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previous: Bruce In a nutshell...YES.To a ce... -- 11/29/2000 9:24 PM View Thread

Re: Max Bias supply needed

11/29/2000 10:18 PM
Re: Max Bias supply needed
Since you mentioned KT88's, you might want to provide a little more negative range. Like Bruce said, you do not need to go that close to zero, if you select the bias variation as he indicated it will also have a protective effect in case the control is dimed accidentally.  
Looking at the Sylvania data book from pre-1959, I found a list of example conditions for audio tubes, on one page, man what I could do with a scanner . . .  
Negative grid volts varies from 13.5 for EL34 to 35-45 volts for various guitar tubes and oddballs as well. The 6550 (at the time 6550A had not been developed) with 600 plate volts and 300 screen volts shows -33 volts bias to yield 100W from a pair into a 5K plate-to-plate load.  
I have been working with GE 6550A in a single end breadboard, and found the Sylvania conditions good starting points (same as GE actually): 400 plate & 225 screen with -16.5 bias to yield 20W into a 3K load. In practice you get 10 hifi clean watts, 20 watts true power I would say available for fairly intelligible announcing service (or maybe heavy metal guitar), popular guitar falls about in the middle, estimate 15W for blues or classic rock service. When I raise the screen it then takes more negative bias to keep the tube from running too hot. The 6550A will take more abuse than the 6550, so I anticipate increasing voltages all around.  
These conditions for P-P vs. SE are drastically different, but there could be a trend for the P-P to need more negative range.  
When they show the KT88 it stands out because of the -65volt bias, but they have 450V on both the plate & screen and it is ultralinear. They also only show 65W from a pair into a 3800ohm load like this but it is with an ultralinear OPT.  
I get the idea that they exaggerated the output of the 6550 and chose example conditions for the KT88 that provided a lower output for comparison. I think Sylvania made 6550's but not KT88's so that might have influenced the way they wanted the data to appear on a comparison table.  
Hope this helps,