McIntosh 40 MC40 Can I use KT88 or 6550?
- Thread starter Hi-FiGuy
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all on YAMAHA untill 1990
Hello Hi-fiGuy
1) KT88 and 6550 are not the same tube and don't have same specs . 6550A is identical to KT88 and no other one .
2) MC40 use 6L6GC , so do not try non recommended tubes like Pio1980 advise you to do .
3) do you think that using KT88 in an amplifier will increase the output power ?
1) KT88 and 6550 are not the same tube and don't have same specs . 6550A is identical to KT88 and no other one .
2) MC40 use 6L6GC , so do not try non recommended tubes like Pio1980 advise you to do .
3) do you think that using KT88 in an amplifier will increase the output power ?
Pio1980
AK Member
You might be able to use KT66 as an alternative to GL6GC, certainly none of the other KT series for any advantage.
http://audiokarma.org/forums/index.php?threads/kt66-can-they-be-used-in-a-mc240-or-not.213834/
http://audiokarma.org/forums/index.php?threads/kt66-can-they-be-used-in-a-mc240-or-not.213834/
Pio1980
AK Member
I wouldn't recommend using them for that reason alone, too many acceptable alternatives in the 6L6 group.
I know this is an older thread but figured I'd throw my 2 cents in just in case anyone is watching.
I have a pair of MC40s that I have recapped with MIT Multicaps (same as my VLT preamp uses) I've rebuilt the supply with bigger caps and use Gold Lion KT66 reissues. The amps sound wonderful. The input tubes are NOS Telefunkin and I believe the others are all RCA. I have run these amps this way for several years now with no problems. The power transformers are never more than warm to the touch. They seem to be overbuilt enough to handle it.( It may be different in a MC240 where one transformer has to handle 2 channels of filament power.)
I had late issue RCA 6L6 tubes that came with the amps that sounded OK but I feel the there is more bottom end power and control with the KT66 and more air overall.
I also tried Sovtek KT66 but like the Gold Lion better. (Sovtek KT66 is obsolete now anyway) Amps sounded good with the Sovtek, maybe a bit better bass control but a bit dry and sterile. The RCAs were warmer than the Sovtek but the Gold Lion seem to have more control overall and the amps seem to breath easier with plenty of warmth. Not quite the midrange magic that some EL34 amps manage but very close.
I don't think I'd try the KT88 or 6550. That might be pushing it a bit too much. That would put another .6 amp on the filament winding over the KT66.
And I don't see much additional advantage given the MC40 power supply isn't built for the kind of power they can handle.
Also being that the amps are self bias a KT88 might bias on the lean side without circuit changes and you might get an increase in distortion especially at low levels. If you make circuit changes to pump the bias current up then that's even more load on the transformer.
If you feel you need the extra power look for a pair of MC75s or a MC275 that can use those tubes to their best advantage.
Pio1980
AK Member
That might be worth ca 1.5-1.8 Amp margin per amp.
BillWojo
Addicted Member
CruzBay, the PN on the PT on my set of MC40's is M225 and all the pictures of the PT on the MC240 that I've seen pictures of is M207A.
Interesting bit of info, the OPT on my MC40's are both M224 but the OPT on the MC240 is 210A or B. I wonder if the OPT are not the same? I suspect the PT is different.
I had an E-mail conversation with Jim McShane about using KT66 in my amps and he recommended against it. He cited the higher amperage draw and ended with "Do I want to gamble on 50 year old irreplaceable transformers?" Considering how sweet they sound with vintage GE and RCA 6L6GC tubes I decided it's not worth gambling with.
The PT does seem to loaf along as after a few hours it barely gets warm and I suspect that much of the warmth is from the power tubes sitting close by.
BillWojo
Interesting bit of info, the OPT on my MC40's are both M224 but the OPT on the MC240 is 210A or B. I wonder if the OPT are not the same? I suspect the PT is different.
I had an E-mail conversation with Jim McShane about using KT66 in my amps and he recommended against it. He cited the higher amperage draw and ended with "Do I want to gamble on 50 year old irreplaceable transformers?" Considering how sweet they sound with vintage GE and RCA 6L6GC tubes I decided it's not worth gambling with.
The PT does seem to loaf along as after a few hours it barely gets warm and I suspect that much of the warmth is from the power tubes sitting close by.
BillWojo
ajauwen
Active Member
I would tend to go with Terry DeWick's take on the issue..!! Terry has been an authorized McIntosh repair guy for several decades and probably knows more about them than the next 10 guys. As Terry has said, these amps were made to also provide filament voltage to an external preamplifier, so the power transformers were made to handle the extra current.
ajauwen
Active Member
My apologies..!! You are correct, the MC40 & MC240 have a connector on the front panel that outputs the speaker hookups and also 25v & 70.7v, but no filament voltage. I had the MC30 & MC60 on the brain. That said, I still put a lot of faith in what Terry said about using the KT66 in the MC240, which should also carry over to the MC40. I've used the KT66 in each of these amps with no noticeable issues, however, I believe I will be doing some detailed measurements in the near future to help convince myself for sure..!!
The first version MC240 had two octal sockets, I believe one for a preamp. 2nd and 3rd versions did not.
My MC240 power transformer can gets hot enough to be uncomfortable to the palm of my hand (don't remember the temp, seems like from memory 175F?) just running 6L6GCs for a few hours (68* room, ceiling fan), I'm not comfortable with running it hotter if I can avoid it. How warm does the MC40 PT run typically?
My MC240 power transformer can gets hot enough to be uncomfortable to the palm of my hand (don't remember the temp, seems like from memory 175F?) just running 6L6GCs for a few hours (68* room, ceiling fan), I'm not comfortable with running it hotter if I can avoid it. How warm does the MC40 PT run typically?
My pair of MC240 are they very early version and have dual sockets. The PTs just get a little warm after running them for 6 hours or more. They're usually on for 8-10 hours a day and they aren't hot.
Couldbfishin
Couldbfishin
I have a pair of MC40s and they both have McIntosh 6L6 tubes in them.
2x’s
Ears for Beers
The Tung Sol kt66 new production only pull .9A like the old school GEC kt66. So no issue with current draw. The new Gold Lion is not suitable for the mc240, they pull 1.2A.
