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Author Topic: help troubleshoot homebrew 6T9 transmitter  (Read 7440 times)
N6AF
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Posts: 9




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« on: January 03, 2014, 12:04:27 PM »

I built the "160 meter phone/CW" rig in the 1970 ARRL handbook.  This uses a 6T9 compactron tube as a combination oscillator/amp.  I can only get about 2 Watts out of the thing and the highest dipped plate current is only about 25 mA.  But the design should have about 5 Watts out and a dipped current of perhaps 50 mA.  So the basic issue is the loaded output level is too low.  This is definitely running into a 50 ohm known good load and measured with a Bird Wattmeter.  The other hint is that the plate current is only 50 mA when the oscillator is not running while it should be 100 mA.  This is stated in the handbook article.  I checked the plate voltage right at the plate and all the other DC levels.  The plate power supply transformer is a monster Thordarson that can easily provide 200 mA or more.  Everything looks fine.  I tried a second 6T9 tube and got the same result.  Both 6T9 tubes checked good on a tube tester.  I'm hoping someone else is familiar with this particular transmitter design.  Any suggestions highly appreciated.

thanks  73   N6AF   Chuck
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G3RZP
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Posts: 4625




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« Reply #1 on: January 03, 2014, 12:45:46 PM »

I can't find characteristic curves for the 6T9. What are the volts relative to ground on pins 9, 8, 10 and 11? I take it that with 'oscillator not working' you've unplugged the crystal?
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VU2NAN
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Posts: 248




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« Reply #2 on: January 03, 2014, 10:10:25 PM »

Hi OM Chuck,

Assuming that the +B voltage (275V) is okay, the problem may be with the screen dropping resistor (value much higher than 15K).

73,

Nandu.
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N6AF
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Posts: 9




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« Reply #3 on: January 28, 2014, 10:03:43 AM »

Following up on this, I discovered that the filament current on these 6T9 tubes is nearly 1 amp each (each 6T9 is a dual tube package).  There are two 6T9's in the design along with a third tube.  So figure something on the order of 2 1/2 amps of filament current.  I believe the filament voltage was dropping down to less than 5 Volts due to the high filament current and my mistaken decision back in 1971 to use the largest guage wire in the PS-to-rig cable harness for the B+ rather than the filament.   Big mistake.  Once I swapped wires in the harness to deploy the heavier wire for the hungry filaments, the filament voltage came up to about 6.5 Volts at the tubes and the RF output is up to nearly 4 Watts with 275 V on the plate.  No wonder I was perplexed.  I figured it had to be a bad component, a weak tube, or something complicated.  Never would have considered filament voltage at the tube.  Just happened upon it when I was checking all the DC levels.  Sometimes the 'bug' is so basic it hides in clear sight.  Just for grins I ran the little rig into my SB-220 !  10 dB gain.  So now I can run 40 Watts output RF using the homebrew exciter and a pair of 3-500Z's !  I am a favorite client of the local power company.  40 Watts RF output and about 160 Watts for filaments between the two 3-500Z's and the 6T9 rig.  I transmit using the CW mode.  The power company responds using the 'BILL' mode.

Thanks for the suggestions.  Nandu VU2NAN, you got me looking at the DC levels and that's when I saw the low filament voltage.
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VU2NAN
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Posts: 248




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« Reply #4 on: March 02, 2014, 08:49:24 PM »

Hi OM Chuck,

Great show!

73,

Nandu.
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N6AF
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Posts: 9




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« Reply #5 on: March 11, 2014, 12:48:17 PM »

Nandu,

I just worked Japan with the 4 watts on 40 meter CW using a windom antenna.  Amazing!  Most contacts are within California and the neighboring states, as distant as Texas.  Having a blast with this little rig.  More fun than the K3 and SB220 into a 4 element yagi.  Where's the challenge in that?   The nemesis is conquered and the fun continues.

73

N6AF  Chuck
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VU2NAN
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Posts: 248




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« Reply #6 on: March 11, 2014, 07:15:59 PM »

Hi OM Chuck,

Congratulations!

Now the fun has begun.

73,

Nandu.
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W9WQA
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Posts: 135




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« Reply #7 on: March 14, 2014, 05:16:09 PM »

we spend so much time with transistors we forget about filaments!!

i'm laughing as i say this!!

funny, a while back i wired up the filaments of a 6z10 just to see them light up.
 i need a project. maybe this will get me going.
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WX7G
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Posts: 6079




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« Reply #8 on: March 18, 2014, 05:27:20 AM »

It's filamentary, my dear Watson.
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