On Sat, Mar 30, 2024 at 06:27 AM, Clyde K2UE wrote:
<Generally speaking there are two types of tube transfer characteristics:
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<The optimum characteristic can be described as two straight lines: constant transconductance at high input levels, and a second straight line of transconductance going from zero to the large signal value at low input levels.? When biased <at the half max transconductance point ideal class AB operation is obtained: for small signals there is half transconductance for both positive and negative going inputs, and for large signal there is twice transconductance for positive <going signals and cutoff for negative, i.e. the same time-average.? Triodes often have this characteristic, as the decreasing plate voltage keeps the gm (transconductance) from rising at high drive levels as the load line decreases the <instantaneous plate voltage.
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<A non-optimum characteristic would be one where the transconductance is constantly changing, with no large constant regions.? This typical of many tetrodes, for instance, where the screen allows the gm to keep rising at hight currents <and the device is more constant current.
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<In the latter case feedback can be very helpful in improving distortion, and without it IM tends to be awful no matter the bias point.? But in the former case the biggest factor is the bias point, and there will be a distinct dip in IM as the bias <is adjusted.? The 3-500Z is the former type, and you can see from the data sheets of various manufacturers that IM of -40dB or better can be obtained if the bias point is optimum (the 1500V zero bias case and the 3500V -15V bias <case).? An old-school analysis of the the tube transfer curves leads to the same conclusion.? So for the 3-500Z family choosing the optimum cathode bias voltage can be more effective than feedback in optimizing IM.
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<Clyde Washburn, k2UE
## NFB really only works for grid driven tetrodes, since they typ have high gain to begin with.? We measured 22.7 db gain on buddy's grid driven tetrode (tube with handles).? It used a 1k ohm grid termination. Had instability on 40-15m, so used a 500 ohm grid termination...and driven by an adjustable pi network. Parasitic suppressor installed right at the grid. After that, rock stable, and gain reduced to 20 db.?
The issue with say the heath SB-220 is..... it uses a grid choke on each socket, and the DC resistance of each grid choke is 25 ohms.? ?With 300 ma of grid current flowing, that's 150 ma per tube.? .150 X 25 = 3.75 vdc
That extra 3.75 vdc of UN wanted, and yo-yoing bias, is in addition to the oem 5 vdc zener bias.? ?Now the bias is swinging wildly from 5.0 vdc to 8.75 vdc.? That alone just screwed up the load line really bad.? The B+ regulation on the SB-220 is nothing to write home about...and sags badly.? With the UN regulated B+ dipping lower on peaks, that too just screwed up the load line.?
On my 4 x L4B's, the grids are bonded to the chassis with cu straps.? I also installed 10 x 1N5408 diodes for bias.? ( it idled at 220 ma? with Drakes zero bias scheme).? ?I tapped the diodes with a spdt-center off mini toggle..to obtain 3 x positions of bias.? On my HB metal tube amps, I use 50? x 6A10 diodes, with a 20 position rotary switch.? To make the diode bias scheme rock solid regulation, I wired a large value lytic across the entire string of bias diodes.? Does not budge. Without the cap, bias V will increase by 10-12%? between? idle current....and max plate current.?
These days, on the drake amps, I use all 10 diodes when on the SSB / 2650 vdc position? (and also the lower 1900 vdc position).? This dropped the idle current from 220 ma....down to 100 ma...and imd is superb? with 1290 watts pep out on 75m.? ?On CW position with 1900 vdc, idle is now just 40 ma.....and IMD is still superb with 625 watts pep out.? Aprx 7.3 vdc bias used. Slightly more drive required to overcome the 7.2 vdc of bias.?
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The limiting factor is the IMD of the xcvr driving the GG amp.? No xcvr is good for -40db for IMD3.? An elecraft? K3 is good for -23 dbc for IMD3.?
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The GG amp has it's input and output 180 degs out of phase. (cathode driven negative = grid driven positive).? That alone will? typ result in as much as a 6 dbc improvement in IMD.? The cathode on the GG amp is common to the input and the output.? With the PI tuned input installed, that feeds the cathode, the IMD can be substantially improved.? On my hb amps, on one of em,? I can adjust the Q between some wide extremes.? It uses a pair of air variable broadcast caps + a 0-4 uh roller coil (made from 8 ga wire). Both caps are padded on 160m only.? Eye opener to say the least.? Optimum linearity results with a total network Q of 4 ( input Q = 2..and output Q also = 2). It's a good compromise between IMD, BW, and circulating current in the coil.? The optimized PI tuned input also results in better harmonic suppression on the output side of the amp.
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You can only do so much, but? bonding the grids on the L4B et-all? is a simple mod.? ?Adding a bit of bias to reduce heat is a bonus.? ?I tossed the drake oem pair of series 50k bleeders and the 5 kw? resistor. That dumps? a HUGE load off the supply.? ?Way less heat on the RF deck.?
