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Has anybody needed to have gridstoppers on their 6L6 sockets higher that 1k5? Why did you do that and did it work?
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I feel like I've seen this issue before somewhereOriginally posted by deci belle View Post
Highest I've seen is 10K but it was an amp that had other issues and the builder thought it was great to stabilize it there, instead of making smaller changes along the signal path.
Marshall use stock 5K6 on some models. Never seen Tweed clones need more than 1K5. As to why it's done, the resistor in series with the grid to cathode capacitance (and the lesser grid to plate) forms a low pass filter.
A JJ 6L6 has an input capacitance of 12.5 pF - Suppose you used a 5K6 resistor in series with it, the cutoff frequency would be:
f = 1 / 2 * pi * RC
= 1/2 * 3.14 * 5600 * 0.0000000000125 (did I count those right?)
= 2.275 MHz
There are other lower capacitances in the tube. So basically you're just filtering out inaudible frequencies, it won't do anything to your tone. But you can't raise the resistor value too much or the bias source impedance will start to cause a sag in bias voltage, which may cause several issues.
It's a healthy addition and as you can see it hardly hurts your tone. -
Man, seeing your math was cool! pi really is pi!! That's a formula that didn't make my eyes glaze over, haha!!
— and plenty without it… so I find it a little depressing.Never seen Tweed clones need more than 1K5
I always thought that 5k6 gridstopper was on account of the EL34 tube itself, not necessarily the circuit. I have never thought to check if old Marshalls with 6L6s or 5881s also have 5k6 gridstoppers!
Does anybody put capacitors between the grid and cathode on the power tube's socket? I know it is done on preamp tubes sometimes. It seems kind of scary!Comment
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Never seen any. The bias voltage makes the grid negative, so if using an electrolytic, remember to have positive facing ground.Originally posted by deci belle View PostComment
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I get something like 2.274MHz
Maybe I can't count either... ha haComment
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Dumb question: for this application, does the gain of the output tube matter? I'm referencing Aiken's site and he specifically calls triode (http://www.aikenamps.com/MillerCapacitance.html), so I'm curious if it applies to pentodes/beam tetrodes. I'm also curious because I can never seem to get an answer when I do the math for this stuff that makes a bit of sense, so I figured I'd ask.-MikeComment
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good question, defaced. you probably need to consider not only miller capacitance there but the PI's output impedance, which I understand to be quite high. so studying the effect of the grid stopper filter on its own doesn't make much sense as far as frequency response goes. the cutoff frequency is likely to be much lower.... thanks for bringing that up.Originally posted by defaced View PostComment
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Well, I tried 4k7 gridstoppers instead of the 1k5 on the 6L6 sockets and that dang sheen is still there.
I guess its something else.Comment
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