813
Posted: Mon Mar 25, 2013 7:53 am
I first saw this tube in 1968 at our Physics Lab at the Rizal High School, and ever-since this tube has become my Holy Grail of sorts.....
so having bought a quad of these from Gerry Sta Maria, tel. 5237551, for 1500 pesos a bottle,(can also be purchased at Watson's for 3,500 each NOS RCA) i have decided to build a set amp, after my 2A3 set success..
plate curves for the 813...
from the curves using incremental plate current, plate voltage and grid voltages, tube characteristics were calculated.....
so that near the operating point chosen, 800volts at 100mA plate current, mu = 7, this is better than the mu of 845, GM70 of about 5,
plate resistance was about 1400 ohms, so that a 5k OPT is fine, i choose instead to design my set OPT at 6.6k:6ohm impedance matching.
operating the trioded 813 at higher plate load results in decreased output power, but since i stopped counting watts a long time ago, this is not so much a concern as far as this build is concerned.
power traffos are split into 2, one for plate B+ with unloaded B+ of about 1100volts, with its own 6CK3 damper tubes as a hybrid tube rectifier,
the other power traffo will supply the filament power for the 813, the filaments will be dc and a toggle switch is provided to reverse poalrity so that the filaments wear out evenly ...... this traffo will also supply the front end drive...fixed grid biasing will be used to maximise power output..
the driver tube will use the 11HM7...http://www.shinjo.info/frank/sheets/049/1/11HM7.pdf
so having bought a quad of these from Gerry Sta Maria, tel. 5237551, for 1500 pesos a bottle,(can also be purchased at Watson's for 3,500 each NOS RCA) i have decided to build a set amp, after my 2A3 set success..
plate curves for the 813...
from the curves using incremental plate current, plate voltage and grid voltages, tube characteristics were calculated.....
so that near the operating point chosen, 800volts at 100mA plate current, mu = 7, this is better than the mu of 845, GM70 of about 5,
plate resistance was about 1400 ohms, so that a 5k OPT is fine, i choose instead to design my set OPT at 6.6k:6ohm impedance matching.
operating the trioded 813 at higher plate load results in decreased output power, but since i stopped counting watts a long time ago, this is not so much a concern as far as this build is concerned.
power traffos are split into 2, one for plate B+ with unloaded B+ of about 1100volts, with its own 6CK3 damper tubes as a hybrid tube rectifier,
the other power traffo will supply the filament power for the 813, the filaments will be dc and a toggle switch is provided to reverse poalrity so that the filaments wear out evenly ...... this traffo will also supply the front end drive...fixed grid biasing will be used to maximise power output..
the driver tube will use the 11HM7...http://www.shinjo.info/frank/sheets/049/1/11HM7.pdf