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Mysterious connection in kathode circuit to OPT

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Posted on June 21, 2024 at 12:36:03
Gwaltinger
Audiophile

Posts: 5
Joined: December 3, 2023



Hello,

I have a 300B tube amplifier and was curious about the schematic. I measured it myself and quickly have put it to paper.

Attached is an excerpt - the output section only.

Everything very straight forward (heater is DC heated) - however, there is a strange connection of two thin cables in the cathode circuit (on the picture with the question mark) which both lead somewhere into the output transforer. The DC resistance between those two connections is nearly 0 Ohms so I gues its some kind of extra winding in the OPT?

Could it be some kind of negative feedback or is it a protection circuit if no loudspeakers are connected or like that?

Any ideas?

Kind regards,

Günther

 

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RE: Mysterious connection in kathode circuit to OPT, posted on June 22, 2024 at 06:49:40
Paul Joppa
Industry Professional

Posts: 7320
Location: Seattle, WA
Joined: April 23, 2001
 It's not uncommon to use the speaker winding(s) for cathode feedback. Tubelab is a fan, and has many posts on diyAudio.

 

RE: Mysterious connection in kathode circuit to OPT, posted on June 22, 2024 at 07:50:03
Gwaltinger
Audiophile

Posts: 5
Joined: December 3, 2023
 Hi,

When I want to try without Cfb, is it sufficient to shortcut this connection or do I have to desolder the cables (leave the coil essentially open) and then bridge the cathode connection?

Kind regards,

Günther

 

RE: Mysterious connection in kathode circuit to OPT, posted on June 22, 2024 at 08:47:34
Paul Joppa
Industry Professional

Posts: 7320
Location: Seattle, WA
Joined: April 23, 2001
 Whether the winding is the the speaker winding or a third feedback winding, shorting it would disable all the other windings. Desolder and insulate is the only option.

Without seeing the circuit, it's hard to tell what the effect might be.

 

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