Do Black Plate 6L6 Tubes Cause Red Plating in Mac Amps?

Given all of the input, it would seem like the black plate warning is an urban myth. Maybe sasquatch or Santa Claus started the rumor.
 
And so all of the MC30s and MC240s with black plate 6L6GCs went on to delight happy listeners with their alluring sound for many wonderful years without red plating, and they all lived happily ever after...
 
...or 7027(A) in MC240, 1614 (metal casing) in MC30...or 6L6GC (MC240 & MC30) regardless of manufacturer, regardless of whether the plates are colored gray, or black. Yes, all will work in a Mc and work well because it IS a Mc. :)
 
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6L6, 6L6G, 6L6GA, and 6L6GB are all glass bottle early versions that will not work (for very long!) in a Mc amp as they have a Max plate watts of 19, Max plate volts of 360 and Max screen volts of 270 @ 2.5 amps.
6L6GB is Max plate watts of 30, Max plate Volts of 500, Max screen volts 450 @ 5 amps.

Ron-C

Not to nitpick but '6L6', while certainly an early - in fact THE earliest - version, is not a glass bottle tube. This was pointed out earlier in the thread but I just wanted to make sure everyone reading this was clear on it so there is no confusion. But as you said, it won't work in a Mac amp and that was the point.



Sometimes people will put an M after the tube number to designate metal envelope. These are also sometimes called 'smokestacks'. I always wanted to build a PPP amp with a bunch of these - they are pretty cheap as 6L6 goes. But who am I kidding, I never have time to build amps...
 
No. 1614 has a 500V max plate voltage, 6L6 is 400, for one thing. Plate dissipation is also higher in the 1614.
 
Isn't the 6L6 the same as the 1614?

For the most part yes, but.......a 1614 is a “selected” 6L6 for transmitter service and carries slightly higher ratings as a result of selection. Not sure just what parameter(s) is selected for.

From RCA HB3:
Max (CCS)
Plate=375V / 21W
g2=300V / 3.5W

Max (ICAS)
Plate=550 / 25
g2=400 / 3.5

6L6
Max
Plate=360 / 19
g2=270 / 2.5

Curiously, some 1614s show their 6L6 heritage.

Above the flange:
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Below the flange:
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You CAN NOT use 7027A tubes in an MC-30. The tube will short the amp. Incorrect pin out in the tube socket. (Or is it one extra pin used?) Trust me, I know from experience (several years ago). On the other hand, 7027A's work fine in an MC-240 amp.
 
Here is what McIntosh said:

"McIntosh amps will correctly perform with 6L6GC tubes. The earlier 6L6 tubes do not have the voltage rating to work in these amps. Red platting is not an issue using 6L6GC tubes as long as the amp is in proper operational condition."

Perhaps whoever started the red plate rumor was using non-GC tubes.
"Red plating is not an issue using 6L6GC tubes as long as the amp is in proper operational condition" Really? My 240 was rebuilt by Audio Classics several years ago, so that the 6L6GC tube with red plating is no issue? The tube itself (rca black plate) tests. very strong, but it's red plating in less than a couple of minutes.
 

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Be sure the second 12AX7 that is associated with those 6L6GCs has clean pin contacts and that the tube socket pin receivers for the 12AX7 tube are also clean. Any corrosion on that tube's pins or in the socket can cause loss of grid bias on the 6L6GC associated with it and result in red plating.
 
Be sure the second 12AX7 that is associated with those 6L6GCs has clean pin contacts and that the tube socket pin receivers for the 12AX7 tube are also clean. Any corrosion on that tube's pins or in the socket can cause loss of grid bias on the 6L6GC associated with it and result in red plating.
Thank you!!!
Just checked the tube on the far right, and it's 12au7, not 12ax7 !!! Swapped it with the right one. No more red plating! Because the music was great, I didn't know I put the wrong tube there (Telefunken 12AX7 & 12au7 are hard to tell without the lettering). Oh boy, I've been using the amp like that for some time. Anyway it reminds me how important it's to double check.
 

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A 12AU7 in that position will do it too. Glad you got it solved!
But it looks like 12au7 was responsible for the red-plating. Actually I've been wondering why I got this red plating in the 6L6GC tube when it tests so strong. Thanks for your timely reminder. Without it, I would have never double checked that tube on the far right. I feel the sound with the right tube is more lively & dynamic!
 
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