This was a Mac 240, Bear, stereo amp with PP 6L6s.
Miles, that was my finding exactly. At the time, I thought that the stabilizing capacitor in the feedback return might be the sonic culprit, so the tactic I used was to put a pot in the return line, with no bypassing capacitor, and 'turn the feedback up' until the peak at OT resonance (usually 70-80KHz) showed up. I would decrease the value of the pot (increase feedback) until that peak was about 0.5-1dB down from midrange level, and that was the place. In the circuits I tried it in, this would be about 5-6dB of gNFB. One of those amps is still in service, PP EL34s, and that sucker is a good sounding amp, and not flat or dry at all.
Aloha,
Poinz
Miles, that was my finding exactly. At the time, I thought that the stabilizing capacitor in the feedback return might be the sonic culprit, so the tactic I used was to put a pot in the return line, with no bypassing capacitor, and 'turn the feedback up' until the peak at OT resonance (usually 70-80KHz) showed up. I would decrease the value of the pot (increase feedback) until that peak was about 0.5-1dB down from midrange level, and that was the place. In the circuits I tried it in, this would be about 5-6dB of gNFB. One of those amps is still in service, PP EL34s, and that sucker is a good sounding amp, and not flat or dry at all.
Aloha,
Poinz
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hey-Hey!!!,
One useful modification to the circuit would be to get rid of the positive FB around the 12BH7 stage. The 12k resistors can be exchanged with CCS-es; bootstrap headroom preserved, pFB removed.
Substituting a CCS for the 18k tail of the LTP 12AU7 stage is another, as is modifying the heater to run 6CG7( and with balanced/matched plate loads) and IMO replacing the input-stage 12AX7 for a 12AY7( with appropriate adjustments to circuit values ).
cheers,
Douglas
One useful modification to the circuit would be to get rid of the positive FB around the 12BH7 stage. The 12k resistors can be exchanged with CCS-es; bootstrap headroom preserved, pFB removed.
Substituting a CCS for the 18k tail of the LTP 12AU7 stage is another, as is modifying the heater to run 6CG7( and with balanced/matched plate loads) and IMO replacing the input-stage 12AX7 for a 12AY7( with appropriate adjustments to circuit values ).
cheers,
Douglas
"What makes the old McIntosh stuff so good?"
Nostalgia? 😉
But I'll be kind. Certainly agree with the built like a tank comments. Solid stuff.
Had a fun experience once, many years ago. It was a demo of vintage tube amps. Same source (CD) preamp and speakers. Only the amps changed.
Up for listen were McIntosh, Scott, Dynaco, Fisher and some others.
Each amp had it's own distinct sound. Similar to the others, but unique. Except the Mac. It sounded nothing like the others. Big, round, warm, lush. What many folks might call "tube sound." Not neutral, not accurate, but a ton of fun to listen to.
I believe that was a big part of the McIntosh seduction.
Nostalgia? 😉
But I'll be kind. Certainly agree with the built like a tank comments. Solid stuff.
Had a fun experience once, many years ago. It was a demo of vintage tube amps. Same source (CD) preamp and speakers. Only the amps changed.
Up for listen were McIntosh, Scott, Dynaco, Fisher and some others.
Each amp had it's own distinct sound. Similar to the others, but unique. Except the Mac. It sounded nothing like the others. Big, round, warm, lush. What many folks might call "tube sound." Not neutral, not accurate, but a ton of fun to listen to.
I believe that was a big part of the McIntosh seduction.
I was just sorting out the circuit of a 50W commercial PA amplifier manufactured in Australia during the 1960's by AWA. It used a McIntosh circuit configuration, with bootstrapped drivers. Valves were EL34 and B+ was about 450-500V, and the OT had an effective total 6k loading. I don't have the amp going, as it was bought for spare parts, but many of these were made at the time. Not sure how close it comes to the patent, but then again protection would probably not have been international. There is global feedback applied. I'm not sure why they used the circuit - however AWA was quite a large concern and did a lot of military, broadcast and commercial equipment for the asian region.
Abridged schematic at:
http://dalmura.com.au/projects/Output stage PA1002BY.pdf
Ciao, Tim
I was always of the impression that the Unity Coupled Design was a product of Bell Labs, and Frank was simply smart enough to capitalize on it. Now I understand that Frank worked at Bell Labs. So in this which-came-first-the-chicken-or-the-egg relationship, how did it all come down? Was Frank the one that developed the circuit at Bell Labs, part of a team that developed it at Bell Labs, or did he just become aware its development at Bell Labs (but unrelated to it) and capitalize on it? Personally, I've always thought it was a heck of a design, that provided a very HIGH degree of damping with all the feedback the output stage alone possesses.
