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LAN_403

Tonskulus

Thu Feb 19 2009, 02:00PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

New video available if someone is interested:
http://www.youtube.com/watch?v=6IUTKUwtUuY&feature=channel_page

Im very satisfied for this heater because of its reliability. Overheating is sometimes the problem as i still have no
watercooling..

Tonic

Thu Feb 19 2009, 03:25PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Wow, that's a rapid heating :O

Tonskulus

Fri Mar 20 2009, 12:13PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

I just find schematic form RCA transmitting tube manual for induction heater using 833A. But i have a question:
For me, 100nF seems to be way too high capacitance as they are usually in range of 1..10nF. Is there mistake or what?
Schema:
http://www.elisanet.fi/tonskulus/56ih.jpg

Steve Conner

Fri Mar 20 2009, 12:36PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The two 0.1uF's, C2, C5, are DC block capacitors, you size them so that their impedance is negligible at the working frequency.

The tank capacitor is C4.

Tonskulus

Fri Mar 20 2009, 03:47PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Steve McConner wrote ...

The two 0.1uF's, C2, C5, are DC block capacitors, you size them so that their impedance is negligible at the working frequency.

The tank capacitor is C4.

Yes, I know C4 is tank and how hartley basically works. Anyway i have seen two different gridleak methods: cap and resistor in parallel and seriescap+resistor between G1 and cathode.
As we know, in first method, both components are playing important role for biasing. Capacitor holds G1 negative while parallel resistor is for discharging that cap. Lower resistor value or smaller capacitor = less bias.

Second method, where resistor is between g1 and gnd, capacitor value is not important? It has nothing to do with actual biasing? I think it is not important capacitor at all, because there is allready dc blocking cap at plate side of tank circuit. So it is there just because of grid amp meter..?

Steve Conner

Fri Mar 20 2009, 04:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't think C5 is just there on account of the grid current meter. If it weren't there, the tank coil would short out the self-bias voltage, R and L2 would do nothing, and the tube would end up closer to Class-AB than C.

The time constant formed by R and C5 has to fall in a certain range to avoid squegging. You also have to avoid parasitic oscillations at the resonant frequency of L2,C5, and ditto for L1, C2.

All of these factors will have been taken into account by the original designer. Or maybe 0.1uF was the cheapest cap he could get with an 0.6A RMS current rating, back in the days before plastic film.

Tonskulus

Fri Mar 20 2009, 04:18PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Steve McConner wrote ...

I don't think C5 is just there on account of the grid current meter. If it weren't there, the tank coil would short out the self-bias voltage, R and L2 would do nothing, and the grid would get driven a lot harder.

Yes, indeed. But i wonder if the value of grid cap is critical..I think that much smaller value would still be acceptable. Like 10nF.

Tonskulus

Tue Mar 24 2009, 12:57PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Now i wonder where is tapping point for GND on tank coil. Any ideas? For class C operation, i think about 300V p-p gridvoltage is required. My tank coil will be made out of enameled 6mm2 copperwire so testing different tap points is not easy task. Somehow i need to know, maybe 2-3turn accuracy, tapping point. I can make few tap points for tankcoil.
In schematic, tap point is near top of the coil but in practice that is not the case. It should be near grid end.

Steve Conner

Tue Mar 24 2009, 01:45PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'd probably start by tapping it one-tenth of the way along. The tap point may well have been adjustable in the finished product.

You can use a separate feedback coil (made of thinner wire) instead of tapping the main coil, it makes no difference electrically. Either way, it's two coupled inductors connected together at a tap point.

Tonskulus

Tue Mar 24 2009, 05:15PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Yes, my current induction heater has separate grid coil. But hartley oscillator is more stable, feedback voltage is automatically controlled as persentage of tank voltage. And the construction itself is very simple.

If inductively coupled ticklercoil is used, it requires also separate primarycoil if matching transformer is used. So more space is wasted. This system should be as compact as possible. Thus, hartley circuit is used.

Ok, i will make some tapping points for each turn between turns 3 and 6 or so. There are 40turns total and tank voltage may be around 4kV. So, there should be 400Volts at 4th turn.. but we'll see how it works..:)

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