griff10672
Well-known member
Wondering if someone can take a look at the schematic I included for this post and maybe point me in a general direction to resolve a few issues .
I sold this amp on Reverb .... worked fine before it left my house ..... didn't work at all when the customer received it ( yeah ... one of those deals )
when I got the amp back .. it powered up ... all tubes had power .... switching worked ... but no sound at all really .. except some minor noises .
I started to trace the circuit with my scope ... and figured out that the two FETs ( BF245's ) that are actually circled in the schematic were dead ...
I replaced them with brand new ones ..... I also replaced the two 1M resistors that come off of them running into " U FET "
U FET seems to be a little seperate circuit done on perf board epoxied to the top of a relay ....
anyway .... once I replaced these components the amp sounded like it should except for this high pitched squeal / oscillation .... that gets a lot louder when the bright cap is engaged .
2nd problem is the Bias .... V5 and V6 bias up just like they should ( getting 497 for plate voltage and have no problem setting that side to 37 mA )
V7 and V8 would only get around 10 mA .... no matter what set of matched tubes I use ... so I rebuilt the bias circuit ... replaced all the caps and resistors ...
now that side will only give me about 20 mA ....
any ideas to what could be causing either problem ??
when I replaced parts .... I used metal film instead of the carbon film that was used originally for the 1M's running into the U FET circuit ... I thought that might have had something to do with the oscillation .... so I replaced those with carbon film ..... didn't fix the problem .
I rebuilt the bias circuit using exact voltage rated caps ... but did go with metal film on the resistors . I have not tried replacing those with carbon yet .
any help would be much appreciated ...
I sold this amp on Reverb .... worked fine before it left my house ..... didn't work at all when the customer received it ( yeah ... one of those deals )
when I got the amp back .. it powered up ... all tubes had power .... switching worked ... but no sound at all really .. except some minor noises .
I started to trace the circuit with my scope ... and figured out that the two FETs ( BF245's ) that are actually circled in the schematic were dead ...
I replaced them with brand new ones ..... I also replaced the two 1M resistors that come off of them running into " U FET "
U FET seems to be a little seperate circuit done on perf board epoxied to the top of a relay ....
anyway .... once I replaced these components the amp sounded like it should except for this high pitched squeal / oscillation .... that gets a lot louder when the bright cap is engaged .
2nd problem is the Bias .... V5 and V6 bias up just like they should ( getting 497 for plate voltage and have no problem setting that side to 37 mA )
V7 and V8 would only get around 10 mA .... no matter what set of matched tubes I use ... so I rebuilt the bias circuit ... replaced all the caps and resistors ...
now that side will only give me about 20 mA ....
any ideas to what could be causing either problem ??
when I replaced parts .... I used metal film instead of the carbon film that was used originally for the 1M's running into the U FET circuit ... I thought that might have had something to do with the oscillation .... so I replaced those with carbon film ..... didn't fix the problem .
I rebuilt the bias circuit using exact voltage rated caps ... but did go with metal film on the resistors . I have not tried replacing those with carbon yet .
any help would be much appreciated ...