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#1
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solid state 6m amplifier
Hello,
having seen that's really difficult to make a working MOSFET linear amplifier for 6m, I started looking for a bipolar design and components. The MRF448 transistors seem very expensive. Can anyone suggest an alternate part or a different approach (not tubes, I wanted to put this amplifier at the antenna)? thanks Francesco IS0FKQ |
#2
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#4
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Try the ARRL website under TIS. It's called the "Compact 6M brick".
It was published in the OCT 1990 QST. The circuit is very close to the Motorola test circuit for the device with bias for linear operation. FAR Circuits has the board for it. I think they can supply a reprint as well. If thats not enough power two of them with 2 port splitter/ combiners will get you better than anywhere from 150-180W. A Wilkinsen splitter and combiner is fairly easy to do at 6m either as lumped elements or maybe even coax. Allison KB1GMX On Fri, 16 Sep 2005 23:11:48 GMT, "Dan/W4NTI" wrote: Where can I find such a design? Dan/W4NTI wrote in message .. . On 16 Sep 2005 07:16:33 -0700, wrote: Hello, having seen that's really difficult to make a working MOSFET linear amplifier for 6m, I started looking for a bipolar design and components. The MRF448 transistors seem very expensive. Can anyone suggest an alternate part or a different approach (not tubes, I wanted to put this amplifier at the antenna)? thanks Francesco IS0FKQ MRF492A usually under 22$US new and good for 70W or more. There are published designs using it that work well for all modes. Most of the VHF power mosfets are priced around $1US/Watt it seems and are dreadfully expensive compared to bipolars. Allison KB1GMX |
#5
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Thanks, I'll check things out.
Dan/W4NTI wrote in message ... Try the ARRL website under TIS. It's called the "Compact 6M brick". It was published in the OCT 1990 QST. The circuit is very close to the Motorola test circuit for the device with bias for linear operation. FAR Circuits has the board for it. I think they can supply a reprint as well. If thats not enough power two of them with 2 port splitter/ combiners will get you better than anywhere from 150-180W. A Wilkinsen splitter and combiner is fairly easy to do at 6m either as lumped elements or maybe even coax. Allison KB1GMX On Fri, 16 Sep 2005 23:11:48 GMT, "Dan/W4NTI" wrote: Where can I find such a design? Dan/W4NTI wrote in message . .. On 16 Sep 2005 07:16:33 -0700, wrote: Hello, having seen that's really difficult to make a working MOSFET linear amplifier for 6m, I started looking for a bipolar design and components. The MRF448 transistors seem very expensive. Can anyone suggest an alternate part or a different approach (not tubes, I wanted to put this amplifier at the antenna)? thanks Francesco IS0FKQ MRF492A usually under 22$US new and good for 70W or more. There are published designs using it that work well for all modes. Most of the VHF power mosfets are priced around $1US/Watt it seems and are dreadfully expensive compared to bipolars. Allison KB1GMX |
#6
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We pirates have made the common, ordinary IRF 510 MOSFET work much
higher than the 6M ham band-like at 88MHZ! For several months WSQT Guerilla Radio(The Squat in DC) was on 87.9 at 10W with a single IRF 510 as the final. Details are in an old post to alt.radio.pirate. We later went up to a dual 6146 tube final for more range, and are now relocating because FEMA didn't like our Katrina coverage and whined to the FCC ,who only seem to trouble us around things like Katrina or the Inauguration. Four months of 10W and 3 at 40-50W with no trouble, but 1 1/2 weeks of Katrina coverage and they came running! Well,we'll soon be back, but meanwhile I figured you guys would appreciate some design info. Never have I found any article on the IRF 510 at VHF, so I did all my own research. Although I understand many hams don't like pirates, we read your newsgroups and manuals to learn how to build decent transmitters that don't pur out spurs and hash, and I figured maybe you guys would enjoy a return of the favor. Anyway, I've gotten 13 watts from one IRF 510, with a little over 1 watt of drive. This is with tuned input and output circuits, not broadband. I've heard others have broadbanded