a question about mosfets matching

[nigelwright7557]

There shouldn't be a problem if they have source resistors as that will share the current better. If it doesn't have them or they are very low values then matching would be a good idea.
But, if they are bought from same batch they should be close anyway.

If the amp is blowing up then there could be loads of reasons why.

 

[famousmockingbird]

Okay I replaced a ton of stuff on that blown channel

Q202-205 MJE340 MJE350

D206-209 (1N3070)

Q206-209 (MPSA06 and MPSA56)


D210-213 (1N3070 and 1N914)



All the 47 ohm gate resistors

All the 470mOhm source resistors

The 1.5k flameproof resistors

And of course the output Mosfets.

I turned the bias all the way down and turned it on with a light bulb limiter.

No Smoke or funny smells which is good but I am getting 1vdc for offset on the channel that was blown before. I have no load on it and no signal, and like I said bias pots are set to maximum resistance for each channel.

Any ideas?

 

[famousmockingbird]

I think I have to take it off the dim bulb tester..............the Vgs is only 1.8v..........................


B+/B- is 60vdc should be 82vdc


all voltages are low


Still don't know why I am seeing a 1v DC offset on the damaged channel. Which makes me not want to take it off the current limiter.

 

[famousmockingbird]

Okay R219 was way off at 330k and it's supposed to be 47R.


I replaced that and checked more resistors around the bias and everything is good. Turned on and no magic smoke. No DC offset on that side and the Vgs is more what i expected. Now to try and bias it.

 

[famousmockingbird]

Okay bias for both sides isn't really doing much........


Manual states: "Adjust AP101 to obtain an average of 25_ + 5mV across R109 ..................



That's ~50mA per device. We'll see what happens

 

[famousmockingbird]

Can the amp be run without D214-217? They are 10v 1 watt zeners and they are all shorted. Are they for signal clamping? I will install them I just want to pull them out and run the amp with no signal to see if I can bias it up and I don't need anything else to add to the order. Unless anyone near Portsmouth NH has some 1N4740A zeners laying around they want to sell me