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
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
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.
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.
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.
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.
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/ARC_D51_D75_D75A_D76_D76A_manual.pdf
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/ARC_D51_D75_D75A_D76_D76A_manual.pdf
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This is a quote from the Audio Research book "Making the Music Glow", by Ken Kessler, regarding the M300 mono amplifier:
"Most notably, the output transformer design was distinguished from previous designs by featuring three center-tapped primary windings: one for the plat supply, one for the screens and one for the cathodes."
"J Gordon Holt explained in his Sterephile review of the M300: 'The output tubes are both plate and cathode coupled to the output transformed, an arrangement which allows for higher coupling efficiency, smaller windings ratios, and reduced DC magnetization of the transformer core (since current flow is opposite through the cathode and anode windings)'.
"Adds Chris Ossanna, 'Along with the higher efficiency of the output transformer, the use of 8 6550 output tunes in quad push-pull operation allowed the primary impedance ration of the transformer to be only 1,250 ohms from primary to secondary, instead of the usual 5,000 of a typical power amplifier.'"
"Most notably, the output transformer design was distinguished from previous designs by featuring three center-tapped primary windings: one for the plat supply, one for the screens and one for the cathodes."
"J Gordon Holt explained in his Sterephile review of the M300: 'The output tubes are both plate and cathode coupled to the output transformed, an arrangement which allows for higher coupling efficiency, smaller windings ratios, and reduced DC magnetization of the transformer core (since current flow is opposite through the cathode and anode windings)'.
"Adds Chris Ossanna, 'Along with the higher efficiency of the output transformer, the use of 8 6550 output tunes in quad push-pull operation allowed the primary impedance ration of the transformer to be only 1,250 ohms from primary to secondary, instead of the usual 5,000 of a typical power amplifier.'"
ARC seem to have used more than one arrangement for cathode feedback. In the Classic range the cathodes were indeed connected to ground via the secondaries of the output transformers:
As John Atkinson notes in his review of the Classic 60, this introduces a small (up to 30mV) DC offset at the speaker terminals.
Alex
As John Atkinson notes in his review of the Classic 60, this introduces a small (up to 30mV) DC offset at the speaker terminals.
Alex
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Unfortunately, CFB winding does NOT reduce DC magnetization in the OT from each tube. If it were true, then all SE designs would be done that way. Easy to check. AC signal is just the same as more or less DC signal. It has to increase/decrease the magnetization or nothing would go thru the OT.
Or one can use the Broskie technique of moving the tube "thru" the CFB winding until it is below it like a conventional configuration. Just a transposed position series arrangement. The phasing of the two winding sections is such that both contribute to magnetization the same way for the same currents thru the tube.
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An additional note about CFB windings. For P-P arrangements, the two CFB windings must be coupled equally (accurately) to the secondary since they are being used for N feedback. Any error here means permanently damaged output signal symmetry. Many P-P OTs, not intended for CFB, will not have equal leakage L from two such windings, unless a symmetrical split bobbin wind-up is used.
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I think you misread what the quote from the book says. The cathode winding is *separate* from the primary winding, and wired in reverse to the anodes.
If you remove the tube and connect the CFB and plate winding taps back together there, the full winding would have no turn reversal in it.
So one can mentally reduce the turns in one section (with the tube re-inserted) while increasing the turns in the other section with no effect on operation, except on the amount of N Fdbk. Putting all the turns into the plate winding and zero turns into the CFB section returns it to a conventional grounded cathode set-up. None of the variants cancels DC magnetization for a single tube. They are series equivalent as far as DC is concerned. Likewise for AC.
So one can mentally reduce the turns in one section (with the tube re-inserted) while increasing the turns in the other section with no effect on operation, except on the amount of N Fdbk. Putting all the turns into the plate winding and zero turns into the CFB section returns it to a conventional grounded cathode set-up. None of the variants cancels DC magnetization for a single tube. They are series equivalent as far as DC is concerned. Likewise for AC.
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It seems to me that for push pull output transformers . . .
When the DC plate currents are equal, their magnetic fields cancel.
(if the turns for plate 1 = turns for plate 2).
When the DC cathode currents to the cathode feedback windings are equal, their magnetic fields cancel.
(if the turns for cathode 1 = turns for cathode 2).
Of course for single ended, or for a single tube, the magnetic fields do not cancel, agreed.
And for Pentode and Beam Power tubes, Plate current does Not equal Cathode current (close perhaps, but not equal).
the Cathode current includes the Screen current, the Plate current does not.
This is a push pull circuit.
Now what was that about magnetic fields not cancelling?
That is why I always advise designers/builders to make sure to provide for a way to match the DC currents in the push versus the pull windings.
The key to being able to use smaller laminations is that the algebraic Total of All Ampere Turns cancels, regardless of how you do that.
Let's try and not confuse the Newbies.
When the DC plate currents are equal, their magnetic fields cancel.
(if the turns for plate 1 = turns for plate 2).
When the DC cathode currents to the cathode feedback windings are equal, their magnetic fields cancel.
(if the turns for cathode 1 = turns for cathode 2).
Of course for single ended, or for a single tube, the magnetic fields do not cancel, agreed.
And for Pentode and Beam Power tubes, Plate current does Not equal Cathode current (close perhaps, but not equal).
the Cathode current includes the Screen current, the Plate current does not.
This is a push pull circuit.
Now what was that about magnetic fields not cancelling?
That is why I always advise designers/builders to make sure to provide for a way to match the DC currents in the push versus the pull windings.
The key to being able to use smaller laminations is that the algebraic Total of All Ampere Turns cancels, regardless of how you do that.
Let's try and not confuse the Newbies.
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Yep, P-P DC is fine.
Was just finding an issue with the article quote, which might be out of context in some way anyway.
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CFB should provide a nice improvement in winding capacitance, since two ends of the windings are at AC ground, which could be placed near the secondary. 50% CFB would seem optimum.
If the CFB portion IS the secondary, then primary turns get reduced and the coupling increased. Also helpful.
Was just finding an issue with the article quote, which might be out of context in some way anyway.
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CFB should provide a nice improvement in winding capacitance, since two ends of the windings are at AC ground, which could be placed near the secondary. 50% CFB would seem optimum.
If the CFB portion IS the secondary, then primary turns get reduced and the coupling increased. Also helpful.
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50% CFB is sometimes called the Circlotron, the McIntosh or the Norman Crowhurst variation on the McIntosh. Folks looking to explore will find these interesting. Partial G2 loading/feedback is also possible, as used in the QUAD. All date to the Truman or Eisenhower administrations in USA time parlance. Lots of cool ideas.
YOS,
Chris
YOS,
Chris
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