these things that high, but I can't possibly see how with all that input C! I've never sen an article on how they did it, only a comment to my original research data and article an alt-reaido.pirate. That 13 watts could in turn be used to apply greater drive to another 4 of these things either in parallel push-pull(possible oscillation problems but I've done this with the 2N2222 for a driver), or with resonant 3 winding combiner transformers. Estimated output: 60 watts if you get it right! Given that I can make the IRF 510 operate at 88MHZ, it should operate at considerable greater efficiency and gain(better input Q and higher capacitive reactance)at 50-54 MHZ. What I want to know is this: can any of you guys one-up me and get at least 8W from one IRF 510 on 2 meters? If we pirates can make this device operate at 87.9MHZ, surely one of you can get it to work at 144! Best thing about the IRF 510-it costs less than $2 and is available at any Radio Shack! |
#7
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On 28 Sep 2005 19:04:34 -0700, "WSQT" wrote:
We pirates have made the common, ordinary IRF 510 MOSFET work much higher than the 6M ham band-like at 88MHZ! For several months WSQT Guerilla Radio(The Squat in DC) was on 87.9 at 10W with a single IRF 510 as the final. Details are in an old post to alt.radio.pirate. We later went up to a dual 6146 tube final for more range, and are now snipped Several questions. Operating mode? Class A, B, C, D, E? Stage gain? Answer makes a difference. SSB required at worst class B and class C would cause way to much distortion. While FM is typically class C as there are no amplitude variations. This makes a great difference is operating characteristics and power out. Since SSB and most amateur uses are narrow band by nature any spurious outputs are undesired. Amps using the IRF510 at HF have been built but their purity (IMD and harmonics) is only adaquate to barely so and tends to degrade as frequency is increased. FYI: to feed an IRF510 as broad band you need to match the 50ohm input to less than 2 ohms reactive at VHF. This is hard to do for greater than octave frequency ranges as it is all reactive. With that I've run an IRF510 at 6m (50mhz) linear (class ab1) with a stage gain of around 9db and with 24V around 4W. It was difficult to match, unstable tending to oscillate, and prone to thermal runaway (case has high termal resistance). A scrap final from a CB (2SC1307) gave me 7W, clean at 12V and far less stability and matching issues plus cleaner IMD. For the same effort a MRF492 at 50mhz easily delivers 90w at 12V and 11db gain as linear. That same 2sc1307 class C will deliver 12w and would drive a MRF492 to 100w class C. To put it bluntly, for the effort to get 10W out of a IRF510 I can do far better with a 22$ transistor and still be ahead. Allison KB1GMX |
#8
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I run the IRF 510 essentially at the "class B threshold"(leakage idling current but nowhere near linear range), simply because I could not drive it hard enough to run it without bias like in the AM BC band transmitter(Oct 03-Jan 05). Envelope distortion is not a factor when frequency modulation is in use, of course. Stage gain with 1W drive and 10-13W out is essentially 10 db-not great, but I've seen a hell of a lot of RF transistors speced with gain from 10-13db. If you want to use the IRF 510 for SSB, I have no idea what the IMD will be,but I've used them at audio as modulators for the old AM rig. If anything, your stage gain will be the same or greater. I used 3V bias, generating leakage current only in the FM rig. For good linearity at audio(and presumably for SSB as well), you need to bias them further forward than threshold(they are NOT like darlingtons at audio for this!), and you will get more heat. I've had them on the scope at audio with sine wave drive, and they needed to go well into AB1, so much so that they made measurabgly more heat than darlington bipolars did for an audio amp. No way will they be MORE linear at RF than audio, so run them in AB1 if you want AM linear or SSB. CW or FM, just set the bias pot for maximum OUTPUT, and you will get about 3V when running in the VHF range, as turn-off time seems to lower output with more bias. As a result, they like to be "centered" between "on" and "off" for maximum efficiency, which still sucks at VHF. I would not be entirely suprised if an IRF 510 amp proves a CW and FM only deal at 6M, and I would bet on