Audio Research amp design question

peterVsi55 · 24th November 2020, 03:30 PM

schematic noob here....

I'm trying to understand the 'partial cathode coupled' implementation ARC uses. For example, in the VT100, schematics show the primary of the OPT sending signal back to the 2nd grid of the 6550 pentode.

What is the purpose, electrically? Any does this infer a split primary on the OPT? At what turn/percentage?

I'm not trying to replicate this design, necessarily, just understand it.

TIA

dgta · 24th November 2020, 03:52 PM

The G2 feedback is called Ultralinear mode. Google it and there are tons of articles on how it works. Not sure what you call a "split primary", it's just another tap. The % depends on the tube, different tubes have different preferred ratios.

peterVsi55 · 24th November 2020, 06:31 PM

Thanks for the thorough explanation.

As a follow-up, in ARC M300, there clearly is a winding on the OPT tied to the cathode of the output tubes (a sperate winding is tied to the grid for ultra linear operation).

What purpose does this serve?

rayma · 24th November 2020, 06:59 PM

ARC has always used partial cathode coupling in their output stages. The center tap of the OT secondary
winding (4 ohm tap) is grounded, forming equal and opposite voltages on the 0 ohm and 16 ohm taps.
These are connected in the cathode circuits of the output tubes with a polarity that gives negative feedback.
While lowering distortion, this does require higher grid drive voltage.

PRR · 24th November 2020, 10:20 PM

When plate and screen may run at same DC voltage we just use a tap on the plate winding. This is by far the most common deal.

At very high power/tube it may be better to have screen DC much lower than plate. This can be done with an additional winding.

egellings · 24th November 2020, 11:15 PM

Cathode connection of the secondary provides a local feedback loop for the output stage and does not contribute to overall loop instability.

peterVsi55 · 9th December 2020, 12:44 AM

Sorry for the long pause.

There seems to some that think the cathode of the output tubes is connected to a separate winding on the primary, the purpose being to a) minimize the impedance of the entire circuit, lower the windings, less inductance, etc and b) lower the DC entering the transformer, lowering the saturation level. By most accounts, this is a more direct coupling of the tubes to the speakers. It also taxes the driver stage & runs the output stage hard.

That said, it's still difficult to determine, mathematically, what the particulars on that separate winding for the cathode are.

rayma · 01:51 AM

There is no separate cathode winding, it is the same secondary that feeds the speaker. See the schematic.
All of their amps were the same in this respect ever since their Electronic Industries days and the original D-100.
https://www.arcdb.ws/Database/D76/AR...76A_manual.pdf

peterVsi55 · 9th December 2020, 01:42 AM

The M-300, which is more recent design than the D-100, does have a primary winding connected to the cathodes (reversed). This is a more recent design than the D-100.

rayma · 9th December 2020, 01:50 AM

That winding is indeed separate from the speaker winding, but it is still a low impedance low voltage
secondary winding, like the McIntosh and Luxman transformers also have.

24th November 2020, 03:30 PM	#1
peterVsi55 diyAudio Member Join Date: Aug 2020	Audio Research amp design question schematic noob here.... I'm trying to understand the 'partial cathode coupled' implementation ARC uses. For example, in the VT100, schematics show the primary of the OPT sending signal back to the 2nd grid of the 6550 pentode. What is the purpose, electrically? Any does this infer a split primary on the OPT? At what turn/percentage? I'm not trying to replicate this design, necessarily, just understand it. TIA

9th December 2020, 01:32 AM	#8
rayma diyAudio Member Join Date: Apr 2011	There is no separate cathode winding, it is the same secondary that feeds the speaker. See the schematic. All of their amps were the same in this respect ever since their Electronic Industries days and the original D-100. https://www.arcdb.ws/Database/D76/AR...76A_manual.pdf Last edited by rayma; 9th December 2020 at 01:51 AM.

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24th November 2020, 03:52 PM	#2
dgta diyAudio Member Join Date: May 2011	The G2 feedback is called Ultralinear mode. Google it and there are tons of articles on how it works. Not sure what you call a "split primary", it's just another tap. The % depends on the tube, different tubes have different preferred ratios.

24th November 2020, 06:31 PM	#3
peterVsi55 diyAudio Member Join Date: Aug 2020	Thanks for the thorough explanation. As a follow-up, in ARC M300, there clearly is a winding on the OPT tied to the cathode of the output tubes (a sperate winding is tied to the grid for ultra linear operation). What purpose does this serve?

24th November 2020, 06:59 PM	#4
rayma diyAudio Member Join Date: Apr 2011	ARC has always used partial cathode coupling in their output stages. The center tap of the OT secondary winding (4 ohm tap) is grounded, forming equal and opposite voltages on the 0 ohm and 16 ohm taps. These are connected in the cathode circuits of the output tubes with a polarity that gives negative feedback. While lowering distortion, this does require higher grid drive voltage.

24th November 2020, 10:20 PM	#5
PRR diyAudio Member Join Date: Jun 2003 Location: Maine USA	When plate and screen may run at same DC voltage we just use a tap on the plate winding. This is by far the most common deal. At very high power/tube it may be better to have screen DC much lower than plate. This can be done with an additional winding.

24th November 2020, 11:15 PM	#6
egellings diyAudio Member Join Date: Aug 2010 Location: CA	Cathode connection of the secondary provides a local feedback loop for the output stage and does not contribute to overall loop instability.

9th December 2020, 12:44 AM	#7
peterVsi55 diyAudio Member Join Date: Aug 2020	Sorry for the long pause. There seems to some that think the cathode of the output tubes is connected to a separate winding on the primary, the purpose being to a) minimize the impedance of the entire circuit, lower the windings, less inductance, etc and b) lower the DC entering the transformer, lowering the saturation level. By most accounts, this is a more direct coupling of the tubes to the speakers. It also taxes the driver stage & runs the output stage hard. That said, it's still difficult to determine, mathematically, what the particulars on that separate winding for the cathode are.

9th December 2020, 01:42 AM	#9
peterVsi55 diyAudio Member Join Date: Aug 2020	The M-300, which is more recent design than the D-100, does have a primary winding connected to the cathodes (reversed). This is a more recent design than the D-100.

9th December 2020, 01:50 AM	#10
rayma diyAudio Member Join Date: Apr 2011	That winding is indeed separate from the speaker winding, but it is still a low impedance low voltage secondary winding, like the McIntosh and Luxman transformers also have.