The formula is for Class AB amps with adjustable bias:
Max plate dissipation of tube type/plate voltage, multiplied by desired % of max plate dissipation.
For example, for a 400 volt plate voltage running EL34s, 70% would be 44ma: 25 watts/400 volts=62.5ma x .7 = 43.77ma
A 460 plate voltage running EL34s at 70% would be about 38ma
The % of max dissipation you think your amp sounds best is a matter of taste, but it will probably be between 65% and 72%
This ma figure is per tube. For some like 100watt DSLs it is per side with two tubes per side so it would double that per side.
Max Plate dissipation of typical octal power tube types:
6V6=14 watts
5881= 23 watts.
6L6wgb=23 watts
Kt-66 = 25 watts
EL34=25 watts
6CA7=25 watts
KT77=25 watts (30 watts absolute max)
E34L=25 watts ( on the JJ data sheet, but many say 30 watts in practice)
6L6GC=30 watts
7581=30 watts
7581A=35 watts
6550= 35 watts
6550A=40 watts
KT88=42 watts
Silver Jubilees tend to run higher plate voltage than typical for post 60s Marshalls. Some Plexi Marshalls from the 60's ran more than 500 volts. Late 70s JPM 50 watt Marshalls often run 400 volts or less. If you run a variac you will need to measure actual plate voltage and re-bias accordingly.
Max plate dissipation of tube type/plate voltage, multiplied by desired % of max plate dissipation.
For example, for a 400 volt plate voltage running EL34s, 70% would be 44ma: 25 watts/400 volts=62.5ma x .7 = 43.77ma
A 460 plate voltage running EL34s at 70% would be about 38ma
The % of max dissipation you think your amp sounds best is a matter of taste, but it will probably be between 65% and 72%
This ma figure is per tube. For some like 100watt DSLs it is per side with two tubes per side so it would double that per side.
Max Plate dissipation of typical octal power tube types:
6V6=14 watts
5881= 23 watts.
6L6wgb=23 watts
Kt-66 = 25 watts
EL34=25 watts
6CA7=25 watts
KT77=25 watts (30 watts absolute max)
E34L=25 watts ( on the JJ data sheet, but many say 30 watts in practice)
6L6GC=30 watts
7581=30 watts
7581A=35 watts
6550= 35 watts
6550A=40 watts
KT88=42 watts
Silver Jubilees tend to run higher plate voltage than typical for post 60s Marshalls. Some Plexi Marshalls from the 60's ran more than 500 volts. Late 70s JPM 50 watt Marshalls often run 400 volts or less. If you run a variac you will need to measure actual plate voltage and re-bias accordingly.
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