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I've been playing my homebrew 5G9 for about 5 months now. I love everything it has to offer, but something I've noticed is that the bass response is monstrous. Generally, I love this, but it's a little overpowering when I'm trying to turn the amp up loud enough to get tone flowing, but also trying not to make the neighbors mad. If it's possible, I'd like to make a coupling cap switchable between two values so I can have a bass reduction mode. Is there a particular capacitor in these tweed circuits that has the most influence on shaping the bass response? Or would I need to make lots of the coupling capacitors switchable? Any help on this would be awesome.
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First cathode bypass cap is a good site. You can also separate the two cathodes and use different cap values to shape the bass as you want. -
Numero uno.Originally posted by Pedro Vecino View Post
Standard is 25uf/25v.
The standard 25uf will keep the ac signal from going into degeneration.
I keep an an assortment on hand. 15, 10 & 4.7uf.
The lower the value, the less bass will pass.
If that doenn't work you have coupling caps & the tone control to play with.Comment
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Thank you both for the info. Would you expect the 10uF to be a significant difference from 25uF, or 15uF compared to 25uF even? I love how it sounds as is. But, I'm not afraid to include a configuration that makes the bass pretty controlled at higher volumes even if the "correct" tone isn't left intact. I can feel the walls rumbling with the original specs.Comment
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To hear a significant difference you'll probably need to go to a 2.2uf or smaller. This is because the stock cap is so large as to bypass frequencies well below what's audible. You probably won't hear much if any difference between 10uf and 25uf. A bypass cap on a typical preamp cathode gives about 6dB boost to the bypassed frequencies. So by reducing the size of the cap you'll get a 6dB cut on bass frequencies below the caps knee frequency.
If it's a switch you want, you could solder in the 2.2uf as the full time bypass cap. Then wire a 22uf cap (or even your stock 25uf cap) in series with a 100k resistor parallel to the 2.2uf cap. The switch should close the 100k resistor. This arrangement should minimize switching noise. Try smaller caps (1.5uf, 1uf and .68uf) for a different effect and choose what sounds best to you."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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Had the same problem in a tweed twin, ended up lowering the bypass caps, but the final change was to change the cap to the bass control to an .02 and use a Mallory and not a Sprauge. The Mallory's seem to be a brighter cap and gave the bass a noticeable change for the better, Much more alive.Comment
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Let me make sure I'm understanding this. Currently the first bypass cap is the 25/25 parallel with an 820R. You're suggesting adding a 2.2uF cap in parallel with a 22uF which would replace the 25/25. Also adding in a 100k resistor in series with the 2.2uF. One side of the switch allows current to flow through the 820R, 22uF cap, 2.2uF cap, and 100k resistor all together. The 2.2uF cap and series 100k resistor act as one unit parallel with the 22uF cap, or is the 100k resistor in series after the two parallel capacitors?Originally posted by Chuck H View Post
Does this setup basically make the amp see "one" capacitor very close to 25uF when both are switched in?Comment
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Interesting. I haven't done much reading on altering tone controls, but I suppose something could be done to change the way a single "Tone" control shapes the sound?Originally posted by pops View PostComment
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No. The 2.2uf cap is the bypass cap. Then the stock cap in series with a 100k resistor would be installed parallel to that. With a 100k resistor series with the bigger cap all the circuit will see is the much lower impedance of the 2.2uf cap. Then a switch wired across the resistor can be closed to parallel the bigger cap with the smaller one. This arrangement keeps both caps charged to reduce popping.Originally posted by wcrankshaw View Post"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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Sorry, i forgot your amp has only a tone control. You could remove the bypass caps altogether for a flatter response and warmer sound. You will have less power so you can crank your volume more, less bass and warmer highs.Originally posted by wcrankshaw View PostComment
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Like this
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The coupling caps coming from the plates of the the preamp tube (12AY7) are they .1 and what brand? You could change them to .02 and use a Mallory they seem brighter to my ears.Comment
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I use the same circuit but with the switch closer to the ground point. Simplifies connections and facilitates the use of coaxial cable if is needed.Originally posted by Jag View Post
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For mine, I made the V1 cathode bypass cap 2.2uF and the 'bright' channel coupling cap .047uFBuilding a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- EnzoComment
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The best place to eliminate excessive bass response is in the cathode cap of the first stage. THis will tighten things up AHEAD of any distortion products to eliminate the "rolling" sound of excessive IM distortion when you turn things up and it gets crazy. You can make the cathode cap selectable with a pull switch on the pot, a nice, nondestructive method. I suggest something like .68uF, and then use the pull switch to parallel a much larger cap across it.Comment
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