Hi again!
As I've written in other threads, I have been enjoying a McMartin MA-50 tube amp. But being a relative newcomer to tube amps, I would welcome some assistance in troubleshooting a problem.
Lately this amp has started to develop significant audio pops and scratches in the main output section, and ultimately blows the 0.5A fuse in the bias circuit on the cathodes. I've checked the bias and it's still at 100mA per the spec. The clicks and pops are pretty loud, and happen even when the preamp gain and master gain are set to minimum. This leads me to believe it's in the output section.
I've posted the schematic at: http://frontiernet.net/~whitfield/McMartin_MA50-w_cap.jpg
You will note a 47uF cap (marked in red) on the 470V output of the OD3regulator tube (and Grid2 of the 7027A output tubes). I installed this to remove some significant 60Hz hum.
All tubes test fine. I can't test the OD3 regulator on my tube tester, but I've swapped it with another with no change. Is my addition of a 47uF cap on regulator causing trouble? Is it's rated voltage (500V) too close to the operating voltage of 470V on the grids?
So where should I look to find the cause of the scratching/pops and the bias fuse on the cathode circuit? I welcome all ideas and help in fixing this. While I'm an EE, please keep this basic so I can learn more about this fascinating world of tubes.
Best regards,
Art
As I've written in other threads, I have been enjoying a McMartin MA-50 tube amp. But being a relative newcomer to tube amps, I would welcome some assistance in troubleshooting a problem.
Lately this amp has started to develop significant audio pops and scratches in the main output section, and ultimately blows the 0.5A fuse in the bias circuit on the cathodes. I've checked the bias and it's still at 100mA per the spec. The clicks and pops are pretty loud, and happen even when the preamp gain and master gain are set to minimum. This leads me to believe it's in the output section.
I've posted the schematic at: http://frontiernet.net/~whitfield/McMartin_MA50-w_cap.jpg
You will note a 47uF cap (marked in red) on the 470V output of the OD3regulator tube (and Grid2 of the 7027A output tubes). I installed this to remove some significant 60Hz hum.
All tubes test fine. I can't test the OD3 regulator on my tube tester, but I've swapped it with another with no change. Is my addition of a 47uF cap on regulator causing trouble? Is it's rated voltage (500V) too close to the operating voltage of 470V on the grids?
So where should I look to find the cause of the scratching/pops and the bias fuse on the cathode circuit? I welcome all ideas and help in fixing this. While I'm an EE, please keep this basic so I can learn more about this fascinating world of tubes.
Best regards,
Art