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Step 11'Mod B'--the next step
Mod A was pretty good, but I still wanted a bit more 'crunchy' from the amp. Mod B is my best effort (yet.)
Mod B has the following improvements:
--Following the example of Silvertone amps with similar tubes (1430, 1448, etc.) the first coupling cap was changed from .05 to .01uF, resulting in a good boost to the high-end.
--Along the same lines, the preamp cathode bypass cap was changed to 0.022 uF (smaller cap values here tend to lessen bass response.)
A second, larger cap (0.068uF) was added with a cut-out switch, so a fuller, jazzier tone can be added at will. The difference is subtle, but noticeable. I find it's difficult to be objective about the tonal effects of cathode bypass caps, but changes seem to alter the response, or 'tightness' of the attack, and to a lesser extent the overall tone...as though the note 'envelope' itself were subtly changed.
--The preamp-to-power-stage coupling cap was changed from .005 to .001uF. Again, more high-end boost.
--The volume control POT upped to 1 Meg (from 500K.) Since it also functions as a grid leak resistor for the power amp, this extra resistance ups the gain. Well, less signal attenuation, anyway. The additional distortion is noticeable...
Replacing with a 2 Meg POT, or moving the volume control to the preamp are other options for punching up the power stage.
--The power amp cathode bypass cap is now 10uF, further enhancing the high-end response (or actually, reducing the bass.)
Mod B has the following improvements:
--Following the example of Silvertone amps with similar tubes (1430, 1448, etc.) the first coupling cap was changed from .05 to .01uF, resulting in a good boost to the high-end.
--Along the same lines, the preamp cathode bypass cap was changed to 0.022 uF (smaller cap values here tend to lessen bass response.)
A second, larger cap (0.068uF) was added with a cut-out switch, so a fuller, jazzier tone can be added at will. The difference is subtle, but noticeable. I find it's difficult to be objective about the tonal effects of cathode bypass caps, but changes seem to alter the response, or 'tightness' of the attack, and to a lesser extent the overall tone...as though the note 'envelope' itself were subtly changed.
--The preamp-to-power-stage coupling cap was changed from .005 to .001uF. Again, more high-end boost.
--The volume control POT upped to 1 Meg (from 500K.) Since it also functions as a grid leak resistor for the power amp, this extra resistance ups the gain. Well, less signal attenuation, anyway. The additional distortion is noticeable...
Replacing with a 2 Meg POT, or moving the volume control to the preamp are other options for punching up the power stage.
--The power amp cathode bypass cap is now 10uF, further enhancing the high-end response (or actually, reducing the bass.)
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General rule is that the smaller the cap value, the more it blocks low frequencies.
(jeeze, I see that I have several duplicate component names-- two C3, two C2, etc. Bear with me, I hope this makes sense with the lousy schematic..)
How that works to filter frequencies depends on how it's oriented in the circuit:
-- caps inline with the signal, like the 0.01uF (upper most cap labeled C3), are "high pass" filters. They block low frequencies from passing forward. These are called "coupling caps", and also block any DC voltage offsets from messing with the next stage.
Changing from .05 to .01 blocks more lows from passing.
-- caps that shunt or bleed signal to ground, like the 0.001 that's attached to the tone POT (lower C2) are "low pass" filters. Those caps block low frequencies from passing to the ground--which means the the high frequencies are now escaping away from the signal path.
That's the way a passive guitar tone control works--the cap cuts the high frequencies by letting them pass through to the ground.
So in each case, small caps block more lower frequencies. It's just a matter of how they are used. Here's a simple calculator and illustration of high vs. low pass.
The other caps are cathode bypass and control grid caps for the tubes, and they're a little more tricky to explain...
One thing tubes do is color sound with even-order (mostly second-order) distortion. It's a type of distortion closest to the fundamental sound itself, and is generally found to be pleasing to the ear. Mosfets can do this too, but somewhat differently.
But a good guitar tube amps also have odd-order distortion (third, fifth, etc.), which is the "bite" or "crunch" we love.
Frequency response itself is interesting, since a guitar (or harp) amp doesn't reproduce sound faithfully--as a stereo system they sound like crap. Plus a guitar speaker normally doesn't have a frequency response above 5kHz or so. But you will hear higher frequencies--they come from the speaker cone and the cabinet / baffle board vibrating at higher harmonics.
Just to clarify, when I wrote: small caps block more lower frequencies, that's a little misleading.
Caps block frequencies below the cutoff freq. Smaller caps don't block more lower frequencies--the cutoff frequency moves higher, so more mid-range cut is added to the already blocked frequencies below.
In the chart here of a high-pass filter, the cutoff would move to the right, as the cap value gets smaller.