Hello
Presently, I rebuild monoblocks Melos amplifiers. The output stage tubes are 6KG6/EL519 tubes driven by a mosfet follower. Between the mosfet followers and 6KG6 grids, there's 20K standard wire wound white ceramic resistors. This is the first time I see such a high value grid stopper resistors. Why they use 20K resistors ? (oscillation problems ?) During the rebuilding, I thought using standard value like 1k non inductive resistors. What do you think ? Thanks.
Presently, I rebuild monoblocks Melos amplifiers. The output stage tubes are 6KG6/EL519 tubes driven by a mosfet follower. Between the mosfet followers and 6KG6 grids, there's 20K standard wire wound white ceramic resistors. This is the first time I see such a high value grid stopper resistors. Why they use 20K resistors ? (oscillation problems ?) During the rebuilding, I thought using standard value like 1k non inductive resistors. What do you think ? Thanks.
Not a common value, but there's only one way to find out if it's necessary.
Does the Mosfet have a gate resistor?
Does the Mosfet have a gate resistor?
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Are you shure they are grid stoppers ?
I think it is the resistor between source mosfet and negative supply.
Mona
Then I supose it's an amp with screen drive and the control grid on ground level.
Mona
Seems it would cause high distortion in screen drive, which should be with a low source impedance.
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That's correct. A driven screen must look into a small driver impedance to ensure screen current at all levels. A 20k resistor is contradictory in this instance.
Best regards!
Best regards!
No, but at least it explans the type of resistor.
Perhaps it's a mix of screen and control grid drive ? Without a schematic hard to tell.
Mona
Post the schematic.
Otherwise, it is like stabbing a gorilla in the dark.
And remember, not all schematics are correct.
Otherwise, it is like stabbing a gorilla in the dark.
And remember, not all schematics are correct.
If you wanted a direct drive on G1 to prevent blocking distortion or keep loading on a phase splitter to a minimum but didn’t want to drive too far into AB2 during periods of overdrive one might use a sizable grid stopper. I’m looking at that now driving 6550’s with 12BH7’s. 16.5k tail, 10k stopper. Why you would want a 5+ watt ceramic sand cast resistor would be a mystery unless it’s to be good and sure it never fails open and lets your tube redplate.