On 2015-11-23 09:42:31 +0000, Jim Lesurf said:

In article , Arthur Quinn
wrote:

Apropos the brightness - a spice simulation of the tone control stage
with both controls centred shows a rise of about 2.6 dB between 30Hz
and 3kHz caused by the loading of the feedback network on the split
collector resistor.

Since you've said that I'll add that a further complication is that some of
the actual tone control components chosen meant that the actual response
varied a bit from one example to another. Question of tolerances and type
having a real effect. Example of how the reality can depart from the
diagram! :-)

...And also to some extent, with time and use, particularly for the
electrolytics. The caps were actually selected and paired, but would still
vary in value.

Then add in that the pots - particularly the old 'AB' ones - didn't always
get the 'flattest' response when the control showed to be at "12 o'clock".
This also varied from pot to pot. Precision only really arrived with the
better Alps pots, which arrived on the scene long after the 600 was
launched. (One of the changes I made was to use the better pots. etc, from
Alps.)

The 600s were made via waterfall flow soldering onto boards. But when you
factor in the component selection and that some components would then be
changed 'on test' you could argue they were all hand-made or at least
hand-tweaked.

And then in practice the bulk of any slight 'brightness', etc, could
usually be corrected by a slight tweak of the controls. As shown by the
measurements many reviewers made that found the response could be a *lot*
flatter than 2.6dB. At the time it was assumed that any sensible user would
simply adjust the controls to get the results that they liked best. Alas,
reviewers may not always fall into the "sensible user" category. :-/

TBH I came to the conclusion years ago that the reason 'subjective'
reviewers whined about tone controls "degrading the sound" was that they'd
found that a tiny twitch of the tone controls changed their main
"perceptions' of how amplifiers "sounded different". Thus risking putting
them out of a job. 8-]

Jim

Yes, while that stage can't be made completely flat it can be made
nearly so, with a dip of under 1dB around 200 Hz.

It is remarkable how the quality of components has improved since those
days and how the effective cost has fallen.


Arthur

--
real email arthur at bellacat dot com