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[TCML] Re: Finally finalizing my 304er VTTC

Hi Dick and all,

To answer the first question:

And give some VTTC basics for those that do not know...
And possibly clear up some (but not all) of the "mysteries" of these

The most common VTTC oscillator circuit is self rectifying when feed with
AC. This ends up being half wave as the power/oscillator tube can only
conduct on positive cycles. So it basically has equal on/off time periods
since off on negative, so overall current draw is lower and everything runs

When rectifying full wave (center tap type or bridge type), the power/osc
tube will now conduct near nonstop with the slight exception of the very
short trip back to 0 then right back up because of ripple. Current draw
goes up of course and everything tends to run hotter.

If you filter this full wave output to smooth ripple, the power/osc tube
now runs nonstop and so does current and again everything tends to run

And now finally answering Dick's first question :^)

Technically no, on AC power.
It is a level shifted supply (or this is sometimes called a DC restorer).
This output waveform is all positive with twice peak input and of course a
lot of ripple here. It does have a longer period of falling to 0 and back
up compared to full wave, but it is not much overall. Because of twice the
peak voltage, current draw goes way up of course and in most ways,
everything tends to run hotter just like full wave. My level shifter is
currently set up with a solid state diode and a charging capacitor, which
is the method used on VTTC's by most builders for the level shifter when

Now this level shifter is a form of a voltage doubler, but we do not want
to use the "traditional" voltage doubler circuit. We want this one with the
"horrible" amount of ripple. This positive waveform has very nice long 60hz
"ramps" that should be taken advantage of with an add on circuit. In my
opinion, this should not be run full CW on VTTC's for very long. Everything
gets hot quite fast, the same as full wave (I have experimented in the past
with full wave without filtering).

We really should take advantage of these nice long ramps with an
interrupter circuit which "switches" the cathode of our power/osc tube at
the right time. I have found that our traditional interrupter circuits are
"switching" in late when used on level shifter supply. This traditional
interrupter circuit was originally designed for normal AC input as the
trigger part of the circuit is off a normal AC waveform. While this does
work some for level shifted supply it is not optimum, as I said it does
switch late. I have been experimenting with these the past couple of years
and learning of a few different ways to better switch for the full extended
ramp time this level shifted supply waveform offers. At lower break rates
in interrupter circuits, this achieves the best longest, straightest swords
that many desire, including me :^) This does make a difference.

And as many know, these interrupter circuits are great for...at reduced
break rates...for everything to run much cooler, which means greatly
extended run times :^)

But a word of caution...as you increase break rates with this circuit...you
are now once again approaching full CW... which means more heat and
possible problems if things pushed too far...as can be seen in my acrylic

Please correct me if anything is wrong here, but I think I have everything
correct as I understand it. I can take it from everyone...if something is
not right :^)

And now to finally answer Dick's second question. I am really good at
stalling... ain't I  :-D

No, I am not using a pair 3B28 xenon gas diode tubes in a full wave center
tap. This MOT transformer already has one side of the high voltage
secondary inner wire attached directly to the transformer core, so at
ground potential of course and I am using the above mentioned level shifter.

I do have several of these gas diode tubes and other mercury vapor tubes
and a few high vacuum type diode tubes. But I have a problem of finding the
required low voltage, but very high voltage insulation type filament
transformers. I only have 2 that I have gathered over the years in my now
"mountain of junk". They do not come by very often these days. Seems, when
they do, the seller knows what they have and I can not justify the high
price. I often wonder where heck they all are, unfortunately I think many
got scrapped over the years.

So even though I am "mainly a VTTC tube guy", I am using a solid state
diode in the level shifter for a few reasons. One high voltage insulation
filament transformer I have is a 10V 10A  and has no nameplate on voltage
insulation spec, so it is just a guess on it. Also it is quite large and
heavy. It is 4-5 times larger than a regular 10V 10A filament transformer.

I do have a transformer for 3B28's, a 2.5V with 10,000 volt rated
insulation, but again quite large and also an unfriendly 240V input, so
another added transformer would be required for 120V input and another
variac(I like to have a variac to control filament supply).

For this coil build I want everything as light as possible and 120V input
and very easily portable.
So...I have a solid state diode...still have the main tube of course :^)

Ok... some damaged coil measurements coming out of curiosity and will post
when I can...back to work here on this project.

Chris Reeland
Ladd Illinois USA

Sent from my Samsung Galaxy Tab® S

On Sat, Apr 24, 2021, 4:54 PM <hooverrl@xxxxxxxxxxxxxx> wrote:

> Chris, are you running your VTTC 304 on AC power? Or are you using a pair
> of 3B28's for a full wave rectifier first?
> Thanks,
> Dick Hoover
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