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In Reply to: RE: The McIntosh Unity coupled circuit. posted by Michael Samra on August 31, 2013 at 01:02:59
...In this case there is ~ 600 ohms loading both the anode and the cathode but if you refer the cathode load to the anode as if this were a more conventional output stage, the equivalent impedance is ~2,400 ohms P-P. 2,400 ohms while seemingly low is probably on the mark for class B 6550s.
Follow Ups:
I understand that part but,wouldn't we add the cathode impedance to plate impedance to get the total load impedance the output tubes would see?
The Deuce and the Macs are honest.Keeping it real!!
Edits: 08/31/13
If we look at the 600 ohm winding in the plate circuit as being in series with the 600 ohm winding in the cathode circuit, windings in series don't add that way.1/2 of a winding is 1/4 the total impedance so 600 + 600 = 2400
Now I'm not saying that those 2 two 600 ohm windings are in series, because there is a tube in the meddle, but if they are then it's 2400 ohms.
Let me put it this way.
If the center tapped cathode winding was 2 separate windings and we took one of them and wired it in series with one half of the plate winding (and did the same for the other side) and then built a normal PP amplifier with the cathodes grounded, the load for each Class B tube would be 2400 ohms and the P to P impedance would be 9600 ohms. (the Class A load for each tube would be 4800 ohms)
I have a pair of Freed output transformer from Krone-Hite LDS-102R amplifiers and the cathode windings are separate and are the same impedance as the separate plate windings and can be used the way I described and the impedance will be as I have described.
The same goes for the 2 identical secondary windings.
This transformers are great because you can wire the windings any way you want. Right now I have all the primary windings in parallel and both secondary windings in series and a 13 ohm load on the secondary reflects 325 ohms for my circlotron output stage. 4 6as7gs give about 20 watts Class A. Not BS Class A, but real Class A.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/31/13
...It seems clear to me that the equivalent P-P load for the stated conditions is ~ 2400 ohms. My logic: We know that the cathode and anode windings have a 1:1 turns ratio compared to each other and by measurement, a 6.1:1 turns ratio referenced to the 16 ohm secondary. If the two primary windings are placed in series, the turns ratio pri:sec is now 12.2:1 and the equivalent primary impedance reflected from the 16 ohm tap = (12.2)^2*16ohms = 148.8*16 = 2,380 ohms. This analysis appears to be in full agreement with that given in RDH4. Since both you and Paul come up with 9,600 ohms under the same conditions, I must be missing something really fundamental here.
" If the two primary windings are placed in series, the turns ratio pri:sec is now 12.2:1 and the equivalent primary impedance reflected from the 16 ohm tap = (12.2)^2*16ohms = 148.8*16 = 2,380 ohms."And if those two primary winding are placed in series with the other two primary windings we get 12.2 + 12.2 or 24.4:1 24.2 squared is 595.36 * 16 = 9525.76 ohms across the whole primary.
P.S. I should re-read Mike's post but I understood him to say that each half of each winding (plate and cathode) measured 600 ohms impedance.
If I'm wrong and each whole winding measures 600 ohms then that would be 2400 ohms for the whole thing (the whole plate winding and the whole cathode winding) in series.
Edit, I re-read Mike's posts and found this "When I measured plate to plate and cathode to cathode,it was 600 ohms total for the plate and 600 ohms total the cathode"
That means it's 2400 ohms for the whole thing and only 600 ohms for each tube.
I'm not sure how that works, a 6550 in Class B loaded at 600 ohms?
That doesn't sound right to me but I don't spend much time with anything but true triodes without feedback.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/31/13 08/31/13
...while the bifilar wound unity coupled scheme utilizes four separate physical primary windings, schematically Mac draws them as two center tapped primary windings: cathode to cathode and anode to anode. IOW there are two 600 ohm composite windings, each built up from two physical windings of 150 ohms each. What Mike measured @ 600 ohms was the value of two physical windings in series: From cathode to cathode Mike measured ~ 600 ohms and from anode to anode he also measured ~ 600 ohms. So ... There are either 4 x 150 ohm primary windings or 2 x 600 primary windings depending on how you choose to look at the situation. Mike measured from the 2 x 600 ohm perspective and I'm in agreement with his numbers. From this I arrive at the 2,400 ohm figure for the equivalent, single end to end primary impedance.Note that the cathode to cathode winding "center tap" is grounded and anode to anode winding "center tap" is fed from B+.
Edit: see schematic of similar MC240. Mike measured from "yel to yel/wht" and from "blu to blu/wht".
Edits: 08/31/13 08/31/13 08/31/13
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Actually it's 2400 plate to cathode, for each tube. That would be 9600 ohms plate to plate if it were in class A, but it's a class B amp basically so the per-tube impedance is where the tube is operating.
Sorry all; I thought the 600 ohms was from one end of a winding to the centertap of that winding. I understand now, it's from one end to the far end.
As for the windings in series, I simplified by assuming class B operation, so when one side is conducting the other isn't doing anything. The power supply has essentially zero impedance at audio frequencies, so B+ is the same as ground - at audio frequencies. Then the signal current loop ia B+ to half the plate winding to the plate through the tube to the cathode to half the cathode winding to ground which is B+. So the two half-windings are in series.
"So the two half-windings are in series."
Yes and, the way I understand it now, those two half windings are 150 ohms each so each output tube is loaded with only 600 ohms assuming Class B operation.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Paul
How did you measure from plate to cathode to come up with 2400 ohms? The cathode winding is inductively coupled to the secondary as is the plate but are both primary windings..How could we effectively measure,a summation of cathode and plate windings at the same time.When I measured plate to plate and cathode to cathode,it was 600 ohms total for the plate and 600 ohms total the cathode.Are you working in the FB winding also?
The Deuce and the Macs are honest.Keeping it real!!
... My understanding of the Macintosh unity coupled output circuit is derived from the Radiotron Designers Handbook volume four. The Macintosh circuit is discussed (and somewhat dismissed) in some detail on pages 594 through 596. On page 595 it seems to be fairly clearly stated that the overall equivalent plate to plate impedance seen by the output tubes is four times that of either the individual plate to plate or cathode to cathode windings. I can verify that Mike's raw measurements are essentially correct for the 75/275 @ 600 ohms/overall cathode or plate winding, thus my 2,400 ohm equivalent overall plate to plate figure. Have I misinterpreted the RDH4 discussion or did they get it wrong?
Edits: 08/31/13
That's what I thought. But I looked and the windings are not in series, per se.
It's 2 separate windings. One center tapped for the plates with the center tap feed by the B+ and a second center tapped for the cathodes with the center tap grounded.
Does that put them in parallel? Or are they in series with the tube as part of the chain? What does that do to the load seen by the tube?
No answers, just questions.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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