Hello,
They gave me an amp from the late 70s and it is an Inkel AK-650. The device is very cool but at the having been in a carpentry shop for 10 years I have had to clean it thoroughly. The fact is that going through the circuit of the amplifier stage there are burned resistors (nothing serious) and the output transistors of a channel are also kaput. They are a Bjt B557 pnp and its complementary D427npn, totally discontinued. In the channel that works, I have made some measurements on the disassembled output transistors that work well:
B557 hfe 45 D427 hfe 51 * Are matched *
Vbe 573mv Vbe 570mv
Ic 6.1mA Ic 6.2mA
By equivalence I will use the MJ221193G pnp transistors and their complementary MJ21194G npn:
MJ21193G hfe 78 MJ21194G hfe 62
Vbe 556mv Vbe 554mv
Ic 6.2mA Ic 6.2 mA
The difference in hfe in this last couple is clear and my doubt is that if this difference in gain could cause any oscillation or malfunction.
Thanks in advance and greetings
They gave me an amp from the late 70s and it is an Inkel AK-650. The device is very cool but at the having been in a carpentry shop for 10 years I have had to clean it thoroughly. The fact is that going through the circuit of the amplifier stage there are burned resistors (nothing serious) and the output transistors of a channel are also kaput. They are a Bjt B557 pnp and its complementary D427npn, totally discontinued. In the channel that works, I have made some measurements on the disassembled output transistors that work well:
B557 hfe 45 D427 hfe 51 * Are matched *
Vbe 573mv Vbe 570mv
Ic 6.1mA Ic 6.2mA
By equivalence I will use the MJ221193G pnp transistors and their complementary MJ21194G npn:
MJ21193G hfe 78 MJ21194G hfe 62
Vbe 556mv Vbe 554mv
Ic 6.2mA Ic 6.2 mA
The difference in hfe in this last couple is clear and my doubt is that if this difference in gain could cause any oscillation or malfunction.
Thanks in advance and greetings
Output stage is unity gain.
But Hfe will drop under load.
More beneficial if Hfe remains high under load.
Since higher Hfe requires less current to drive the base.
which then, Requires less current and strain on the driver transistors.
Hfe will will chance depending on current so min and max ratings give you a better
idea.
2SD/B 557 and 427 have Hfe min 45 and Max around 140
21193 /21194 have Hfe min 25 and Max around 78
So overall the actual current under load needed to drive the 93/94 is higher.
Stability is more with the Ft and what the amplifier compensation is set to.
The original transistors are slightly faster around 5 to 8 mhz
93/94 are high current / higher voltage transistors and are relatively slow at around
3 to 4 mhz
I dont think you need 250 volt transistors, the originals are 120 volt.
Its likely you can find 120 to 150 volt T03 replacements
which might have a closer transition frequency around 6 to 8 mhz
Far as Hfe, T03 package are roughly the same
under high current they all sag down to about 25 to 35
The originals were relatively high performance for the time.
they maintained 40 to 45 Hfe at high current. When most sag down to 25 to 30
Overall not a big deal, TO3 replacement is rather easy, and all rather slow around 2 to 8 mhz
you get in more trouble with stability with higher Ft amplifiers around 30 mhz
But Hfe will drop under load.
More beneficial if Hfe remains high under load.
Since higher Hfe requires less current to drive the base.
which then, Requires less current and strain on the driver transistors.
Hfe will will chance depending on current so min and max ratings give you a better
idea.
2SD/B 557 and 427 have Hfe min 45 and Max around 140
21193 /21194 have Hfe min 25 and Max around 78
So overall the actual current under load needed to drive the 93/94 is higher.
Stability is more with the Ft and what the amplifier compensation is set to.
The original transistors are slightly faster around 5 to 8 mhz
93/94 are high current / higher voltage transistors and are relatively slow at around
3 to 4 mhz
I dont think you need 250 volt transistors, the originals are 120 volt.
Its likely you can find 120 to 150 volt T03 replacements
which might have a closer transition frequency around 6 to 8 mhz
Far as Hfe, T03 package are roughly the same
under high current they all sag down to about 25 to 35
The originals were relatively high performance for the time.
they maintained 40 to 45 Hfe at high current. When most sag down to 25 to 30
Overall not a big deal, TO3 replacement is rather easy, and all rather slow around 2 to 8 mhz
you get in more trouble with stability with higher Ft amplifiers around 30 mhz
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Thanks White Dragon! You have given me a great answer that helps me to reinforce my knowledge of electronics. Since I have bought a 93/94 pair (measurements in my previous message) I will mount it in the circuit and adjust bias and offset, I think it should work without problem. He did not know that hfe fell with load connected😉
All the best
All the best