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KT66 can they be used in a MC240 or not?

vettedrummer

vettedrummer

vettedrummer
Hello all. I was just looking at some tubes and I saw a listing for KT66 tubes and the listing made it look as if a KT66 will replace a 6L6 in a McIntosh 240 amp. :scratch2:Does anybody here know if it will or not. Is there any other tube that will replace the 6L6 tube in the MC240? What are you running in your MC240's? Assuming you have one. Let me know if you know! Cheers, Frank vettedrummer
 
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6L6GC or 7027A only please just like it says next to the tube socket. All others will be big trouble.

Thanks,
Ron-C
 
Have run KT66 in MC240 with no excessive heating of the power transformer, these were original Genelex and sounded much better than the EH6L6s that were in the amp.. Have done this in several MC240s and MC30s with no ill effects and superb sound. The Tungsram tube manual data shows them the same as a 6L6 except filament current of 1.27A for the KT66 and .9A for a 6L6.
 
Terry, are you shilling your tube stash? Ha. Have you tried this with the current production "KT66" tubes?

Thanks,
Ron-C
 
Had 4 old genelex in my collection, they tested just minimum so I hung on to them for about 20 years, a local wanted to try them in his 240 and borrowed them for a few weeks, he wanted to buy them but I quoted an eBay price, he said 'no thanks' so I put them on ebay with a buy it now, sold them in less than 10 minutes from posting, stash of KT66 is now gone... have not tried with the new ones, need to get someone to loan me a set...
 
I have vintage Mullard EL37 tubes in my Quad II amps, which of course use KT66 as original tubes. No mods needed for this switch (AFAIK), and the EL37 tubes sound great to me. Not sure about WE350B tubes.
 
This certainly sounds like an opportunity to verify, for sure!

My MC-30s are nowhere ready for being test pigs, although I'd be willing to use them for this test with the following caveats:

- The Mac tube amp has been fully confirmed to be operating correctly with the recommended tubes first.

- The Mac amp, if a vintage unit, be fully rebuilt and back into operating specs first.

- The operating bias voltages on the vintage Mac tube amp has been set up per the recommendations in the Hints and Kinks sticky here. I would want to use the KT-66s under test to set the bias voltage to ensure they are used under spec'd conditions.

- The tube sockets are clean of corrosion and the pins all make good, solid contact.

Once those are done, then I'd be willing to sit back and try a set of KT-66s in my amps without worrying about red-plates, arcing screen grids, overheated tranformers or fire-and-smoke of some kind. Think of it as a "Trust but Verify" approach.....

Cheers,

David
 
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dshoaf said:
This certainly sounds like an opportunity to verify, for sure!

My MC-30s are nowhere ready for being test pigs, although I'd be willing to use them for this test with the following caveats:

- The Mac tube amp has been fully confirmed to be operating correctly with the recommended tubes first.

- The Mac amp, if a vintage unit, be fully rebuilt and back into operating specs first.

- The operating bias voltages on the vintage Mac tube amp has been set up per the recommendations in the Hints and Kinks sticky here. I would want to use the KT-66s under test to set the bias voltage to ensure they are used under spec'd conditions.

- The tube sockets are clean of corrosion and the pins all make good, solid contact.

Once those are done, then I'd be willing to sit back and try a set of KT-66s in my amps without worrying about red-plates, arcing screen grids, overheated tranformers or fire-and-smoke of some kind. This of it as a "Trust but Verify" approach.....

Cheers,

David
Click to expand...

I can verify from experience the viability of using KT66s and EL37s in a properly operating MC240...at least for short term operation. Electrically, both are plenty close enough to a 6L6 to be subbed w/o issue. For the long term I'm a bit concerned about the ability of some MC240's PT to provide the additional filament current these tubes require. Some early 240s were equipped to provide preamp power inc filament. For this version I wouldn't hesitate to use the KT66/EL37 subs long term. I don't know if Mac downgraded the 240's filament current capability when the aux power socket was removed so I'd be more hesitant to use the subs long term w/o verification. Since my 240 is the later, non-aux power version and I haven't been able to verify the PT capability, I've stuck with th 0.9A 6L6/7027A tubes for the long term.
 
"...big trouble"?
I have to respectfully disagree with Ron and agree with Terry. I've used NOS Genelex KT66 tubes in my 240 for years without any problem. A careful search of the 'Asylum or Audiogon Forums will show the same.
 
