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Mosfet finals = not for SSB rigs? sound like sh**.
- Thread starter Staybolt
- Start date
Because something is not being done right either in the selection of the MOSFET itself or the installation or operation of them. MOSFETS have been used for years in amateur and commercial SSB equipment without any issues. Most of those however require either 24 or 50 volts to operate however. Not all but most.
What were you trying to gain with the conversion? There can't be that much of a difference in power output to make it worthwhile.
What were you trying to gain with the conversion? There can't be that much of a difference in power output to make it worthwhile.
I agree, all new radio's have mosfets and an RCI 2950 and Magnum 257 HP sound great on SSB. You must have done something wrong. There was a learning curve for the good cb shops several years ago. Through trial and error they got the mosfet mods down and now offer them as service sales work.
I am not a big mosfet fan but let's face it, we are all stuck with them like it or not. The first rule that too many people don't seem to get is you don't take a WORKING RADIO and try a mosfet "upgrade". This type of a mod is best used where say you blew the dual 1969's and can't find any at a fair price.
I am not a big mosfet fan but let's face it, we are all stuck with them like it or not. The first rule that too many people don't seem to get is you don't take a WORKING RADIO and try a mosfet "upgrade". This type of a mod is best used where say you blew the dual 1969's and can't find any at a fair price.
That was not a slam. I guess my point was when mosfets first came out guys like DTB & Doug at Custom cb had to do the trial and error thing until they discovered what worked. Even the forum members that like to tinker probably made some mistakes as well.
I think cbtricks.com has step by step mosfet conversions on their site someplace. The mod was probably posted by one of those guys that tinkered enough with it until it worked good.
I guess you can either keep at it and see what went wrong or just proclaim mosfets sound like sh** on SSB. That's up to you.
Since no one has mentioned it yet, I figured I would. The mosfets that C.K. mentioned in Ameture equiptment are designed to amplify RF.
The 520's, 530's, and the 2030 were NOT ment to amplify Rf. They are used in switching power supplys and will not tolerate any abuse whatsoever. I haven't heard of any of these bi-polar final radio "upgrades" pan out as in being equivilent to the bi-polar version radios. And if they did manage to get them to "work" they did so for only a short time and are "margonal" at best, Being that the operating circumstances remained perfect . They are extreemly finicky with the adjustments ( bias) And sensitive to SWR and if not set perfectly there will be issues.
Ever wonder why you can convert a bi-polar rig to mosfets, But Not the other way around?
Most of the guys over there on CBT gave up on those experements long ago. Many of them frustrated after "Doing everything right" and it still not come out right......And gave up.
I agree with all of the previous posts except one. The 1969 can be still had, and ruining these rigs with mosfets because of the price of replacement is blastfomous at best. The only mosfet that I have seen that produced the best results was the 7030 and even they are sensitive to otherwise inperfect operating cicumstances.
The three before mentioned mosfets are cheap, plastic transistors that do not conduct heat very well, and the only reason that they are being used is due to the cost per device since the bi-polar transistors went extinct and no readily available replacement. They can be made to function in place of the bi-polars but thats about it. They could never be as reliable, Or as robust as their predicessors were.
I am sorry to hear that you had bad experences with the conversions Staybolt, But you are not alone.
9c1driver I disagree, We are not all stuck with them. Those like myself that hold onto the older equiptment due to this issue are quite happy to do so. It is only those that want all the eye candy and bells and whistles of the newer products are stuck with them.
Nevertheless it is what it is. And what the real problem is here is instead of putting big $$$$ into all of the flashy crap designs in the new equiptment of today.....Why not design a mosfet that is ment to amplify Rf at 12-14 volts that could also be a realible replacement for the older bi-polars? If Eleflow can do it so can the radio manufactures.
I wish you luck in your future converstions.
73's
M.S.
The 520's, 530's, and the 2030 were NOT ment to amplify Rf. They are used in switching power supplys and will not tolerate any abuse whatsoever. I haven't heard of any of these bi-polar final radio "upgrades" pan out as in being equivilent to the bi-polar version radios. And if they did manage to get them to "work" they did so for only a short time and are "margonal" at best, Being that the operating circumstances remained perfect . They are extreemly finicky with the adjustments ( bias) And sensitive to SWR and if not set perfectly there will be issues.
