[78-L] RCA dealers' phono? (OT?)

[J. E. Knox]

Greetings from FixitLand!

Malcolm Rockwell wrote:

> If the 5Y3 is the rectifier and the 6K6s are the push-pull power  
> amp (they are),
> then it only stands to reason that the other two (6SQ7s?) are very  
> probably the preamp stage.

Nope -- the schematic is quite clear: 6J5 is the first (preamp)  
stage, one 6SQ7 is a 2nd voltage-amplifier stage, the other 6SQ7 is a  
phase inverter. One could perhaps say that the three tubes together  
comprise a "preamp" feeding the push-pull output stage, but that term  
usually refers to an amplifier that brings a phono-level or low-level  
stage up to a line level. Here, we have a crystal cartridge which is  
already pretty much at line level; the 6J5 medium-mu triode doesn't  
add a bunch of gain but it is the first stage of the amp. It's also a  
metal-shell tube (rather than the 6J5-GT glass tube), and the shell  
is grounded, providing shielding. The 6SQ7s' shells are grounded too.  
That unit would likely hum up a storm if unshielded glass tubes were  
used.

Take care,


J. E. Knox
"The Victor Freak"
Engineer, MetroEast Community Media
829 NE 8th Street
Gresham OR 97030-5643
(503) 667-8848 x309
E-mail: rojoknox at metroeast.org