Mosfets and "power" claims

[Two Pack]

So I have put mosfet finals in two radios so far, a cobra 148 and a Cobra 29.

I see claims all over the internet of people getting 30-40watts out of them

I've only been able to get about 18watts out of either of my radios, irf530 and 13n10.

So where's everyone hiding all the magic juice! Hahaha

 

[nomadradio]

Sounds a bit like adding all the hot bolt-on gadgets to the motor on your daily driver. Next question is why you don't see all those extra horsepower they promised. More than "just one" possible cause there.

First thing I wonder is how you read peak power from your radios. If you use a so-called "passive" peak-reading meter, that can affect the modulated power you'll see.

All wattmeters should show the exact same reading on a dead carrier, into a dummy load.

What they'll show on modulation can vary a lot from one meter to the next.

A "peak-reading" meter with a 30 year-old electrolytic capacitor behind the "peak" switch can result in low peak readings, too.

More than one possibility here.

73

 

[ExitThirteen]

I really don't think there's any "magic juice" to MOSFETS... a person may get a couple extra watts out of the radio, but in the case of my mods, they're not designed to "squeeze" every last watt out of a radio, they are designed replace bipolar transistors which are starting to become obsolete. Getting a couple extra watts out while retaining a clean wave on the 'scope and proper longevity is a bonus.

The radio would have to put out 4 times the power to get any real noticeable improvement (1 "S" unit), so it's best to install them with the mindset to keep the radio functional and not to get "Mo' watts".


~Cheers~

 

[Low_Boy]

I think some people are getting crazy watts out of these mosfets by cutting limiters. I just did a connex 3300 with a IRF520 driving a IRF520. No companion parts, changed the parts in the radio exactly the same way the instructions show on CB Tricks. I am getting 15W low power and 18W high power. I was hoping someone that did this Mod. would answer me on my post and tell me what there output power was but I have not heard from any one yet. I have tried the mosfet conversion on cobra 29's with a bit more power than a factory final. Sometimes I get more power out of a IRF520 than a 13N10. I am wondering if anyone has perfected this change over yet. I am not good enough at electronics to do this.
Thanks for all your hard work.

 

[jon666]

cheap dosey meters

 

[rabbiporkchop]

Should have stuck with the bipolar 2SC1969 transistor. More gain less heat.

 

[kaos513]

i hear that some mods have to be done to get more juice not just install them.

 

[nomadradio]

Mostly I stay out of "MOSFET-mod" arguments.

Nobody wants to hear my opinion, and that's all good.

One thing I observe that's not an opinion is a particular controlling factor in how any MOSFET behaves, no matter where it's used.

The DC gate-bias voltage on conversion-job MOSFET mods is a bit of a wild card. Too much and you get really high wattmeter readings, followed by one of two things.

1) Blown fuses, or

2) Plastic face blown off the MOSFET when it overheats and fails.

Too little DC-bias voltage on the MOSFET gate and it's just weak.

Factory-installed parts and circuits found in a radio built with MOSFET finals is the result of an engineer's tweaking, to get a consistent result from thousands of units. If you make a slavishly-indentical perfect copy of that circuit in a conversion radio, it should perform the same.

A one-off conversion might fall neatly into place and function just great. Or, it can fall victim to factors nobody can predict.

I have a suspicion that radios frequently get this mod because the power was below par in the first place. If that problem was caused by reduced drive upstream from the original final transistor, the new MOSFET will also be working with reduced drive. Too many possible causes of that to make any simple rule.

Except one.

Only convert a radio that's delivering normal, healthy transmit power.

A radio that's already weak, compared to normal power for that model, may still be weak with the conversion in it.

But if it's giving you the normal, proper power with the stock transistor, where is the incentive to modify it?

But yeah. The radio most likely to give you good power is a radio that doesn't really need the conversion.

And a radio that gets converted because it only did half or 2/3 the wattage it should may have gremlins in it that hold back the new MOSFET final as well.

Toss of the dice.

73

 

[OldTech03]

Mostly I stay out of "MOSFET-mod" arguments.

Nobody wants to hear my opinion, and that's all good.

