Re: [TCML] valve coil problems

[<lightningfor@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Carl,
No problem I must have got mixed up with someone else I saw on the list.
They had built a valve coil with a driver on the grid, and a nifty
arrangement for the grid bias.
I can't remember who it was...
Anyone remember?

Yes I have the data sheet for the valve.
It is roughly as you said, about 200-300 watts dissapation, but that
normally means you need at least twice this power to drive it.
Unless I used a biasing Power supply with it somehow.

Definately a nice way to go if I can get enough info.
I don't have much time for R&D though, because I am up against a time line
on this project.

Kind regards,

Carlos

On Wed, 02 Nov 2011 07:27:47 -0700, Carl Noggle <cn@xxxxx> wrote:
> Hi, Carlos,
> 
> The coil I did was in 1956 when I was in high school, and it was very 
> small.  I can't remember the tube type, it was an 8xx.  I got a huge 1 
> 1/2 inch corona out of it.  I could get a similar arc one third of the 
> way up the secondary, but nothing at two thirds.  I guess we gotta start

> somewhere.  My next coil was with NSTs and got a three foot arc.
> 
> I would hope you could make a driver with a half-bridge of FETs.  Do you

> have characteristic curves for the control grid?  The driver will have 
> to take the grid several hundred volts negative for cutoff, and positive

> enough to saturate the plate current.  This will require lots of current

> flow from the grid, but from the sketchy data I could find on the net, 
> the grid dissipation is only 200 watts, so it should not be too 
> difficult to build.
> 
> Is there a full data sheet available?
> 
> ---Carl
> 
> 
>> Hi Carl,
>> Thanks, I will try it out.
>>
>> I am definately interested to try the driver idea you mentioned though.
>> Can you suggest a source of info to take some of the R&D out of it?
>>
>> If I remember correctly you did a similar valve coil recently...
>> Do you think it could be scaled up, given that I would need about 500
>> watts of RF power to drive the grid into Class C.
>>
>> Thanks, Carlos
>>
>> On Tue, 01 Nov 2011 12:58:37 -0700, Carl Noggle<cn@xxxxx>  wrote:
>>> Hi, Carlos,
>>>
>>> If you have a scope, you could shock-excite the secondary and measure
>>> its resonant frequency.  Take it away form the primary, then put a
>>> one-turn coil around the bottom (bottom of the secondary is grounded)
>>> and hook the one-turn coil up to the scope.  then flick a 9V battery
>>> across the leads to the one-turn coil.  With persistence you should be
>>> able to get a damped oscillation and measure the frequency.
>>>
>>> As a fix, try resonating both the primary and the grid coil at the
>>> operating frequency.  Or you could make a series-resonant circuit to
put
>>> across the grid coil--the L should be 2 or 3 times that of the grid
>>> coil, with capacitor to resonate at the third harmonic
>>>
>>> Or for that matter, you could try running it at the higher frequency.
>>>
>>> I still suggest driving the grid with a low-impedance square wave. 
Then
>>> you can run in class C.  Also, make sure that your screen grid supply
is
>>> well-bypassed.
>>>
>>> Good luck--sounds like a cool coil.
>>>
>>> ---Carl
>>>
>>>
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