2 mrf150 driving 26,,,,,, What do you think ??

[fm969]

That's right 2 driving 26 mrf150's

2 pro built boards, heat sinks and 3 power supplies.

Looking for mrf 150 expert...

Nashville Tn.


Have lots of pictures when i find the right person...

Lets Talk.









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[W5LZ]

Is there any particular reason for it?
- 'Doc

 

[mackmobile43]

26 is an odd number when stage combination's used so are used so you may want to consider adding 6 more or taking 2 away and if you're smart enough to figure out how to do this otherwise then you can probably do it your self.

 

[Shockwave]

Two of these transistors could never drive a stage with 24 or more finals adequately. These transistors also operate on 50 volts so that takes away the ability to use a standard power supply unless you string four in series. At this point I ask if there is really a need for an amplifier with 7000 watts of dissipation available it's probably wiser and easier to do this with a single tube.

 

[MrSuburban]

26 is an odd number when stage combination's used so are used so you may want to consider adding 6 more or taking 2 away and if you're smart enough to figure out how to do this otherwise then you can probably do it your self.

Not when it comes to fet's

 

[Shockwave]

Not when it comes to fet's

When it comes to combining configurations, the FET is no different then a bipolar transistor. It's the input and output impedance that is most different between the 2SC2879 and the MRF150. The MRF150 is still run in the typical broadband transformer push-pull configuration on HF. That means two port and three port combiners would still be common in most amplifiers. Of course it is possible to design non standard combiners with more ports but that applies to both bipolar and FET's alike.

 

[mackmobile43]

Not when it comes to fet's

Look up MrSuburban, I didn't think I needed to explain that part but the only configuration that you could use 2 transformers would be 2 stages of 13 and I think we all know there aren't any transformers that would stand that much rf voltage.

So 24 finals would be a more realistic combination scenario and I know that you can run odd number of fets in single stages but no amount of odd number of fets (26) will work out to an equal number of transformers to be combined unless you happen to be of the **POWER school of thought.

 

[dudmuck]

Two of these transistors could never drive a stage with 24 or more finals adequately.

Sure?
The MRF150 have at least 17dB gain on 11m.
On a single device 3w in gives at least 150w out. It means a single MRF150 would be more than adequate to drive 24.

These transistors also operate on 50 volts so that takes away the ability to use a standard power supply unless you string four in series.

There are standard telecom supplies that do this.
If using a transformer directly from the AC mains, its alot smaller because the current is so much less.

 

[Shockwave]

Sure?
The MRF150 have at least 17dB gain on 11m.
On a single device 3w in gives at least 150w out. It means a single MRF150 would be more than adequate to drive 24.


There are standard telecom supplies that do this.
If using a transformer directly from the AC mains, its alot smaller because the current is so much less.

Granted the MRF150 has more gain then the 2SC2879 but if you look at some HF amps using the MRF150 like the IC-PW1 that uses eight, the drive requirement is about 100 watts. You would be working a driver with 2 much harder then the final stage with 24 to come up with reasonable drive. Four drivers would be more practical.

Good idea with the telephone company power supplies. The secondary winding on it's transformer could be of smaller wire since the transistor operates on 50 volts. The transformer size is not really based on this though. That would be according to it's KVA rating based on power.

 

[Shockwave]

Dudmuck, I decided to look into this further and do the math. Three watts into a stage with 17db gain is indeed 150.356 watts output. However in a practical circuit like the IC-PW1 even the minimum drive requirement for 8 transistors is 60 watts with a maximum of 100 watts. That's still well over double the 3 watts drive per transistor. I also suspect that anyone putting an amp together with approximately 28 of these transistors is not going to be running them at a conservative 120 watts output per device like the IC-PW1. More output will obviously require more drive.

Update: I think I may have figured out why there is such a difference in gain. The 30 Mhz test circuit for the MRF150 used to arrive at the 17db figure uses no negative feedback. Almost any HF amp using these transistors would apply negative feedback to stabilize the amplifier. This in turn would lower the gain of the stage.