[dude_500]

Not exactly an audio device, but I figured it is essentially a really high power amplifier and I knew it would require tubes for this voltage, so why not ask here. I'm looking to create a modulator that will take a -50kV negative input and modulate it to up to 100mhz from a wave-form generator. I would want it variable frequency from the generator, but I presume any amplifier design for this situation would be. The current demands are in the range of 0-50mA with 50 being a peak, average operation in the 10-20mA range.

Does anyone have any tips on what tubes to look at as a basis for beginning the design? And yes, I know what I am doing with these voltages and have been using them for a while, I'm just not sure how to modulate it.

 

[Dougigs]

So you're building a 2,500 watt FM transmitter aimied squarely in the middle of the commercial band (100MHz). Does the FCC know about this?

Since you have experience with this, you'll know that any of the larger water-cooled triodes will do the trick for you. Check any broadcast-tube manual (there are several online). These are often available on the surplus market for a few thousand each... though the delivery will likely set you back more than that.

 

[Uncle Erik]

I can't comment on the technical aspect, but this is a killer project.

If you pursue it, please post updates - I'd love to see how you implement it.

As for the FCC, yes, I think they'd be interested. You're allowed to build one transmitter per year. Unfortunately, an amateur radio license will only allow 1,500W of output. I think you'd be OK with a GROL.

Again, very cool!

 

[dude_500]

I can't comment on the use right now, but it's not an FM transmitter. Let's start the guessing game! lol

I'll be sure to come post how it turns out, hope to be done in the next 9 months but the budget is limited and I'll need to research a lot about high power tubes.

 

[Dougigs]

If you're modulating a 100MHz signal with another frequency, then it is an FM transmitter, no matter what other role it may play, and you'll have to be very careful in your design (is it HF welding?) to avoid having the FCC burst into your house or workplace.

You can try something like this: http://www.g8wrb.org/data/Svetlana/pdf/3CX15000A3.pdf - - widely used for such purposes, and sold by a number of suppliers. It's half your desired supply voltage (and not capable of the 2,500 watts peak you desire) but is forced-air cooled... remember that aside from your HV supply, you'll need a 160 amp, 6.3 volt filament supply for these babies.

 

[Dougigs]

Actually, here's a n ice guide to all the major relevant triodes... as you can see, your specs are taking you into the water-cooled end of the chart. But some fun reading there.

Data sheets on RF Power Triodes

 

[dude_500]

I can't just modulate the primary of my 120>50KV 500ma peak transformer can I? I presume at such a high frequency the core wouldn't magnetically saturate enough to produce any current. Am I correct in this thought?

Thanks for the tube links, I'll check that out

 

[Fixz8]

Assuming it's an iron-core transformer, it isn't designed for operation at that high of a frequency. The transformer windings will eat the modulated signal due to skin-effect copper-loss, and the core will eat the rest of it due to eddy-current losses in the core. After that, it might explode.

is the 500mA peak on the 50kV side of the transformer, or the 120V side?

 

[dude_500]

It's on the 50kv side, of course I never run it that high because I don't have that kind of power access in my house

 

[dude_500]

It looks like thyratrons are designed for switching the positive anode side. Is there any way to switch a high voltage negative polarity with a thyratron setup? It would be unsafe to leave the high voltage negative connected always and modulate the ground, I need to leave ground connected and modulate a negative polarity of about 50kv.

I essentially need a p-channel thyratron correct? But they are only made in the n-channel configuration it seems.

 

[dbfreak]

Something like these?

Specs in PDF format available upon request.