Ever wonder why you can convert a bi-polar rig to mosfets, But Not the other way around?
Most of the guys over there on CBT gave up on those experements long ago. Many of them frustrated after "Doing everything right" and it still not come out right......And gave up.
I agree with all of the previous posts except one. The 1969 can be still had, and ruining these rigs with mosfets because of the price of replacement is blastfomous at best. The only mosfet that I have seen that produced the best results was the 7030 and even they are sensitive to otherwise inperfect operating cicumstances.
The three before mentioned mosfets are cheap, plastic transistors that do not conduct heat very well, and the only reason that they are being used is due to the cost per device since the bi-polar transistors went extinct and no readily available replacement. They can be made to function in place of the bi-polars but thats about it. They could never be as reliable, Or as robust as their predicessors were.
I am sorry to hear that you had bad experences with the conversions Staybolt, But you are not alone.
9c1driver I disagree, We are not all stuck with them. Those like myself that hold onto the older equiptment due to this issue are quite happy to do so. It is only those that want all the eye candy and bells and whistles of the newer products are stuck with them.
Nevertheless it is what it is. And what the real problem is here is instead of putting big $$$$ into all of the flashy crap designs in the new equiptment of today.....Why not design a mosfet that is ment to amplify Rf at 12-14 volts that could also be a realible replacement for the older bi-polars? If Eleflow can do it so can the radio manufactures.
I wish you luck in your future converstions.
73's
M.S.
We still see Cobra 148 GTL NW/ST's coming in to the US here equipped with 1969 transistors. Guess the Chinese have their own cache or are mfg their own real copies; one or the other - or both - must be true.
BTW MS - The ERF-2030's are mfd by EKL for the expressed purpose of RF applications. Sam Lewis of RF Limited co-owns EKL. There was a conversation about this subject a couple of years ago on this forum.
Many new amateur radios being built today are built around the MOSFET output design.
IIRC, there is a MOSFET device that is capable of handling 1kw. There is a thread on this forum somewhere about that story too.
Getting a MOSFET to operate at CB freqs isn't beyond the capacity of these devices. If you read the spec sheets, they you can see that they can handle amplifying signals at those freqs - and higher freqs too.
BTW MS - The ERF-2030's are mfd by EKL for the expressed purpose of RF applications. Sam Lewis of RF Limited co-owns EKL. There was a conversation about this subject a couple of years ago on this forum.
Many new amateur radios being built today are built around the MOSFET output design.
IIRC, there is a MOSFET device that is capable of handling 1kw. There is a thread on this forum somewhere about that story too.
Getting a MOSFET to operate at CB freqs isn't beyond the capacity of these devices. If you read the spec sheets, they you can see that they can handle amplifying signals at those freqs - and higher freqs too.
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I personally don't care for mosfets.....in my experience bi polars are still the best for gain and ability to take abuse.
I can remember back about 8 years ago when i started seeing mosfet power modules in commercial rigs. I think they were vx2500 vertex ltr radio i started seeing big problems with...they made a cheap ltr trunking radio that was a cheaper radio than lenwoods. They all came with mosfet power modules....every one we sold was retrned because of a burnt out pa...I started putting in bipolar and they held solid.
Even now...most of the Tad radios i fix are blown mosfet power modules.
They just can' t take any abuse.....way to sensitive.
I can remember back about 8 years ago when i started seeing mosfet power modules in commercial rigs. I think they were vx2500 vertex ltr radio i started seeing big problems with...they made a cheap ltr trunking radio that was a cheaper radio than lenwoods. They all came with mosfet power modules....every one we sold was retrned because of a burnt out pa...I started putting in bipolar and they held solid.
Even now...most of the Tad radios i fix are blown mosfet power modules.
They just can' t take any abuse.....way to sensitive.
I've never really messed with IRF520N devices in SSB CBs but here's my take on it based on looking at the device datasheet and playing with a Spice model.
