[Hallicrafters] reverse engineering of the SX-28 LF amplifier

[Emanuele Girlando (IW1DHI)]

Dear Gurus,
I am trying to reverse engineer the LF amplifier of the SX-28.
For the 6V6 push-pull everything seems ok (in line with my knowledge)
For the 6SC7 sections, something sounds me very strange.

First question:
The 6SC7 specs state typical operations @ 250V plate voltage with a bias
of -2V leading to a plate current of 2mA.
The SX-28 manual state for plates voltages of 130V(pin2&5) with a bias
of -2V(pin6) that should lead to a plate current of 0.6mA (measured values
almost confirm that). This working condition is far away from the one
recommended by tube manufacturer. Why this choice? Why the choice to stay so
low in all the values?

Second question:
As per original diagram the plate load for the first section is R37 or the
LC circuit CH2-C43 depending on the SW10 position. This configuration leads
to two completely different working conditions and introduces a level of
asymmetry with the second section plate loading (plain R36=R37). Is it
normal? Is it a mistake in the diagram? If correct, how it works?
I don't have an original SX-28 to check with.. my own has been tampered with
everywhere...

Can be of help to have look to the original Halli diagram and a redrawn
version @
http://space.tin.it/clubnet/egirland/sx28/Power%20Supply%20&%20LF-amplifier/
LF-amp.htm

Thanks in advance
Ciao.


Emanuele Girlando (IW1DHI)
http://space.tin.it/clubnet/egirland/
mirror at
http://www.qsl.net/iw1dhi/