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This amp is running 4X6550. After the bias splitters (150K; schem shows 82K), the wires go directly to pins 5 of the two middle tubes and then through a 1K5 to either outer tube. The schematic shows 4 56K grid resistors on each tube. This amp only has a pair of 1K5 going to the outer pair of tubes. It looks like it came from the factory that way.
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I guess my question is why do some Marshalls have grid resistors on only two of the four power tubes?
(Oh, and the schematic is hard to read; they're 5.6K, not 56K, and no, there isn't 4 on each tube ;o)
I've worked on others that were wired this way, and and the schematic matched. -
Best guess is it's a holdover from GEC schematics published in the 50's. Still doesn't answer WHY, I know.Originally posted by Dave Curtis, dB AudioTech View Post
Purpose of "stopper" resistors leading to output tube grids is to minimize the possibility of ultrasonic/RF oscillation, right? And it's good practice to use carbon comp resistors in stopper service. So the mystery deepens, why leave some out?
I've run across old Marshalls & other UK made 4-output-tube amps that have one screen grid resistor per pair also. It's sensible to make 'em all identical. Doesn't hurt to change those bias distro resistors to 82k or 100k to keep a tight control on those output tube grids.This isn't the future I signed up for.Comment
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The reason to have one resistor between screens of one side is to prevent the tubes from interacting.Education is what you're left with after you have forgotten what you have learned.Comment
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Yup.
The later 60's Plexi models had no grid stoppers on the 50watt amps, like my '69.
When they started making 100w amps, they added grids to only two of the output tubes before finally putting them on all four output tubes for 100w amps.
Remember, they were kinda' new at this stuff an experimenting all the time!Comment
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