I am working on a Voice of Music Model 714 tape deck and noticed that the 6V6 output stage is a little different. The voice coil of the output transformer is in series with the Cathode to ground. There is also a -20v bias supply that is hooked to the control grid circuit. I have not seen this arrangement before. Can someone explain the ratianale behind this?
Thanks

I've never seen this before but it is a way to get negative feedback into the output stage for lower distortion. The -20V supply is fixed grid bias for the output stage. Common on high-power amplifiers but uncommon with a single-ended 6V6 stage like this, which would usually use cathode bias.

-David

That is an interesting and odd way to add negative feedback. Usually, its fed to the cathode of a previous amplifier stage.

As they are using the cathode circuit for the NFB, they added a bias supply to properly bias the tube.

Quite different

-Steve

Could you post the rest of the circuit? My TR-1 just has the 710, not the later one you have.

The 710 output stage is different in that they provide the audio current fed to the
R/P head during record, as well as monitor audio, or not, so the 6V6 has to
work properly over a very wide range of plate impedance .

Here is the complete schematic.

Didn't some McIntosh amplifiers in the 50's or 60's have the audio output cathodes connected to a separate winding on the output transformer as well?

Indeed, somehow my last reply got toasted. VM had some great ways to get a lot of music out of a few parts. Noted for their sturdy record changers, they also made a lot of pretty good and sturdy inexpensive record players which were much loved by kids in th 50's and 60's.

There is another negative feedback path from the tone control through a 100K resistor to the cathode of the 12AX7.
Don

That treble control may affect the recording too. Another question, since some versions are
two track stereo, is the stereo playback staggered or inline?

Those VM units were popular for schools, as they could play turntables and mikes for PA.

I may have a few of these machines that would have been *forgotten* were is not for
the query.

I had one of those when I was a teenager! It was a pretty good unit, and built like the proverbial brick outhouse.

Sure did, and they still command a premium and sound wonderful!

O/P tube cathode resistor (with parallel bypass cap) to ground via the OPT secondary is seen in some tube radios. The NFB achieved is quite small but, of course, free! However, if you want to add global NFB to an earlier stage, e.g. to a triode stage cathode, the OPT secondary may be wrongly phased (choose the global NFB option in this case.) Also, the global NFB negates the typical radio tone control as it's inside the NFB loop... I just take the TC out as who wants top-cut on an AM radio?!
Cheers,
Roger

I have a V-M 710 reel to reel and they do indeed get a lot of sound out of a single 6V6.