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Author Topic: 6AG7 Osc/Driver ?  (Read 3805 times)
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wb6kwt
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« on: January 20, 2007, 12:24:48 PM »

I'm working to re-create a transmitter I first build in the mid 60's, it's from the 1965 ARRL handbook. My first attempt work first time it was turned on but this one has me stumpted. I'm trying to confirm that the oscillator is working but the 6AG7 is getting very hot. You can view this part of the circuit here:

http://www.wb6kwt.com/tubexmtrschosc.jpg

the rest of the circuit is on my website too.

I'm 99.9% sure the circuit is wired up properly. The tube plate measures 400V and the screen is set to 150V. I don't have anything connected past the 470uf coupling cap. I'm thinking I might need to put a load on the plate before I'll get the proper operation. I've tried four different crystals but still can't see any output on the scope.

Do I need to terminate the plate at the 470uf cap with a load? What would you suggest?

Thanks,

Bob
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K1MVP
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« Reply #1 on: January 20, 2007, 07:25:08 PM »

Hi Bob,
 The 6AG7 is esentially the same as a 6CL6, which is usually real easy to get to
 oscillate.
 400 volts on the plate of a 6AG7 seems quite high,IMO
 seems to me 250 to 300 would be more "normal".
 Also,--I would "complete" the circuit by adding a a pi-network to match the
 output to a 50 ohm load,--as you will be using it as a transmitter.
 I think the screen voltage at 150 is ok,--or it might be a bit high.
 I have built both a 6AG7 rig and a 6AQ5 rig,--es I have "pushed" the
 6AQ5 at 400 volts on the plate,--to get 5 watts rf out.--the tube ran
 hot, but it was useable.
 I could never run 400 on the 6AG7,--as the tube would "self destruct",
 I could ony realize about 2 and a half to 3 watts max out of the 6AG7.

                                  73,s and good luck, Rene, K1MVP
                                 
 
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WA1GFZ
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« Reply #2 on: January 20, 2007, 10:09:25 PM »

Sunds like you need some bias on the tube. Try putting a hunderd ohms in series with the cathode.
Measure the voltage across the resistor to determine if current is right. Increase the vale to reduce current. Yes 400v is a bit high. You could also put negative bias on the 100 K grid resistor but more work. Sounds like the tube is just sitting there at full bore saturated.
Go to Bama and download schematics of the V2 or ranger for ideas. I haven't run a 6AG7 oscillator since about '67 but just found my tube.
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The Slab Bacon
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« Reply #3 on: January 21, 2007, 08:31:38 AM »

It sounds like it is not oscilating for whatever reason. I question the purpose of the 22 ohn reziztor in the grid circuit. it is prolly there to limit the crapstal current. However quite often a small carbon reziztor can have an isolation / decoupling effect at radio frequencies. Dont forget that in order to have the full effect of the grid leak biass the tube should be oscillating. This looks like a basic  coldpiss oscilator with the feedback through the classic capacitive divider. Make sure that your wiring matches the skizmatic. If you dont have a feedback path the thing wont oscilate. If it wont oscilate the tube will prolly hit the saturation point at 400v on the plate. At that point it would run VERY hot. Also the 470Pf coupling cap from the plate seems a little large.

Also If memory serves me correctly the crapstall should be in parallel with the 100k grid leak resistor instead of having one leg grounded. (also try losing the 22 ohm resistor)

Also try reducing the plate voltage. With 400v on the plate tha plate current may just be too high and the tube too saturated to oscilate. Try something between 150 and 250v.

I have a similar oscilator built in an aluminum minibox with a smaller tube and 150v on the plate that I use as a crapstal checker. I pipe the plate outpoot into a freq counter to check crapstals.

                                                                       The Slab Bacon
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"No is not an answer and failure is not an option!"
K1MVP
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« Reply #4 on: January 21, 2007, 09:28:30 AM »

There is an old saying,--according to murphy`s law, that an oscillator will
not oscillate when it should, and an amplifier WILL oscillate when it should not.

I am wondering if you should not have a bybass capacitor at the bottom
of your plate RF choke to tie it to "rf ground"? --I dont see any.

A good 6AG7 circuit can be found in the "55"--"56" handbook,--"a novice
6AG7 transmitter", which works real good,--you might want to check it out
to get some ideas.

