plate current metering

[kb3ouk]

For my homebrew rig I want to be able to monitor the plate current of the 814s, but without putting the meter in the B+ line to the plates. If I monitor the cathode current, is there a way to read the true plate current and not the combined plate+screen+grid current? I remember reading once that if you have the grid return directly to the cathode, the meter would read the true plate current, but I believe this was in reference to a triode final, so in my case I would probably still be reading the plate+screen current. The other possiblility is putting the meter in the negative side of the plate power supply, would this work?

[w4bfs]

the simplest way to do this is to put the plate current meter in the cathode return to the B- lead, assuming you are using a separate screen supply and not a dropping resistor off the plate supply, just like a grounded grid amp ... whatcha got, Shelby ?

[kb3ouk]

I was planning using a dropping resistor for the screens, because the 814 data sheet mentions that the screen voltage is preferably obtained from a dropping resistor from the modulated plate voltage, but I also see that it only requires 300 volts, which I can get from my power supply for my exciter. That power supply is going to put out somewhere around 325 volts, so I can just feed the screens from a resistor (which should also provide some current limiting) that drops it from 325 or whatever volts my exciter power supply works out to be, then just allow the screens to self modulate through a choke. I'm already going to use the -45 volt bias from the exciter supply to provide some fixed bias for the 814s in the final, so I might as well use the exciter B+ for another purpose too.

[KA2DZT]

I was planning using a dropping resistor for the screens, because the 814 data sheet mentions that the screen voltage is preferably obtained from a dropping resistor from the modulated plate voltage, but I also see that it only requires 300 volts, which I can get from my power supply for my exciter. That power supply is going to put out somewhere around 325 volts, so I can just feed the screens from a resistor (which should also provide some current limiting) that drops it from 325 or whatever volts my exciter power supply works out to be, then just allow the screens to self modulate through a choke. I'm already going to use the -45 volt bias from the exciter supply to provide some fixed bias for the 814s in the final, so I might as well use the exciter B+ for another purpose too.

How do you plan to put the xmtr in standby??

[K7LYF]

The Hallicrafter HT9 uses a single 814 for a final. Check out the schematic for the circuit.

[kb3ouk]

I was planning using a dropping resistor for the screens, because the 814 data sheet mentions that the screen voltage is preferably obtained from a dropping resistor from the modulated plate voltage, but I also see that it only requires 300 volts, which I can get from my power supply for my exciter. That power supply is going to put out somewhere around 325 volts, so I can just feed the screens from a resistor (which should also provide some current limiting) that drops it from 325 or whatever volts my exciter power supply works out to be, then just allow the screens to self modulate through a choke. I'm already going to use the -45 volt bias from the exciter supply to provide some fixed bias for the 814s in the final, so I might as well use the exciter B+ for another purpose too.

How do you plan to put the xmtr in standby??

I'm keying the power supplies, transmit=turn power supplies on, standby=power supplies off. I have a setup with a few relays that handles that, which are also part of the exciter power supply.

[KA2DZT]

OK FB,  reason I asked is the screen supply has to go off before or at the same time as the plate supply.  You can damage the screen very quickly if you have screen voltage without the plate voltage.  That's why it is better to use a screen dropping resistor off the modulated plate voltage.  You can't make a mistake with that method.

Another thing to consider,  you can turn on to transmit and the low voltage supply comes on (screen voltage present) and the HV supply fails to fire up for some reason.  You would need some sort of safety relay that trips off the HV supply voltage that will connect the screen voltage to the screen.  Whole thing starts to get complicated.  Your xmtr will end up like my HB 6146-813 xmtr that has some 30 or more relays in it

Fred

[w1vtp]

Fred

How about some pictures of this fabled 813 transmitter.  I cannot remember the last time you were on the air

Al

[kb3ouk]

OK FB,  reason I asked is the screen supply has to go off before or at the same time as the plate supply.  You can damage the screen very quickly if you have screen voltage without the plate voltage.  That's why it is better to use a screen dropping resistor off the modulated plate voltage.  You can't make a mistake with that method.

Another thing to consider,  you can turn on to transmit and the low voltage supply comes on (screen voltage present) and the HV supply fails to fire up for some reason.  You would need some sort of safety relay that trips off the HV supply voltage that will connect the screen voltage to the screen.  Whole thing starts to get complicated.  Your xmtr will end up like my HB 6146-813 xmtr that has some 30 or more relays in it

Fred

That's why I'm leaning more towards just having the screen voltage come from a dropping resistor off of the modulated B+. I looked at the HT-9 schematic (I've been using it as a reference for several things during this build) and they got the 814's screen voltage coming from a voltage divider off of the unmodulated B+, then ran that through a tertiary winding on the mod iron to modulate it. The plate current meter on the HT-9 is in the negative side of the plate supply. I think I will do the same thing with the meter on the negative side of the plate supply, which will give me the plate+screen current, and just feed the screens from a dropping resistor. I have a separate meter for the grid current. I was originally going to try and monitor the screen and grid currents with the same meter by switching it between the two, but I don't really think it is that necessary.

