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Re: [TCML] Some VTTC tube rolling results...got surprised twice!

Hi all,

It has been a while since I posted anything... been lurking though and got
some updates here...

I have made a decision on my small 304 VTTC about 3 weeks ago. Inspired by
Steve White's 833A and Dave's 810 to be easier to set up and be much more
portable I have made several changes. I will admit it is not a thing of
beauty, but I am mostly concerned with being functional.
I have also greatly simplified it. Removing several extra monitoring
meters. I am now confident in how all is running, that I don't need them.
Only have two meters now. A meter to monitor filament voltage is a must in
my opinion. And I have a current meter monitoring input on the variac, to
monitor efficiency, which is something I have also made a major decision on
this coil. Also removed extra different type of tube socket, simplified my
experimental extra adjustable grid leak setup and several other small
changes. So back to the efficiency monitoring... I have decided even though
I have achieved a maximum of 42" with this small coil, the efficiency is
terrible at 25A, even with several PF caps added. I can't even imagine what
it would draw with out them, if the 120 line was capable of higher amps and
proper breaker. The other efficiency issue and I also consider a health
problem is the MOT. I was using a fairly large 20 amper variac with 140V
input. The variac handles this no problem. The MOT now though, is being
pushed very hard, no bad sounds, but definitely being pushed. Run times
even at reduced rates limits run time to about 5 minutes max. MOT is then
extremely hot, even with the full time fan I have mounted above it. Takes
at least 35 minutes to cool way down and it is still hot some! Like I said,
extremely hot! The tube handles the low rate just fine though. The run
times are even shorter if rates are up on MOT, and now tube is starting to
be pushed hard. Also extremely finicky and hard to tune in for maximum of
42". Most times I can only tune to 36" while it still draws 25A w/PFC. So I
have decided to make this coil much more usable run time wise and much more
efficient running. I have gone back to the original 10 amp variac using the
120 setting. It also has a 140 setting, but my variac and the MOT does not
like this as I have run into this in the past quite a bit ago. Both made
unpleasant sounds, and again efficiency was really terrible again. Don't
remember exactly how bad amp wise and how much PFC was tried but it was not
good. So it is strictly now only using 120 input on this 10 amper. Since
this is smaller and with the changes to coil layout, I am able to squeeze
this on the coil board attached with wing nuts, so it can be removed in
about 2 minutes if I need to borrow it for something else. Did the same for
a variac on the filament transformer. Same wing nuts and can be borrowed if
needed. So now strictly limited to just 120V on a 10A variac, I also made a
decision to not use any PFC if possible to make things simpler.
So all this, also means that I need to retune the tank capacitance. Did
some crude calculations, and decided on what I need approximately uF wise.
Still keeping and using the same vacuum variable cap in conjunction with a
fixed tank cap for fine tuning. The calculated mica cap required I did not
have even though I have a whole mess of them. Seems like you can never have
enough on hand :-) So I searched around online and found a couple. Had no
choice on package type for the micas. It is a 292 type. I was using a F3
type mica for the tank and another for the BP. Decided to get the 292 for
the tank and another for the BP, so they match package wise. Next thing to
figure out was how to mount this different type of mica. Decided to make I
guess I will call a RF deck made out of a piece of scrap metal laying
around. This greatly simplified connections, while mounting securely. I
started to make all these changes 2 weekends ago. Just worked on, on and
off, a bit at a time with some small tests at times. Last night got done
with all changes physically. And it was time for running new complete
setup. Very pleased now, with just some minor tuning so far. I just picked
a random 304 tube that I have to run. I am going to roll all 304's that I
have again, as they all have to be retuned to new setup. I will post
rolling results later eventually when I get through them all.  Also
recently picked up more also. The tuning required on the one tube that I
have tried so far is completely different. It is much easier to tune in for
maximum output and efficiency. Much more flexible also for achieving
maximum efficiency with slightly reduced output. I can now also, it seems
like, I can run this coil indefinitely so far... I ran this coil non-stop
for 1 hour and 20 minutes at different rates at times. The MOT when I shut
down was just kinda hot, not bad at all. Checked MOT 7 minutes later while
the fan was still on and it was already just lukewarm.
Thrilled about this. Not going release the magic smoke now...
Here are a few numbers on performance currently that will probably change
just a bit as I "play" with this more.

Maximum output length now currently is 31"@12A

Best efficiency tuning wise so far is 28"@9A

And yes, I am not going to bother with any PFC, achieved this goal...

Did not expect this kind of efficiency, I will admit, and thrilled about
running non-stop. A much more portable and useful coil and takes up way
less on the bench:-) While not a 36"-42" coil anymore, it is a better small
coil now. Bigger streamers is the goal on the next VTTC project that is
nearly done. Everything is better setup for this also. Just need to wait
for Spring. I don't like coiling outdoors in the Winter. Can not
concentrate as well and more likely to make a major mistake as well in my
opinion, as this one must be run outside. Don't have anywhere big enough to
do indoors for this one. Also I have been gathering parts up still for the
next bigger VTTC, no real work on it yet, still hunting parts... getting
there though... piece by piece...
Another reason to change this coil is to have something to do while waiting
for Spring. I can now spend a lot time experimenting now since it seems
like I can run indefinitely if I want to.
No more short run times, with very long cool down times and possibly
ruining the MOT.

Here are a few links to pictures of new setup. Again, I will admit not very
good to look at, but very functional.




Hope this all makes sense...tried to edit all mistakes I could find
here...may have missed a couple of "strays":-)

Chris Reeland
Ladd Illinois USA

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