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> Triode Oscillator Help
adamsimpson
Posted: May 15, 2008 04:44 pm
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Could anyone help expain this tuned plate tuned grid triode oscillator? I don't really know very much at all about tubes but am trying to build the following oscillator for a project.

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I understand the the tuned grid circuit, although I am not sure how to figure out the capacitance for the grid leak part. I am mostly confused by the tuned plate part of the circuit though, as normally you just see a simple LC parallel circuit but here it's not quite that simple. If someone could help explain the workings of the plate circuit it would be greatly appreciated. Thanks
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tekwiz
Posted: May 15, 2008 06:45 pm
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The reason for any tuned plate circuit is to eliminate the effects of the plate capacitance, which tends to short out any RF. Making the plate capacitance part of a tuned circuit overcomes this difficulty. If you view your circuit in terms of plate capacitance, it actually is a parallel tuned plate circuit.
As for the grid leak capacitor, the idea is to have enough capacitance so that the circuit has a low impedance at operating frequency. Typical value would be 50-100X the capacitance of the tuning capacitor, but it's not critical. cool.gif


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Sch3mat1c
Posted: May 16, 2008 08:25 pm
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So the plate capacitor is in fact a much larger coupling capacitor, and presumably there is a tuning capacitance on the plate coil (left side) as well?

If that's the case, then the three fixed capacitors (I feel like calling them condensers considering the age of this drawing!) can be roughly 0.01uF, 500V or 1kV ceramic disc types.

A typical grid leak might be 10k to 470k, depending on oscillator power output and tube type. Sometimes a value is given in datasheets (class C self-excited amplifier / oscillator in this case).

Tim


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adamsimpson
Posted: May 17, 2008 12:05 am
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Thanks for the help so far. The large inductor on the left runs directly into a high impedance load that has a capacitance of about 60 pf. I am a bit unsure about how the power is output in the circuit though. I have seen the load inductively coupled to the inductor on the plate circuit before but am not sure how this configuration is set up really. Am I right in assuming the inductor on the top of the diagram is part of the plates tuned circuit? Thanks again for the help.

This post has been edited by adamsimpson on May 17, 2008 12:05 am
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CWB
Posted: May 17, 2008 03:47 am
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a little more information can be found here :
http://www.antiqueradio.com/Oct02_Corkutt_gridleak.html
http://www.vias.org/basicradio/basic_radio_15_05.html

if you can find one of the old arrl handbooks it will contain a wealth of practical quick-n-dirty information on how many turns , tuning and such .
a small lightbulb can be used to tune the output ... a number 47 is good up to about 5 watts . series-parallel connection of four lamps (soldered into a compact package) will get you to about 20 watts . the indcutance at low (hf) rf is negligible .

as far as coupling to the tank (plate) coil goes ...
loose (proximity) coupling would be about the easiest way to go .
this isolates (electrically) the antenna circuit from the transmitter .
changing the spacing between the two coils will change the loading of the plate circuit and requires a slight tweaking of the tuning . heavy loading causes "pulling" of the oscillator , shifting it's frequency . interaction city !
some coupling circuits used a series variable cap to the antenna to change the resonance/loading .


This post has been edited by CWB on May 17, 2008 03:54 am


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tekwiz
Posted: May 17, 2008 07:40 pm
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QUOTE (adamsimpson @ May 16, 2008 03:05 pm)
Am I right in assuming the inductor on the top of the diagram is part of the plates tuned circuit? Thanks again for the help.

Nope. The inductor at the top of the diagram is the plate choke, that allows DC to flow to the tube, but blocks RF from getting out thru the plate supply. The big coil to the far left is the tuned circuit inductor. It appears that this configuration is designed for direct coupling of the output, which is taken directly from the tuned circuit. cool.gif


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adamsimpson
Posted: May 18, 2008 05:37 am
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Thanks a lot everyone, I think I understand it now. Now to order the parts and see what I can make happen.
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adamsimpson
Posted: May 18, 2008 04:40 pm
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Actually I have another question as it has come to my attention we may not have an appropriate DC power supply available. In the diagram I am confused over the (seemingly?) lack of a rectifier circuit, as I said though I know little about these circuits, or any circuits for that matter, so I am sure it's something I'm just missing.

This post has been edited by adamsimpson on May 18, 2008 04:40 pm
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CWB
Posted: May 18, 2008 06:05 pm
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this is one of those "self rectifying" circuits ... dirty as hell .

i think these type of circuits were designed for induction heating ... put it in a faraday cage .

you are going to need a separate filament xfmr .

This post has been edited by CWB on May 18, 2008 06:06 pm


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tekwiz
Posted: May 18, 2008 07:09 pm
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Yeah. That circuit will only work on the positive half cycles of the mains frequency. The other half cycles, it will do nothing at all. The output will consist of pulses of the operating frequency, modulated by the mains frequency. This type of circuit is used where a clean, steady output is not necessary. It is used because it is cheap...there are no costs for rectification & filtering circuitry. cool.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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