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Just out in the shed rummaging through some bits 'n pieces, and came across an old commercial VHF amplifier, almost intact.
The thing that caught my eye was an MRF492, which is the final output device.
Anyone had experience in using these devices in 6 meter amplifiers? They are good for about 70 watts or so.
Before the 'Gee it's an old piece of gear' brigade starts up, yes I know its old but, I am not interested in a commercial amp, I am going to home brew my own.
Just looking for past experiences from those who have been there, done that.
Ron
VK4BVI
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Ron
From a data sheet :
The MRF492 is a 12.5 V low band VHF large-signal power amplifier applications in commercial and industrial FM equipment.
That is not to suggest that you couldn't run it in AB mode so that it is fairly linear for SSB use but it was designed for FM use. I didn't find any full specs on a quick search but it might be difficult to get the input and output impedance matching networks designed for best operation without the full data sheet.
What are you going to drive this device with ?
Specs suggest 11d-13dB stage gain so about 7-10 watts of RF drive required from your 6M driver, allowing for some losses due Z mismatches..
Most modern HF/6 rigs already do around the 100W mark on 6M, though I do have a modified 27MHz CB that is channelised on 50MHz SSB and I have a modified CB linear for after that. Pity I haven't had power on it for at least the last 5 years as it was great for low-power-consumption monitoring of 50.110 at the old QTH. (
https://www.vk4adc.com/web/vhfuhf-projec...conversion )
Doug
Doug VK4ADC @ QG62LG51
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Doug
Thanks for the feedback.
I am using my trusty TS-680S (10 - 12 W. output) as the driver for the proposed amplifier.
Having a trawl through the internet, I came across this article from DL5DBM which holds promise. (Google is still getting a bashing looking for information!).
http://dl5dbm.darc.de/6mpa100.pdf
This is a definite home brew project. ALL parts will be sourced from the junk box.
Ron
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Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
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Doug
I still work for a living
, work gets in the way of my amateur radio life.
The junk box is coming up trumps for the various bits, so maybe it will.......not be ready for the field day.
Thanks for the feedback.
(Another couple of posts to keep the forum ticking over)
Cheers
Ron
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Hi Ron
The MRF492 is rated for a maximum output of 70W. It loses linearity if used above this power level.
I have modified DSE K6349 100W PAs that use two MRF492s from 100W to 130W output. This involved replacing the PA coil with a much heavier gauge wire and likewise the filter coil and capacitors. It also greatly improving the bias circuit which is not just a simple diode based one.
A single MRF492 will not get you a clean 100W especially if a simple diode based bias circuit is used as per the DL5DBM article.
73s
Igor
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(02-12-2018, 08:47 AM)VK6ZFG Wrote: Hi Ron
The MRF492 is rated for a maximum output of 70W. It loses linearity if used above this power level.
I have modified DSE K6349 100W PAs that use two MRF492s from 100W to 130W output. This involved replacing the PA coil with a much heavier gauge wire and likewise the filter coil and capacitors. It also greatly improving the bias circuit which is not just a simple diode based one.
A single MRF492 will not get you a clean 100W especially if a simple diode based bias circuit is used as per the DL5DBM article.
73s
Igor
Hi Igor
The transistor being used in the amplifier is now a 2SC2879. After receiving some excellent advice I discarded the MRF492.
Between natural disasters, I am wiring up the switching relays, so almost ready to go.
73
Ron
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(13-12-2018, 04:09 PM)VK4BVI Wrote: (02-12-2018, 08:47 AM)VK6ZFG Wrote: Hi Ron
The MRF492 is rated for a maximum output of 70W. It loses linearity if used above this power level.
I have modified DSE K6349 100W PAs that use two MRF492s from 100W to 130W output. This involved replacing the PA coil with a much heavier gauge wire and likewise the filter coil and capacitors. It also greatly improving the bias circuit which is not just a simple diode based one.
A single MRF492 will not get you a clean 100W especially if a simple diode based bias circuit is used as per the DL5DBM article.
73s
Igor
Hi Igor
The transistor being used in the amplifier is now a 2SC2879. After receiving some excellent advice I discarded the MRF492.
Between natural disasters, I am wiring up the switching relays, so almost ready to go.
73
Ron
Hi Ron
The 2SC2879 is a better transistor but it is listed as a 2-30 MHz device so may not work too well at 50 MHz.
73s
Igor
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(14-12-2018, 09:41 AM)VK6ZFG Wrote: (13-12-2018, 04:09 PM)VK4BVI Wrote: (02-12-2018, 08:47 AM)VK6ZFG Wrote: Hi Ron
The MRF492 is rated for a maximum output of 70W. It loses linearity if used above this power level.
I have modified DSE K6349 100W PAs that use two MRF492s from 100W to 130W output. This involved replacing the PA coil with a much heavier gauge wire and likewise the filter coil and capacitors. It also greatly improving the bias circuit which is not just a simple diode based one.
A single MRF492 will not get you a clean 100W especially if a simple diode based bias circuit is used as per the DL5DBM article.
73s
Igor
Hi Igor
The transistor being used in the amplifier is now a 2SC2879. After receiving some excellent advice I discarded the MRF492.
Between natural disasters, I am wiring up the switching relays, so almost ready to go.
73
Ron
Hi Ron
The 2SC2879 is a better transistor but it is listed as a 2-30 MHz device so may not work too well at 50 MHz.
73s
Igor
Hi Igor
I did note that on the datasheet, but it was in the junk box, so nothing to lose. All in the name of experimentation.
Thank you for your interest in the project.
73
Ron
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wonder how the amp building went i am very interested
cheers John
VK5KAF
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(20-02-2019, 01:29 PM)VK5KAF Wrote: wonder how the amp building went i am very interested
cheers John
VK5KAF
Well the amp is finished, complete with case, but I haven't had much time to finish wiring the relays.
So far the bias supply works, but haven't applied power or RF to the actual amplifier.
Work and domestic duties keep interrupting my radio routine.
I will try and make an effort to finish the wiring this weekend, and give it a smoke test.
Welcome to Queensland, beautiful one day, natural disaster the next, and another one coming.
Cheers
Ron
VK4BVI
