screen / Hi voltage shut off.

[Crusher]

I am in the process of starting to put together a small tetrode amplifier. I would like to install a circuit so that if I loose plate voltage, it would automatically shut down screen supply. Anyone know of a link or suggestion to build this kind of circuit? Going to start off small. Either a 4cx250B or maybe the russian equivalant of a 4cx800a. Would like it so that if it loose plate at tube. So even if I still have plate, but glitch resistor opens. Figured have to sample after glitch, maybe drop down voltage to 120 or so maybe 12 or 24vdc and use that to activate relay. If glitch opens, drops voltage and opens relay for screen supply. Sound correct? New to tetrode stuff but not triode. So any and all ideas welcome.

73
Crusher

 

[Crusher]

Figure if I can drop it down to managable voltage, could use hi watt zener and or voltage regulator to keep voltage stable before I use it to control relay. Of course necessary bypassing is a must to keep rf off zener/ V reg circuit. Just tossing idea around. Don't know if I'm on right track or way out in left field. I realize there will be some voltage drop when load is applied to plate so would have to take this into account.

 

[exces3]

I would use some kind of, voltage divider arrangement,
fed from B+. Along with the usual string of 5 W zeners
and a switch for screen V Hi/Lo

 

[Crusher]

Yeah, that's what I was thinking myself. Using a string of resistors to drop the voltage to a managable voltage to work with. And the screen, a string of zeners like you mentioned so that I can have recommended voltage and then tap at a lower point as you said for Hi/ lo feature.

 

[Shockwave]

The biggest risk here is preventing high voltage from entering the low voltage relay protection circuit. One common method is to use an opto-coupler between the two circuits to provide this isolation. The opto-coupler is a simple device with an LED and photo transistor in one package. When the LED is conducting it shines into the photo transistor and causes it to conduct with no electrical connection between the two.

If we wire the LED to the HV supply through a voltage divider, we can use the photo transistor side of the device to drive the protection relay. The resistor values for the voltage divider should be chosen to provide just enough current to keep the LED on under normal full load conditions. This will prevent delay in triggering the protection relay waiting for the filter caps to discharge.

Don't forget to shunt regulate that screen supply too. Smaller tubes work OK with the typical 50 watt zener diode string. Consider using beefy transistors with smaller zener diodes across the base / collector junctions for larger tubes. You can make a 200 watt zener diode with a $1 five watt zener and a $2 high dissipation transistor. One 50 watt zener will cost you $20 and not have the same degree of regulation or dissipation.

 

[Crusher]

That is a great idea with opti coupler. And yeah a 10W zener and a 3055 or better yet 2n5686.

 

[exces3]

keep in mind that a 5 watt zener can only get rid
of 5 watts of heat, if the leads are clamped to an
infinite heat sink, which is not the usual case.
...........

how much collector voltage,
can a 2n3055 or 2n5686 tolerate?

 

[Crusher]

Good call on V rating of pass transistors. Not sure on the 3055 but the 2n5686 are only rated for 80 V so would need a bare minimum of 5. Going to have to look and see what else I can come up with. This isn't even on paper yet. That's why I asked the pros. Because this is new territory for me. The other transister I may look up is a TIP35C. Easier package to work with. I have some more reading before I start building. I appreciate all the comments thus far.

 

[Shockwave]

The 5 watt zener diode is all you need across the base / collector junction of the transistor because the transistor is providing the main current path rather then the diode. I normally work with the MJW3281A transistor. As I recall it's an NPN with a breakdown voltage of 1000V and a power dissipation of 200 watts.

While this is a beefy transistor, you will still have to observe power dissipation when working with very big tubes that could draw upwards of 200 ma of screen current. In this case you stack the transistors in series by connecting the collector of one to the emitter of the next. For currents in this range, I only use 100 volt zeners to limit the voltage drop and dissipation each transistor will have to handle.

Smaller tubes like the 4CX250B can have all the screen voltage handled by a single zener and transistor where the larger ones will require a series string. Since each transistor has its own base voltage established by the zener, the voltage across any one transistor is always the same as its zener. Therefore, you can stack as many as you want to add voltage and the only part that will experience the increased high voltage will be the insulators behind the transistors, isolating them from ground.

The transistor is in a TO-3P case and mounts with a single screw. The case has an internal insulator for the screw itself but the metal back still must be insulated from the heatsink. In any application where there is more then a couple of hundred volts, I highly recommend using thick ceramic insulators and not plastic or mica here. It's also a good idea to RF bypass the base / emitter junctions of each transistor by installing a .01 cap across each one.

Being that this is a shunt regulator, it can be used in the cathode bias circuit on your typical triode amp too. Replacing the zener or string of rectifier diodes we often see. This idea of using a transistor to buffer the current load on a smaller zener also improves the degree of regulation and reduces thermal drift on the zener. The MJW3281A transistor is available online from Allied Electronics at about $2 each. I'd only use the Radio Shack transistors for low voltage cathode biasing on smaller amps.

 

[Crusher]

That's good stuff right there. Thank you everyone and just in time. I picked up a homebrew amp that was built by one of the better known CB type amp builders. It is a single 4cx250B amplifier. Has 2500vdc on plate. Amp is unstable to say the least. 512vdc on screen and bias is adjustable from 60-135V. The way the bias and screen are done, is 2 AC lines come in to full wave bridge rectifier. The positive side goes to a cap and is the screen, the - side is the bias. The - of cap cap goes to 10k resistor. Other side goes to positive of cap. Its a mess considering that tube only needs 325vdc for screen. More to come later. But I got a little work in front of me.

 

[Shockwave]

Instability can unusually be traced down to one of two problems in the tetrode amp. Either oscillations caused by improper neutralization or instable screen voltage due to no shunt regulator. Oscillations are usually noted as abrupt shifts in power or current. Often associated with small changes in the plate tune adjustment. Where screen voltage problems can often be identified as showing a slow rise in output power the longer you are keyed. As secondary emissions occur within the tube, this drives the screen voltage higher and the power output follows.

 

[Crusher]

The amp I have, as soon as it is turned on, goes right to 515V instantly. Think something let loose, but this thing is ugly. Gonna get new filament transformer and i have a seperate 250vac/50vac dual secondary transformer i am gonna use. the only regulation, is a 10k ohm resistor from screen+ to chassis. there is nothing else. case is so tight wont even fit a t0-3 transistor in amp. Might be able to to use a string of zeners. The bias uses some transorb style zeners, rated at 28.5v. 3 in parallel and 2 sets in series. bias goes from 60-135v.

 

[Shockwave]

I hear you on the little room to work with inside. I've been tasked with upgrading a few of these 250B boxes to add regulators in cramped areas also. With cases like this I add the heatsink on the back of the amp and mount it with angle aluminum to add standoff clearance to mount the required parts on the heatsink. You can put your transistors, zeners, and bypass caps all on the outside heatsink this way.

 

[Crusher]

I found the mjw3281A npn transistors you were talking about. They are in a TO-247-3 case. Vceo -230v max. might have to run 3 in series and figure out a good zener to use. might use 100v, will give me 300v. As the max screen is 325. Rather give it just a tad less. Bad enough 2500V on plate. So it already has extra gain to say the least. But it least in this style case, easier to mount in small compact area.

 

[Crusher]

I didnt find the MJW npn transistor, but i did find a FJL6920TU ( 800V @200W dissipation). Figured i'd use 2 150V@5W zeners stacked and use that with the pass transistor. As you said, should only take 1 to do the job. I really do appreciate your help on this.