Glitch Diode in Plate Supply Negative side.

[Opcom]

Is this the correct way to use a diode to protect the meters and whatever else from an arc-over or flash in the transmitting tube? D1 is a 600V 80A diode I found a few of. Vf=0.4 @ max current.

The upper wire from the rectifier is B+ (going onto the filter choke), the lower wire is the negative side, usually would be grounded.

[KD6VXI]

Correct.

But that isn't there to protect the meters.  That's to prevent the chassis from soaring to B+ if the metering resistors fail.

To protect the meters back to back diodes across the meter terminals.  Size according to voltage needs (IE, if it's 1.2 volts full scale, two diodes in series and back to back those).  Ideally you don't want them conducting anywhere near the meters operating range as it will skew it at the tip end.

--Shane
WP2ASS / ex KD6VXI

[WB6NVH]

How does DC go through C4 and C5?

[K8DI]

What Shane said is right, one other detail though…Vf of the diode in your schematic also needs to be above the grid meter voltage so as to not partly bypass the meter under normal operation..may need two in series there.

I note too that W8JI in his metering discussion calls for that bypass cap to be like 50 volts so it fails if the diode fails.  You show it at 200v, maybe a bit high..

Ed

[KA3EKH]

Harris had the plate current meter for the old HT series FM transmitters installed on the low side of the plate supply. There was also a shunt resistor that carried most of the current. If the shunt failed to keep the low side low they had a ten or twenty-volt zener diode bridged across the shunt resistor that would short out so the transmitter stayed on the air but the plate current read zero. Disconcerting to look at the output meter and see three thousand watts, full plate voltage but no current. The problem was that every time we had a lightning strike it would short the diode so the plate current meter stopped working. At one point had a couple HT-3.5 but the HT-20 and 25 never had that problem.

[Opcom]

How does DC go through C4 and C5?

----------

There should not be DC going to the other side of those capacitors. C5 is a coupling cap from the anode to the PI coil, and C4 is a coupling cap from the input tuned circuit to the tube cathode. I have not shown any cathode bias circuit either.
But they are only there to clarify the RF input and output of the amplifier tube.

[Opcom]

Correct.

But that isn't there to protect the meters.  That's to prevent the chassis from soaring to B+ if the metering resistors fail.

To protect the meters back to back diodes across the meter terminals.  Size according to voltage needs (IE, if it's 1.2 volts full scale, two diodes in series and back to back those).  Ideally you don't want them conducting anywhere near the meters operating range as it will skew it at the tip end.

--Shane
WP2ASS / ex KD6VXI

-------------------

Can the chassis get charged like that even with a utility (mains) ground?
Or is it meant that it can be charged with B+ in rellation to the many components inside?

How 'fast' should the meter protection diodes be? Just asking, since the meters do have internal shunts and I would prefer to use those if it's possible. On the other hand, the intended meters are not likely replaceable, but are darn good-looking.

[Opcom]

What Shane said is right, one other detail though…Vf of the diode in your schematic also needs to be above the grid meter voltage so as to not partly bypass the meter under normal operation..may need two in series there.

I note too that W8JI in his metering discussion calls for that bypass cap to be like 50 volts so it fails if the diode fails.  You show it at 200v, maybe a bit high..

Ed

---------------

There are maybe 6 of those big diodes here. Diodes are inexpensive thankfully.

I can reduce that capacitor's voltage. It's not installed yet.  What type is preferred so that it will be likely to short rather than open?

[K8DI]

I can reduce that capacitor's voltage. It's not installed yet.  What type is preferred so that it will be likely to short rather than open?

My limited capacitor knowledge/experience indicates that ceramics are likely to short while film/metalized types are likely to fail open. I’d be looking for some part that isn’t tiny..a .1/50 that is intended to bypass a digital IC with 0.1” lead spacing would seem more likely to explode when it shorts, thereby becoming unshorted…  an old school 1/2” diameter Z5U like I’ve occasionally replaced due to being shorted across an AC line seems about right.

Ed

[KD6VXI]

Use a ceramic cap, then use a non resonant shut L at the end of the C load cap.  Then if any DC appears on the Tune side of Block it will be shorted to ground.

I prefer to use a couple in parallel to share rf currents.  Then again, I usually deal with larger rf currents.

As to how fast do the diodes need to be....  They are in the DC area of the circuit.  I use 6A10 or 10A10.  With 400 amp surge capabilities they usually last in case of an event.

If you float the b+ for metering purposes then there is the ability to lose ground reference if the metering resistor blows. unless you have a separate ground on the rf deck and power supply. 

But, since you have to lift the B+ supply a few ohms above ground for metering, you do want a clamp. Two diodes in series means 1.3 or so volts vs hundreds to thousands?

Cheap protection.


--Shane
WP2ASS / ex KD6VXI

[Opcom]

I believe I am coming to understand that the diode is not there for a tube flash-over or other short/arc to ground, but only to protect, or to keep voltage relationships between HV (-) and GND somewhat normal in case a meter shunt in that same area opens. I don't know why I thought it was there to protect the unit against tube arcs (like 3CX3000 internal arc-over).

Probably was getting mixed up over the various things that could go wrong in something of goodly size. The 3cx3000 project is still sitting here and needs restarted. I hope it will be in 2024. So there is that at 3500-4800V.

Protections:
meter shunt opening protection = the diode or series diodes as needed to cover the normal shunt voltage drop

HV short or tube arc = #30 wire as HV fuse - between HV filter cap and tube anode

meter movement olverload  = back to back diodes right at the movements.

Limiting the arc current in the RF tube = Glitch resistor

[KD6VXI]

When a tube arcs, it will blow the metering resistor.  This disconnects the HV - side from chassis ground.

A diode across it will absorb the surge and limit it to appx seven tenths of a volt.  Add times N for multiple diodes in series.  This gives the fuse in the B+ side time to work and extinguish any arc.  A surge limiting resistor in the B+ lead helps a lot

Think about it, a hypothetical 100A surge that is limited to  .7 volts is a heck of a lot less damaging than one at hundreds of volts.

Hope that helps.

--Shane
WP2ASS / ex KD6VXI

[Opcom]

It's very clear now!