Electronics > RF, Microwave, Ham Radio

RF Triode Filament Monitoring Circuit

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I have a ham radio RF amp (Alpha 87a) running two 3CX800A7 ceramic triodes (all else solid state) with a lot of protective circuitry.  I have been getting intermittent faults on the filament monitor for the 3CX800A7’s showing the filament current is low  - suggesting that one of the tubes has an open filament.  I know this is not the case as it is intermittent fault and when not faulted I can get full output power so I think it’s an issue with the filament monitoring circuitry.  I understand the basic concepts of current monitoring using a parallel resistive shunt load and feeding that voltage drop into an op amp to provide a corresponding proportional voltage that could be read by a microprocessor analog input.  My amp does not require precision, it is designed to shut down the amp if the filament current is too low (<2 A) or too high (>4 A).

Attached are the two schematics that show the filament monitoring circuit.

The transformer’s 13.6 vac go via lines labeled ‘FIL’ and “FIL RETURN” through the Low Voltage Board, these are passed through to the Tube deck along with a 3rd line called  FIL_SENSE GND.    On the Tube Deck,  FIL is connected to one side of the filaments and the other side of the filaments are grounded.  FIL_SENSE GND is connected directly to ground  and FIL_RETURN is connected to ground through a series 0.3 ohm, 5W voltage dropping resistor.  At first I figured the filament monitoring circuit is sensing the current through this 0.3 ohm dropping resistor but both sides of the sensing points (FIL_SENSE_GND and FIL_MON are at or near ground and nothing is shunting current from the FIL and FIL_RETURN lines.  I cannot figure out how the monitoring circuit is working and would appreciate some advice.  I hope someone can volunteer to help me on this issue.

Many thanks in advance

Jerry NY2KW

Simple enough, it gets the AC voltage across the sense resistor, around 0.9VAC or so, and this is passed via C27 to a simple voltage multiplier comprised of D19,20 and C25,26,27 so that a DC voltage roughly proportional to filament current is present across R35. U8A and D26 does some gain and clips it so that you only get a DC voltage applied to the filament monitoring pin which is clipped so it will be 5V if the current is ok and 0V if not.

At a guess you should first change C26,27 with new ones, as these are likely getting elderly, and low value electrolytics are always suspect. As well you can monitor U8 pin 1 and it will show you an analogue value that should be reasonably steady at around 4-8V, with no noise showing.

Of course you have checked for dry joints on the filament connectors and that the wiring and interconnects are all fine, and no dry joints on the sense resistor.

Thanks SeanB but I don't think it's that simple.  The sense circuit input is between FIL_RETURN and FIL_SENSE_GND.  One is tied directly to GND and the other is tied to GND via R7.  Where is the potential difference coming from between these two points?

Jerry NY2KW

The voltage is applied from the transformer to the filament return ground and the filament.
One of the filament pins is directly connected to ground and then to filament return ground thru the resistor.
In order to get a complete path to the filament transformer the current must flow thru the resistor.
I agree change the caps.

It is as simple as SeanB explained it.

And yes change those caps....


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