The first place to look is at the curve traces for Vgs vs Id vs Vd and you can see that this device needs to be biased quite hard to give acceptable linearity. i.e. it looks like it needs to have just over 4V bias at the gate to get the device in the most linear region. This will mean the drain current at idle will be quite high. eg over 1A and this means that these transistors will run hotter than a conventional NPN device that can idle at just 50mA.
So you probably can get the MOSFET devices to run fairly linear if you bias them hard enough but the cost is a huge degradation in overall efficiency with typical human speech on SSB. Presumably if you were to key up a MOSFET radio on SSB and just let it idle with no speech then the radio will get much, much hotter than a radio with the classic 2SC2166 + 2SC1969 combo with both devices sat idling at 50mA.
eg the bipolar devices will have a combined bias of 100mA so the Pdiss at idle is 1.2W from a 12 v supply.
With the 2 MOSFET devices the combined idle current could be 2A so the Pdiss for two devices is 24W. A huge difference.
Obviously nobody keys up an SSB radio for long without speaking but I'd still expect to see a significant difference in average power consumption with normal SSB speech. For this reason I'd expect the MOSFET transistors to run much hotter. However, I'm basing all of the above purely on the datasheet graphs for the device.The datasheet does suggest the MOSFET has better thermal transfer from junction to case than the bipolar devices so this will help them cope with the extra Pdiss.
By contrast if you look at the datasheet for a 2SC1969 and look at the graph for Ib v Vc v Ic the graph looks so much nicer with good linearity across all the (nice and flat and evenly spaced) Ib curves even with low Vc and low Ic. The curves for the 2SC2166 aren't quite as nice as the 2SC1969 but then this device is only used as a low power driver.
I would definitely go for the bipolar devices over the MOSFET whenever possible and I can't see why people class the MOSFET as an 'upgrade' unless the only goal is to produce a bit more power at the expense of linearity and efficiency.
The first place to look is at the curve traces for Vgs vs Id vs Vd and you can see that this device needs to be biased quite hard to give acceptable linearity. i.e. it looks like it needs to have just over 4V bias at the gate to get the device in the most linear region. This will mean the drain current at idle will be quite high. eg over 1A and this means that these transistors will run hotter than a conventional NPN device that can idle at just 50mA.
So you probably can get the MOSFET devices to run fairly linear if you bias them hard enough but the cost is a huge degradation in overall efficiency with typical human speech on SSB. Presumably if you were to key up a MOSFET radio on SSB and just let it idle with no speech then the radio will get much, much hotter than a radio with the classic 2SC2166 + 2SC1969 combo with both devices sat idling at 50mA.
eg the bipolar devices will have a combined bias of 100mA so the Pdiss at idle is 1.2W from a 12 v supply.
With the 2 MOSFET devices the combined idle current could be 2A so the Pdiss for two devices is 24W. A huge difference.
Obviously nobody keys up an SSB radio for long without speaking but I'd still expect to see a significant difference in average power consumption with normal SSB speech. For this reason I'd expect the MOSFET transistors to run much hotter. However, I'm basing all of the above purely on the datasheet graphs for the device.The datasheet does suggest the MOSFET has better thermal transfer from junction to case than the bipolar devices so this will help them cope with the extra Pdiss.
By contrast if you look at the datasheet for a 2SC1969 and look at the graph for Ib v Vc v Ic the graph looks so much nicer with good linearity across all the (nice and flat and evenly spaced) Ib curves even with low Vc and low Ic. The curves for the 2SC2166 aren't quite as nice as the 2SC1969 but then this device is only used as a low power driver.
I would definitely go for the bipolar devices over the MOSFET whenever possible and I can't see why people class the MOSFET as an 'upgrade' unless the only goal is to produce a bit more power at the expense of linearity and efficiency.
G0HZU:
Perhaps the SPICE model needs to be compared to a radio that uses the IRF520N as an OEM part:
http://www.cbtricks.com/radios/galaxy/dx959_mosfet/graphics/dx959_mosfet_sch.pdf
The MOSFET does get hot; but the output and audio quality seems much better in the 959 than those that use the MOSFET as an upgrade.
Sooo . . . How does this Galaxy 959 schematic differ from your SPICE model?
