Author Options:

how can you use a tetrode in a vttc? Answered

I have this ww2 nos 803 pentode tube that can be used as a tetrode tube. It has a 350 watt rating and i want to use it in  a class c armstrong oss. How would i do that, and what resistors should i use, the power supply is a 2kv voltage multiplier



Best Answer 8 years ago

Steve is correct about "trode mode" for a pentode (connect the screen to the plate)...but as you point out, the screen voltage for the 803 can't be as high as the plate (I've seen this before with other pentodes.)

But why use the 803 in triode mode anyway? Using a pentode as a triode reduces it's power handling by about half. Why not just use it as designed--as a pentode?

I don't personally have an interest in HV coils or similar "sparky" things, and I don't have time to dig into their design... Why not emulate this project which uses a pentode driven VTTC?

Of course, a 6L6 can have a higher screen voltage relative to the plate then a 803. So you'd have to tailor the screen supply voltage / current to prevent any 'redplating.' (Hint--there's a screen resistor on the above link.)

which one are you talking about ? if you mean the first one with the 6l6 tube the i see to problems with it. first as he said the secoondary and the primary are grounded which is not good also one of the grids are connected to the cathode. i dob't think that is good because the current rating for the screen is lower than the cathode. so i think it is better to connect that grid to the other grid to make a tetrode. he is using it like a tetrode in the plan, right. so am i right or am i missing something. thanks,


Like I noted afterward, only the external link uses the 6L6.

The other projects, excluding GMI-90, use pentodes. You'll have to adapt those for your tube.

As far as the screen voltage, he's using a variac to adjust that, so even he can't give you exact figures on that value. It's probably at least partly dependent on the coil windings.

Actually, he details that his pentode drivers worked better (bigger sparks) than the tetrode driver--the GMI-90.

One grid connected to the cathode? Not good? That's what makes it a pentode in the first place!

It's the "suppressor grid", and is almost universally connected either to the cathode, or to ground-- In fact, in many pentodes (like the 6L6), it's connected to the cathode internally. In every one of those (pentode) plans, the suppressor grid is connected to the cathode, or to GND.

If you're stuck on using the 803 as a tetrode, then don't connect the suppressor grid to anything. Disconnect it, and it's a tetrode.


your very helpful.

( that guy has realy bad luck!)

i just have one more question. how does the amount of voltage applied to the 2 grid affect operaction ? also do you apply ac or dc


Paul (i am using the plan with the gu 81m when he is powering the 2 grid with a varic )

I expect the "2 grid" is the screen.

The other grid (suppressor grid) probably should be tied to the cathode.

Screen voltage has a large effect on the operation. Plate current is pretty much a function of the screen voltage.

I'd look at other 803 designs and see where they take you.

Start with a safe, lower screen voltage. Personally, I'd insert a 1 ohm, 1% resistor (like 10 or 20 watt) between the cathode and the ground--you can read the voltage dropped across that resistor, and figure the current draw (it's a standard way to set the "bias" on amps.)

Either that, or just watch the tube carefully and see if it begins to "redplate..." ;-) (might be too late if it does.)

DC voltage for screen supply. A "bare" AC variac wouldn't do the trick... Unless AC is needed for the VTTC oscillation--I'm not really into HV circuits (above 500V, anyway) so I don't know much about a VTTC. But his earlier designs used a more typical screen resistor off the HV rail, so I doubt AC is called for...

so what your saying is this- i take my rectified variac and connect to a resistor to make sure that not to much amps are sucked in by the screen. then i connect my amp meter to the plate and turn on the coil. then i slowely rase the variac looking at the amp reading on the plate then when the max amount of amps is read on the meter i stop raising the variac. so what you are saying is the higher the voltage on the screen the higher amps on the plate right ? question- does the screen take all the amps given or is it self regulated and how does current affect screen operation?

Note: that link refers to a external page with a 6L6 driver, but he's using other pentode, just like you're trying...

Try it, connect the screen grid connected to the anode. Watch the device ratings.


the screen grid is rated to 600volts .015 amps so it will not stand up to anode voltages. i thought you would connect the screen grids to a resistor like the 1 grid like it says on 4hv's page on vttcs. http://wiki.4hv.org/index.php/Vacuum_Tube_Tesla_Coil but were would you connect the resistor to ground? also what would the resistor be, 5kohm? I know that you have to use the data sheet to see the rated current for it, but what is the voltage that flows to the grid ? is the same as the input voltage ,2kv?


What's that voltage rating relative to ? If its the anode you're OK.


what do you meen by relative to? soyou want me to connect the screen to the plate?

Where is the "maximum screen grid voltage" measured to and from ?

it is from the data sheet but i think you are asking what the votage is crrelating to.  it is reletive to ground. it is the max votage you can charge the screen to.

I shall have to ask an expert I know tomorrow - my father.


ha ha my father knows nothing about electronics. i lecture him.


My father started playing with electronics in the 1940s, and is still up to speed in the 2010s.


wow, my dad lived in korea ( im korean) in the 50s and back then there were no electronics back then not even washing machines or radios. so he knows nothing about tech

The best way to approach this is to get some old school electronics text books. The classic "Electronic and Radio Engineering" by Fredrick Terman goes RIGHT back to valve (tube) design.


do you have a copy ? all i want is a plan for using a tetrode or pentode in a class c armstrong oss. please answer quickly, spring break starts today and my 803 tube came! vintage 1943 !



o and can you ask your dad how to use a pentode or tetrode in a vttc, i can't find info anywere.

when finished my single 803 vttc will make 12 in sparks if i figure out how to wire the tube up!

Thanks for helping me so much,