Also some good suggestions,-- here from others,--that the tube is probably
"saturated" and running "hard" DC wise,--so what might be your bias?

                                        73, K1MVP

P.S,--make sure you dont have any "cold" soldered connections,--I know
       that has happened to me a lot when I hastily put together a circuit.
       I just "know" this "stupid" simple circuit HAS to work, and then I
       find a "stupid" cold soldered joint.

   
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wb6kwt
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« Reply #5 on: January 21, 2007, 01:25:34 PM »

Thanks all for your suggestions. I've looked at this thing so much I could be missing something. This circuit does work as drawn because I've built it before and it worked the first time it was turned on. My real question is, do I need a load on the plate for it to function properly. I built the entire transmitter last time and then tested it, this time I wanted to ensure the oscillator was working before I finished up.

As some have stated, the tube is VERY hot so it appears to be conducting full bore.

Bob
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k4kyv
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Don
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« Reply #6 on: January 21, 2007, 02:25:36 PM »

Have you measured plate, screen, grid and/or cathode current?  Calculate the DC input to the tube.  How does that compare with the rated plate dissipation?

I have had the same problem with 6V6 audio amplifiers.  Even running at proper specs, they would run too hot, and after a few months even N.O.S. ones would test weak.  This doesn't seem normal, since good tubes should run within the manufacturer's ratings for many years before testing weak.  But because tubes are in dwindling supply, I reduced plate/screen voltage and increased cathode resistance to cool them down, sacrificing a little output power for increased tube life.
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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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WA5VGO
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« Reply #7 on: January 21, 2007, 04:04:19 PM »

This is a tried and proven circuit right out of QST. It should work as shown. The 6AG7 will work well up to 500 volts on the plate. WRL and Johnson ran them at about 450 volts in some of their novice rigs.

Darrell, WA5VGO
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K1MVP
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« Reply #8 on: January 21, 2007, 05:28:44 PM »

This is a tried and proven circuit right out of QST. It should work as shown. The 6AG7 will work well up to 500 volts on the plate. WRL and Johnson ran them at about 450 volts in some of their novice rigs.

Darrell, WA5VGO

True,--this may be a "tried and proven circuit" out of QST,--BUT it is also working
INTO a grid circuit of a 6146 (in the article)--not "open-ended".
I would assume it should see some kind of a "load.

Ever try to get a 6AG7 transmitter-oscillator to work without "dipping" or "tuning"
to resonance the pi-net in the plate circuit,--It will NOT work, and the plate
current will go through the roof.

AS far as 4 to 500 volts on the plate of a 6AG7,--good luck,--I HAVE tried running
400 volts on a 6AG7,--and  it WILL NOT take it,--I ran a comparison with a 6AQ5,
a more rugged tube, and the 6AQ5 will take 400 volts,max, but it will run hot.

400 to 500 volts on an oscillator tube,--is too much,IMO--250 to 300 is more in line.

400 to 500volts is usually run on a final amp tube such as a 6DQ6, or 6L6.
 
                                          73, K1MVP

P.S., I would either complete the transmitter with the 6146,--or build a small
       pi net, and "load" the oscillator
 
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wb6kwt
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« Reply #9 on: January 21, 2007, 07:03:25 PM »

Thanks everyone, I found the problem and it was a bad crystal. I didn't think to check the crystal because I had used it before and it was ok. The tube is running hot so I'll try to reduce the plate voltage some and see how that works out. It would probably be ok in CW but this little rig will be used for AM too so it will be conducting for long periods of time.

The output looks a bit strange on the scope but then it isn't terminated yet, however I hear a nice clean sounding signal on my receiver.

Thanks for all your suggestions.

Bob
wb6kwt
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K1MVP
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« Reply #10 on: January 21, 2007, 07:42:12 PM »

Hi Bob,
 Glad to hear you found the problem,--(a bad xtal)--I actually "blew" a good
 crystal, trying to get "max power" out of an oscillator,--the oscillator was
 working great one minute, and it just up and quit,--I felt the xtal and it
 was warm,--7040 as I recall.
 So I decided I might just want to reduce the plate voltage, and run the
 oscillator a bit "cooler".
                                     73, K1MVP
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WA1GFZ
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« Reply #11 on: January 21, 2007, 08:43:31 PM »

You will need a tank on the output to clean it up. Also lower voltage may help.
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