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Okay, bear with me here. I have this old silverface Bassman 10 that I thought was working pretty well, until I started actually playing bass through it at more than low volume. With volume at 5 or more and bass at 10, everything below the open A string farted out badly. After checking the routine things and finding nothing wrong, I start thinking maybe this is normal behavior for this amp.
So I decided to "upgrade" the 6L6 tubes to 6550s, along with additional filament transformer and boosted filter caps (a la Gerald Weber's book). This didn't help. (And yes, I biased the 6550s to run at about 65 ma each with no signal.)
I have eliminated as culprit every single component, including the output transformer, speakers, and power supply components, by substitution.
I have found two ways of improving matters: 1) cathode biasing the output tubes with no bypass cap, and 2) putting a very large (25k) resistor between the power supply and the two 470 ohm screen grid resistors on the 6550s to bring the screen voltage down by 100V or so. Trouble is, it seems like these things are addressing the symptoms rather than the cause. I mean, I wonder if I'm just reducing the gain.
There are amps that work properly with 6550 screen voltages close to plate voltage, right? I mean, even though data sheets say that screen voltage should be in the neighborhood of 300V?
This thing is driving me nuts. I'd appreciate any suggestions. My kingdom for an oscilloscope!
So I decided to "upgrade" the 6L6 tubes to 6550s, along with additional filament transformer and boosted filter caps (a la Gerald Weber's book). This didn't help. (And yes, I biased the 6550s to run at about 65 ma each with no signal.)
I have eliminated as culprit every single component, including the output transformer, speakers, and power supply components, by substitution.
I have found two ways of improving matters: 1) cathode biasing the output tubes with no bypass cap, and 2) putting a very large (25k) resistor between the power supply and the two 470 ohm screen grid resistors on the 6550s to bring the screen voltage down by 100V or so. Trouble is, it seems like these things are addressing the symptoms rather than the cause. I mean, I wonder if I'm just reducing the gain.
There are amps that work properly with 6550 screen voltages close to plate voltage, right? I mean, even though data sheets say that screen voltage should be in the neighborhood of 300V?
This thing is driving me nuts. I'd appreciate any suggestions. My kingdom for an oscilloscope!
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