A puzzling thing about the Mc240 is the cathode follower 12AX7s, which sit at +430 VDC on the anodes, and output valve bias voltage, maybe -35 or -40 VDC on the cathodes, design center at 115 VAC mains. That's a lot of joltage, but it's common to see machines with their factory original Telefunken ECC83's still working fine. It's a puzzlement.
All of the Mc amplifiers operate very close to class B and distortion at low signal level improves dramatically with some increase in idling current. Adjustment is made by changing fixed resistors, and they do (of course) run warmer, so really only applicable to DIY on one's own machine. But well worth the effort, if possible.
All good fortune,
Chris
All of the Mc amplifiers operate very close to class B and distortion at low signal level improves dramatically with some increase in idling current. Adjustment is made by changing fixed resistors, and they do (of course) run warmer, so really only applicable to DIY on one's own machine. But well worth the effort, if possible.
All good fortune,
Chris
Hi Chris,
Agreed!
Exceeding the H-K voltage isn't something I would ever do, I'm sure failure could be spectacular.
Yes, my feelings exactly about bias. I have never attempted to mess with that since I don't own one.
Agreed!
Exceeding the H-K voltage isn't something I would ever do, I'm sure failure could be spectacular.
Yes, my feelings exactly about bias. I have never attempted to mess with that since I don't own one.
Once had an Mc240 about 50 years ago, probably borrowed from a friend because sure didn't have that kind of money, with a pair of BR LaScala's. I liked them best run in mono with the LaScala's stacked vertically tweeter-to-tweeter and homemade (of course) cardboard mouth extenders. It was the 70's so I don't feel all that guilty. As Joe Walsh said, it was better to be 20 in the 70's than 70 in the 20's.
Always good fortune,
another Chris
Always good fortune,
another Chris
Oh yes, those great and heavy boat anchors, designed by Mr. Mile Nestorovich. Eight big color TV sweep tubes as finals, and, note, an OPT with pentafilarly wound primary. Anyway, AFAIK the Grateful Dead's Wall of Sound featured SS power amps exclusively, but MI-350's we're part of the Woodstock public adress system.
As said above, the spirit of Frank McIntosh's/Gordon Gow's Unity Coupling Design is that it allows to run the finals in true, relatively cold biased pentode mode by almost eliminating the distortion caused by switching the stray inductance in the crossover region.
Note that the patent paper shows triodes as finals, though...
Best regards!
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The Owlsley Wall of Sound was an evolutionary branch that died on the vine, way too expensive, too unique, way too far ahead of its time. For folk who haven't yet heard of it, it was a combination of the historic everybody-brings-their-own-amps Beatles era band model with a giant line source speaker array (an idea only relatively recently repeated, in a different context). No floor kicks, no separate house and stage mixes, each band member stood in front of their own giant line source speaker and heard the same sound as the audience. Mixing was in the acoustic free space between the stage and the house.
This was in contrast to the modern PA model, where everyone is mic'd and the mic's are (electrically) mixed separately for house and for each performer's personal kick (stage monitor, pointed up at them, also called foldback), or even more modern, in-ears. Very different vibe.
Vocals couldn't really survive being in the downwind range of the speakers without feedback, so paired B&K mic's were used in anti-phase*, and the results are audible on the gazillions of live recordings from that era. Doesn't really bother me, but
Can't comment on the use of MI350s, except to reference John Curl who was there and would know better than me.
All good fortune,
Chris
*yes, of course it's anti-polarity. forgive me my sins
This was in contrast to the modern PA model, where everyone is mic'd and the mic's are (electrically) mixed separately for house and for each performer's personal kick (stage monitor, pointed up at them, also called foldback), or even more modern, in-ears. Very different vibe.
Vocals couldn't really survive being in the downwind range of the speakers without feedback, so paired B&K mic's were used in anti-phase*, and the results are audible on the gazillions of live recordings from that era. Doesn't really bother me, but
Can't comment on the use of MI350s, except to reference John Curl who was there and would know better than me.
All good fortune,
Chris
*yes, of course it's anti-polarity. forgive me my sins
Hey Chris,
That sounds much like an opportunity to be a hot mess! It would only work if everyone cooperated.
Live sound evolved like that for a reason! lol! And feedback ... yeah, huge issue. They tried everything before anti-phase mics.
I can't see them trucking around huge tube amps. Too much damage, too long to set up, power requirements through the roof. I imagine they tried it once. Never did see the Dead in concert, it would have been a blast!
That sounds much like an opportunity to be a hot mess! It would only work if everyone cooperated.
Live sound evolved like that for a reason! lol! And feedback ... yeah, huge issue. They tried everything before anti-phase mics.
I can't see them trucking around huge tube amps. Too much damage, too long to set up, power requirements through the roof. I imagine they tried it once. Never did see the Dead in concert, it would have been a blast!
I bought mine in a store in San Francisco, 1978, used, Mac Clinic'ed, with all new RCA & TFK tubes, for $275, or about $1327 of today's $.
Not cheap, but still affordable. Thanks to RCA who announced they stop making tubes, there was a selling frenzy among the owners at that time. 🙂
I replaced mines in 1998, they were tired but still serviceable. Never saw any flash over in 20 years. But they were TFK's.
