Driver replacement bjt -> mosfet in broken Accuphase

[couguarek]

Hi,

I have broken power amp in integrated Accuphase E460. Power mosfets are obsolete 2SJ618 / 2SK3497. I replaced it with FQA36P15 / 28N15 used by Acu in newest E800 integrated amp. The main problem is with biasing. New mosfets need more biasing voltage (Vgs(th) is 2x higher). So I change the R11 to higher value. Everything works but in this case drivers (2SA1837/C4793) dissipating too much and have about 70*C (38*C with standard biasing voltage). I thought to change them to mosfets like IRF610/9610, they have the same pinout but Vgs(th) similar to new output mosfets. It could work without problems ? I will be grateful for help. Schematics below are from E550 not E460, but they are the same like in 460, only rail voltages are 2x higher.

 

[nigelwright7557]

If its getting hot then bias is probably to high.
I would set bias current to about 20mA through each source resistor on the output.

 

[couguarek]

I done this at 15mA per output transistor. Still too hot.

 

[nigelwright7557]

What voltage did you set it to across source resistor ?

 

[couguarek]

3,5-4 mV @ 0,22 ohm. Keep in mind that rails are 2x higher (~80VDC).
There is about 16V (100mA) at R23.

 

[Bensen]

What if you try to increase R23?

 

[couguarek]

I will try it today

 

[chip_mk]

+1(increase R23)

 

[as_audio]

Where did you find the schematic ?
Thanks.

 

[richie00boy]

Swapping the 2SA1837/2SC4793 for BJT will do nothing for the heat as IRF610 is same package. Heat (power) is a product of voltage across device and current through it, which will not change whatever device you put there. You need to look at reducing voltage or current to lower the temperature.

 

[couguarek]

Where did you find the schematic ?
Thanks.

From my friend It’s not easy to get this. If you need I can send it to you via email.

 

[couguarek]

Changing R23 to 240 ohm resolved the problem. Thank you for help !

 

[Bensen]

Glad we could help.

Looking at the output bias (60mV//0R22), this will give you something like 15W classe A into 8Ohm, nice, heatsinks must be quite large.

Does anybody has an idea what the actual benefit is of paralleling 2 input stages as in the circuit of post 1?

 

[AlexRivera]

Changing R23 to 240 ohm resolved the problem. Thank you for help !

Hello Couguarek,

Do you think the same FQA36P15/FQA28N15 would work in an E-530?

Do you think I'd need to change R11 to a higher value, can you please share what you used?

Thanks in Advance!
Alex

 

[couguarek]

Hello Alex,

Yes, you need to increase R11. You have 2k stock value. In E460 AB class amp R11 was 1,5k and I changed it to 2k. I hit the correct bias at 3/4 turn potentiometer. You need more idle current (class A) so I suggest about 2,5-3k. Remember, always turn bias potentiometer CCW before power up the amp.
You will also need to increase the R23 like me.
Also check all resistors, because I have broken almost every piece from drivers to output mosfets.

 

[AlexRivera]

Great Thanks for your help, I will try!

I've been looking at the equivalent Newest Version of the E-530 which is the E-650 and in that one they use IRFP240 and IRFP9240 like most of the PASS Amps, I'm wondering if they would work as well...

 

[couguarek]

They will work. Only have bigger case, but it could fit FQA have about 2-3x more current than irfps, so you don’t need very close matching.
Don’t forget to change insulators (kapton or keratherm are best).

 

[couguarek]

Keratherm Transistor Insulators – diyAudio Store

 

[stocktrader200]

small clip on heat sink may work TO-220p 20w power dissapation

 

[EUVL]

You replaced MOSFETs on only one channel or both ?
If only one, do they sould the same, left to right, afterwards ?


Patrick