Dave
Dave
MI350s
Hello all,
I have a pair of MI350s which I rebuilt from the ground up. Designed all new FR4 double sided PCBs, modified the amplifier circuits considerably, new power supply for the screen grids, all input stages use fully regulated supplies, DC on all the front end heaters, custom pots for the bias controls.
I have heard many tube amps over the past 40 years and in my opinion no tube amplifier comes close to the MC3500/MI350s.
After modification my amplifiers exceeded the Mac specifications by a huge margin.
I had the opportunity some years ago to repair a MC3500 which I compared to my MI350. In every aspect my MI350 was better.
The two biggest problems with McIntosh (and Marantz) was of course the power supplies were not well designed in terms of capacitance and regulation and those old passive parts were of very poor quality.
Steve Mantz
Zed Audio
Hello all,
I have a pair of MI350s which I rebuilt from the ground up. Designed all new FR4 double sided PCBs, modified the amplifier circuits considerably, new power supply for the screen grids, all input stages use fully regulated supplies, DC on all the front end heaters, custom pots for the bias controls.
I have heard many tube amps over the past 40 years and in my opinion no tube amplifier comes close to the MC3500/MI350s.
After modification my amplifiers exceeded the Mac specifications by a huge margin.
I had the opportunity some years ago to repair a MC3500 which I compared to my MI350. In every aspect my MI350 was better.
The two biggest problems with McIntosh (and Marantz) was of course the power supplies were not well designed in terms of capacitance and regulation and those old passive parts were of very poor quality.
Steve Mantz
Zed Audio
Yep, that's it. Without big Irons or Lundahl there is no chance for good sound.
Was the Mc3500 recapped and working correctly, way back when they were untouchable IMO when compared to other tubes. The cost of 16 6LQ6's would make you groan everytime..🙂
I'm not one for commercial 'high end' kit, but I heard a McIntosh tube amp (don't remember which one) at a show with a decent CD player and some fancy speakers, and the subjective performance was truly enjoyable.
It was only a sample of one, so not possible to draw general conclusions. However based upon the demo I investigated the brand a little and it seems as if good quality output transformers and global feedback feature in their amps and contribute significantly to the performance.
It was only a sample of one, so not possible to draw general conclusions. However based upon the demo I investigated the brand a little and it seems as if good quality output transformers and global feedback feature in their amps and contribute significantly to the performance.
If you take into consideration the speakers that were available back in 1940 I'd say these amps were very close to as good as it gets. There was no computer modeling or spice or any real help in the aftermarket. These guys did everything themselves. Most of the trannys were wound in house by women from the neighborhood, not some guys who chose this as there lifes career.
Most if not all of the early Mac gear was built to military standards. Extremely rugged chassis and very easy to access repair. Frank Mcintosh was a decendant of the Bell Labs era. These guys did things off the charts. Remember the Bell hygrogen peroxide single man flying rig. Ever seen anything since. Much bad rap can be tossed at the Mac logo. I think any company which has endured with excellence for 7 decades is due a little respect.
Frank was keenly aware of overuse of feedback and many other tweaks which enhance specifications. If he and the company chose to do their products the way they did they must have had very good reason to do so. These are not fly by night engineers these guys really do think before they leap. Just employing Miles Nestorvic is a genius choice. That guy was very smart. He knew exactly what needed to be done and the company left him alone to do it. There will never be a commercial Mi350 or MC3500 amp. No one can afford them. And this is truly sad.
Long live tubes.
Tad
T
Most if not all of the early Mac gear was built to military standards. Extremely rugged chassis and very easy to access repair. Frank Mcintosh was a decendant of the Bell Labs era. These guys did things off the charts. Remember the Bell hygrogen peroxide single man flying rig. Ever seen anything since. Much bad rap can be tossed at the Mac logo. I think any company which has endured with excellence for 7 decades is due a little respect.
Frank was keenly aware of overuse of feedback and many other tweaks which enhance specifications. If he and the company chose to do their products the way they did they must have had very good reason to do so. These are not fly by night engineers these guys really do think before they leap. Just employing Miles Nestorvic is a genius choice. That guy was very smart. He knew exactly what needed to be done and the company left him alone to do it. There will never be a commercial Mi350 or MC3500 amp. No one can afford them. And this is truly sad.
