Questions about RF chokes used in McIntosh tube amps.
Since 1960 production, McIntosh MC240 and MC275 have used RF choke 1.5uH and 2.7uH that
1. connects KT88’s plate to transformer primary plate out - this is understandable to avoid high frequency oscillation.
2. connects upper KT88’s screen to lower KT88’s plate? And another choke to connect upper KT88’s plate to lower KT88’s screen? Why is that?
3. Why two different choke values? Aren’t push pull supposed to be symmetric at high frequency cut off?
Since 1960 production, McIntosh MC240 and MC275 have used RF choke 1.5uH and 2.7uH that
1. connects KT88’s plate to transformer primary plate out - this is understandable to avoid high frequency oscillation.
2. connects upper KT88’s screen to lower KT88’s plate? And another choke to connect upper KT88’s plate to lower KT88’s screen? Why is that?
3. Why two different choke values? Aren’t push pull supposed to be symmetric at high frequency cut off?
Attachments
1) Plate stoppers to break up any high frequency resonances. Never used KT88's myself, but the STC 807 Report recommends plate stoppers for 807s (and likely other 6L6-oids as well). These should include parallel 100R (C-comp, 2W, installed inside the coll) to de-Q the coils so's they don't ring at their natural frequency. Used the same stoppers with 6BQ6s, even though there's no explicit recommendation, but being top cap types, the run between the plate caps and OPT are longer than normal, so more possibilities for parasitic resonances.
2) Cross connected lNFB. Use this cautiously since it's the same connection of a multivibrator. You don't want your finals operating as a multivibrator.
3) Stability hack? Plate stoppers should be symmetrical
The values depend on winding differences in the output transformer. That is why these are different in value. I wouldn't call it a hack, McIntosh engineers knew exactly what they were doing. Output transformers rarely have equal impedances between windings, and leakage inductances matter. Interelectrode capacitance also varies.
What to see unstable? The 6146B.
What to see unstable? The 6146B.
Great insights, thanks Anatech. What’s your thoughts the #2 of upper tube plate and lower tube screen connection?
As a complete guess, negative feedback. It is opposite to what you would normally see, and it is actually connected that way in real units. Don't forget they drive the output transformer with the cathodes as well.
So I don't know the why. I have had these units with the output transformers connected incorrectly, so I know they are supposed to work the way you see it drawn from actual bench experience.
So I don't know the why. I have had these units with the output transformers connected incorrectly, so I know they are supposed to work the way you see it drawn from actual bench experience.
Thanks.
My guess is NFB too and it should pair with the output transformer’s cathode winding to function properly.
My guess is NFB too and it should pair with the output transformer’s cathode winding to function properly.
Most (including these) McIntosh amps operate full pentode, but with half of the output valves' load from cathode to ground and half from plate to ground. Screens (G2) are connected to points where their signal voltage is equal to each valve's cathode signal voltage, the definition of pentode operation. Some amplifiers have separate bifilar screen (G2) windings, allowing different, lower, G2 DC voltages, but if the output valves don't mind equal plate and screen DC voltages, the plate windings of the opposite polarity valves are the right place for pentode operation.
Maybe interesting: if the screens (G2) were instead tied to B+ (signal ground) the output valves would operate 50% triode mode, roughly "Ultra Linear".
All good fortune,
Chris
Maybe interesting: if the screens (G2) were instead tied to B+ (signal ground) the output valves would operate 50% triode mode, roughly "Ultra Linear".
All good fortune,
Chris
Hi Chris,
I haven't analysed their amplifiers. They work very well, and I'm sure some folks have studied them extensively.
I will say that UL operation is generally about the best. No end of positives and that is how I design mine. 43% taps as they always have, for good reason. Beam power tubes of course.
I haven't analysed their amplifiers. They work very well, and I'm sure some folks have studied them extensively.
I will say that UL operation is generally about the best. No end of positives and that is how I design mine. 43% taps as they always have, for good reason. Beam power tubes of course.
Very interesting read. Thanks Chris.
In short, in a push pull pentode configuration, one output tube’s screen connects to opposite polarity tube’s plate is the full pentode PP operation.
I guess then it has nothing to do with NFB here? Since screen is the control grid for current flow and it doesn’t take AC swing as input?
The McIntosh transformers (or a Circlotron) let you do 50% taps but at reduced G2 DC voltages, which would allow sweep tubes as output valves.
Mc did something related in their really big, Grateful Dead size, amps, using another (trifilar!) primary winding for screens, keeping full pentode operation. I guess that was just the McIntosh way, and Devil take the hindmost.
50% cathode loading is a lot of negative feedback and requires heroic efforts to maintain enough drive voltage, including positive feedback ("bootstrapping") to the driving valves' plate load resistors. It's a tricky thing to pull off, and not for the faint of heart, but the crazy things are mostly all still running these many decades later, so...
All good fortune,
Chris
Mc did something related in their really big, Grateful Dead size, amps, using another (trifilar!) primary winding for screens, keeping full pentode operation. I guess that was just the McIntosh way, and Devil take the hindmost.
50% cathode loading is a lot of negative feedback and requires heroic efforts to maintain enough drive voltage, including positive feedback ("bootstrapping") to the driving valves' plate load resistors. It's a tricky thing to pull off, and not for the faint of heart, but the crazy things are mostly all still running these many decades later, so...
