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Question about vaccum tube Fmax

tip120

Sep 24, 2010
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I'd like to build a couple of RF amplifiers for different amateur radio bands. However, I've never messed with vacuum tube equipment before, and I have a question about the Fmax.

For most of the tubes I'm considering using, the datasheets have something like this:

Frequency for maximum ratings: 110Mc

http://w5jgv.com/4-400A_600M_amplifier/4-400A.pdf

My question is, what happens when you exceed 110Mhz? Is that the freq limit only with the maximum plate voltage/current? What about 75% plate voltage/current?

Anyone know how this works?
 

duke37

Jan 9, 2011
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There are several reasons why a maximum frequency is specified. Power valves like the 4-400A are large and so there is a significant inductance in the internal wiring of the valve. The input is to the grid which has a damping resistance which goes up with frequency and also has a capacitance to the anode which is more significant the higher the frequency.

The whole business is quite complicated but you could get more information from the ARRL handbook. If the circuit oscillates instead of amplifying, things can go badly wrong.

If you have never used high voltage circuits, remember that they have the advantage that they kill you only once.
 

tip120

Sep 24, 2010
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Sep 24, 2010
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There are several reasons why a maximum frequency is specified. Power valves like the 4-400A are large and so there is a significant inductance in the internal wiring of the valve. The input is to the grid which has a damping resistance which goes up with frequency and also has a capacitance to the anode which is more significant the higher the frequency.

The whole business is quite complicated but you could get more information from the ARRL handbook. If the circuit oscillates instead of amplifying, things can go badly wrong.

If you have never used high voltage circuits, remember that they have the advantage that they kill you only once.

I've done a lot of HV work in the past, and have worked with valves before as well, there are just some of the more advanced valve concepts that I've never needed to understand for what I've been doing. The reason for me wondering about this is I have a couple of 4-400As, and 4-1000As, and since these large broadcast tubes tend to be hard to find or out of my price range, I wanted to know if there was a (safe) way I could use these tubes to build a 2 meter linear amp, I just didn't know if going to 144Mhz when tubes fmax was 110 was a good idea.
 

duke37

Jan 9, 2011
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As the requency rises, the efficiency drops, partly due to electron transit time and partly due to inductance and capacitance in the valve. Going up from 110MHz to 144Mhz is not too big a step but the 4-400A would be happier at 30Mhz.

As the efficiency is lower, there is more dissipation in the valve so the HT voltage and current should be reduced. You will have difficulty in getting an efficient output matching circuit with the excessive output capacitance.

At 144MHz valves such as the 4CX250B are preferred as they are much smaller but need blown air cooling.
 
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