One thing I observe that's not an opinion is a particular controlling factor in how any MOSFET behaves, no matter where it's used.

The DC gate-bias voltage on conversion-job MOSFET mods is a bit of a wild card. Too much and you get really high wattmeter readings, followed by one of two things.

1) Blown fuses, or

2) Plastic face blown off the MOSFET when it overheats and fails.

Too little DC-bias voltage on the MOSFET gate and it's just weak.

Factory-installed parts and circuits found in a radio built with MOSFET finals is the result of an engineer's tweaking, to get a consistent result from thousands of units. If you make a slavishly-indentical perfect copy of that circuit in a conversion radio, it should perform the same.

A one-off conversion might fall neatly into place and function just great. Or, it can fall victim to factors nobody can predict.

I have a suspicion that radios frequently get this mod because the power was below par in the first place. If that problem was caused by reduced drive upstream from the original final transistor, the new MOSFET will also be working with reduced drive. Too many possible causes of that to make any simple rule.

Except one.

Only convert a radio that's delivering normal, healthy transmit power.

A radio that's already weak, compared to normal power for that model, may still be weak with the conversion in it.

But if it's giving you the normal, proper power with the stock transistor, where is the incentive to modify it?

But yeah. The radio most likely to give you good power is a radio that doesn't really need the conversion.

And a radio that gets converted because it only did half or 2/3 the wattage it should may have gremlins in it that hold back the new MOSFET final as well.

Toss of the dice.

73

Amen brother, I can't find any disagreement with the above and although I could add more the scenario I'm happy with it as it stands. But as they say in the halls of Congress I reserve the right to revise and extend my remarks.

Oldtech

 

[ExitThirteen]

Nomad,

You're spot-on with your last post. MOSFETS can be pretty tricky, and it seems like most people do it for a wattage gain.

When I first started experimenting with them, I was also sucked into the "Mo' Watts" mentality, but I quickly learned that it's not about making more power.

I began to realize that MOSFETS can potentially serve a better purpose as a viable replacement for bipolar transistors once the supply dries up. Once I took on that mindset, the ballgame changed for me. Now I wanted to see if I could come up with a way of installing these devices and not only making them sound good in a radio, but also have have good longevity.

I have done quite a bit of testing, and I have come up with some good information. And while I know that these MOSFETS are not ideal for RF applications, they can work well if they are installed and biased correctly, and will last for many years.

Over 90% of the MOSFET mods I have done to the vintage radios over the years are within a watt or two of the radio's original output once the MOSFET has been properly installed, and biased. I have told my customers that MOSFETS won't necessarily put out more power, but instead allows the customer to resurrect an old rig for 1/4 of the price (of the final).

It's not about power, it's about having a low cost option if the customer does not want to spend 20 bucks for a final. The customer has options.

I 100% agree that if your radio is delivering normal output, there is absolutely no reason to mod it to a MOSFET final. None. If the radio has a working bipolar transistor for a final, keep it in there.

~Cheers~

 

[Low_Boy]

Should have stuck with the bipolar 2SC1969 transistor. More gain less heat.

Going back to stock as soon as I get some time.

 

[Robb]

I've done a few MOSFET replacements.
But I did it because I am CHEEEEEP . . .

The OEM replacement transistors are NOT cheeeep

ALWAYS seen power gains.
You have to re-peak the TX coils and make sure you have the right amount of coupling capacitance between the driver and final.

That works - too.

 

[Low_Boy]

Nomad... I always value your opinion and observations.
I sort of think you hit the nail on the head with my radio. It was putting out decent power but not what other connex ang Galaxy 33's were doing so I thought this would wake it up. I have a couple beat old galaxy 33's that put out correct power. Possibly I will switch things and see what happens. Time is limited so it probably will not be soon.

 

[Low_Boy]

10-4, exitthirteen and OldTech03.

 

[sonoma]

a guy on another forum that goes by redlight has had some 1969 finals for a decent price. I bought 25 of them a few weeks ago and they put out just like the old 1969 finals. by buying 25 of them at one time I got a real good deal on them. my buddy that has a shop here in the city took 15 of them and they have worked good for him also.