Quote:
Originally Posted by Donut
Hello!
My friend is working on his preamp and he shows me the circuit lately. The preamp named Grounded Grid and claimed that the circuit is GG as well.
However, some people called it a mu follower.....SO what is this?? GG or mu follower?
http://i2.photobucket.com/albums/y48...e/ggpreamp.jpg
I'm really curious!!
thx! |
None of the grids in the subject circuit are at AC ground, so no grounded grid circuits here. Two that could be are at V1B & V2B, but they have AC NFB from their upper triodes. Hope I got that right, the schematic is a bit tough to read. The stacked triodes are connected as mu-followers.
Grounded grid circuits using triodes are common as RF amplifiers for VHF & UHF circuits. The benefit is isolation of the output circuit from the input, thus reducing the risk of positive FB resulting in instability & oscillation. The grounded grid provides the necessary shielding of the output from the input within the tube. The rest is up to the designer to provide a good physical layout.
The other benefit is the good match to a low impedance source. Matching to an antenna at 300 ohms (or 75) with low standing wave ratio (SWR) is important to maximize the signal transfer from the source (antenna) to the RF amplifier.
For audio, the circuit can be used to match a magnetic pickup on a phonograph or microphone to the high impedance of a normal tube circuit. But there would then be a DC biasing of the pickup or matching transformer, not the best idea.
Another use that was fairly common with amateur radio operators was to put a carbon microphone (telephone type) into the cathode of its amplifier. Something like one triode of a 12AU7 works well. I’ve done that one for a power line amplifier running at about 600 KHz. It worked very well where I was working at the time (1960).
I am a P.Eng (Electrical), hope that does not disqualify me! Got my tube design experience, 10 years in a research lab as a random designer of random circuits. My experience since then is in power & communication.
Cheers to all, John L Stewart