Long live tubes.
Tad
T
It would be interesting to compare one of my Pyramids with McIntosh tube amps... I believe mine would beat them because voltage regulation today is much cheaper, and military tubes are better than what was used then for hi-fi.
Some folks here seem to be confused about the nature of the McIntosh output transformer... you don't need "Lundahl" or similar. In fact the McIntosh output transformer is more or less "scramble wound". The key to it is the matching. That changes everything.
Now, on to the circuit presented by trobbins is decidedly not a McIntosh style output section. Interesting design, but not a McIntosh.
Anatolily, I don't know what ur Pyramids will do or not, but the McIntosh design should not be confused with an implementation of the design. The original amps, as noted, lacked all sorts of things that today we would all want to include... you can start with the carbon comp resistors! 😀
As far as tubes go, I wouldn't count on modern military tubes (ruskie, I presume?) being better than production products post WWII here in the USA - they were very good!!
Also many of the standard tube types were available in ruggedized and long-life versions.
😀
_-_-bear
Now, on to the circuit presented by trobbins is decidedly not a McIntosh style output section. Interesting design, but not a McIntosh.
Anatolily, I don't know what ur Pyramids will do or not, but the McIntosh design should not be confused with an implementation of the design. The original amps, as noted, lacked all sorts of things that today we would all want to include... you can start with the carbon comp resistors! 😀
As far as tubes go, I wouldn't count on modern military tubes (ruskie, I presume?) being better than production products post WWII here in the USA - they were very good!!
Also many of the standard tube types were available in ruggedized and long-life versions.
😀
_-_-bear
Yes, they were VERY GOOD for mass-produced tubes. But tubes especially designed and manufactured for military are still better: they did not cut corners. As was said in one Broadway play, "I know who won Cold War. Generals". "Military generals?" "No. General Motors, General Electric, and so on...".
You may compare for example Allied gear for hams with Collins gear for military. Military production during Cold War used better engineers, better materials, better manufacturing, better quality control...
Today I see on eBay very expensive highest quality Cold War debris dirt cheap... Resistors, capacitors, tubes, relays, tube sockets, you name it... If to combine it with parts made using modern materials and components non-available yet then you can build killing machines. I mean, machines that kill their elder brothers and sisters by quality of sound reproduction.
For example, KT-88 is a very good tube. It always was. But it is a fatter version of cheap old RCA 6L6. GU-50 that I use in power stages of my Pyramids is repackaged copy of Telefunken LS-50 that is still better: better design, better materials, better (stricter) QA. Many times cheaper today... It is not a ruggedized version of a cheap civil tube. It is a tube made especially for military that did not count money.
Ok, ok... there's no need to argue about it. Just pick what you like. The way something sounds bears little relationship to the price or the way it is built (when it comes to tubes at least)... as far as ham gear? Collins is Collins... sure the military gear from the WWII through the end of the Cold War was built very well, not sure what the point is or how that relates back to audio?? You do know about TMC (radio gear)?? 😀
You like the 4D-32? 😀
I like Russian tubes, so don't miss the point I am trying to make.
I'm just saying that I prefer to take each device for what it is, and select what is going to do what I want... from whatever is available.
_-_-
You like the 4D-32? 😀
I like Russian tubes, so don't miss the point I am trying to make.
I'm just saying that I prefer to take each device for what it is, and select what is going to do what I want... from whatever is available.
_-_-
What makes the old McIntosh stuff so good?
Company can still service all their gear and I heard 80% of what they produced is still working. Any other manufacturer can offer that kind of service?? . Gear looks cool ,have great reputation, and sounds pleasant. I'm not a big fan although I have/had some vintage pieces. My friend has newer SS integrated and CD combo.He is happy. The cheapest Mac amp wiped a floor with my prized at that time (and now costing arm and leg ) Bow Technologies ZZ-0ne.
Company can still service all their gear and I heard 80% of what they produced is still working. Any other manufacturer can offer that kind of service?? . Gear looks cool ,have great reputation, and sounds pleasant. I'm not a big fan although I have/had some vintage pieces. My friend has newer SS integrated and CD combo.He is happy. The cheapest Mac amp wiped a floor with my prized at that time (and now costing arm and leg ) Bow Technologies ZZ-0ne.
Good for you. But did we discuss your personal preferences?
My point was and is, McIntosh made the name building great stuff compared to the rest available on the market there and then. Now this name keeps the boat floating, here and now, while better equipment was, and is, available...
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