MC240 with NOS tubes question…

[Lingonberry]

Just had my MC240 recapped by the guys at HiFi Haven (did a great job), and just received some NOS input tubes (all Telefunken little tubes) and RCA black plates for the output tubes. They aren’t even close to being broken in, and I can tell an immediate difference in the sound; much louder on the same volume as my other tubes (all JJ new tubes for both the input littles and the output 6L6GC).

My question is, after about 3hours of listening, the power transformer got very hot. I know they get warm to the touch, and did when I was running the JJs, but this seemed a bit warmer. I could always keep my hand on the power transformer before, but it was tough to do with the NOS tubes due to the heat. Also, the two unity coupled transformers would stay very cool with the JJs, but seemed a little warm with the NOS tubes. I was running the amp at pretty low volumes, just trying to break in the tubes.

I don’t know if this is something to be expected. I usually listen to the amp for a couple hours every day at relatively low volumes, so may have just been because I was listening for a longer duration in an attempt to break in the NOS tubes. But the power transformer got so warm it concerned me and I shut down the power after the 3 hours.

Thanks for any input. I know this topic has been discussed before (about the MC240 power transformer getting warm), but I was more interested if anyone had noticed if different tubes could make the effect worse, particularly NOS tubes.

 

[62caddy]

Specific tube models have defined operating characteristics so I can't see what difference it would make whether a given tube came from Brand X or Brand Y as far as power draw (and PT heat generated) is concerned.

Otherwise the heat of the PT you describe sounds like normal function- exactly the same as my experience. I never noticed any difference whether the amplifier is just idling with no signal or exercised to its limits. It's the duration of being on is what causes the heat to build but after certain point it will get no hotter.

In his memoirs, Dave O'Brien mentioned an MC240 brought in by a customer who found it "very warm" after not having played the system for weeks. The amplifier had been installed in a closet and due to the amplifier being plugged into the "always on" outlet of the preamp rather than one of the switched outlets, it turned out the amplifier had been on continuously for twelve years. He said all it needed was an output doubler and a set of fresh output tubes to restore it back to spec - both of which it would probably have needed anyway after 12 years.

 

[SaturationPt]

It is possible, that the RCA "black plate" tubes are better at radiating the heat, as that is what the black plates are supposed to do.

It is also possible that it is just concern about the new tubes and you're running normal temps.

The best is to use a quality thermometer and measure the temperature, both on the tube side and the non-tube side (mine run measurably warmer on the tube side). Hard to assess without hard numbers (will need to know the temperature above the ambient (room) temperature).

You can also try a sheet of aluminum foil between the tubes and transformer to see if the transformer still runs warm, just be careful to keep it from slipping under a tube where it can short the pins.

It continues to amaze me how there is a constant supply of matched NOS RCA black-plate tubes even 60 years after they went out of production.

 

[Dave_in_Va.]

Coincidentally, I recently bought a recapped 240 and replaced the GE tubes with a set of ANOS Black-plate RCA 6L6GCs. I also replaced the first shielded 12AX7 with a ANOS Telefunken. The other tubes were replaced with nice ANOS tubes except for one 12BH7 (McIntosh branded, probably original) that tested very strong.

I was also worried about the power trans. getting hot but not because of the tubes.
My speakers are 6 ohm "nominal" and I was told I could use either the 4 or 8 ohm tap on the 240. The 8 ohm tap sounded better (and louder - these speakers spec for 40 watts minimum).
So I bought one of these...

https://www.amazon.com/dp/B00DMI632G?psc=1&ref=ppx_yo2_dt_b_product_details

and my power trans. is right at 107 - 108 F. In one of my searches on this sight, I read that around 110 was good.

So I'd say pick up the digital thermometer for the peace of mind.

 

[sberger]

It continues to amaze me how there is a constant supply of matched NOS RCA black-plate tubes even 60 years after they went out of production.

Been easier for me to find these then GE's, which I'd like to try based on what some folks have told me about them.

 

[TSmith8605]

. . .It continues to amaze me how there is a constant supply of matched NOS RCA black-plate tubes even 60 years after they went out of production.

Me too given that NOS means New Old Stock meaning sitting on a shelf for 50 years and never used. Used tubes are more common.

 

[Dave_in_Va.]

I edited my post to change "NOS" to "ANOS".
When you get right down to it, it's nearly impossible these days to confirm that a tube has never been used at all. Those RCAs I installed in my 240 had (as I was told) about 50 hours use and were bought from dealer as NOS, but who knows? But they tested very strong so close enough. Ditto, the Telefunken 12AX7 which came from a reliable dealer in Serbia. It tested like a new 12AX7 and he said it was NOS (it was in the orig. box) but again, who knows? There are still stashes of new tubes in the original boxes out there but those (the usual guitar amp/hi-fi tubes) are pretty scarce. Much scarcer than even ten years ago.
I'm an old guy so these tubes will still be working fine when I'm long gone. I doubt that I'll ever have to buy another 6L6GC (unless I buy another 240....nah).

 

[TSmith8605]

I edited my post to change "NOS" to "ANOS". . .

A distinction that is more important in classified ads for tubes (where I see the term abused often) than here.

 

[SaturationPt]

I collected tubes for HiFi, at one point I estimate the total (including non-hifi) was over 4,000 tubes. 95% or more of these were in "original boxes", many came in tube caddies. Most also were "NOS" (and actually appeared to be so).

However, so many tubes I bought and/or looked at buying are used tubes put back in the box where the new tube came out. I even bought an EV speaker pair that had an excellent 12AX7 Telefunken (old style) box in it (empty unfortunately).

Tube counterfeiting was such big business decades ago, and I believe is again today. Further, so many tubes that were rejected at the manufacturer went home with employees (I knew a guy who retired from an RCA tube plant, was QC, took home boxes he told me).

NOS is a great concept, but I suspect that a very small percentage of these 60+ year-old "NOS" tubes that are hitting the market are truly as advertised, and there are very few people left who can tell.

 

[Lingonberry]

Specific tube models have defined operating characteristics so I can't see what difference it would make whether a given tube came from Brand X or Brand Y as far as power draw (and PT heat generated) is concerned.

Otherwise the heat of the PT you describe sounds like normal function- exactly the same as my experience. I never noticed any difference whether the amplifier is just idling with no signal or exercised to its limits. It's the duration of being on is what causes the heat to build but after certain point it will get no hotter.

In his memoirs, Dave O'Brien mentioned an MC240 brought in by a customer who found it "very warm" after not having played the system for weeks. The amplifier had been installed in a closet and due to the amplifier being plugged into the "always on" outlet of the preamp rather than one of the switched outlets, it turned out the amplifier had been on continuously for twelve years. He said all it needed was an output doubler and a set of fresh output tubes to restore it back to spec - both of which it would probably have needed anyway after 12 years.

So I took Dave_in_Va.’s advice, and purchased a laser guided thermometer. When I check the temperature of the tube side of the power transformer, the temp reads 146 F; when I check the top and non-tube side, the temp reads 110 F; the outside of the unity coupled transformer, the temp reads 91-97 F. So that seems to be in the range you mentioned, and that the “hotter” touch seems to be from the tubes. This is after running the amp for 3 hours. So all seems well.

And to add to the discussion from above about tubes being really NOS or not, I got my RCA black plates from Brent Jessee at audiotubes.com, so I am pretty confident they are truly NOS tubes…

Thanks to all that gave advice in this thread!