Yes, they invented crossover distortion 🙂
It was so bad I couldn't listen to them at first, until I upped the standing current from 20 or so to 65mA. Then they were OK, and the delta temp. was about 30°C at the top of the power trafo. Subsequently, I backed down to 55mA. Not a trivial hack though, with 4 individual bias pots.
That was part of my 2nd wave of hacks to make these guys sound better: easy peasy: just remove C9, C10, C19 & C20. I've nothing but contempt for PFB.
Funnily, JC was the guy who gave me the final motivation to buy the MC240. He told me: "They have the best output transformers in the world; Marantz tried to get these, but Mc refused". Wonder why? 🙂
What you call PFB is just bootstrapping. It is necessary to achieve the big output tube drive signal swings.Without it, output power gets rather limited.
Best regards!
Best regards!
Sorry, last century for me was pen, paper, and a Radford DMS 4 that worked most of the time; IOW, I didn't log everything. As an example, I didn't record the mains voltages at the time of the tests, so any max power is to be taken with a big spoonful of salt.
The Mac circuitry is intrinsically low in distortion (for a tube amp), in the 0.x% range, so I'm not losing any sleep over THD. But other things WILL get in the way. Basically, the mods are, by order of diminishing return:
Beware: if you mod, your resale value will be negative!
The Mac circuitry is intrinsically low in distortion (for a tube amp), in the 0.x% range, so I'm not losing any sleep over THD. But other things WILL get in the way. Basically, the mods are, by order of diminishing return:
- Higher bias
- Bypassing all the input pots (mines were worth as much THD as the amp itself)
- Stiffer power supply (I use 900uF)
- No PFB
- No input cap
- Add a trim pot to balance the phase splitter
- ...
Beware: if you mod, your resale value will be negative!
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Hi Zung,
Great idea for a new thread!
As for mods and distortion, anything that adds to the distortion whether it be IMD or THD - or noise, those things should be returned to factory configuration. As for input pots. may as well change them to a mute switch. That is their typical use anyway. Stiffer power supply? It isn't a problem, but regulating the input stage is a plus in every way. Your original capacitors may be fine, just look at the ripple waveform. Only your scope needed, very fast check.
I'll disagree with you on the input capacitor, strongly. Use a PP, you cannot hear it. Balancing the phase splitter is always good. If you use matched output tubes, you really only do need one bias control. Please don't drill holes in the terminal PCB. That is destructive and forces you to mount parts in different locations. That is a strong negative. I've had to restore a couple McIntosh amps that an idiot did that to. For bias and balance, a pot can easily be mounted off board. May as well make it accessible from the outside.
-Chris
Great idea for a new thread!
As for mods and distortion, anything that adds to the distortion whether it be IMD or THD - or noise, those things should be returned to factory configuration. As for input pots. may as well change them to a mute switch. That is their typical use anyway. Stiffer power supply? It isn't a problem, but regulating the input stage is a plus in every way. Your original capacitors may be fine, just look at the ripple waveform. Only your scope needed, very fast check.
I'll disagree with you on the input capacitor, strongly. Use a PP, you cannot hear it. Balancing the phase splitter is always good. If you use matched output tubes, you really only do need one bias control. Please don't drill holes in the terminal PCB. That is destructive and forces you to mount parts in different locations. That is a strong negative. I've had to restore a couple McIntosh amps that an idiot did that to. For bias and balance, a pot can easily be mounted off board. May as well make it accessible from the outside.
-Chris
When it comes time to replace the can caps, I can really highly recommend an outfit in USA called Hayseed Hamfest, who make beautiful new look-like-spun-stainless versions to your spec using 105C Nichicon's internally, and at Ham, not Audiophile, prices. The proof of their sincerity is their clunky website!
All good fortune,
Chris
All good fortune,
Chris
They look great, but I already bought F&T drop in replacements from Germany; they've been in a drawer for years while I try to overcome the fear to attack the taps that are soldered to the chassis. Besides, the original ones are still better than specs, about the same as the F&T in terms of dissipation factor (D).
That guy needs some serious professional attentions: there're plenty of unused original holes in the original board to mount whatever it is you fancy. 🙂
Misc. points:
I disagree with your disagreement 🙂 re. the input cap: the original mylar thingy belongs to the recycle bin.
Also re. "...should be returned to factory configuration...", the factory standard sound was what got me into all these mods: I could not listen to the thing as is.
I did say polypropylene didn't I? Mylar was simply a good option back then, we have much better parts today.
This is what I had to rebuild. The rest of the chassis is worse (believe it or not).
The rebuilt unit sounds amazing. I'm sure you would have liked it very much. I had to replace absolutely everything and rebuild the terminal board. Replaced terminals where there were big holes. Even the wire (most was only 300VDC!!!!).
This is what I had to rebuild. The rest of the chassis is worse (believe it or not).
The rebuilt unit sounds amazing. I'm sure you would have liked it very much. I had to replace absolutely everything and rebuild the terminal board. Replaced terminals where there were big holes. Even the wire (most was only 300VDC!!!!).
