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PostPosted: November 26th, 2016, 5:51 pm 
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All,
Hello, it's Mr. low-self-confidence again, with a question that's answer is probably pretty self-evident. The MC240 is intended for the 6L6 tube, but it was also designed to power the matching pre-amp through one of those octal sockets.
So, can I safely run KT66 tubes in place of the 6L6?

6L6 Heater: 0.9A
KT66 Heater 1.3A
12AX7 Heater (parallel, 6.3V)

4x6L6 = 3.6A
4xKT66 = 5.2A
Delta = 1.6A

Unfortunately, I cannot find info on the pre-amp that was intended to be paired with the MC240. I thought that would be an easy find, but apparently I'm Google-challenged.

If it was a circuit comparable to the dynaco PAS (4 x 12AX7) that would be 1.2 Amps of the margin accounted for, leaving 0.2A over the design-load.

So my question is 2-fold:
a) Does anyone know the designation of the matching pre-amp and it's tube compliment
b) If (a) is unknown, is my PAS analogy safe, and can I run the MC240 filament winding at 0.2A over its design?

V/R
James


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PostPosted: November 26th, 2016, 7:23 pm 
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If you're talking about the jack to the left of the speaker terminals labeled "Line Output" that's not for a pre-amp, that's for a 70v or 140V line out for commercial sound installations. So, you'd be looking at the entire 1.6A being over standard for the tubes...

Roscoe

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PostPosted: November 26th, 2016, 8:19 pm 
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A lot depends on the head room your power transformer has for the filament supply. You're talking a 44% increase in filament current using the KT-66.

That said -- if you plan on using genuine Genelex (vintage British tubes and not copies by Russia or the Chinese) KT-66s -- and your power transformer can take the hit -- you stand to reap the benefits of much improved sound.

I've had a previous love affair with the Genelex KT-66s. WOW!

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PostPosted: November 26th, 2016, 8:28 pm 
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If the standard power transformer cannot handle the filament current, if you are willing to do some work you could use an external larger filament only transformer as a substitute for the filament windings on the main transformer. It would require an external box for the transformer and a cable to bring the power into the main chassis. You would also want to be very careful to not do any permanent changes in case you wanted to return it to original later and sell it.

Tom


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PostPosted: November 26th, 2016, 11:08 pm 
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All,
I have an earlier MC240 that has 2 octal sockets. One, as Roscoe pointed out, is for 600Ohm/70V out, but the one next to it is clearly labeled as "stereo preamp power". Looking at the schematic, it shows the octal plug carrying L & R signal and grounds, and 435 VDC. The remaining 3 pins carry 25.2 VAC from a separate center-tapped winding on the power tranny, which leads me to gather that the intended pre-amp has 12.6V filaments stacked in series.
So I'll show my ignorance again: Is the limit on the power tranny a cumulative thermal one, or are the 6.3V filament windings of a gauge such that they could not carry any more current? In other words, the main filament winding was intended to power only the tubes on the main chassis, but the tranny, as a unit, was sized to also carry the filament load of the pre-amp as well. Can I run higher-filament-current output tubes since the 25.2V windings are unused, or do I risk burning out the windings? I've seen both opinions on the boards, but I don't trust the technical pedigree of those people as much as I do this group.
V/R
James


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PostPosted: November 27th, 2016, 12:52 am 
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Not having schematics I can only speculate -- Even the power-amp tubes' filaments on one set of windings and the preamp power source on another set of windings -- that still brings us back to the original question. Sure the overall transformer thermal performance is given a break without the added burden of a preamp, but the current capacity of the power amp's filament windings still has to have the headroom to carry 40% more current without fusing out. If any of the documentation out there on the WEB can't shed light on the current-carrying capacity of the windings then someone at McIntosh may be able to provide that insight.

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PostPosted: November 27th, 2016, 10:35 am 
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Here is an interesting thread where some have rolled in KT-66s with no issues and others recommend against it.

Hmmm.

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=37592

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PostPosted: November 27th, 2016, 1:32 pm 
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My guess is that you would not have any problem with the added heater current, and here is why. When a power transformer is designed, the IR loss in the primary winding is made similar to the combined secondary losses of all the windings with the expected loading. The power delivered by a power transformer for a tube power amp is usually highest for the B+ supply, so you would not want to increase the B+ loading by 40 %. But the heaters, especially 6L6s, don't come anywhere near the power represented in the B+. Here, you are talking about increasing the load by ten watts out of probably 150 watts overall. But still not sure--there is a way to confirm potential problems. Plug the amp into a stiff source of 120 V ac power with the 6L6s. Measure the ac heater voltage on one of the 6L6s. Then take a resistor or tube heater that represents ten watts at 6 volts and connect it in parallel with the heater voltage AS CLOSE TO WHERE IT COMES OUT OF THE POWER TRANSFORMER AS YOU CAN GET. Observe the measured heater voltage where the probes are attached to one of the 6L6s and note how much the voltage drops. Say the voltage drops by 2%. Then as an approximation you could assume that there will be an additional 2% extra heating in the transformer due to the ten-watt extra load. You should really verify that the 120 V ac mains does not drop when you do this. Also, the ac is probably drifting up and down anyway, so you might wish to repeat the measurement a few times.

David


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PostPosted: November 27th, 2016, 2:36 pm 
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You could also contact McIntosh themselves, they are still in business and may be able to answer your question...


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PostPosted: December 9th, 2016, 7:54 pm 
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So I finally contacted McIntosh tech support about using a KT66 in the mc240. Here is a transcript of the e-mail conversation:

Me: Hey, can I use KT66's in my mc240
Guy#1: I don't think so. Guy #2, what's your opinion?
Guy #2: Sure, I don't see why not, nowadays KT66's are the same as 6L6's
Me: Uh, the KT66 draws 0.4 amps more than a 6L6
Guy #2:"well, there's your answer then"

You'll notice I put the last line in quotes, as he actually said that (though even the lines above it are pretty damn close to the real conversation, too)

V/R
James


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