According to the Scott schematic the 12AX7s filaments are wired in series with the tubes filaments wired for 12.6 each tube. According to the schematic each 12.6 volt tube is receiving about 7 volts filament from the separate power supply supplying tube DC filament voltage and power tube bias.
This makes no sense. Schematic is on HIFi engine site.
This makes no sense. Schematic is on HIFi engine site.
I can't believe the amp would function. If this is true, I will change circuit for full filament voltage by rewiring tubes for 6.3 volts and install a 50 ohm voltage dropping resistor. The circuit will be highly filtered now keeping AC ripple low. The 42 volt transformer winding is listed at 1.2 ohms, it will pick up full tube 300ma current load.
Fired up the Scott 272 bias voltage. Verified 7 volts on 12.6 volt filament. Unbelievable. Good news is bias winding is 53 volts AC. Using a CRC with 3000uF each cap & 5 ohm resistor I get 71.6 volts for 6 tubes in series under load with a schottky bridge. I will modify the bias supply. Same bad news on the Scott 296.
It sat for 40 years not powered up. So, I am replacing all the electrolytic caps and four power tube grid coupling caps before power up. Then after listening I will replace rest of coupling caps with Russian K40Y-9 and hear the improvement. If it does not beat out my KNight KA95 I am also rebuilding and my main amp right now Allied 333 it will be sold. I am using the best Mullard 12AX7s and phono tubes to really hear it. The Scott is built like a tank. All transformers are oversized. 14lb audio transformers and a huge power transformer. It is a beast!
Agree. Do not assume you know more than the Dead Men.
Note *also* that all these tubes run at VERY low plate current. And the noted plate voltages differ from what you expect for 100K+1.5K bias. They worked this out. They could have gone "by the book", but didn't. Why?
Leave it be or be prepared for much-much re-tinkering. Jacking the heater power to 12V will make the tubes draw some more current, drop the already low 160V supply down to ~~100V, cut headroom.
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If you are measuring the filament right after power on, the voltages will be lower than after the amp has fully warmed up. I recently rebuilt a Scott 222C and simply replaced the original FWBR by Siemens with a modern one the results were very good on the amp with slightly more voltage than with the original rectifier but the amp was very quiet and very nice sounding.
150mA heater current. They are in series.
Kerchoff's Law; they are all equal.
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