Hello, I'm starting a new thread as the last one was long and no longer being checked.
Here's my problem:
I changed the two JE1692 power transistors for two 2N6488's.The service manual suggested these are superior parts and being that the amp is 40 years old I thought it would be a good idea. This was to be my final tinkering with this amp as all other issues were resolved with the help of you guys on the MM forum!
The amp is working fine and everything is running smoothly and quietly, but I'm detecting a very faint break up on the clean channel that was not there before I changed the transistors.The volume is reduced and the amp seems to sound kinda flat, with the beautiful upper presence sounding like harsher treble. It sounds like its biased way cold and not what I wanted at all.I measured 19ma and 20ma across R51 and R54,pretty closely matched so that might be a positive benefit of the 2N6488's? but that seems low as they suggest a minimum 25ma I believe? [Correct me if I'm wrong here.] The new 2N6488 power transistors are not shorting out and are installed correctly. Anyway, any idea what could cause this? These amps don't have a bias trim pot but it sounds like very faint cross over distortion to me. Its not bad and probably wouldn't be a problem at all for most players, but the amp definitely sounded much better before I changed the power transistors. I did try several matched pairs of power tubes.
Mon, 06/08/2020 - 00:53
#1
double post
oops
FYI
Ok, the amp is buttoned up and sounds Amazing, so happy with this little head!
However, there is something that I feel needs to be addressed as there is some BAD info out there on this site.
Specifically, I've read in multiple threads that the BC550C is a direct replacement for the ZTX109, stuff like ''The ZTX109 is a weak 100mA allround tranny, replace it with the 200mA BC550C and you are done ! ''
So I bought em' and installed them in Q1/Q2/Q3 and Q4. Unfortunately I did this in tandem with changing the power transistors. While the BC550'S worked in Q3/Q4 they were the cause of my problem in Q1/Q2. I assumed [never a good idea] that the problem was the new power transistors, so put the short legged originals back in. [that was kinda like playing 'Operation' as a kid, where you replace the body parts with the tweezers :-/ ]
The problem was the BC550's in Q1/Q2. I believe the replacement should be NTE123AP''s. Its working fine now with the ZTX109's back in there. Are you guys all swapping BC550C's for the ZTX109's? Am I the only one who has encountered this?
The bummer is I could have left the 2N6488's in there....:-(