Wow, I am surprised you guys have the original KT66s floating around. The little Quad amps were the main users of these and Quad sold a ton of them.
If the 240 power supply is in good shape it should survive the higher heater current draw.
If the KT66s are the same rating as 7581A they should work. Neither one of these tubes however is rated for the plate volts used on the MC240. A 360 volt rating on these is not the same as the 425 volt rating of the 7027A or 6L6GC.
GL KT66s are the coolest looking of all of these tubes.

Thanks,
Ron-C
 
ron-c said:
Wow, I am surprised you guys have the original KT66s floating around. The little Quad amps were the main users of these and Quad sold a ton of them.
If the 240 power supply is in good shape it should survive the higher heater current draw.
Click to expand...

Don't forget the Heath W5. Although the QII was popular worldwide, based on what I've seen, in the US the W5 probably outsold it by a large factor. Over the past 20yrs I've seen scores of W5s at hamfests and swaps with harvested KT66s. I've seen only three QIIs

If the KT66s are the same rating as 7581A they should work. Neither one of these tubes however is rated for the plate volts used on the MC240. A 360 volt rating on these is not the same as the 425 volt rating of the 7027A or 6L6GC. GL KT66s are the coolest looking of all of these tubes.

Thanks,
Ron-C
Click to expand...

KT66 ratings changed somewhat over the many decades it was in production (as did rating protocols). 60s era KT66s carried design max ratings of 500V for both plate and g2 with 25W plate for est life of 8000 hrs. Absolute max ratings were 550V for plate and g2 with 30W. EL37 ratings were a little less.

Also consider that the unity coupled output topology is apparently pretty forgiving WRT electrode voltages. Historically, the MC30 exceeded the voltage ratings of the original issue 1614s and the 50W2 exceeded the voltage ratings of 6L6Gs. Based on this info, I'd conclude that the KT66 and EL37 should survive nicely in an MC240. Based on your comment above, the 240 should survive the KT66s so all is well.
 
ron-c said:
Wow, I am surprised you guys have the original KT66s floating around. The little Quad amps were the main users of these and Quad sold a ton of them.
If the 240 power supply is in good shape it should survive the higher heater current draw.
If the KT66s are the same rating as 7581A they should work. Neither one of these tubes however is rated for the plate volts used on the MC240. A 360 volt rating on these is not the same as the 425 volt rating of the 7027A or 6L6GC.
GL KT66s are the coolest looking of all of these tubes.

Thanks,
Ron-C
Click to expand...

Here is a pic of 7581/A/ KT66, this one of my spares for Quad II amp that uses KT66

Heater current is less than GEC KT66 so safe to use in Quad II. Can't remember figures, but the info is around.

I think I read somewhere that the 7581A is the best of the 6L6 range.
 
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Radfordman said:
Here is a pic of 7581/A/ KT66, this one of my spares for Quad II amp that uses KT66

Heater current is less than GEC KT66 so safe to use in Quad II. Can't remember figures, but the info is around.

I think I read somewhere that the 7581A is the best of the 6L6 range.
Click to expand...

Great photo of a BOX. :D
 
jdandy said:
Great photo of a BOX. :D
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Sorry, but I thought that the important bit was the printing on the box that says "7581A/KT66" showing that General Electric considers the 7581A OK to use where a KT66 is specified. I believe that Quad did supply 7581A as replacements at one time.

Anyway for all you tubeaholics here is another pic.

Apologies.
 
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MT GE tube guide does not show the higher rating on the KT66s. Anyone try the 'new' production KT66s? I have seen MC240s well over 50 watts using the 7027As. Maybe 60+ watts mid band at the onset of clip.

thanks,
Ron-C
 
ron-c said:
MT GE tube guide does not show the higher rating on the KT66s. Anyone try the 'new' production KT66s? I have seen MC240s well over 50 watts using the 7027As. Maybe 60+ watts mid band at the onset of clip.

thanks,
Ron-C
Click to expand...

Here's a page from GEC databook showing higher ratings:

attachment.php


Here's a graphical depiction of the "evolution" of the 7581A with a KT66 thrown in for comparison. All original 7581As I've come across have been of GE mfg regardless of brand. At the end, I believe only Philips was making them and they were probably a redesignated 6L6GC.

attachment.php


Although the 7581 officially carried higher ratings than the 6L6GC, from everything I can tell, by the 80s there was no difference, even to the point of co-designation. The GE shown has "6L6GC" printed on one side and "7581A etched on the other (not shown). As a result, the only 7581As I'd pay a premium for compared to 6L6GC would be the pink and micanol based versions.
 

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I bow to your great knowledge of tubes, nice photos and explanation.

However, if GE mark a tube 7581A/KT66 I would expect it would be up to the spec of a KT66.

It is possible that selected 7581A's were marked like this.
 
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