NEED 2SK225 & 2SJ81 FETs for Sugden amp! Help!

[milosz]

NEED 2SK225 & 2SJ81 FETs for Sugden amp! Help!

I NEED 2SK225 & 2SJ81 FETs for a recently dead channel in my dear old Sugden A25 amp-

These old Hitachi FETs are obsolete- and I cannot find a cross to them.

Can anyone offer a suggestion of a replacement pair?

And, if I replace the dead channel's pair, do I also need to replace the other channel's FETs to keep both left and right sounding the same?

 

[burbeck]

hi try, 2sj162 and 2sk1058

these parts are single hole fixing, so you may have to drill new mounting holes.

also i think that the pin outs are the same, but please check this

the Iq will need reseting on the amp.

 

[AndrewT]

Hi,
does the A25 have a single pair or a two pair output stage?

 

[Mooly]

Hi, Can you first of all confirm what package they are. T03, or flat pack. By the way, have you tried E-Mailing Sugden to ask what they recommend. They are a very friendly bunch, and I had the good fortune of a tour of the factory a couple of years back. Worth a try ? Back to the FET's, there was a similar thread a while back and I made the mistake of assuming the 2SJ/K type were like the laterals we know today, when in fact they are not. Some of the very early FETS have very different transconductance characteristics and the pinch off voltage is significantly different, they are not directly equivalent with the modern types.
Regards Karl

 

[AndrewT]

The 2sj81/k225 are Lateral FETs in MT200 package (two hole flatpack).
The equivalent in To247 package is the 2sj160/k1056, but the higher voltage versions will work just as well, i.e. 2sj161/k1057 or 2sj162/k1058.

 

[aandy]

best use 2sk2221 and 2sj352,as they are more stable than the 1058/162
recent batches of the 2sj162 2sk1058 are a bit trying to say the least.
the bius will not need to be changed with these fets

 

[toprepairman]

Great success with BUZ901P and 906P.

 

[milosz]

Not bad after all

I eventually found exact replacements, some old stock that was in some dusty old box....

Thanks for all the suggestions.

By the way, Sugden kindly supplied a schematic for this amp, and I post it here for reference by anyone needing it.

 

[davidsrsb]

The classic Hitachi lateral circuit yet again

 

[AndrewT]

Notice how Sugden have shown the local decoupling C9 & 10. Quite separate from the supply rail decoupling C11 & 12.

Can you see where C15 & 20 are located relative to the opamps?

 

[davidsrsb]

No output protection for the amplifier or the speakers either, just fuses in the supply.
I think Sugden may have had stability problems, they don't have the usual gate stopper resistors.

 

[Dobs]

I need some help to resolve a problem with the two transistor on the 18v supply (BC639/BC640) They run very hot and finish to burn. I have 30v on the base and collector and 19v on the emitter on both. Should I check the current consumption that goes through the 18v supply to see if it does not overload the BC639/BC640?

 

[milosz]

HOT HEAT

These amps are biased at a pretty high level, the devices do run hot. I put two small fans in mine and a little 12v supply to run them, along with a 70° C thermal switch.

 

[toprepairman]

30V on bases and 19v on emitters is definitely not right, no surprise they're burning up.

 

[Dobs]

30V on bases and 19v on emitters is definitely not right, no surprise they're burning up.

If you can refer to the schematic in post #8 look at the T6 and T7, if you can explain me why it's not right would be greatly appreciated. I have tested all the resistors and diodes related to those transistors and all tested fine.

 

[AndrewT]

T6 & T7 form a pair of regulators.
clip your black DMM lead to Power Ground or to PSU Zero Volts.

Use the Red lead to measure voltages referred to that Zero Volts.

 

[Dobs]

T6 & T7 form a pair of regulators.
clip your black DMM lead to Power Ground or to PSU Zero Volts.

Use the Red lead to measure voltages referred to that Zero Volts.

BC639 E:+19.50v C:+31.50v B:+20.25v

BC640 E:-19.25v C:-31.00v B:-19.75v

 

[AndrewT]

BC639: Vbe = 20.25V-19.50V = 0.75Cbe
BC640: Vbe = -19.75V -(-19.25V) = -0.5Vbe
One is +ve and the other is -ve, OK.
One is 50% higher than the other. This seems suspicious.

 

[davidsrsb]

OP2 is a NE5534 with 100% feedback at high frequencies. This chip is not unity gain stable, so may be oscillating. Should have a compensation capacitor.
Power consumption off the +/-18V rails should be about 15mA, which implies ~200mW power dissipation, fairly hot to the touch

 

[AndrewT]

The max quiescent current for 5534 is 8mA.
Typical is stated as 4mA.
That equates to 144mW
The 5534 is rated for upto 44V rail to rail.

BJT inpt opamps do not make good DC servo.
Use a FET input opamp for DC servo duty.