| GU 81 M VTTC A DIFFERENT APPROACH |
| GU 81 M is a power pentode used in amplificators and oscillators. It is the easiest avalaible tube in Europe for the VTTC coilers. In an VTTC mountage it can be used as pentode as well as triode with a few modifications. This new approach makes the sparks more longer. |
| CIRCUIT MODIFICATIONS -the GU81M pentode is used with a different configuration. The grid 3 is connected with the middle of the fillament and the ground. The grids 1 and 2 are connected together to the feedback coil. -in my precedent VTTC with GU81M used as pentode the fillament voltage was too low (11.8 V from the fillament transformer without load and 10.6 V with the fillament connected), so i wound 11 suplimentary turns ( 0.2 V / turn) for 14.3 V without load and 12.6 V under load (the average fillament voltage rating). Sure, the fillament can be overvolted (with maibe more 1.5-2 V) but this will short the life span of the tube. The tube received one more cooler just to have the glass envelope temperature kept low. -a little birdie on the plate connection prevent parasitic oscillations and protect the tube against eventually discharges between plate and grids (not happened yet). -the feedback coil is mounted UNDER the primary, preventing the sparks which appear between the secondary and the tank coils at high power. The primary and the feedback coils are separated by a plastic plate, the couplage with the secondary is good, the tube enters easily in oscillation. -the grid stuff (cap and resistor) are mounted on the feedback coil end connected to ground (it seems that this configuration allows longer sparks). The grid leakage resistor is now 484 ohms at 100 watts ( a 220 V @ 100 Watts light bulb). -the primary has a big number of turns with tapping points on it. It has different tapping points for the primary cap and the plate of the tube trying to increase the load impedance of the tube for maximum power transfer. I thanks a lot for this idea to Fabio McGyver, his site is here: |
| -the primary cap is connected to the 34-th turn and the plate to the 48-th (the last) turn. It is possible that the spark would be longer with a few more turns for the plate but I need a longer primary form. The resonant tap on the primary is on the 32-th turn, but I tapped at two more turns after to obtain the best spark output (the streamers de-tunne the coil). -the primary capacitance was increased to 2 nF @ 20 kV. I am using now some very good ceramic caps, that warm a little after 1-2 min of run but not too much. One more important thing is that after the impedance matching the MOT doesn't heat so much as it did in the first mountage. -I added some more capacitance on the top of the secondary and sharpened the nail. The sparks appear now at a higher voltage from the variac and sometime I have to start the discharge with a metal rod, but the sparks are healthy, well branched and they grow nice while the variac is turned up. -the longer sparks are obtained with an artificial ground (maibe mine water line i used before it is too old and maibe corodated, so this is not a too good grounding, so i am using now the metalic body of a washing machine. -the spark target is a metalic screen mounted between two copper bars connectd to the artificial ground. -I am using a 220 V / 0-250 V @ 9 A variac to power the MOT (220 V / 2200 V @ 1 kW). The variac allows me to vary the voltage on the MOT' primary between 0-240 vac, increasing the primary cap charge and so the spark lenght. -the capacity in the level shifter is 2 uF @ 6.3 kV, made with two huge foil-paper caps (better than the precedent 3.3 uF @ 4 kV electrolitic cap string) -no staccato used yet |
| SPARKS AND ARCS |
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| See the precedent mountage with GU81M used as pentode |
| GU81M USED WITH DIFFERENT GRID CONFIGURATION AND IMPEDANCE MATCHING ON PRIMARY COIL RESULTS: 18 INCHES SPARKS |
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| Next step: a bigger MOT, increasing the coupling coefficient, more power with staccato |