BLF278, MRF151G, SD2932 What's the difference?

[Elopid]

Hi all,

I'm looking to purchase a replacement 300w output transistor, I would prefer to go with someone like Enigma Components as I know he's got a good reputation.
I have the 300w PCS pallet module and Broadcast Concepts LPF.

I would like to know what the difference is between these transistors apart from the prices as follows?

BLF278 - £85.00
MRF151G - £78
SD2932 - £69

Will the SD2932 work as the same as the BLF278 as they appear to be the same specs?

With thanks, eLoPiD

[Analyser]

In my experience, the BLF and MRF behave virtually the same (work well). The SD tended to be less stable so I would steer clear of that.

[Elopid]

Hi Analyser,
Thank you for your response. I think that means I'll go for the MRF option. I'm hoping that the LPF isn't going to cause a problem as it caused that Spanish board with the double MRF151's to blow, but I will add this to a new posting.
Appreciate your help, eLo

[Analyser]

In that case test the PA without the filter first and turn the bias right down to make sure it's all working fine. That PA board is very efficient and will do a lot of power in to a dummy load but when you connect a filter you'll see the power drop and the current increase. This is normal, but if there is anything wrong with the filter you do risk blowing the transistor. If you can, use a variable power supply and slowly turn up the voltage whilst monitoring the current. If you only have a fixed supply then don't go more than 48v. If you have a way of measuring current then the PA without a filter will normally draw 7-8A at 300w output and after the filter is added you're looking at 9-10A. If you're drawing more than about 11A something is wrong.

[Elopid]

The BIAS voltage had been preset by them and they have painted some kind of red paint glue over the two presets so you cannot adjust them.
I am thinking of replacing transistors with BLF177 as they are cheaper, would I need to re-adjust the BIAS voltages as opposed to the same voltage which he's already set up for the MRF151??
I will turn the power supply right down to test the power amp with the lowest voltage first, I thought by putting close to half of the recommended drive into it then the voltage should be OK, but next time I'll turn the voltage on PSU right down.
I've made a big post about this on another posting. I much appreciate your help Analyer. ELO

[Analyser]

If you replace the MRF151s with BLF177s the bias will need adjusting.

[Albert H]

If you have a way of measuring current then the PA without a filter will normally draw 7-8A at 300w output and after the filter is added you're looking at 9-10A. If you're drawing more than about 11A something is wrong.

I always use a 10A fuse on my BLF278 boards. It gives three huge - money saving - advantages:

1. If the output match through the filter is wrong, the fuse blows

2. You can't over drive the PA - when output power goes up, the fuse blows - before the FET!

3. You can't over bias the FET - the fuse blows!

Another trick for expensive device preservation is to "soft-start" the PSU - apply (say) 20V, and have it slowly ramp up to the full 48V. This has saved devices on many occasions.

You MUST use the right components in your output filters at that kind of power. I actually started making my own mica dielectric capacitors for bigger powers since the good capacitors were getting stupidly expensive.

I had quite a lot of success with the SD2932, but ran them at lower power, with (relatively) low drive so that I had no trouble with them at all.

[Elopid]

If you replace the MRF151s with BLF177s the bias will need adjusting.

Cheers Analyser,

Yes I don't want to be damaging anything again.
Another quick question, I have found MRF150 even cheaper than the 151 and BLF177. What is the difference between the 150 and the 151?

Kind thanks, Elo

[Analyser]

If you replace the MRF151s with BLF177s the bias will need adjusting.

Cheers Analyser,

Yes I don't want to be damaging anything again.
Another quick question, I have found MRF150 even cheaper than the 151 and BLF177. What is the difference between the 150 and the 151?

Kind thanks, Elo

Unfortunately they're completely different, the MRF150 is less gainy and has a different input and output impedance so they can't be dropped into a 151 circuit.