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In Reply to: RE: KBG-MN Incredible as coupling capacitor!!! posted by twystd on April 26, 2017 at 15:01:48
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.
Follow Ups:
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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