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Author Topic: Amp with no tuned input question  (Read 14726 times)
G3RZP
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« Reply #15 on: September 30, 2013, 01:08:12 PM »

The redeeming feature of not using a tuned input on the amplifier  is that the majority of transceivers with a solid state PA have far worse high order IMD products than the last generation of tube PA rigs. Thus, even without a tuner that has a capacitive output, IMD performance is likely to be governed by the transceiver rather than the amplifier.

The mathematical analysis shows that this would be the case...
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WA8UEG
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« Reply #16 on: September 30, 2013, 02:12:36 PM »

The other great feature is a 40+ year old amp will work on the WARC bands also. My MKII puts out full legal limit on 17 & 12.
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KJ7WC
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« Reply #17 on: October 11, 2013, 04:59:49 PM »

Good timing! I just put a Mark 1 in my shack, too. Just got my tuned input board, and need to wind some inductors.

Check out the kit that WD7S just sent to me: http://home.earthlink.net/~wd7s/TU-6B.htm
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K2UE
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« Reply #18 on: October 12, 2013, 01:51:21 PM »

If the driving rig has a low Pass Filter, as most modern exciters do, then it is seeing a resistive load, since the harmonics generated by the discontinuous conduction of the PA can't pass thru the filter, and a tuner is redundant unless the impedance is too far from 50 ohms.
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KJ7WC
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« Reply #19 on: October 12, 2013, 06:47:36 PM »

This is a good point at which to know your rig. K2UE, I would love to know your definition of a modern exciter. While I realize that my FT-897D is starting to get a bit long in tooth, it isn't that old at about eight years. The FT-897D Technical Supplement states: "the transmit signal is passed through a low-pass filter (1.8-29.7MHz)..."

Such a low-pass filter will be unable to meet FCC requirements for IMD, when fed through the front-end of a tube-type amplifier. I recommend reviewing W8JI's article, found here: http://w8ji.com/tuned_input_circuit.htm. Rigs from 40 years ago, running tube finals, had Pi-filters using series inductance on every band. We didn't have to worry about second- and third-order harmonics, because the transmitters dealt with each of the bands independently. I will continue to build my tuned-input board and recommend that other hams with older amplifiers follow-suit.

Another article you may enjoy is by W3EUT: http://www.rfampguy.com/Pages/TUNEdinput.html.
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K1ZJH
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« Reply #20 on: October 14, 2013, 10:20:06 PM »

The matching unit really belongs as close to the cathodes as possible. Otherwise, even a short length of coax will adversely effect the iMD performance due the harmonic content and the impedance mismatch.

Pete
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G3RZP
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« Reply #21 on: October 15, 2013, 02:39:47 AM »

One point that has been noticed in testing transceivers is that the IMD at 1.05:1 VSWR and the IMD at 1.3:1 VSWR can be very different.

Although the transceiver's built in low pass filter isolates the transceiver PA from the harmonics in the cathode circuit of the amplifier, those harmonics still need a low impedance to ground - which as Pete, K1ZJH says, needs to be right at the cathode pin.
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K2UE
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« Reply #22 on: October 17, 2013, 01:23:46 PM »

This is a good point at which to know your rig. K2UE, I would love to know your definition of a modern exciter. While I realize that my FT-897D is starting to get a bit long in tooth, it isn't that old at about eight years. The FT-897D Technical Supplement states: "the transmit signal is passed through a low-pass filter (1.8-29.7MHz)..."

Such a low-pass filter will be unable to meet FCC requirements for IMD, when fed through the front-end of a tube-type amplifier. I recommend reviewing W8JI's article, found here: http://w8ji.com/tuned_input_circuit.htm. Rigs from 40 years ago, running tube finals, had Pi-filters using series inductance on every band. We didn't have to worry about second- and third-order harmonics, because the transmitters dealt with each of the bands independently. I will continue to build my tuned-input board and recommend that other hams with older amplifiers follow-suit.

Another article you may enjoy is by W3EUT: http://www.rfampguy.com/Pages/TUNEdinput.html.

By modern rigs I meant exciters that have an LPF for every band that cuts of below the 2nd harmonic on that band.  Tube exciters with Pi-Network outputs would have been no better than that.

But your point on the need to conduct cathode current harmonics to ground is well taken, since if they are unterminated they drive the cathode out of its desired linear operation, and provide the mixing vehicle for IM generation.  An antenna tuner, unless it provides a capacitor output filter, and is VERY close to the PA is not much of a substitute for a pi-network at the cathode.
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