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Old 5th March 2012, 05:45 AM   #1
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Default RF Voltage Amplifier

Guys anyone have an idea of the right way to make a high frequency voltage amplifier using a pentode?

I am using 1 6146B beam power tube - the lab has given me this, i cant change it. and also a dc power supply of 460V. Following the book "Valve Amplifiers" by Morgan Jones i built an amplifier for 18 gain. I simulated this in LTSpice, it gives just 3.2 gain. The circuit and the pentode curves are in the attached figures. I have been calculating and reculculating for the past few days without avail.
Operating point=200V,100ma
Frequency=20MHz
Attached Images
File Type: png VAmp.6146.png (13.5 KB, 183 views)
File Type: png 6146B-Curves.png (14.9 KB, 177 views)

Last edited by marcusbritanicus; 5th March 2012 at 07:08 AM. Reason: added more info..
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Old 5th March 2012, 08:20 AM   #2
Doz is offline Doz  United Kingdom
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Possibly because you've built an audio amp, and are expecting it to function at 20MHz!

Have a look here : Lineairs

Take as a start the EL519 single amp, and look at the output "tank" circuit.
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Old 5th March 2012, 09:54 AM   #3
DF96 is offline DF96  England
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What is the application? Do the people who have insisted that you use a 6146B know what they are doing? BTW I think that is a tetrode, not a pentode. Frequency, bandwidth, linearity, required efficiency? How much experience do you have of RF power design and construction? Any RF design and construction?

I don't want to rain on your parade, but I fear this project will end in tears. Even experienced people sometimes have trouble with valve RF PAs. You don't sound experienced.
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Old 5th March 2012, 11:42 AM   #4
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Hi Marcus. The power amplifier you ask, has some drawbacks. First, normally the act in resonant plate load, in which RDC is very low compared to RAC, so the model fails, also usually in RF circuits there exists a way to adjust the optimum plate load, example, a PI network, a tank with a link, etc.

Second, in normal operation, it works in Class C, that is, the plate current flows during a short period of time compared to the cycle. In that condition plate wasted power is much lower than in Class A. This, except in linear operation (For SSB) in where it works in Class AB usually.

And third, in power amplifier it is often more important power output or plate yield than signal gain. Finally, the power output also depends on frequency, and Q of the plate loading.

Good luck.
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Old 5th March 2012, 12:22 PM   #5
TheGimp is offline TheGimp  United States
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Here is a link to an article that is a pretty good expatiation of how an RF amp works.

ftp://ftp.ham.hr/Books/QEX/QEX20013.pdf

Start on page 32.

Here is a SE output class C RF amp with the schematic, and full details of the build.

Note the tuning network C3, C4, L3, L4 with L4 having taps for gross tuning.

The AA8V 6146B Amplifier - Amplifier Schematic Diagrams and Circuit Descriptions
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Old 6th March 2012, 05:07 AM   #6
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Hey guys... I want to build a voltage amplifier, not a power amplifier. This is a physics project, the amplifier takes an input of 10V at 20MHz and should be able to give a 200V output at the same frequency. It is to be used as a driver for an electro-optic modulator. Could you please guide me on this?

I do not have any experience in RF design and construction.
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Old 6th March 2012, 05:40 AM   #7
kevinkr is offline kevinkr  United States
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You'll still need a tank circuit on the plate of that pentode tuned to 20MHz, and you need to know the impedance of what you are driving - despite the fact that you claim to be building a voltage amplifier you will need to deliver some power to the load in order to provide the 200V swing you need. First you need to figure out what the impedance of your electro-optic modulator and what are the components of that impedance both real and imaginary (reactive) - that will determine whether or not you need to deliver significant power to it. The fact that they gave you a specialized RF beam power tetrode is probably NOT a coincidence. You probably are building a linear amplifier... Take a look here: 6146B Beam Power Tube and Data Sheets
(Lots of RF power amplifier design information here)
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Old 6th March 2012, 10:46 AM   #8
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This should not be too complicated. Just let us know the output impedance of the signal source and the input impedance of your load ( electro-optic modulator ) you use. Are both standard 50 ohms ?
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Old 6th March 2012, 11:00 AM   #9
DF96 is offline DF96  England
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Does it have to go down to DC i.e. are you building an RF amp or a wideband video amp? The circuit techniques would be different, especially in the anode circuit.
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Old 6th March 2012, 11:09 AM   #10
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He told: "the amplifier takes an input of 10V at 20MHz "

As soon as I know the input and output impedances, the case is not so difficult.
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