the series R) has to be selected to minimize the difference between key-up/key-down to justify their use, and in my experience w/6AG7s xtal osc, it's just more trouble than it's worth, and more stuff to fail. Just look at the available circuits - the Heath AT-1 comes to mind - and none use voltage regulation on the 6AG7 osc.
Dave - WA6VVL


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 6:23 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 837
Location: Central Pa, 17044
Ok, Them Zenier diodes are 1N5377B and are rated at 91V@5W each
and I have 3 in series and am getting 310VDC which I am happy with,
however they are getting very hot. Could I use a higher voltage and
higher wattage to get the desired 300+/- voltage?
All voltages are no load, just the meter.
As for the B+ 1 The 807 plate voltage can be 800+, But I'm getting
925VDC. If the two 40K 10W power resistors were increased, would
that lower the out put voltage to more in the 750/800V range?
Can you buy 100uf caps at a higher voltage than 450VDC?
Sure hope I can save this PS and don't have to put the old Xfmr
back in.
Gary


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 6:50 pm 
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Location: Livermore, CA
Gary

5 watt Zeners will get hot. What current are they carrying? They should be able to handle 50 ma. Having 3 in series and getting 310 volts might mean you are drawing more current through the Zeners.

Adding resistance in series will reduce voltage but only when current is being drawn. With no load series resistance won't reduce voltage. Could add shunt resistance (bleeder). This type of resistor draws current all the time and reduces voltage.

Less expensive to have 450 volt caps in series. Maybe use 3 in series? You can buy 500 volt caps. Much higher and they are most likely made with two in series inside.

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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 9:53 pm 
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Joined: Aug Tue 28, 2007 6:44 pm
Posts: 455
Location: Tustin,CA
Hello,
Well, we still don't know how much current is going through those zeners. You need to measure the key-up voltages on both sides of that 5K series R. Another interesting factoid is that those zeners are 5%. So, the worst-case B+ s/b 286.7 Vdc at their indicated Izt of 15mA. 310 Vdc tells me that the zener current is >> 15 mA. If all that weren't bad enough, these so-called 5W zeners aren't 5W zeners. The 5W rating has some very stringent "rules" regarding the length of the leads, their heatsinking, and the 5W rating is at a lead temp of 25C. Most of this is ignored by most folks that use them with reckless abandon at their 5W rating - and they will fail sooner/later. The VR tube is actually a better choice as Norm pointed out, but you have to operate the tube(s) within their 5-30 mA window. That means that with the proper selection of series R, that the 6AG7 circuit can draw no more that 25 mA max key-down. That might be a deal-breaker. In 6AG7 tests that I did several years ago, there is a wide variation of 6AG7 plate currents even among tubes of the same mfr and date-codes. The difference expands as you add different mfrs to the mix. It would not be surprising to find that 6AG7 circuit drawing well past the 25 mA limit using VR tubes. Bottom-line? Get rid of the zeners (or VR tubes) and just run the 6AG7 from the LV B+.
Dave - WA6VVL


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 Post subject: Re: 807 CW Transmitter
PostPosted: Aug Fri 17, 2012 10:27 pm 
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Location: Texas. USA
I was going to post the same thing about those zeners and the "5 watt" rating. The 3/8" lead temp, however, is 75C, which is not trivial heatsinking since one usually has to consider an up to 50C ambient.

What they'll do free air is not specified.


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