On Nov 24, 9:00*am, flipper wrote:
On Sun, 21 Nov 2010 03:34:51 -0800 (PST), Patrick Turner
wrote:
snip
You might want to review this article I found.
http://www.hpl.hp.com/hpjournal/pdfs...Fs/1959-04.pdf
According to them the 'standard' receiver test is with 30% modulation
and, as you can see from the graphs (about midway down), the 606A is
not capable of doing 100% modulation to 20Khz at 'rated max'
(apparently 3%) distortion.
Thanks for the link to the .pdf above.
The two 6CL6 work with low bias currents in mainly class B.
Their modulation AF signal is applied to their commoned cathodes via
a 12B4 in series with both cathodes and 0V.
The grid signal to the 12B4 is derived from a differential amp using a
6AW8 triode-pentode tube. The AF oscillator is applied to the triode
side of the 6AW8, and the detected AF signal from the AM output signal
is applied to the 6AW8 LTP side as a series voltage negative FB
signal. The differerential gain of the LTP is about 60, and is reduced
to about unity with NFB so any distortion in the envelope shape,
detector network with diode, or the LTP itself is cleaned up by the
NFB. But because the PP 6CL6 have so much distortion without NFB, the
distortion can only be reduced so much by FB.
The FB loop employed is very prone to parasitics being generated. To
overcome the likelhood of parasitic instabilities the open loop
bandwidth of the LTP diff amp has to be carefully limited and the
detected signal must be fairly free of phase shift as AF mod F rise
past 2kHz.
Yesterday, I spent some time re-designing my own smaller 2 band AM
modulated RF gene so that it includes a diff amp with incoming AF
applied to one side and the detected signal to the other. After much
time spend experimenting with some counter intuitive ideas which I
didn't think would work, then finding that they did work, I was able
to get similar performance to the 606A just using a single grid
modulated 6EJ7 as the modulator, with Ea at 220V, and Ia at about 2mA,
and working into a single tuned RF tranny which I'd made 13 years ago.
I used a 12AT7 as the LTP. Distortion becomes high when full
modulation is tried above 6kHz like the 606A. But at low mod, say 30%,
then I can get mod to over 20kHz and flat which allows testing of
radio pass bandwidth. The distortion at 98% mod using 1 kHz is at
least as good as the 606A, and at 75% it is probably less than 0.2%.
Usually the distortions in recovered AF generated in old radios is
much worse than what is produced in the sig generators so if there is
5%THD at 1 kHz AF at 75% mod, you know the radio could be doing a lot
better, and IF alignment and detector design could be improved
perhaps.
Patrick Turner.