[VE3AJM]

If you want to measure plate current and not have high voltages across the meter, why not build a simple voltage divider network? There are lots of circuits out there to do this.

Al VE3AJM

[kb3ouk]

If you want to measure plate current and not have high voltages across the meter, why not build a simple voltage divider network? There are lots of circuits out there to do this.

Al VE3AJM

how do you do that? voltage divider off of plate B+ and then the plate current meter goes between the voltage divider and ground? On the HT-9 schematic, Hallicrafters has the plate iron CT, filter cap, and ground for the screen voltage divider all tied together and passed through the meter to ground, which is why it reads the combined plate and screen current.

[Opcom]

The HT-9 puts the plate current meter in the B- lead after the filter cap, to GND.
It does take G2 volts off the plate supply via a tap on the bleeder so you would need a separate screen supply if doing it that way.

The pic shows how to get the result you want.

[kb3ouk]

That's kinda what I was looking to do.

[N2XD]

Sure would like to know more about your 814 rig. I pick one up about a month ago and I am planning to restore it and but on the air. any drawings by any chance? Thanks. 

John N2XD

[w4bfs]

The HT-9 puts the plate current meter in the B- lead after the filter cap, to GND.
It does take G2 volts off the plate supply via a tap on the bleeder so you would need a separate screen supply if doing it that way.

The pic shows how to get the result you want.

just remember to add back to back diodes across the meter, assuming that they would not conduct in normal operation ...this protects you if the meter movement should open (same problem with meter in + leg)

[Carl WA1KPD]

Sure would like to know more about your 814 rig. I pick one up about a month ago and I am planning to restore it and but on the air. any drawings by any chance? Thanks. 

John N2XD

John,
Do you mean a Hallicrafters HT-9 or a HB 814 rig? If it is the HT-9 I have had one for about 15 years and love it.
Carl
WA1KPD

[W1ADR]

Good Evening. As previously noted and conveyed, might I offer an old field expediency wherein You use a  discrete length of the B+ plate feed lead itself as a shunt element in conjunction with a 50 or 100 microamp meter and the appropriate series meter resistance. Not sure what current levels You are interested in, but the procedure is essentially that used when a single meter is used to monitor multiple functions with the only difference herein is not installing individual shunts and/or multipliers, but using the resistance of the appropriate lead, or leads,  to satisfy the basic shunt requirement. A distinct advantage is that a common power supply could suffice to handle both the screen and plate requirements without having to provide for a protective bypass in the event of an in-series plate meter failure as well using the meter to read either the plate current or screen current by switching one of its leads to either the appropriate plate or screen feed points.

Anthony,  W1ADR

[kb3ouk]

The HT-9 puts the plate current meter in the B- lead after the filter cap, to GND.
It does take G2 volts off the plate supply via a tap on the bleeder so you would need a separate screen supply if doing it that way.

The pic shows how to get the result you want.

just remember to add back to back diodes across the meter, assuming that they would not conduct in normal operation ...this protects you if the meter movement should open (same problem with meter in + leg)

What way should the diodes be? cathodes towards or away from each other or doesn't it matter?

Sure would like to know more about your 814 rig. I pick one up about a month ago and I am planning to restore it and but on the air. any drawings by any chance? Thanks. 

John N2XD

No schematic yet, I haven't finished building it. most of the design is in my head right now, not really based on anything else. As far as the rest of it, the modulator uses a pair of 811As (right now I actually have a pair of 572Bs in it, my 811s weren't behaving right). The exciter is a Millen 90800.

[w4bfs]

What way should the diodes be? cathodes towards or away from each other or doesn't it matter?


ok ... back to back diodes would be in parallel with the cathode of one connected to the anode of the other .... this forms a bipolar clamp of 0.7V or so and should protect your meter from a huge overload if a tube shorted or took off on a massive parasitic osc and would keep the meter close to ground potential if the meter winding should open ...

[kb3ouk]

ok gotcha. I was thinking in series for some reason. Any particular voltage rating? I have a bunch of 1N4007s and 1N5408s. I probably don't need the current rating of the 5408s but the voltage probably should be a little higher than 1kv, since thats right around where the rig will be running.

[w4bfs]

you can use almost any silicon diode for this purpose ... piv is not a consideration because the largest voltage seen across the back to back pair is whichever diode is forward biased at the time

[KD6VXI]

It's Isurge that matters.   I used 6A10 diodes until I ran out,  then went to 10A10s.

You want the diodes to survive the surge event.   If they blow open during the surge,  you've accomplished nothing

--Shane
KD6VXI

[kb3ouk]

I'll go with the biggest ones then, I believe the 5408s can handle something like 3 amps. My modulator has a meter on the center tap of the filament transformer to measure the plate current, I should probably put the back to back diodes on it too.