Wil,
If you change the secondary wire diameter and therefore
the required primary tune point, this changes the amount
of feedback needed, and also changes the tank circuit Q.
All these interactions can make it difficult to judge the
effect that changing just one parameter is having (multiple
parameters tend to change). In experiments I've done
however, I didn't see much difference using different tank
Q values (Q of 10 vs. Q of 20, etc.).
There are some rules of thumb and formulas at Steve
Ward's website. Some of these are originally from
Duane Bylund, some from Ed Phillips, some are various
relationships I've observed, etc.
I think the dynamic aspect of VTTC operation (spark
growth and loading, varying voltages, etc.) makes it
hard to perfectly match anything and it doesn't seem
to make much difference anyway.
Basically the critical elements are to tune the coil
as well as possible and to use the right amount of
grid feedback (too much or too little will reduce the
spark length). Spark length tends to track directly
with input voltage. Doubling the HV input voltage will
tend to double the output spark length, and that
will require four times the input power. You have to
retune as needed as the voltage is increased, otherwise
the spark length won't increase.
VTTC's can be tricky beasts to adjust, some designs
seem to tune up easier.
I like to use about 800 turns on the primary (26 or
28awg), and about 23 turns on the primary, and
the proper amount of capacitance to tune. Usually
the required calculated primary tune point will be quite a lot
lower than the secondary resonant frequency due
to the effects of capacitive spark loading. The amount
of required grid feedback will depend on the type of vacuum
tube(s). The amount of feedback can be adjusted by
sliding the feedback coil closer or farther away from
the primary.
You can get good results without an oscilloscope.
Cheers,
John
-----Original Message-----
From: William Howard <snakeprior@xxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Mon, May 26, 2014 9:15 am
Subject: [TCML] VTTC primary and feedback guidelines
Dear Group,
I understand that when building a VTTC we are impedance matching coils but
I have been unable to find guidlines on how to do this.
Is there a way to calculate how many turns would be needed to get the
perfect or near perfect match between primaries and secondaries and
feedback coils?
I noticed I was getting less length in arcs after tuning roughly two
secondaries. Both with the same amount of windings but different guages. I
got 40cm off the .25mm wire with 800 turns and 15cm off 1mm with 800 turns.
I don't want to keep winding secondaries and feedbacks and would like a way
to calculate it.
Also I noticed some feedback coils turn the tubes on more powerfully than
others even with physically moving them. I only have basic tools. A
multimeter! I need a variac and oscilloscope but have no budget for them.
I have used tc calculators but they focus on spark gap coils.
Sorry if this has been addressed previously.
Cheers!
-Will
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