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In Reply to: RE: Cathode bypass cap removed and I like it posted by dave slagle on January 10, 2017 at 17:50:47
Yes, Dave, I am still using the Quad 63s. The reason why I like the sound better without the cap is because of the transparency and faster transients. The overall frequency sounds very even and rich. I listen 95% of the time to chamber music and solo violin/cello. So, richness and tonal purity is important.
Over the past couple of months I have tried the following cathode bypass capacitors:
50uF ASC Oiler
175uF ClarityCap TC
65uF ClarityCap TC
10uF Russian PIO K75
I even tried to bypass these caps with a 0.47 Russian PIO and Jupiter Condenser Copper Foil. The bypassing certainly helped (at least to my ears).
All of these caps imparted its own signature sound, depending upon the listeners one might like a given cap better than the other, but there was no real standout winner. Thus, why I removed the cap completely.
Also, the previous hum in my amp is nearly gone without the bypass cap. Another side benefit.
Follow Ups:
OK.
I am not intimately familiar with the ESL63 but in general an ESL panel is a pure capacitance and the thing that makes them "difficult to drive" is actually a function of the step up transformer and the crossover that follows it. By removing the cathode bypass you have dramatically altered the behavior of the tube essentially increasing the RP from about 800 ohms to about 6K. This means your output impedance is now probably greater than the load it is driving and interestingly enough, some speakers actually like this situation.
The logical thing to do would be to use a mic and take frequency response measurement sweeps with and without the bypass cap and see if anything jumps out at you. My first guess would be that you would attenuate the HF info because of the higher output impedance working against a fixed capacitance. Unfortunately the 63's have a delay line in addition to a crossover so I fear simple theory goes out the window.
dave
Dave,
Thanks for the response. I don't have a mic to take frequency response info. Also, I have been using the 3.5K tap on my Hashimoto OPTs. The OPTs also have 2.5K tap, but after reading some of the replies to my post, it appears that I want to continue using the 3.5K tap, correct?
I am confused on fixed bias: by remove the cathode bypass cap, does this imply I have implemented fixed bias? Since I wont be tube rolling the 2A3s, should I consider fixed bias?
Pat
I'm not so sure how much it will help but you could do a frequency plot at the input of the speakers under both conditions and see if anything shows up. I have a hunch that the improvement you hear is due to the fact that you have drastically altered the behavior of the 2A3 and all of the suggestions of fixed / filament bias are attempting to solve a problem that may not exist.
dave
"Fixed bias" is a funny term.
Fixed bias is when you have a negative supply, that is usually adjustable, feeding the bottom of the grid resistor placing a negative voltage at the grid and the cathode is connected directly to ground (in your case the CT of the filament transformer would be connected directly to ground)
Self (or "auto bias", or cathode bias) is what you are doing (with or without a bypass cap).
The DC plate current flowing through the cathode resistor places a positive DC voltage on the cathode while the grid is at zero voltage DC.
The tube is now biased because the grid is negative WRT the cathode.
In the end the tube doesn't really care how the bias is accomplished as long as the grid is negative WRT the cathode by the correct amount.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre,
I found this schematic to help me understand fixed bias. What do you think of using a simple battery to set the negative bias voltage? I currently use a battery on my driver tubes, so I have no adversity to batteries. Unless you advise otherwise.
I am intrigued by the fixed bias method and want to hear what it sounds like. I just need to make sure I understand where the negative voltage should come from and how much negative voltage I should be using for the 2A3 tubes.
Pat
The amount of negative bias is dependent on how much plate to cathode voltage you will have and how much current you want flowing through the tube at idle.The schematic left out the grid resistor. The grid resistor would go between the grid and the top of the battery.
If you change from cathode bias to fixed bias on your 2a3 there will be more plate to cathode voltage because the cathode will be at zero volts where as now it's at +45 volts or so.
That will change the amount of negative voltage you will need to reach the idle current that you want.
If you use a negative supply (instead of batteries) that delivers more negative voltage than you need then you can make it adjustable and you can set it where you need it.
I hope that made sense.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/11/17
"The logical thing to do would be to use a mic and take frequency response measurement sweeps with and without the bypass cap"
That's really an excellent idea, and not too difficult if the primary concern is midrange and higher. Take the amp and one speaker to the backyard and have at it!
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