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Re: VTTC output
Original poster: "Herwig Roscher by way of Terry Fritz <twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>
Ed Phillips wrote:
> > Trying to max out my system I connected a 5 Ohm resistor
> > between the base of the secondary and rf ground. I scoped the
> > voltage across this resistor and found it to be 20 Vpp at low input
> > power levels.
> > Increasing the input power real streamers started growing from the
brush, resulting in about 40 Vpp across the resistor. Obviously
streamers have a lower resistance than the brush.
>From 40 Vpp an effective current of about 2.5 A is calculated and I'm
wondering when the AWG 26 will become hot.
> Probably won't get hot. Your secondary has a lot of area and, when you
> calculate the total dissipation and divide it by the area, you will find
> the power density (watts/square inch, centimeter, or whatever) is quite
> low and easily cooled by convection.
I've just finished another series of tests: Using 15 kOhm/1.6 nF as
grid leak combination and a Vg2 of 1200 V, the plate current was
480 mA at a plate voltage of 8600 Vdc. Seems that I reached the
limits of my tube ;-)
The tank caps became barely warm and the primary coil reached
about 30 deg C after a few minutes of testing with various loads.
But the lower part of the secondary became so hot that I hardly
could touch it (more than 60 deg C ?).
Trying to find a possibility to "measure" the relative output power
instead of guessing it by eye, I've used
- the a.m. 5 Ohm resistor
- a pickup coil of 4 turns wound on the form of the primary at a
distance of 3.5" and connected to a 230 V/100 W bulb. The voltage
across this bulb is 130 Vpp and nearly independent(!!) on the power
I was really surprised that the system is so insensitive. Could the
reason be that I'm using a tetrode which has a very high plate
resistance in comparison to triodes?
If I calculated the tank voltage from 67 turns of primary / 4 turns of
pickup coil and multiply it by 130 Vpp, I get 2200 Vpp. IMO this is
low, taking into consideration the plate voltage of 10 kVdc.
Any hints or comments?