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Plate & Load

Joe WB9SBD All Messages By This Member #40571 开云体育 ?Been an AMP user for almost 50 years. But have a question. The Plate/load/coil match the tube(s) output impedance to the desired antenna impedance typically 50 ohms. Just like a tuner changes a non 50 ohm antenna to 50 ohms. But I just thought of this. in the antenna tuner the two caps can be and usually are all over the place. And their tuning is typically rather broad. the caps are not super tight. But in an AMP The Plate control is very critical and narrow in optimum setting. And It's level of "C" is almost predictable. You KNOW the 40 meter will have less "C" than 80 meters. And so on through the bands. The load cap is almost in the same position and in almost every amp I have had the optimum was rather broad. Why? Joe WB9SBD
Bob All Messages By This Member #40572 Joe, I can't answer like some of the guys here but I do know on the lower bands the plate is extremely broad and narrows as you go up in frequency. Bob W4JFA toggle quoted message Show quoted text On Tue, Sep 24, 2024 at 10:44?AM Joe WB9SBD via <nss=[email protected]> wrote: ?Been an AMP user for almost 50 years. But have a question. The Plate/load/coil match the tube(s) output impedance to the desired antenna impedance typically 50 ohms. Just like a tuner changes a non 50 ohm antenna to 50 ohms. But I just thought of this. in the antenna tuner the two caps can be and usually are all over the place. And their tuning is typically rather broad. the caps are not super tight. But in an AMP The Plate control is very critical and narrow in optimum setting. And It's level of "C" is almost predictable. You KNOW the 40 meter will have less "C" than 80 meters. And so on through the bands. The load cap is almost in the same position and in almost every amp I have had the optimum was rather broad. Why? Joe WB9SBD
Jim VE7RF All Messages By This Member #40573 The amp uses a PI network.? ?Most ant tuners use a T network.?
Eric All Messages By This Member #40625 Some possibly demented thoughts...... ? In (nearly) all cases, if the frequency of operation, and impedances at both sides of the matching network are known, the needed component values in the network can be calculated ? In a tube amp, the input impedance, and the output impedance are fairly constant despite changes in frequency, so the only thing that needs to be taken into account when designing the network...is the frequency....and those "make sense" as far as less capacitance or inductance is needed as the frequency goes up. ? With an antenna tuner, the impedance at one end is usually 50 ohms with no reactance while the other end of the antenna tuning tuner is presented with a wildly varying impedance, so the component values in the tuner need to take into account the both the impedances as well as the frequency.? ? Eric ?
Jim VE7RF All Messages By This Member #40630 The plate load Z of the tube is fixed.? ? The 50 ohm output is fixed.? ?The PI net just matches the plate load Z to 50 ohms.? ?Knowing the typ load Q you want, the rest is easy....... as you can see on the GM3SEK PI spreadsheet.? The tube can't source anything, it can only sink current.? In class AB2? (think SB-220, with it's 5vdc zener), the tube only conducts for 210 degs....and is cutoff the remaining? 150 degs.? ?To drive the grid positive, the cathode is driven negative.? ?Meanwhile the output is positive.? ?The input and output are in series....and 180 degs out of phase.....which is the reason that a tube in GG typ has 6db better IMD? vs the same tube that is grid driven.? The input Z of the tube is sky high? during 150 degs....and low for 210 degs.? ?The PI tuned input provides for some flywheel action, and smooths out the overall input Z.? ? The tune and load cap, in some cases, is padded on the lower bands.? ?On my drake L4B amp, the load cap is padded on 80m.....(700 pf mica).? ?The load cap is a 4 x section air variable.? ? All 4 x sections are used on 40m only.? On 20-10m, only 2 of the 4 x sections are used.? ? ?The load cap on the drake does not have a 6:1 vernier......so by only using 2 of the 4 x sections of the load cap, the load control is less touchy.? ?The tune cap on the drake has a 6:1 vernier.....and tuning is not touchy at all.?
Jim VE7RF All Messages By This Member #40631 Typ overall ( OUTPUT ) network Q in an amplifier is 12.? ( input Q of 10..and an output Q of 2).? Typ that Q is used so the harmonic suppression is good.? Lower Q = less harmonic suppression.? On a PI tuned input for an amp is low, like overall Q of 3....and an input and output Q of 1.5 On any ant tuner, we are not interested in harmonic suppression, just matching, so typ a low Q is used.?

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