gain ratings on transistors....

[bsco]

Awhile back I had repaired a Fender Frontman 212........it is back again with the output transistors blown.....I can't find any other defective parts there.....I have ordered replacement transistors and have noticed that they have different gain ratings......one of them has a "Y" gain rating and the other has an "O" rating.....the original transistors had a gain rating of "Y".......so will the new replacements cause a problem??? I know that when dealing with stereo equipment this can make a big difference depending on the circuit it is used in.....but will it make a difference in the frontman 212 amp.....

[The Dude]

They should be matched. That is the main concern. Did you buy the first set of outputs from a reliable source? Maybe they were Chinese fakes. It's a good idea to check bias after a SS repair. If there is too much current in the output stage, it is inevitably going to come back. Even if bias is not adjustable via a potentiometer, checking bias will often reveal that a problem still exists in the amp.

[bsco]

They should be matched. That is the main concern. Did you buy the first set of outputs from a reliable source? Maybe they were Chinese fakes. It's a good idea to check bias after a SS repair. If there is too much current in the output stage, it is inevitably going to come back. Even if bias is not adjustable via a potentiometer, checking bias will often reveal that a problem still exists in the amp.

I didn't replace the original outputs..I replaced a broken pot and got rid of the noise when no jack was inserted.....I used that resistor mod on the mute ckt.....for some reason the outputs popped after the owner used the amp for awhile...he said there was a loud hum and then shut off.....the outputs were shorted so I ordered these replacements....I will be installing them tomorrow.....I can't remember off hand if there is a bias adjustment there.....I'll get back to you after I take a peek tomorrow......thanks for the reply...

[bsco]

just getting back at this unit again......I did a check on the gain ratings of these replacement transistors.....the 2SA1294 has a gain rating of "Y" (70-140) and the 2SC3263 has a gain rating of "O" (50 to 100) the individual replacements that I have picked out are close......the 2SA replacement transistor has a gain reading of 108 and the 2SC replacement unit has a rating of 83.....that is the closest I can get......will this be sufficent or will I need tighter specs.....I have some used ones here but those leads are too short and would have to be lengthened which would not look very good.....(the gain readings are closer.)....anyway, just looking for some expert advice before I install them........I found a crack in one of the mica insulators and have obtained a couple of replacement ones which are good.....

[Jon Snell]

Basicaly the Hfe rating of a transistor means one thing;
if there is 10mA of drive current available (R91 + R92) and the output stage produces 100W into a 4R load, That equates to a 5A output capability. So to get 10mA up to 5A the Hfe or current gain required is 500.
So the driver transistor Q18 would have at least an Hfe of 25 (25 x 10 = 250) meaning the output transistor Q20 will need an Hfe of at least 20 (250 x 20 = 5A) which they have.
As they have more Hfe than required this is where Q16 comes in to play, as the voltage rises, with current output, across R109 it will switch Q16 on and stop the drive to it, hopefully protecting the transistors.

The critical part is the Vbe. If the Vbe is less than desired then the transistor will be switched on more at rest meaning a higher quiescent current will flow meaning possible overheating. There is no adjustment!
Select with care.

[bsco]

Ok. I had a set that were matched......didn't realize I had them.......got them installed and so far so good...will complete reassembly this weekend.....cheers...

[bsco]

Just heard back from the owner..he said the amp is perfect........