READER FEEDBACK Continued from page 9
state gear, well designed and made tube equipment can
keep this to inaudible levels.
Audiophiles (people who desire near perfect sound
reproduction) sometimes also use tube preamps and
power amps, and they would not tolerate these noises at
any volume level. What sounds really absurd is that guitar
players would generate several to many watts of audio
power, only to dissipate it all as heat and only use a small
sample of the signal voltage to drive another amplifier
input. More needs to be done to understand the
distortion of overdriven tube power amps and duplicate
this on a preamp or line level circuit.
The output transformers used in tube guitar amps are
smaller than those in high fidelity amplifiers using the
same output tube types and having the same power
capabilities.
As a result, their cores saturate more at lower
frequencies, making the lower notes even more fuzzed
than the higher ones. Do the current distortion processers
simulate this too?
Michael Kiley
Crestwood, IL
Response: I agree that not all tube equipment is inherently
noisy. I have a tube-based 'audiophile' headphone amp
that is simply amazing.
However, when it comes to guitar amps, someone
interested in clean, pure tones isn't going to invest in a
tube amp. Solid-state amps are a better choice. It's for this
reason you won't find many tube-based amps for acoustic
guitars. Electric guitar players after 'vintage' tones rely on
vintage designs, including the easily saturated output
transformers.
I've worked with a number of high-end distortion
processors over the years and — although they're getting
closer — they have yet to achieve the same complex sound
quality provided by a tube amp running fully saturated.
The practical question, and one that I've grappled with
more than once, is how much is the gap in sound quality
worth?
Bryan Bergeron
CHIRPER CHATTER
How can the basic test circuit (Figure 17) for the
November 2012 Chirper project be made adjustable
within a small range for both time on and cycle rate? My
objective is to make my own flasher controller that can
be adjusted from slow (0.5 Hz) to fast ( 2 Hz), and the
duty cycle for the turn signal light to be on from 25% to
75% of the flash rate. Some of this range may not be
suitable, but I want to see what this could be like. Thanks.
Tedd Lovell
The Dalles, OR
76
February 2013
Response: As it turns out, changing the frequency and/or
duty cycle of a 555 based astable oscillator is as easy as
changing two resistors. For the moderate changes you're
talking about, you wouldn't need to change the capacitor
(C2).
If you want continuous variability and you have some
means (an oscilloscope) to be able to measure the
waveform, I'd add a couple of potentiometers. I'd put a
200K pot in series with R3, and replace R4 with a 4.7K
ohm resistor in series with another 200K pot. You would
have continuously variable 'on' time from about 65 ms to
about 2. 8 seconds, and continuously variable 'off' time
from about 32 ms up to about 1.4 seconds. The pot
adjustments are interactive, so you'll need to play with
them to get the exact timing you want to see.
The 555 won't allow duty cycles below 50%. For that,
use an inverter on the output (I'd use an NPN transistor
and a couple of resistors); that'll reverse the output so that,
for example, a 60% duty cycle becomes 40%.
A trick for observing the waveform using a non-storage
oscilloscope is to scale the time with the capacitor. For
example, using a .01 µF capacitor in place of the 10 µF will
speed it up by a factor of 100. Instead of a one second
cycle, you can view it at 10 ms.
You can tweak the adjustments at this faster speed,
then replace the capacitor and scale it back to one
second. Be aware, though, that unless you use precision
capacitors, the scaling factor won't be exactly 100 as it will
be affected by the capacitor’s tolerances.
Check out TI's LM555 datasheet. It's available online at
www.ti.com/lit/ds/symlink/lm555.pc.
Good luck and have fun!
Mike Huddleston KJ4LN
CONTEMPLATING IN VANE(S)
I would like to add something to Bryan Bergeron’s
discourse on Crookes' radiometer in his January 2013
editorial. I don't think there is even a partial vacuum
inside.
Radiation falling on the black side of the vanes is
absorbed, the black side warms up, and consequently
warms the adjacent air which expands and in so doing,
pushes on the black side of the vanes. This does not
happen on the shiny side of the vanes as the radiation is
reflected. With no expanding air pushing on the shiny
side, the radiometer vanes will rotate towards the shiny
side.
Now to the interesting part. Imagine that you could
pull a really good vacuum inside the radiometer. Also
imagine that you could get rid of bearing friction (think
magnetic levitation).
Radiation has momentum. The relation between the