Perhaps the SPICE model needs to be compared to a radio that uses the IRF520N as an OEM part:
http://www.cbtricks.com/radios/galaxy/dx959_mosfet/graphics/dx959_mosfet_sch.pdf
The MOSFET does get hot; but the output and audio quality seems much better in the 959 than those that use the MOSFET as an upgrade.
Sooo . . . How does this Galaxy 959 schematic differ from your SPICE model?
i have not had much to do with the switching fet final rigs, i have yet to hear one that sounds as good as a correctly setup bipolar radio to my ears,
the anypile at5555 & its sisters have the most heat per output watt and worst pa efficiency of any cb i ever came across,
swapping from 1969/2312's to five n dime switching fets in ssb sets is far from an upgrade, its just fashionable for meterbaters.
the anypile at5555 & its sisters have the most heat per output watt and worst pa efficiency of any cb i ever came across,
swapping from 1969/2312's to five n dime switching fets in ssb sets is far from an upgrade, its just fashionable for meterbaters.
Hi Robb
All I really did with the Spice model was to generate a series of Id vs Vgs traces across several drain voltages.
i.e. I just looked at the linearity of the Vgs vs Id at DC and it shows the usual poor linearity from a FET device with the response varying in a non linear manner from low Vgs to high Vgs.
The best linearity was with it biased just over 4V to give just over 1A drain (idle) current. I'd imagine that if you tried to bias this device for a low idle current eg 100mA it would not sound good on SSB.
All I really did with the Spice model was to generate a series of Id vs Vgs traces across several drain voltages.
i.e. I just looked at the linearity of the Vgs vs Id at DC and it shows the usual poor linearity from a FET device with the response varying in a non linear manner from low Vgs to high Vgs.
The best linearity was with it biased just over 4V to give just over 1A drain (idle) current. I'd imagine that if you tried to bias this device for a low idle current eg 100mA it would not sound good on SSB.
there's a couple posts here that 'hit on' the problem. after a discussion with chris holland & a couple others about the original 1st gen BLUE FACED mosfet magnum 257, dealing with WHY they had a detectable wobble on SSB, they all had the same answer. there were technical aspects as to WHY, but the main part was that these export radios are not DESIGNED around mosfets like HF rigs. the mosfets in exports are SUBSTITUTED in place of bi polar transistors. oh yes they absolutely DO work, BUT they are so touchy in bias & nearby parts values, that if 1 assembler exhaled & another inhaled while soldering a resistor, the added-or reduced-resistance would cause totally different sounding radios. well maybe a bit of an exaggeration, but you get the point. they told me that ranger & magnum needed to design an entire transmitter section around the PROPER mosfets, not just slap some odd mosfets in the holes & add a few complimentary parts. not necessarily a comparison to 'sweep tube' amps-they worked, too, but not really the best way to build a quality amplifier.
so yes, mosfets or just mosfet swaps are NOT an upgrade, just a different part. kinda like champion spark plugs vs those dual gold tipped jobbies....that never worked right in my durango. or xj for that matter.
so yes, mosfets or just mosfet swaps are NOT an upgrade, just a different part. kinda like champion spark plugs vs those dual gold tipped jobbies....that never worked right in my durango. or xj for that matter.
Hi,
I use Metal film resistors (1%) - 1 Watt. I take a bunch of them and take a DVM and make an matched pair. You got "identical" resistors with 1% tolerance.
Carbon film is 5% tolerance plus values change with age and heat. This work very well for me when I work with dual finals.
Maybe some detractors arise, but the thing work well for me.
SHHHHHHH Metal film resistor are more costly.
<gotproof>
Greetings from the Caribbean!!!: wink:
I use Metal film resistors (1%) - 1 Watt. I take a bunch of them and take a DVM and make an matched pair. You got "identical" resistors with 1% tolerance.
Carbon film is 5% tolerance plus values change with age and heat. This work very well for me when I work with dual finals.
Maybe some detractors arise, but the thing work well for me.
SHHHHHHH Metal film resistor are more costly.
<gotproof>
Greetings from the Caribbean!!!: wink: