GU50 question

[Leevi]

The power tube cathode is grounded through the OT secondary/speaker?Is that some HiFi way to do it or is there fault in the schematic?
/Leevi

[Latole]

First time I see that. We alway learn.
sluckey may know .

[Esquirefreak]

Feedback loop?

[Latole]

Feedback loop?

It make sense.

[Leevi]

Feedback loop?

I checked the amp and it has been wired as described in the schematic.Yes, it is most likely a feedback, have never seen that before in the SE amp.
/Leevi

[kagliostro]

Ciao Risto

I asked and an italian friend (engineer passionate about tube amplification) told me:

it is a form of UL applied to the cathode instead of g2. Much lower percentages are enough to linearize the curves of the final (triodize the curves, so to speak) and improve the operation of the transformer, because it is driven by a lower rp. If I remember correctly you should also partially cancel the magnetization of the output transformer due to the power tube current, giving it a little more headroom.

Franco

[Leevi]

Thanks for the clarification Franco. I could not have imagined this.


/Leevi

[kagliostro]

Me too 😄


Franco

[danhei]

If you have Merlin’s excellent “Designing Tube Preamps for Guitar and Bass, 2nd edition”  (http://www.valvewizard.co.uk/Book1.html) cathode feedback is briefly covered in section 10.8 Global Feedback: Single-Ended Amplifier Example (Fig. 10.8).


I’ve used cathode feedback with success, showing noticeable improvement in bass response and low-frequency extension, although it probably depends on having a good enough output transformer.


In any case, it’s super easy to try in a single-ended amp.