Bypass the cathode resistor (or not)

Propeller Head Plaza

Technical and scientific discussion of amps, cables and other topics.

Hi,
This was part of a subthread down below discussing whether or not
to bypass a cathode resistor because it changes the harmonics. It
was an interesting question posed by John Curl ( I always read him
carefully ;)
In thinking about this, one question is: what are we trying to
accomplish? If we believe Cheever (and to some extent Hiraga),
then I think the answer is: we want to achieve a certain harmonic
pattern from our amps.
The pattern that Cheever proposes is one that matches the natural
non-linearity of the ear. This is as good a pattern as any to start
with. Incidentally, the data for this pattern was apparently gathered
from an unknown work done post Wegel and Lane. No one so far has
been able to find the reference aricle for the Cheever data
(Fig 2-2), including Cheever himself (I asked him).
One interesting side note about this harmonic pattern...I
looked at data at the website referenced below. This site has plots
of distortion for various tubes using a simple circuit. There was
only one tube out of over 50 tubes graphed that had a harmonic
pattern even close to the pattern resembling Cheever Fig 2-2.
I wanted to see if I could duplicate the Cheever pattern.
I ran some simulations of a simple circuit using a FET
in a simple voltage gain stage with a load resistor. Bias
was with a combo of battery bias and source resistor (feedback)
that I could change the value of.
What I found is that it is tough to duplicate this pattern.
Not really successful at doing it.
Using pure battery bias by itself, and the harmonic pattern dropped
off too rapidly and too uniformly. Using the pure source resistor
(unbypassed), had a pattern similiar to Cheever (the delta between
harmonics decreased with increasing harmonic), but it never matched
the deltas in the Cheever data.
Some future work might be to see what circuits and devices best
match the Cheever harmonic pattern best. Need to dig up a model
for a 45 tube.
Anyway, what I got out of this exercise was that maybe this
harmonic pattern notion is a good one to pursue. It doesn't look
like it is very common or easy to achieve. It might require a few
twists or variations on some common circuits. Maybe it will yield
some good results.
If this harmonic pattern notion looks good, then the question
about a particular modification should be: does it bring the amp
closer or further away to the pattern?
Mike

http://users.rcn.com/joepage/tubelist.htm (Open in New Window)

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

Topic - Bypass the cathode resistor (or not) - mfc 21:31:03 05/10/03 (0)

You can not post to an archived thread.