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In Reply to: RE: Tube coupling caps reduced from 2.7uF to 1.5uF posted by Mendel on July 16, 2024 at 21:32:33
What is the value of the grid resistors that the caps feed into?
For fun let's just say they are 100k.
2.7uf into 100k makes a high pass filter with a -3db point of .59 Hz
1.5uf into 100k makes a high pass filter with a -3db point of 1 Hz
In this example there would be no audible difference caused by the capacitance value change.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Hi Tre'
Thanks for the response.
I can't answer your question about the grid resistors. The tech that did the cap change says that the amp is flat to 10Hz. I'm sure that he noted the value of the resistors.
The company that did the mods is not some tech working in his basement, rather it is a large parts supplier and repair depo, run by a guy who used to manufacture a well known line of amplifiers and preamps. So they would have a full set of test equipment and should know what they are doing.
The silver/oil caps are probably emphasizing highs, so the bass doesn't sound a pronounced right now, but give it time, they will settle in and you will adjust to the sound.
Not stressing it. Just never heard of a 2.7uF cap being replaced by one with almost 50% less capacitance at 1.5uF. Didn't know you could do that.
And I appreciate the reassurance from those in the know. Time to break in those caps...-...-
The coupling capacitor value in conjunction with the input impedance of the following stage together form a low pass filter, as Tre' pointed out. He showed that on the assumption that the grid resistor of the following stage is at least 100K ohms, which is reasonable, the low frequency cut-off is around 1Hz, even with the 1.5uF coupling capacitor value. This is much more than adequate to assure a flat response down to 10Hz. Even if the grid resistor is 50kHz, you are still in good shape. What I think may account for the slightly shrill sound quality is the presence of the .01uF Dynamicap in parallel with the Mundorf. I personally never bypass coupling capacitors because the bypass capacitor almost inevitably adds a hi-fi-ish sound that has a negative effect the sense of realism I seek. If you know how to do it, you might try disconnecting that bypass capacitor to see if you like the sound better. It's not needed, that's for sure.I see that Madisonears essentially wrote the same thing, regarding the possible negative effect of the bypass capacitor in a coupling application. I agree with him.
Edits: 07/22/24
Thank you Lew for the response.
I personally never understood this "bypass" thing myself. The only reasons I did it is because in their upgrade package for my amp Rogue bypasses these cap, and my local tech seemed to be in favour of doing it. This seems to be a contentious issue, some liking to bypass, others not.
That said, I am listening as I type this, the amp sounds superb. Far better than I remember it being before. Not sure if it's all the new caps breaking in or my ears getting used to the sound or a combo of both but I am happy so I'm not changing anything for now.
Easy to remove the bypass caps down the road or even substitute the Mundorfs for something else. My main concern was reducing the cap from 2.7uF to 1.5uF. Since that seems to be a nonissue, it opens up more options down the road. May even be able to fit a 1.5uF film cap in that spot now.
Isn't the Mundorf SG already a film and foil cap as opposed to metallized film?
They are not exactly tiny, so I imagine you could easily find a film and foil cap that is about the same size, in that value. You could probably get away with 1.0uF without losing any significant bass response, if you can confirm that the input it drives presents at least 50K ohms or better. Also, I don't categorically believe film and foil is necessarily superior sonically to a very good metallized film type, which the SG certainly is. I also like Dynamicaps and Auricaps, in that order.
Could never fit a film 2.7uF on the board. But maybe a 1.5uF film might fit.
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