i like genuine mrf454's, 20a & 250w pdis,
they make more power & hold together better than the 455, 1446 or 2290 in the right circuit.
they make more power & hold together better than the 455, 1446 or 2290 in the right circuit.
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2SC2879 vs. 2SC3240
- Thread starter Shockwave
- Start date
Aren't the MA/COM 454's the exact same as the original MRF454? I never dug into it, but have replaced pairs of bad Motorola MRF454's with the MA/COM's and they did great, exact drop in replacement. I like transistors that are stable. MRF454's and SD1446's seem to be pretty stable in my experimenting.
My only experience with buying transistors from eBay that ship from China, was that I had a IC-706 non-MK that needed both the VHF PA and HF PA repaired. I bought a MRF255 (for the HF PA) for $50 from the most reputable seller I could find at the time (Polida? something like that), and a MRF5015 or whatever it was (funny shaped 10 or 15W Bipolar VHF PA transistor) for about $15. The MRF255 was a fake, barely worked at all, the 5015 or whatever on the other hand worked better than new, put out about 22W and I ran it hard, never failed.
I hear what you are saying though about the oddball Mitsu part and demand. Just hate buying any obsoleted transistors from China sellers.
I hear what you are saying though about the oddball Mitsu part and demand. Just hate buying any obsoleted transistors from China sellers.
I just realized that the MRF454 has a collector - emitter breakdown voltage of 25 volts! With 7 more volts of "headroom" than the 2SC2879, no wonder the MRF454 is so rugged. Even the dissipation is the same 250 watts that the 2SC2879 has. The only weak link with the MRF454 is the 20 amp collector current, being 5 amps less. Therein lies the key to your statement regarding "in the right circuit".
Isn't it ironic how the best RF transistor you could buy 35 years ago, is also the best one to still survive and be in production today? In the mid 1980's, most of the biggest HF solid state amps were using the MRF454. Several years later when the Toshiba hit the market, it really didn't take too much work to get those old circuits to match up with the new part.
Therefore, it would be very easy to go back one step and rework 2SC2879 amplifiers, to increase their input and output impedances to once again, match the MRF454. This makes me wonder if there is not a market to replace the 4 junk, TO-220 transistors we see in high power exports, with a matched pair of MRF454's? I can't imagine it would be too hard to fit them on the board, rewrap the input and output transformers and then scrub off 75% of the bias voltage with a 4:1 resistor divider...
From a guy who knows he has no business inside a radio case (nevertheless) , Wow!
That’s a possibility I hope pans out.
It seems like this 2SC3240 transistor is very popular with Japanese CB'ers. I found several videos on Youtube showing everything from 8 transistors doing 2.6 kw, all the way to a 48 transistor amplifier. Most of them had horrible RF layout with like 8 feet of unshielded, non Teflon wire feeding the combiners.
The link below was the best build quality out of the ones I viewed. We can see he is using 3 wraps on the input transformers with a 1000pf cap across the base secondary winding. Also notice the output caps are one on each transistor, that go from the collectors to ground rather than across the collector primary winding. Now for the bad news, all of the discussion he provides is in Japanese.
The link below was the best build quality out of the ones I viewed. We can see he is using 3 wraps on the input transformers with a 1000pf cap across the base secondary winding. Also notice the output caps are one on each transistor, that go from the collectors to ground rather than across the collector primary winding. Now for the bad news, all of the discussion he provides is in Japanese.
I think that's the same transistor that Contra Electronics used in the final version of their cp-100 amplifier,
early versions used mrf454 then 2sc2290 & the last version used the strange shaped mitsubishi 2sc32xx, The best 50w rated cb amp available over here in the 80's.
I have one here that I had given with no transistor due to been wired backwards & the guy been unable to find a replacement mitsubishi,
I used srf3749 which works about the same in that circuit. 100w or a little more FM @14V.
early versions used mrf454 then 2sc2290 & the last version used the strange shaped mitsubishi 2sc32xx, The best 50w rated cb amp available over here in the 80's.
I have one here that I had given with no transistor due to been wired backwards & the guy been unable to find a replacement mitsubishi,
I used srf3749 which works about the same in that circuit. 100w or a little more FM @14V.
He was mostly complaining about issues with the transistor mounting. Also some complaints about the negative feed back circuit and biasing. I've gotten a little rusty since I lived there 40 years ago.
73
Dr_DX
This forum never ceases to amaze me. I wasn't expecting to have a member able to translate the bulk of that video and thank you for this input. I'm not really seeing any difference in this transistors mounting as it seems to have the same mounting tab and pinout of the 2SC2879. Bias idling current is probably different and with almost none of our circuits having any adjustability, that may need to be added. Since it has less gain than a 2879, it may require a lot of drive in class C or with low bias.
The negative feedback used here on the other hand, is more aggressive than the resistor values we see in most other circuits. Values used seem to be anyplace from the high 30's, up to 68 ohms, as opposed to the standard 100 ohms. That confuses me since less stage gain, should mean less negative feedback is required... One thing is for sure. With its higher collector dissipation and breakdown voltage, it does appear to be the most powerful 12 volt HF, RF bipolar transistor ever made.
