Dear RATs,

some weeks ago I started a thread Error in schematic?
or something like that.

Some answers had been very interesting, some had been below
the belt ... anyway, as I wrote I duplicated/breadboarded
the circuit:

http://www.ndh.net/home/schlangen/ro...et/bench02.jpg

since the author didn't/couldn't respond to my
questions due to an overflowing mailbox above quota.

Meantimes, I did a lot of measuring and calculating on that
circuit, and yes, there is at least one error in the schematic,
but that isn't the topic of this article and left to the
reader to find out himself, if interested to do so ;-)

Also, I did a lot of reading RDH4 and calculating thru quite
some grounded cathode combined gain/driver stages to be found
on the web in quite similar GC circuits to the
one I breadboarded. I duplicated lots of them using
6SL7 or 12AX7 for roughly comparable gain, too, and
calculated and measured and compared thru them.

There is something I am wondering about, and I couldn't
work out the reason for so far or find it in RDH4:

Quite some GC gain/driver stages, mostly newer designs, use
very low anode current ratings in actual designs; e.g 6SL7
(one system) below 1mA, also for ECC83/12AX7, below 1mA
per system, too.

Looking at datasheets, these low current operation points
are often way below suggested values for these tubes, and
datasheet nomograms show less-than-ideal curves for such
low anode currents: These curves sometimes warp like
snakes (ahem) in low current ranges.

I can't measure distortions (I don't have the measuring
gear to do so), but the result of such low current
operation points should result in quite higher
distortion values than possible/optimal, am I right?

I suspect one reason being that many "newer" designs go
for as-high-as-possible gain values in such stages, with
very high value anode resistors to get most gain (thus
resulting in low plate current), and the authors don't
thinking about negative side effects, like higher distortion
due to nonlinearities.

Is there a reason for excessive low plate currents
in GC driver stages I overlooked?

Tom

--
Okay, maybe i am paranoid. But that doesn't mean
they are not out to get me. - unknown