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In Reply to: RE: I got the info I wanted on the Mac output stage posted by Michael Samra on May 06, 2016 at 22:26:22
he said you can't add the readings together because the windings are in parallel but if they were in series you would and he explains why..
After designing transformers and winding MC output for around 30 years, I can tell you that the output stage and OT do not have real voltage gain...
Voltage Gain is approx .75 .....
This is due to the plate winding and the cathode winding are in PARALLEL and have identical AC signals across them during dynamic operation...if not then there would be cross currents in the windings that would over heat the windings...Those little inductors on the plates take up the small difference between plate and cathode AC volts to keep the windings balanced and equal, thus preventing AC cross currents...The little plate inductors account for around 4 to 5 AC Volts...
If the winding were in SERIES, then there would be a considered gain..then the plate load would not at 1/4...
The reason you see signal on the OT with no output tubes is because the follower has been enhanced with "Bootstrapping" from the Cathode winding...when it is properly operating the cathode will inject signal current into the cathode resistor center point to bootstrap it and make it appear larger then what it really is, the effective AC impedance of the follower load increases significantly, thus linearizing the follower signal.... With no output tubes the follower will drive the cathode winding through this connection and appear in the output windings..no harm will be done...
Early MC30's and MC60's do not bootstrap the same way as they inject in parallel and thus have a tougher battle since they inject through a 220K .... later circuit revs will use the big .47uF caps to AC couple the bootstrap, in series and provide even more feedback for better bootstrapping affect..ie more linear drive stage..
When measuring these circuits you need a isolated differential probe such as a Tektronix P5205 .... You cant use a single ended scope probe unless your measuring only the cathode winding and in that case you need to float the ground on the MC amp to prevent ground loop with the scope...
michaelsamra ...your MC60 OT like all MC outputs is just a single C-core...no double C-cores... I would post a picture of a real MC60 OT ...but dont have a URL of it ..only a photo in my drive..
Last edited by cerrem; Today at 06:41 AM.
A friend you get for nothing,an enemy has to be bought
Follow Ups:
First of all, the windings as loaded are in series...as to the rest, I'll just put this guy on the list with Doc, and hopefully, only for the time being, you Mike.
He is right about needing an isolated diff probe set though....
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"
A friend you get for nothing,an enemy has to be bought
Got much better things to do than worry about that guy, who's not posting here. That circuit is not all that complex. It was developed to run full pentode rigging of the power finals while delivering low output Z and linear output. The additional complexity was conjured to deal with the low gain that particular circuit has.
There is nothing so special there it can not be understood. Actually it is the opposite; it is a very elegant means of solving that problem.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The new source you're quoting is tough to follow. His description of the function of the plate chokes seems to be incorrect. Additionally, not all Mac amp designs used them. The later versions of the MC225 had none and other models used simple solenoid chokes of equal value.
The stated reason for not adding the cathode winding voltage to that of the anode winding also seems wrong. Both windings contribute to the output and both need to be considered.
The description of the connection of the bifilar windings doesn't make sense and is inconsistent with the diagram in Mac user manuals. I haven't reverse engineered any Mac OPTs so I can't verify from direct experience who might be correct but the Mac diagram makes sense.
The source also mentions that a differential probe is necessary to properly measure a-k and g1-k voltages. A properly insulated battery operated voltmeter is inherently "differential" so I have no idea why a special differential probe would required. A scope isn't necessary to make a gain measurement.
I take it from all this that you've made no measurements from a-k and g1-k as previously discussed, yes? Regardless, I'm going to re-measure one of my Macs and report back. Think an MC40 is representative enough?
Steve
Sometimes it's easier to talk to the individual than derive things from a 2 paragraph post.Many of us don't always translate our actual thoughts to paper in the best way as we leave a lot of things out that we take for granted that the reader nay already know..I many times leave thoughts out and I have to go back and reedit..
A friend you get for nothing,an enemy has to be bought
Steve
The choke explanation would be between the lines a bit he didn't say all Mac amps used them.We were talking about the Mc30 when he chimed in..It seems as those that wind the Mac opt trafos seem to have a different perspective from those that don't wind the transformers including Doc Hoyer.
Give him a call when you get a chance and let him explain it..I see from a point of a view where this could have a 2 to 1 gain when you look at from the aspect of being similar to cathadyne PS but since there are other variables at play,you may not be able to do that..I don't know who is right or wrong at this point.
A friend you get for nothing,an enemy has to be bought
Edits: 05/08/16
"Those little inductors on the plates take up the small difference between plate and cathode AC volts to keep the windings balanced and equal, thus preventing AC cross currents."Who are you quoting, Mikey? Those are parasitic suppressors, roughly 2uH. In no way do they balance the windings.
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Edits: 05/08/16
Jack
Yes they are for parsitics but mentions they also have another purpose..
The guy is a transformer engineer and his forum stage name is posted by edit..He has wound mac transformers as well.
A friend you get for nothing,an enemy has to be bought
Mikey, they have no other purpose. 2uH at 20kHz looks like 0.25 ohm. At 50 Hz, it's 0.0006 ohm. These chokes aren't intended to have any effect whatsoever at audio frequencies. I'm highly skeptical whether your transformer winder knows any areas of electronics outside his field of expertise.
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Buy Chinese. Bury freedom.
Edits: 05/08/16
That was my guess it was wrong because when you remove the choke and put a jumper in,it oscillates even at idle with no signal.The guy doesn't appear to be stupid but we may misinterpreting what he is saying.There may be some balancing affect because why the inductors different values on the two tubes?
A friend you get for nothing,an enemy has to be bought
Edits: 05/08/16
"There may be some balancing affect because why the inductors different values on the two tubes?"
It's likely the design team had a criteria for stability that covered a certain range of variables. The variable(s) might have been plate voltage, grid voltage, or something else. Whatever it was, they probably would have varied those parameter(s) over whatever window they deemed necessary while hunting for spurious responses. The two different values might simply have been chosen empirically as those that produced the most stable amplifier. In fact, the need for different values might be the result of a slight imbalance in transformer winding C, but that doesn't mean such an imbalance can be corrected in this way. Clearly, the chokes are only what was required to make the amplifier stable. One thing interesting - I was reviewing the values of the chokes this afternoon online, and it appears Mac made them the same in later products.
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Buy Chinese. Bury freedom.
I got one from rat shack out of the multipacks they had for like 3.00 and it worked..I broke it pulling it out putting in a new socket but it was a much better quality choke than was in it from the factory as far as build..Not that you can tell much from a tiny plate choke..
A friend you get for nothing,an enemy has to be bought
Edits: 05/08/16
About cerrem
Biography
Analog IC Chip designer, Power supply and magnetics engineering, Defense contractor
Location
San Diego, CA
Interests
SCUBA, Skydiving, Paramedic, M-14 rifles, chics, single malt scotch, TUBES
Occupation
Electrical Engineer Aerospace
Country
United States
A friend you get for nothing,an enemy has to be bought
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