The negative feedback used here on the other hand, is more aggressive than the resistor values we see in most other circuits. Values used seem to be anyplace from the high 30's, up to 68 ohms, as opposed to the standard 100 ohms. That confuses me since less stage gain, should mean less negative feedback is required... One thing is for sure. With its higher collector dissipation and breakdown voltage, it does appear to be the most powerful 12 volt HF, RF bipolar transistor ever made.
The mitsubishi transistors are the same tab & pinout as 2879's,
I can't understand a word he says apart from transistor & tapping,
Watching him drawing he seems to be talking about how the transistors are screwed to the heatsink & the problem with heat causing the tabs to bow so the tab loses contact with the heatsink, common to see in our amps that are run flat out on FM,
its not just bowing that occurs, the the round part of the tab goes concave,
I have sanded dozens of bent & convex tabs flat using grades of wet & dry paper on a sheet of plate glass,
European amps use sheet metal screws that distort & pull material out of the hole causing major seating issues if you don't countersink the holes & check flatness before you refit the transistors,
Japanese made cb amps I have owned always use tapped threads & machine screws with spring washers,
they don't cause a mole hill around the screw holes
The two caps are collector emitter caps, He has no compensation cap on the output transformer primary windings,
I don't know what he says about the bias tracking diodes but they should be central on the transistor & some thermal compound on them, Japanese amps & hf sets always use compound on the diodes,
The cap & resistor feedback setup he draws does the opposite of what a feedback network should be doing to flatten gain & impedance excursions as you move down in frequency,
proper amps use inductors in the feedback network to increase feedback,
I wish I could understand what he's.
I can't understand a word he says apart from transistor & tapping,
Watching him drawing he seems to be talking about how the transistors are screwed to the heatsink & the problem with heat causing the tabs to bow so the tab loses contact with the heatsink, common to see in our amps that are run flat out on FM,
its not just bowing that occurs, the the round part of the tab goes concave,
I have sanded dozens of bent & convex tabs flat using grades of wet & dry paper on a sheet of plate glass,
European amps use sheet metal screws that distort & pull material out of the hole causing major seating issues if you don't countersink the holes & check flatness before you refit the transistors,
Japanese made cb amps I have owned always use tapped threads & machine screws with spring washers,
they don't cause a mole hill around the screw holes
The two caps are collector emitter caps, He has no compensation cap on the output transformer primary windings,
I don't know what he says about the bias tracking diodes but they should be central on the transistor & some thermal compound on them, Japanese amps & hf sets always use compound on the diodes,
The cap & resistor feedback setup he draws does the opposite of what a feedback network should be doing to flatten gain & impedance excursions as you move down in frequency,
proper amps use inductors in the feedback network to increase feedback,
I wish I could understand what he's.
Perkin Elmer had a similar issue with the RF exciter deck used in a Gas Chromatograph Mass Spectrometer. This RF generator used an MRF422 to drive an 8877. The MRF422 shares the common 211-11 package as the 2SC2879 but has a total dissipation of 290 watts! Under sustained carrier in class AB, they would distort the mounting flange no matter how robust the heatsink was.
The fix was to install a small 3/8th inch thick copper spreader under the transistor. It was surprisingly small, about the size of a book of matches. That increased the reliability of the exciter "drawer", to the point where the only real concern was the unreliable 1980"s 3CX1500A7 supply. It got so bad with defective tubes failing in the middle of critical test procedures, that the entire RF generator line, was redesigned around a different tube.
I must have been typing half asleep convex/ concave, The tabs go concave with heat,
When you rub them on a flat abrasive surface you are only sanding the outer rim of the round part & the 2 lugs, new transistors start out flat or very close to flat,
Nobody I know over here bothers to check how flat the tabs are, If the transistor measures ok they blob compound on & use it, not much good if you can see light between tab & sink which is often the case,
sanding works, refitting them & not addressing the screw holes & tabs makes for an amp that won't last long abused on FM,
Not sure what the tabs are made of, it looks like copper under the plating,
It makes perfect sense to use a copper spreader with similar expansion & much improved thermal conductivity,
The only time I have seen copper used in CB amps was my CCM using copper heatsinks in his keydown amps.
When you rub them on a flat abrasive surface you are only sanding the outer rim of the round part & the 2 lugs, new transistors start out flat or very close to flat,
Nobody I know over here bothers to check how flat the tabs are, If the transistor measures ok they blob compound on & use it, not much good if you can see light between tab & sink which is often the case,
sanding works, refitting them & not addressing the screw holes & tabs makes for an amp that won't last long abused on FM,
Not sure what the tabs are made of, it looks like copper under the plating,
It makes perfect sense to use a copper spreader with similar expansion & much improved thermal conductivity,
The only time I have seen copper used in CB amps was my CCM using copper heatsinks in his keydown amps.
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