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In Reply to: RE: KBG-MN Incredible as coupling capacitor!!! posted by banpuku on April 22, 2017 at 14:02:06
I use those 30uf 160V MBGOs a lot for cathode bypass, when I can't eliminate the need for cathode bypass. I parallel 4 of them to bypass 300Bs. You'd be fine with 2 in parallel. It'd help to know what bias voltage you have across them, for instance the 300Bs were biased at 75V. When you parallel them you reduce their ESR (a good thing).
twystd
Follow Ups:
My 2a3 tubes are biased at 50V, so I should be fine. I like the idea of using only 1 for cathode bypass duties, but I will order 2 per channel so I can parallel them just in case. thanks for the input, as I have been looking for a capacitor to replace my ClarityCap TCs which is more suited as a power supply cap.
Since capacitors are basically many plates in parallel, there is no harm in paralleling them. 50uf is very low for bypass caps on a 2A3. No harm if you are running subs, but if you are running them full range they will be bass shy. For full range, parallel 3 or better yet 4, they are cheap enough.
twystd
What do you think about the following for a cathode bypass cap parallel combo:
30uF MBGO
30uF MBGO
10uF K75-10
Would the 10uF (being the smallest value) have a discernible difference versus using 3x 30uF MBGO? The sonic qualities of the K75-10 are very very good. I am not sure if the MBGOs are of similar quality or not.
Usually bypassing caps is done if there is a roll off of highs, and the value is significantly lower than you propose (~.1uf). Also the K75-10s have a wonderful mid range, but they do roll off slightly in the highs, so don't know what you'd hope to accomplish with the k75. Of course you could always try it. I'd compare with 3 (better yet 4) of the 30uf MBGOs. Of course it would help if I knew if your amp runs full range or not. Even 3 MBGOs in parallel will probably still introduce some phase shift in the bass. Again working in a vacuum here.
To know the optimum value bypass cap it'd be nice to know your cathode resistor value, the operating points of the 2A3 (so I could refer to the plate curves to determine Rp), the impedance of your output transformer, and the lowest frequency your speakers can reproduce.
twystd
My amp schematic is attached. Here are my operating points:
driver tube
RCA 12bz7
plate voltage: 208V
grid voltage: 2V
current: 1.5mA
output tube
JJ 2A3-40
plate voltage: 320V
VDCp-k: 270
grid voltage: 50V
current: 50mA
Output Transformer
Hashimoto H-20-3.5U
Can be set to either 2.5k or 3.5k Ohms
my speakers are quad ESL-63s. They go to 40hz -3db. Subjectively, the amps and speakers play low bass (100-50hz) fairly well. Since I listen to chamber music almost exclusively, I don't get much bass below 50hz. I have not tried any cathode bypass caps with a greater value than 65uF. So, I am not sure if 2x or 3x or 4x 30uF MGBOs is the way to go. Any input would be appreciated.
Looks like your transformer is set up for 2.5K, that's awfully low for a 2A3. 2.5K will give you more power than 3.5K but it will have more distortion. Your quads aren't all that efficient, who knows you may prefer more power rather than lower distortion.
twystd
Replace your .47uf K40Y coupling cap with a smaller value ~.1uf. I'd really recommend a SSG silver and mica. Too big of a coupling cap is as bad as too small. In my experience big couplers reduce speed and microdynamics, with that 330K grid leak resistor on the 2A3 .1uf should be fine. Second make sure that the cathode bypass cap bypasses the cathode resistor by being connected to ground on one lead, and the top of the 1K cathode resistor with the other lead. I have calculated the value to be ~80uf to not have a phase shift at 40hz, so I'd parallel 3 of the 30uf MBGOs. I'd be willing to bet that you will like what you hear.
Update: I replaced the 0.47uF K40Y coupling cap with a 0.22uF K40Y cap. While the 0.22uF cap needs to be broken in, there was a perceived improvement in the dynamics and transients. Sharp attacks appeared my startling and noticeable.
I also discovered that using multi-stranded silver coated copper hook-up wire is a no-no in my system. I switched over to using solid 99.99 silver and the sound improvement was very noticeable. No more glassy, etchiness in the upper mids and lower treble region. For whatever reason, my amp does not like the silver coated copper hook-up wire.
I think you will find a similar improvement in coupling if you go to .1uf, and really, the SSG silver and micas are worth a try. BTW, I don't like to use stranded silver coated hookup wire either, my preference is for dead soft 99.99% solid silver as well (teflon tubing for insulation). Dead soft copper solid wire works well too, when you can't afford silver. I have just soldered up 33 step TVCs and switches. That took right at 50' of wire, just couldn't justify the cost of silver there, especially since the windings themselves are copper.twystd
Edits: 04/28/17
Are you sure about that schematic. Looks to me like one end of the cathode bypass would go to ground like you are showing, however the other end should be at the top of the 1K cathode resistor. Even if you were doing some type of ultrapath amp, the cap would go from the power supply and transformer node to the top of the cathode resistor.
twystd
You are correct, the schematic incorrectly shows the cathode bypass cap going to ground when it should be at the top of the 1k resistor, my bad.
I have a 0.22uF cap that I can use to replace the 0.47uF cap. I will give it a try.
Easy enough to try 3x or 2x 30uF MBGO caps.
Thanks for all the input and time. Great stuff!
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