Hello,
I've got an interesting case of excess voltage in power supply on the bench right now - it's my pal's mistreated 1972 Super Reverb AA270. It's got a whole bunch of problems but the most interesting one is that it the voltage is way too high. When I first turned it on, the Chinese 6L6WGS were dissipating 26 watts with 540V on the plates! I had to modify the bias supply in the first place to get enough negative voltage to stop them from everheating, but they should probably be replaced with proper 30W 6L6GC's as well. Rectifier tube was GZ34 even though the tube chart calls for 5U4GB. I put one in and we are now at 500V which is probably still too much for those cheap tubes. A correct rectifer does help the voltage problem a bit, but there's 7,2V AC on heaters under full load and the voltage selector is already in the 240V position. I assume this gives away that the PT could have developed a small short in the primary winding and needs to be replaced. The owner is hesitant, though, as he's worried that the unique - as he say - sound of his amp could change.
Are silverface Fenders notorious for the voltage creep up problems? I know the network mains voltage is now higher than 50 years ago, but in case of this amp the difference is too big anyway, and it's not even running on the 230V selector setting either. It's obvious that voltages in old amps are usually higher than specified in the schematic, but I'm sure we're not talking about 540V on 6L6s' plates.
On the side note, only the filter caps were replaced in this amp at some point in the past and probably the tech was aware of the higher voltage as they are now all doubled in series, not only the first one. No resistors across the converted ones, unfortunately. It's definitely the AA270 model judging by the bias supply arrangement and a few other details, but values of the resistors in the supply doesn't match the schematic which calls for 1K and 4,7K after the choke and I have 2,2K and 10K instead.
All other caps including filter caps in the bias supply are still original from 1972. I could observe the bias voltage go rapidly up or even disappear as I was lightly tapping the bias board - frightening, so I had to replace the associated caps immediately. The 12AT7 driver tube turns off and on by itself when it feels like it, which is always fun.
Cheers,
Tomi
I've got an interesting case of excess voltage in power supply on the bench right now - it's my pal's mistreated 1972 Super Reverb AA270. It's got a whole bunch of problems but the most interesting one is that it the voltage is way too high. When I first turned it on, the Chinese 6L6WGS were dissipating 26 watts with 540V on the plates! I had to modify the bias supply in the first place to get enough negative voltage to stop them from everheating, but they should probably be replaced with proper 30W 6L6GC's as well. Rectifier tube was GZ34 even though the tube chart calls for 5U4GB. I put one in and we are now at 500V which is probably still too much for those cheap tubes. A correct rectifer does help the voltage problem a bit, but there's 7,2V AC on heaters under full load and the voltage selector is already in the 240V position. I assume this gives away that the PT could have developed a small short in the primary winding and needs to be replaced. The owner is hesitant, though, as he's worried that the unique - as he say - sound of his amp could change.
Are silverface Fenders notorious for the voltage creep up problems? I know the network mains voltage is now higher than 50 years ago, but in case of this amp the difference is too big anyway, and it's not even running on the 230V selector setting either. It's obvious that voltages in old amps are usually higher than specified in the schematic, but I'm sure we're not talking about 540V on 6L6s' plates.
On the side note, only the filter caps were replaced in this amp at some point in the past and probably the tech was aware of the higher voltage as they are now all doubled in series, not only the first one. No resistors across the converted ones, unfortunately. It's definitely the AA270 model judging by the bias supply arrangement and a few other details, but values of the resistors in the supply doesn't match the schematic which calls for 1K and 4,7K after the choke and I have 2,2K and 10K instead.
All other caps including filter caps in the bias supply are still original from 1972. I could observe the bias voltage go rapidly up or even disappear as I was lightly tapping the bias board - frightening, so I had to replace the associated caps immediately. The 12AT7 driver tube turns off and on by itself when it feels like it, which is always fun.
Cheers,
Tomi