I wanted to mention that the driver on the Mc60 is bootstrapped as well and that is using the dual C Core but I did the measurement on the Mc30..Isn't the Mc60 bifilar and the Mc275 trifilar?
If there is any voltage gain it is very little comparison to the conventional output stage.
I measured this huge rms voltage at output grids of the Mac and on the Dynaco I measured a very small number..I don't doubt you can make the output stage amplify voltage in this type circuit,I just don't get why MAc would over stress the driver just to get better numbers because that can be done in the output stage.
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That's why I adviced to read more into the subject.
McIntosh designed the unity coupling concept with a reason; please read this:
http://www.cieri.net/Documenti/Sche...s of a new 50 watt tube amplifier circuit.pdf
It is about what they call "notch distortion" in a class B amplifier.
McIntosh designed the unity coupling concept with a reason; please read this:
http://www.cieri.net/Documenti/Sche...s of a new 50 watt tube amplifier circuit.pdf
It is about what they call "notch distortion" in a class B amplifier.
Thanks Pieter,,Will do. I was going to try one more thing and take the signal generator set up to deliver 2 volts to the grid, take a diff probe and measure anode-cathode on either of the finals.Lets say I pull the 12Bh7 out as don't want it to influence the reading in anyway.Would that be a possibility?
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..
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..
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MC30, MC40: each of the primary windings is 1K ohms, end to end.
MC60, MC75: Each of the primary windings is 600 ohms, end to end.
MC3500, MI350: Each of the 5 primary windings is 150 ohms, end to end.
I've had all of these OPTs at one time or another, and have done measurements on them all.
A DIYaudio member named "Funker" wound a couple of MC3500 outputs from scratch. He may not have gotten the exact McIntosh winding specs correct (his were even more intricately wound than the originals) but his homebrew amps have output specs at least as good as McIntosh's original amps. Search this forum for his thread; an interesting read.
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In the same thread you will find my experience with the shorted MC275 EI output transformer.
This transformer has 480 + 480 windings in the plate circuit and 480 + 480 windings in the cathode circuit.
Number of secondary windings 80 (4 ohm), 120 (8 ohm) and 160 (16 ohm).
Secondary FB has 40 windings.
I'd say this presents a composite load for the output stage of 2k3 (each of the primary windings 576 ohm end to end).
This transformer has 480 + 480 windings in the plate circuit and 480 + 480 windings in the cathode circuit.
Number of secondary windings 80 (4 ohm), 120 (8 ohm) and 160 (16 ohm).
Secondary FB has 40 windings.
I'd say this presents a composite load for the output stage of 2k3 (each of the primary windings 576 ohm end to end).
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FYI: All MC outputs are 9 Ohm taped not 8 ohm...... only labeled as 8 ohms..
The way the winding geometry works out....
This is valuable info. So under dynamic conditions,you are saying there is no real voltage gain in the output stage? I know if you look at it as a cathodyne phase splitter,it would appear to have a 2 to 1 voltage gain because as I look at it more,it made sense what Pieter was saying. There seem to me so many variables and different opinions depending on whom you ask but if you break it down piece it sort of comes into place. It sounds as tho you can't add the separate readings being the windings are in parallel but if they were series connected they would add.
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A friend of mine ownes some McIntosh amplifiers, among them MC30 and MC275.
I just made an appointment with him for coming Tuesday; then we will do some reliable measurements.
He is a former colleague and qualified audio technician.
I just made an appointment with him for coming Tuesday; then we will do some reliable measurements.
He is a former colleague and qualified audio technician.
Please post photos so we can see exactly the method he is using. I can measure grid to plate and grid to the cathode with an injected signal but if you can't add the results together,what would we expected to see?
Well, I think I've stated this dozens of times already in other threads, but I think that the only way to measure/calculate the gain of this stage that makes any sense = Delta Va-k / Vin. Others have stated that only the plate voltage swing or only the cathode voltage swing should be the "Vout" of the stage. I don't agree. They are working in series so should be added together. It makes no sense to consider them in isolation. They are both working into the load, and a Unity-Coupled amp with two 1K primaries will present a 4k plate-to-plate load to the tubes (because the primaries are being driven in series).
If you have a high-gain output device, the gain will approach 2. The only proper way to measure this is to place a differential probe from anode to cathode of an output tube and measure the output voltage there.
Now if you measure input voltage as the voltage differential between the two grids, you have to double the Va-k output voltage because that differential grid voltage is causing twice as much delta in Va-k, because it is driving two tubes. You will still get the same answer, though.
If you have a high-gain output device, the gain will approach 2. The only proper way to measure this is to place a differential probe from anode to cathode of an output tube and measure the output voltage there.
Now if you measure input voltage as the voltage differential between the two grids, you have to double the Va-k output voltage because that differential grid voltage is causing twice as much delta in Va-k, because it is driving two tubes. You will still get the same answer, though.
This is what I don't get. The people that wind the MAC Opt trafos seem to come to the conclusion that there is no voltage amplification on and depending on whom you ask,you get different opinions.
If the windings are in parallel,these are what's loading the output so how can you legitimately add the voltage readings together?
If the windings are in parallel,these are what's loading the output so how can you legitimately add the voltage readings together?
They can be wound in parallel but driven in series. That's how.
While it is true that there is no gain from the point of view of a single primary winding (and of course they mirror each other exactly with a DC offset of hundreds of volts), you don't draw load lines and calculate an output tube's gain from the point of view of one of the transformers windings when there are two windings that are being driven in series (even if they are wound in parallel).
I agree with SpreadSpectrum.
Besides, if the output stage of the MAC didn't have any voltage gain that configuration would just be silly because a power cathode follower would be much easier, economical and would have even better performance in terms of driving difficult loads. How could one possibly get a patent out of such a silly idea???
Besides, if the output stage of the MAC didn't have any voltage gain that configuration would just be silly because a power cathode follower would be much easier, economical and would have even better performance in terms of driving difficult loads. How could one possibly get a patent out of such a silly idea???
My thought is to measure the voltage differential between the two grids and the voltage differential between the two plates.
Ideally this should give a 1:2 voltage gain, but as the output stage is loaded I expect something around 1.5.
But we will see shortly.
Ideally this should give a 1:2 voltage gain, but as the output stage is loaded I expect something around 1.5.
But we will see shortly.
But even if it's driven in series
Very few people can wind the Mac OPT and the ones I have talked to all seem to come to the conclusion that the opt stage is not a voltage amp..Simple logic when looking at this a cathodyne would definitely show a 2 to 1 gain if this were a typical phase splitter voltage amp but it obviously is not and the way he explained it made sense why because of the output windings being in parallel.
Can you still add the total at the output of plate and cathode that is in parallel? It doesn't sound like voltages would add in this case..Whether there is voltage gain or not, I think all that matters is that it works.
Very few people can wind the Mac OPT and the ones I have talked to all seem to come to the conclusion that the opt stage is not a voltage amp..Simple logic when looking at this a cathodyne would definitely show a 2 to 1 gain if this were a typical phase splitter voltage amp but it obviously is not and the way he explained it made sense why because of the output windings being in parallel.
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