All good fortune,
Chris
Hi Chris,
Yes, most McIntosh amps I see are fine unless messed with. Normal stuff like OOT resistors and failing capacitors.
One thing to watch for is in earlier amps (some) they regulated the first amp stage plate supply - and exceeded the H-K voltage of the 12AX7. Later changes eliminated the section of the 12AX7 used for regulation. These days we could stick a mosfet in there, and disconnect that 12AX7 section.
True enough about the challenges of cathode drive. They made great transformers and those made their amplifiers difficult to copy. It isn't fun when someone changes the transformer wires and messes up the connections. Yes, I have seen that because we all know Teflon wire sounds better - right?
Yes, most McIntosh amps I see are fine unless messed with. Normal stuff like OOT resistors and failing capacitors.
One thing to watch for is in earlier amps (some) they regulated the first amp stage plate supply - and exceeded the H-K voltage of the 12AX7. Later changes eliminated the section of the 12AX7 used for regulation. These days we could stick a mosfet in there, and disconnect that 12AX7 section.
True enough about the challenges of cathode drive. They made great transformers and those made their amplifiers difficult to copy. It isn't fun when someone changes the transformer wires and messes up the connections. Yes, I have seen that because we all know Teflon wire sounds better - right?
Chris, what were these amps? Maybe it's a longshot but it sure would be nice to have some schematics or some sort of visual to put to these. Nice information here, both of you. Thanks
Loren
Loren
Hi Loren, it’s in the title of this thread. McIntosh MC240 and MC275.
Here is the schematic of MC240:
https://www.drtube.com/schematics/mcintosh/mc240-service-manual.pdf
Here is the schematic of MC240:
https://www.drtube.com/schematics/mcintosh/mc240-service-manual.pdf
For some fun, folk might check out the MI350 and Mc3500, both with banks of sweep tubes as output valves. Don't even think about picking one up - Grateful Dead Wall of Sound size stuff.
All good fortune,
Chris
All good fortune,
Chris
The MC60 I think in my example.
Often there are running production changes in models from every brand. You may find one or two schematics, but there may be more.
Hi Chris,
For sure!!! Imagine the cost of retubing it today!
The Conrad Johnson Premier One is another (not as big). 6 x KT88 per channel. Bob Carver also made some large tube product, among others.
Often there are running production changes in models from every brand. You may find one or two schematics, but there may be more.
Hi Chris,
For sure!!! Imagine the cost of retubing it today!
The Conrad Johnson Premier One is another (not as big). 6 x KT88 per channel. Bob Carver also made some large tube product, among others.
The way John Curl tells it the Dead were happy enough to move to Mc2300's when they became available, and that's what you see in most photos. Tube amps were just too fragile for such beat-on-every-other-day duty, even Mc's. The Wall of Sound was two identical systems leap frogging to get set up one show ahead. By the time I finally got to see them in 1981 they'd long gone back to conventional PA, with the upstage all percussion instruments, Bill Kreutzmann and Mickey Hart running around playing them almost Kodo style.
All good fortune,
Chris
All good fortune,
Chris
Hi Chris,
Yeah, road equipment lives a very hard life. I would never consider using a stack of tube amps. Their "heads", sure. That's their sound. The PA, no way!
We had several amp racks come in that fell off the truck over the years. Amps don't like that! lol! Look at any MI amp compared to a consumer amplifier and see the differences. They still smash PA amps.
The cost of transporting heavy amplifiers is also a big concern for the touring company. That and heat dissipation in the rack. Now the main issue is supplying enough AC power.
Yeah, road equipment lives a very hard life. I would never consider using a stack of tube amps. Their "heads", sure. That's their sound. The PA, no way!
We had several amp racks come in that fell off the truck over the years. Amps don't like that! lol! Look at any MI amp compared to a consumer amplifier and see the differences. They still smash PA amps.
The cost of transporting heavy amplifiers is also a big concern for the touring company. That and heat dissipation in the rack. Now the main issue is supplying enough AC power.
Don't forget Bob Heil RIP. I use one of his mic's on my Flex SDR.
Although he was mostly a mixing/speaker guy. Also invented the talk box used by Frampton etc.
I apologize if I've drifted too far from the OP topic.
Although he was mostly a mixing/speaker guy. Also invented the talk box used by Frampton etc.
I apologize if I've drifted too far from the OP topic.
I apologize for the miscommunication. I was referring to the amplifiers Chris had mentioned. Now I can look them up, thanks Chris. And thank you as well, I do enjoy looking at the 240 schematic. Also, your original question...Don't think I had seen this before (the different values not the method). It does make perfect sense, with both sides of the primary having their differences.
Loren
I think the best sounding McIntosh tube amps are the MC240 and the MC60. I've restored many over the years. MC30 is okay, MC40 not so good.
Transformers are physical things. They are pretty good, but not perfect. A toroid output would be really bad, I can't see how they could possibly be consistent as there is no way to control the wind in the centre. A Hammond transformer engineer specializing in audio products explained it to me. Some stuck.
Transformers are physical things. They are pretty good, but not perfect. A toroid output would be really bad, I can't see how they could possibly be consistent as there is no way to control the wind in the centre. A Hammond transformer engineer specializing in audio products explained it to me. Some stuck.