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Author Topic: ARB-704 / Tuner Port REALLY a safe alternative?  (Read 4471 times)
KF7CG
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Posts: 810




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« Reply #15 on: December 26, 2012, 11:09:37 AM »

KB1TJY,

You are missing something in the discussion. When the amplifier is in operate mode, the radio's antenna tunner only tunes out any mismatch between the radio and the amplifier. The amplifier very effectively prevents the radio tuner from seeing the antenna. This causes the swr from the amplifier to the antenna to be what it will unless there is an antenna tuner between the amplifier and the antenna.

Believe it or not, amplifier tubes are quite good at isolating the antenna impedance from the rig output. In fact they transform the 50 ohm input impedance to something like 3k to 5k and that is reduced to 50 ohms again by the amplifier output circuits if it is a tube type amplifier. For transistor amplifiers
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K8AC
Member

Posts: 1465




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« Reply #16 on: December 27, 2012, 06:14:40 AM »

If you think about what needs to be accomplished to switch the amplifier, it isn't hard to figure out.  See the diagrams on page 8 of the FT-450 manual.  To switch the amplifier, you need to ground the PTT line of the amplifier.  Both the Data and Linear jacks include a pin named: TX Gnd Out.  That line is connected to nothing else on those two connectors - it just goes to ground when the transceiver goes to transmit.  Page 13 gives the limits on the voltage and current that the switching transistor in the 450 will handle.  The safe thing to do would be to measure the voltage on the amplifier PTT line, and then measure the current as you short the PTT line to ground with the ammeter in series.  If both the current and voltage observed are within the limits of the transistor, then you don't need the 704 to protect the 450.   It's not unusual at all for a transceiver to provide the amplifier keying line on more than one rear panel connector.  If you have something else already plugged into the Tuner jack, open the connector and add a wire to the TX Gnd Out pin.

73, Floyd - K8AC
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WX7G
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Posts: 5917




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« Reply #17 on: December 27, 2012, 06:34:48 AM »

What amplifier are you using? The ARB704 is not needed with any modern amplifier than I am aware of.
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KC9TNH
Member

Posts: 304




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« Reply #18 on: December 27, 2012, 10:29:04 AM »

What amplifier are you using? The ARB704 is not needed with any modern amplifier than I am aware of.
The Amp is an Ameritron AL-80B.

Lyman
KB1TJY
FT-450D, AL-80B (with a 17-yo Eimac  Grin) and AT2K tuner here. No buffer, no issues, no unnecessary interconnections.

- Amp tuned the way taught by W8JI and WB2WIK, settings for amp & tuner on a cheat sheet on the desk. (Thank you gentlemen for the grain from alot of old thread-chaff.)

- Have the 450D Pnl/C.S. front-panel push-button programmed for SWR function which allows a quick 10w carrier to check whatever the AT2K's  been set to w/amp in standby; takes less than a second.

- AT2K switched to a direct output to which is connected a 1.5K dummy load. Amp on, verify output to dummy load so as not to bother the folks on a band.

- Flip AT2K back to antenna selector; play radio. Takes longer to type than do.

You have a great combo, it will serve you well.
Smiley

« Last Edit: December 27, 2012, 10:46:23 AM by KC9TNH » Logged

73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
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