• Please note that there are a few updates and clarifications made in the Audiokarma Rules, mostly relating to advertising and the addition of the new "Paying it Forward" & "Giving back" forums in the AudioKarma Audio Marketplace section.

12ax7 Voltage Question For New Mac

Matt The Cat

Matt The Cat

Prowlin' At 78 RPMs
Hi,

I'd like to replace my stock tubes with NOS 12ax7s in my MA2275. I've heard that older tubes (50s and 60s) used a lower voltage and it might not be a good idea to use NOS tubes in NEW McIntosh gear? What's the general consensus on this? I know Mac (Ron-C) will say that we should only use the stock tubes and I appreciate that. But I'm wondering if I could do any "harm" by using an NOS 12ax7 in a new MA2275?

Thanks,
Matt The Cat
 
Register to hide this ad
If you buy from a reputable dealer & the tubes test good with no shorts, leaks, etc., then there is no harm. The only older tube that displaces less gain, or significantly less gain than a comparable 12AX7 version is the RCA 5751 (70% less); which some like because the lower gain reduces noise and microphonics.

When I had my MA2275, the best bang for the buck & best tube to replace was the 12AT7 (driver tube for the KT88's). Telefunken ECC801S or the Siemens ECC801S were pure magic in those slots. The 12AX7's are located in V4/V5 slots for phono and V6/V7 for tonal. Some claim to replace the ones in V6/V7 also improve dynamics, etc., -- but I always had the tone control bypassed.

It's a fantastic integrated; no doubt.

P.S. I've never used stock tubes -- upon opening of the box(es), the stock tubes go back inside. The stock tubes are great -- especially from a mass produced aspect -- but there are better tubes, NOS, out there for the individual user who only needs 2 or so of a particular type.
 
Last edited by a moderator:
GREAT! Thanks for the info. I was wondering how important in the sound chain the 12AT7s were and you answered my question.

Is the 12at7, the final tube the signal hits in the pre-amp stage before going to the power amp KT88s or is the 12at7 actually part of the power amp? Please explain what the 12at7 does? I know it's a lower gain tube from the 12ax7.

Thanks,
MTC
 
The 12AT7's drive the grids & cathodes of the KT88's. In an amplifier other than a McIntosh (there may be exceptions - I know of none), the driver tube only drives the grid of the KT88. McIntosh, on the other hand, takes power from both the plate (thru the grid) and the cathode of the pairs of KT88's and sends their output to each of the two parallel primary windings in each output transformer. That's how McIntosh gets nearly 100 watts from a pair of KT88's verses 50 from other amp manufactuers. Your MA2275 is probably kicking out, nearly 90 watts per channel. The conservative rating by McIntosh is 75 watts -- but they always understate their output tube data.
 
Vintage Tube,

I recently got myself a new MC275 mk IV (of which I can't speak highly enough) and your post got me interested. I see you have MC275 as well. Would you say the biggest bang for the buck is replacing the 12AT7 in the 275 as well? While I'm not yet ready to start rolling tubes I have seen several comments highlighting its benefits, so I'm wondering what should be the starting place, come the time.

Thanks much.
 
LewinskiH01 said:
Vintage Tube,I recently got myself a new MC275 mk IV (of which I can't speak highly enough) and your post got me interested. I see you have MC275 as well. Would you say the biggest bang for the buck is replacing the 12AT7 in the 275 as well? Thanks much.
Click to expand...

When the time comes, the best bang for the buck tube replacement in the MC275 is the 12AX7's in slots V2 & V5. They are gain multipliers.

I consider V1 to be important as well, especially if you are using RCA connections. This 12AX7 is a splitter/inverter. If you use XLR's, then any 12AX7 can be used to satisfy the circuit -- because the V1 slot is only used for RCA's.

After those come the 12AT7's located in V3/4 & V6/7 as these drive the grids/cathodes of the KT88's.

Finally, the KT88's themselves.

If you do not have a tube tester -- suggest you inquire about tube purchase from a reputable tube dealer.
 
12AX7a tubes can handle higher voltages than a 12ax7. The non 'a' version has not been made for about 40 years though. 5751 is a drop in replacement for the 12ax7a in McIntosh tube gear. The 5751 is a General Electric designed '5 Star' military low noise version of the 12ax7a. the 5751 has a slightly lower gain in some circuits but this would be very hard to measure. The amplification specs are 70 for the 5751 and 100 for the 12ax7a..

Thanks,
Ron-C
 
ron-c said:
12AX7a tubes can handle higher voltages than a 12ax7. The non 'a' version has not been made for about 40 years though. 5751 is a drop in replacement for the 12ax7a in McIntosh tube gear. The 5751 is a General Electric designed '5 Star' military low noise version of the 12ax7a. the 5751 has a slightly lower gain in some circuits but this would be very hard to measure. The amplification specs are 70 for the 5751 and 100 for the 12ax7a..Thanks,Ron-C
Click to expand...


Ron - Your expertise & clarification is ALWAYS well appreciated.:thmbsp:
 
Not so much my expertise as reading out of the the book, 'Essential Characteristics' by GE. Available at ARS as a reprint.
Testing 12ax7a and 5751 tubes in the Hickoks, they measure about the same. The 5751s do draw a scosh more heater current which is not a problem in a Mc but may be if another design was marginal in the power supply.

Thanks,
Ron-C
 
I know this is subjective, BUT I just replaced the stock tubes in my line stage with 1962 RCA gray plate 12ax7s and HOLY S&*T what a difference. I'm serious, it's night and day. The RCAs just blew me away.

