Sorry if this has been beaten to death or anything, but I've only started posting here recently and haven't yet gotten a feel for "what's been done a million times" and what hasn't...
Anyway, last night I installed a switch on my JVM410h so I could switch quickly back-and-forth between using the stock R106 resistor (measured at 458 ohms) vs. a 10H choke (Mercury Magnetics MC-10H). I routed the switch out through the front panel, next to the stand-by switch, so I can access it quickly from the front. After a few hours of playing here's my initial impression of the choke vs. stock resistor...
The difference is very subtle, but in a "feel" and "dynamic" way moreso than a significant tone difference.
With the choke: There seems to be a very slightly reduced gain and not quite as sharp a high-end or attack.
With the resistor: Slightly more gain and a little edgier attack - a little sharper.
Again the tone differences are subtle, but there seems to be
a little less attack with the choke and that might give a slightly different playing feel. Overall, the differences are quite subtle and can largely be compensated for by tweaking the knobs. To sum it up I'd say with the choke the amp is a shade "spongier" and with the stock resistor it's a little snappier. Application-wise, I'd say the choke is better for a more classic, vintage response and the resistor better for higher-gain, tighter metal, but it's certainly not an extreme difference either way. Based on my experience with the two immediately back-to-back, I'd say Santiall was completely forthcoming when he said a choke wasn't used on the JVM not for cost purposes but because there isn't significant enough a difference to warrant one. Of course, for purists looking for a truly "vintage feel" the choke may be a step in that direction, but you'd need to be pretty particular to get the difference in a "blind" test.
But here's the confounding factor that might negate these perceived differences: switching to the resistor vs. choke or vice versa appears to
shift the bias a mA or few. That change in bias could very well be responsible for the perceived differences between choke vs. resistor. When I get time I'll pay more attention to this and see if the bias eventually settles back to where it was after a switch from choke to resistor. If it does I'll rule out possible bias change as an influence and attribute the perceived subtle differences as truly due to the choke or resistor. If the bias does shift then I'll use the switch to conveniently adjust from a hotter bias to a slightly lower one "on-the-fly".
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