I have built the following amp using the schematic below:http://diyaudioprojects.com/Schematics/ ... Amplifier/
I breadboarded one channel of this amp prior to starting my build and it worked well. Since building it onto a plate I am seeing the following problems:
1) The feedback resistor on one channel will scorch within a matter of about 1 minute.
2) No audio output
I tested both the 6sl7 tubes and the 6v6 tubes.
1) The 6sl7 tubes test as new
2) The 6v6 tubes test low to replace. One or two of them test as "ok"
I printed out the schematic and numbered the tube pins on the schematic. I color coded the wiring and marked such on the schematic.
The only difference that I made to the build is to make this an integrated amp. I built the 6n7 "heavy metal" preamp into the same box and that is actually working properly. I saw the amplified signal on the output of the pot that I was running into the amp (cd player). That feeds directly into the 10k/470k input of the 6sl7.
The output transformers are old hammond output transformers. I used one of them when I breadboarded the amp (1 channel) and it worked fine. I should have tested the other.
I went back over the schematic/build tonight. I verified the pin numbers from the datasheets to the schematic, and then traced each connection from the schematic to the amp build. There were no issues there that I found. I checked each solder connection and resoldered any that looked remotely questionable.
I am wondering if I am actually seeing the result of two different issues here...... a bad OPT and bad tubes in the output. I can't think of a reason that the feedback resistor would be drawing current like that unless there were a shorted winding. Then again, the 6sl7 cathode in the driver doesn't have a direct path back to ground for DC, only AC signal.
I have no more 1k resistors to replace the bad component with at the moment, so I will need to order some.
Anyone have any ideas as far as the feedback resistor? Do I sound like I am on the right track for t/s the problem?