it at 2! On the other hand, efficiency at 6M will be better than at 3M, so you should be able to stay within acceptable dissipation, even if AB1 bias is CUTTING output with the same drive as "threshold B" bias. ****-SSB's a low duty-cycle mode, while at FM the duty cycle is 100%! This should compensate for the extra heat of AB1 operation necessary to make the IRF 510 linear at ANY frequency, even audio. When I used the IRF 510 in the AM band as a drain-modulated Class C pair, they could make 55W carrier but only sustain 35-40W carrier for 30 minutes operation. Years ago, there was a well-published design for a broadband HF aming this device, accepting as much as 80W pep at 40M! Spurious outputs, BTW, are as undesired for what we do as they are for amateur work. If you get spurs and they bring the FCC, you can probably resolve the issue. For a pirate, spurs can cause mean MUCH more serious "difficulties!" I have a lot more trouble chasing them out of exciters than amps, but always check out both very carefully. Voltage used in the FM rig is 30V, the AM rig was 17 volts B+, going to 34 at 100% mod. With tuned input and output circuits stability is excellent, and unlike multi-2N222 drivers I've never had to chase hash or oscillation with the IRF 510. Of course, a big bulky heatsink serves as a groundplane, with all parts screwed to it. Glad to hear I'm not the only one to push the IRF 510 above 30MHZ! Several questions. Operating mode? Class A, B, C, D, E? Stage gain? Answer makes a difference. SSB required at worst class B and class C would cause way to much distortion. While FM is typically class C as there are no amplitude variations. This makes a great difference is operating characteristics and power out. Since SSB and most amateur uses are narrow band by nature any spurious outputs are undesired. Amps using the IRF510 at HF have been built but their purity (IMD and harmonics) is only adaquate to barely so and tends to degrade as frequency is increased. FYI: to feed an IRF510 as broad band you need to match the 50ohm input to less than 2 ohms reactive at VHF. This is hard to do for greater than octave frequency ranges as it is all reactive. With that I've run an IRF510 at 6m (50mhz) linear (class ab1) with a stage gain of around 9db and with 24V around 4W. It was difficult to match, unstable tending to oscillate, and prone to thermal runaway (case has high termal resistance). A scrap final from a CB (2SC1307) gave me 7W, clean at 12V and far less stability and matching issues plus cleaner IMD. For the same effort a MRF492 at 50mhz easily delivers 90w at 12V and 11db gain as linear. That same 2sc1307 class C will deliver 12w and would drive a MRF492 to 100w class C. To put it bluntly, for the effort to get 10W out of a IRF510 I can do far better with a 22$ transistor and still be ahead. Allison KB1GMX |
#9
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Cut to the chase. The only thing the IRF5xx series has going for it is it's dirt cheap. The problem is that coaxing them to deliver is more pain than better devices. Good devices in reasonable circuits have one advantage, repeatability of performance. As to MW am service, I'd use the heavier devices and run them class E in the several hundred watts to KW range. Even the IRF510 in class E is good for more than 50W for a pair and should run reasonably cool. Even the really big E amps only need a few watts as excitation. This works because they are being used as switches, what they were designed to do. A class E amp can be "plate modulated" in the usual way but a PWM modulator will be far more efficient. There are a number of people running class E Am phone on 160, 75 and 40m and they sound very good. For decent RF designs that withstand impossible swrs a good RF rated device is hard to beat. The real problem is finding a source that doesn't mark them up 500%. However there are some devices out there that are like the venerable 6146 they do the job and can be had at less than 0.40$/watt. For 6m the MRF492 for example rated power out at 12V of 70W and most deliver 80-90. For the price it's cheap. For 2m I'd just grab a MRF247, easy 75watts at 2m class C or B and a pair with Wilkerson splitter/combiners deliver a heafty 145watts on the Bird. They work well for 2m SSB service with decent IMD. the last pair(matched) I bought as for a commercial amp repair only cost 70$. For that kind of power that's cheap. Allison KB1GMX |
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