I'm taking Vintage Tube's advice and replacing my 12at7s next with vintage Telefunkens and I've got some Tele 12ax7s on the way for the phono stage. I'm doing everything step by step so that I can hear how the improvements sound as I go. I'm not knocking the Mac stock tubes as they do the job and sound pretty damn good as well, but man, when people tell you that NOS is the way to go, I've got to get on board with that theory now. Hearing IS believing and I'd recommend the RCA 12ax7s to anyone and you can get them pretty cheap these days (as compared to the Teles).

-Matt
 
Hey Vintage_Tube,

I just put the Telefunken 12at7s in my MA2275 and WOW! You were right. A def upgrade. The stock tubes were good, but now it sounds GREAT!

Thanks for the advice.

-Matt
 
You're welcomed Matt -- enjoy that integrated -- which I feel is one of McIntoshs' greatest units -- I say that because many ignore or do not investigate the integrated route. A true beauty to boot.:thmbsp:
 
Ron,

Help me out - ARS? What does this stand for (so I can get a copy of this reference)?

Paul

ron-c said:
Not so much my expertise as reading out of the the book, 'Essential Characteristics' by GE. Available at ARS as a reprint.
Testing 12ax7a and 5751 tubes in the Hickoks, they measure about the same. The 5751s do draw a scosh more heater current which is not a problem in a Mc but may be if another design was marginal in the power supply.

Thanks,
Ron-C
Click to expand...
 
Call em and work out the shipping -- I did it on the web at o'dark thirty after a few hefeweizens and they charged me $14 for shipping.:tears:

The photo will make Ron's day.:D


FYI -- there are no in depth detail references to the European tubes.
 

Attachments

  • Book 001.JPG
    Book 001.JPG
    80.9 KB · Views: 23
I use a MC275 MkV with a C2200 preamp in my listening room. The tubes in the C2200 are stock. I am currently using Gold Lion KT88, Philips JAN 5751 (12AX7A), and Philips 12AT7 (EGC8739) in the MC275 with excellent results. I have four strong old stock Telefunken 12AX7 tubes, and four NOS Mullard 12AT7 (CV4024) tubes. I installed Telefunken tubes in place of the Philips JAN 5751, and listened for a few days. There seemed to be a very minor improvement in upper midrange detail, or maybe it was the Telefunken reputation making me think that. I left the Telefunken tubes in, then replaced the Philips 12AT7 tubes with the Mullard 12AT7 (CV4024) tubes. Now I heard a real difference. The enitre midrange stepped forward about a foot (if you can see what I mean). There was a brighter presentation with the Mullard 12AT7 tubes. I listened to the Telefunkens and Mullards for two days.

Then I reversed the process. I left the Mullard 12AT7 tubes in place, but returned the Philips JAN 5751 (12AX7) in place of the Telefunkens. There was very little, if any, difference to be heard. The presentation was still very detailed, with the midrange still forward, or closer to me than with the Philips 12AT7 tubes. I listened for a day. Finally, I returned the Philips 12AT7 tubes in place of the Mullards. The midrange stepped back about a foot, but no loss of detail, sustain, or warmth, just not quite as in your face with the midrange. I found solo piano (Liz Story) to be correct again, without a bite to the upper register.

I have been listening to the Gold Lion, and all Philips configuration now for the past three days. I am still not positive I liked the closer midrange with the Mullards, but I can say that the Philips JAN 5751 (12AX7) holds it's own against the old Telefunken. Both tubes are very transparent.

I may swap the Mullards in again for another listening period, but for now the all Philips and Gold Lion arrangement is making absolutely incredible music.

I realize this was strictly a subjective, unscientific listening test, but felt obliged to offer my results. I may play with the preamp next.
 
To vintage tube......the Andra II is a world class speaker, no doubt, and beautiful, too. I see that you are driving them with a pair of MC275 amps (parallel at 150 wpc I imagine). Does the 88.5 dB sensitivity at 6 ohms work a single MC275 to its limits? I ask because I am taking delivery on my new PMC EB1i speakers next week, and they have a 89 dB sensitivity at 4 ohms input. I am concerned about having enough power to drive the speakers to live jazz club type levels without working the amp to death. Did you initially try to power the Andra II's with one MC275? Are two amps satisfactory for your needs, or are you looking for more power? I'm thinking I may need two amps before all is said and done.
 
jdandy said:
To vintage tube......the Andra II is a world class speaker, no doubt, and beautiful, too. I see that you are driving them with a pair of MC275 amps (parallel at 150 wpc I imagine). Does the 88.5 dB sensitivity at 6 ohms work a single MC275 to its limits? I ask because I am taking delivery on my new PMC EB1i speakers next week, and they have a 89 dB sensitivity at 4 ohms input. I am concerned about having enough power to drive the speakers to live jazz club type levels without working the amp to death. Did you initially try to power the Andra II's with one MC275? Are two amps satisfactory for your needs, or are you looking for more power? I'm thinking I may need two amps before all is said and done.
Click to expand...

I had driven the Andra II's with a MA2275 (integrated version) and was quite happy; but I knew with their design, I needed more than 75 watts. So, the dual MC275's entered the picture & were just what the Andra II's called for. The result was more punch, better overall clarity, and resolution & the soundstage opened up moreso with the extra watts.

Since day one, I have used the 4 ohm tap after seeing the graphs & ohmage dips in stereophile magazine awhile back.

To answer your last part -- I thoroughly enjoy my MC275's; but I'm in love with what I believe the MC2301's can give me and headed in that direction. I'm hoping to capitalize on the 300 watts for additional impact & definition.

Oh, by the way congrats on your new speakers -- have not heard them -- maybe showing at RMAF in Denver in October -- will make a note to check them out (down to 19Hz -- impressive).
 
You must log in or register to reply here.
Back
Top Bottom