# Bridge in the Plate circuit

• posted

Has any one ever inserted a SWR bridge in the plate circuit going to the tank so that SWR could be monitored to prevent disaster while ramping up R.F. power? This is a 100Khz OSC/AMP circuit. The tube is rated for 375K watts but we don't use that much of it .

Just looking for a tempory/Permanet solution to prevent arcing when the load is defective.

I was going to pass the plate electrode lead (#6 wire) through a donut that composes a SWR bridge (potential type bridge) so that standing waves could be measured while ramping up the power.

The OSC is a basic hot cathode tube using a isolated inductor as a feed back with cap in series to set the OSC freq into the grid. The plate/anode is connected to the R.F. transformer primary side.

This system uses B- and is feed at the cathode, so the PRIM R.F. transformer leads, come from the Plate and go to ground. We have a spark gap set for 1/2 " at the moment and when something is going wrong on the load side, which is inside a pressured sealed SF6 vessel, it some times arc, like currently. We are trying to come up with a way to monitor this issue so that the system can be alarmed while ramping up and shut down. The Plate Lead we can get to easily while the primary side-R.F. xformers are inside of the pressured vessel.

"

• posted

I think you're both missing the boat and gilding the lily.

First, SWR meters don't, by and large, measure SWR. What they measure is the impedance of their output port in a way that can easily be expressed as forward and reflected power. While a few of them ("monimatches") do measure the SWR of a section of transmission line, sorta, you're still measuring the impedance at the output port (after all, where are the standing waves in a resistor, or a capacitor?).

Second, you don't care about SWR as a whole. What you care about is the dissipation at the tube, the maximum voltage of the tank (which causes your arcing problem), and probably the maximum plate or cathode current (and other element's currents, if that's a concern). Those are the things that are going to damage your tube or the surrounding circuitry.

Third, if you _did_ care about the SWR, you'd have to care about the SWR at a really weird (and high) characteristic impedance -- so any 50-ohm SWR bridge circuits you may have in mind would need wildly different circuit constants, and may not even function correctly with real-world components.

So why not measure what you care about, instead? At 100kHz you shouldn't have too much trouble measuring all the relevant voltages and currents, and shutting down the amplifier if it's indicated. You won't have to go to the trouble of figuring out an SWR bridge that operates at the tube's loading impedance, and you'll make better measurements of more relevant operating parameters.

--
Tim Wescott
Wescott Design Services
• posted

Think time domain, maybe. A string of schottly diodes, SiC or GaN maybe, could both peak-rectify the plate voltage for sensing, and clamp it long enough to shut down the drive.

Longer term, you could digitize the grid voltage, the cathode voltage, the plate current, and other stuff, and compute dissipation in real time, and apply appropriate controls and protections. We do that on big fet power amps, and it works really well.

What's a tube like this cost?

What's the supply voltage?

John

• posted

The problem is currently, we can't even get past 2kDC on the anode before this arc takes place, the arc is actually in the RF junction box. its not at the tube osc cabinet.. we have a short run of Coax cable that connects the OSC cabinet to the R.F. box. just on the other side of the RF box is the RF xformer that goes to ground on the primary side. This circuit is very simply. It has caps in parallel to make it resonate on the primary side. SO if you can picture the primary side of the xformer with one lead to ground and one lead to the plate of the tube, with the cap connected the same way. I wanted to monitor the circuit from the plate lead up to this tank for non resonant issues. We normally use a scope there to examine the wave form..

On the other side of this xformer is the secondaries that drives the rectifier stack in the SF6 pressured vessel.

The monitor circuit was for future alarm options to be used if an issue is taking place while the operating is ramping up the voltage on the OSC tube. I suspect that we'll need to take the vessel apart and examine the components on the stack.

We also have another issue with the power supply that could also be causing some problems. before I left saturday, I noticed the 100khz carrier was getting modulated to saturation what appeared to be coinciding with maybe a lost phase on the 3 phase supply.. we'll have to look at that tomorrow. Thanks.

"

• posted

Well, I've done some thinking, Since this tube is cathode (B-)driven, I don't see any reason why I can't have a h-Z protected sense bridge circuit. The Bridge would use the (B-) as the reference against to what is on the Plate side. if the voltage should exceed the (B-), this should indicate a problem getting ready to happen.

But I have a sneaky suspicion I know where the problem lye's for the current moment.. Before I left saturday, I examine the 3 phase supply, because I saw what looked like null carrier at the base line that coincides with maybe a lost phase. Since this circuit is highly inductive, I guess if one was to saturate the RF xformer on one cycle and when the other cycle was to start but instead, the tube hits the base line quickly, I guess this could cause the RF xformer to discharge gracefully. And since we are only putting no more than 2kDC in the circuit when this happens with a 1/2 spark gap, this is only way I can see that happening. Oh well, we'll check that tomorrow.

Thanks.

"

• posted

On a sunny day (Sun, 09 Nov 2008 19:39:38 -0500) it happened Jamie wrote in :

Perhaps you can monitor the plate waveform for harmonics?

• posted

On a sunny day (Mon, 10 Nov 2008 09:14:10 GMT) it happened Jan Panteltje wrote in :

What I actually was thinking, was to put a small LC pick up coil, tuned to 2xf0 or 3xf0, close to the thing, and add a simple diode detector. If the waveform changes a lot due to clipping for example, then the harmonic should increase, and the detector output increase. No physical contact needed.

| ------- a DIODE k ------------- || | | | |L C C R Udet || | | | | ------------------------------- perhaps a ferrite rod.

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.