Strange feedback method

[hotbottle]

I have been restoring some old radios (as an aside, what are the best forums on the net for radio restoration discussion?) and found a mantel radio that used a feedback method for the output, that I've not seen before.

The output is a 6M5 pentode, single ended, into a typical output transformer and speaker. They have provided feedback from the speaker to the cathode of the 6M5 by putting the speaker (in parallel with the transformer output winding of course) in series with the a.f. bypass capacitor. So, the cathode circuit of the 6M5 is a resistor to ground, and in parallel, the series combination of the bypass cap then the speaker//transformer_output (with one side of the speaker grounded).

I wonder what the merits of doing this might be? Has it any place in a hifi amp?

[ErikdeBest]

This sounds like 'cathode feedback'. A search will turn up some threads: member Tubelab (George) has done some tests with it. Also Morgan Jones covers it in the design of the 'scrapbox challenge, described in Valve amplifiers, third edition.

[ray_moth]

It allows the AF from the cathode to go through the OPT secondary, while the DC goes through the bias resistor straight to ground.

[tubelab.com]

I wonder what the merits of doing this might be? Has it any place in a hifi amp?

This is one of the methods of accomplishing cathode feedback. I use this exact circuit in the SimpleSE HiFi amp. It is a type of negative feedback applied to the output stage only. In some cases especially with undersized OPT's it will really wake up the bass response without many of the sound killing attributes of global negative feedback.

[hotbottle]

This is one of the methods of accomplishing cathode feedback. I use this exact circuit in the SimpleSE HiFi amp. It is a type of negative feedback applied to the output stage only. In some cases especially with undersized OPT's it will really wake up the bass response without many of the sound killing attributes of global negative feedback.

I just looked up the circuit on your web page. Yes, that is that same arrangement. I like it - easy to implement. In fact, I have some SE amps that I have built over the years, one very similar to your SimpleSE amp, that might benefit from this. Thanks