MC275 MKVI tube identification

arien

Active Member
Hi,

It seems that the MC275 MKVI have different tube position and id, from the label i can see from left to right

V1L 12AT7 V2L 12AT7 V3L 12AX7A V4L/R 12AX7A V5R 12AX7A V6R 12AT7 V7R 12AT7

My question is: which one is the splitter/inverter tube that turns the single-ended input signal into a balanced signal? is it the V4L/R ?

Does anyone have the schematic of the MKVI?

Thank you in advance.
arien
 
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I think the V4L/R 12AX7A.

Last month, my little V5R 12AX7A has broken broken down, but the MC275LE can still play music. Does anyone know why? (I'm running monoblocks)

Thank you.
 
I think the V4L/R 12AX7A.

Last month, my little V5R 12AX7A has broken broken down, but the MC275LE can still play music. Does anyone know why? (I'm running monoblocks)

Thank you.

Thanks, that's what i think as well...


but would like to get confirmation if possible....or a circuit diagram.....

I'm also looking forward to the answer to your question.....

hope Ron-C can respond....cheers
 
What does broken down mean? V4 is the input-preamp tube, V3 and V5 are the phase inverter for the two channels and V2 and V6 are the output driver amps and V1 and V7 are buffers in front of the KT88 output tubes.
Unlike all earlier versions, MC275LE and MKVI have the preamp/input tube in the middle with the amplifiers on either side of it.

thanks,
Ron-C
 
What does broken down mean? V4 is the input-preamp tube, V3 and V5 are the phase inverter for the two channels and V2 and V6 are the output driver amps and V1 and V7 are buffers in front of the KT88 output tubes.
Unlike all earlier versions, MC275LE and MKVI have the preamp/input tube in the middle with the amplifiers on either side of it.

thanks,
Ron-C

Thank you for the information about these tubes :thmbsp: . I mean it (V5R 12AX7A) does not glow or heat, but the amp can still play music normally. The next day, I found it have some cracks on tube glass and the silver coat above the getter has gone white completely.

I call my dealer, and they replace the broken tube, hope it does not damage my MC275LE.


Thanks.
 
What does broken down mean? V4 is the input-preamp tube, V3 and V5 are the phase inverter for the two channels and V2 and V6 are the output driver amps and V1 and V7 are buffers in front of the KT88 output tubes.
Unlike all earlier versions, MC275LE and MKVI have the preamp/input tube in the middle with the amplifiers on either side of it.

thanks,
Ron-C

Thank you very much Ron.
 
What does broken down mean? V4 is the input-preamp tube, V3 and V5 are the phase inverter for the two channels and V2 and V6 are the output driver amps and V1 and V7 are buffers in front of the KT88 output tubes.
Unlike all earlier versions, MC275LE and MKVI have the preamp/input tube in the middle with the amplifiers on either side of it.

thanks,
Ron-C

So, just to be 100% clear, the V4 tube is completely out of the signal path if balanced inputs are used?
 
No, it always in the signal path, in the balanced side.

Thanks,
Ron-C
 
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No, it always in the signal path.

Thanks,
Ron-C

Thank you VERY much for clearing this up. There is an assumption floating around the net that the V4 tube is used only to amplify the unbalanced signal and does not affect the balanced inputs.
Clearly false, then.
 
No, it always in the signal path, in the balanced side.
Thanks,
Ron-C

It appears that in the previous versions of the MC275, such as mkIV, V1 was the phase inverter tube that turned the unbalanced signal into balanced.

There are many posts around stating that V1 is not in the signal path when unbalanced input is used, such as here:

http://www.audiokarma.org/forums/showpost.php?p=6078923&postcount=15

if you're only using the BAL inputs, the V-1 can be any old 12AX7 with working filaments. It won't be in the signal path -- it's only needed to complete the circuit for the V-2's filaments ;~)

You yourself have confirmed it here:

http://www.audiokarma.org/forums/showpost.php?p=2495612&postcount=6

...you skip the first tube when using balanced inputs.

Now, mkVI seems to be a different ball of wax. As you stated above: "V4 is the input-preamp tube, V3 and V5 are the phase inverter for the two channels".

So as V4 does not have the same functionality as V1 in the earlier versions, and is always in the signal path, it is important there is a good quality tube in V4 even if balanced inputs are used, correct?

So, in contrast to the earlier versions of MC275, in mkVI ALL signal tubes are in the signal path regardless whether balanced or unbalanced inputs are used, right?

Sorry for all these questions, but as good NOS 12AX7s tend to cost, I want to be sure I am not wasting them in positions where they can't make any difference.

Thanks,
Alex
 
Alex,

Yes, V4 is the preamp tube. I would not get to caught up in tube rolling as this amp will perform as intended with pretty worn out tubes. If a tube make a big change in the sound it is probably defective.

Thanks again,
Ron-C
 
I would not get to caught up in tube rolling as this amp will perform as intended with pretty worn out tubes.

Interesting. One of the stock KT88s failed on me after 6 months (the sentry protection kicked in, so no damage - a great feature!). So I have put in a new quad of Russian "Gold Lions". The sound difference is dramatic. It might be argued whether it is 'better' or not, but no one could deny a non-subtle difference in sound character. I would have assumed that rolling the small signal tubes would also have potential to alter the sound...

Thank you very much for the reply,
Alex
 
The Gold Lion KT88 tubes have their own sound which some like, I always thought they are OK.

Thanks,
Ron-C
 
The Gold Lion KT88 tubes have their own sound which some like, I always thought they are OK.

Thanks,
Ron-C

Ron,

Interesting…
Are you referring to current Russian GL and if so, are those similar to original Genalex KT88s?

I’ve restored a pair of MC60s that came with Gold Monarch KT88 (circa 1970) but replaced them with Russian GL immediately without and A/B listening.
I wonder if there’s any difference. But then, I might not be able to hear it… :scratch2:

Thank you.
 
The Russian Gold Lions do not sound like the original GEC Gold Lions to me. I think the Shugang KT88-Z are much closer in sound to the Genalex Gold Lions. Yes, I have all of them.

Thanks,
Ron-C
 
I would also add the current JJ KT88s as being close to the classic GEC UK KT88s.

Thanks,
Ron-C
 
I had the center 12ax7 tube on my MC275VI get a bad filament and the music didn't stop. I use balanced inputs. I didn't notice the filament was out at first until I turned off the green LED's. :)
 
I had the center 12ax7 tube on my MC275VI get a bad filament and the music didn't stop. I use balanced inputs. I didn't notice the filament was out at first until I turned off the green LED's. :)
I'll be darned. You are right. The amp plays fine with no tube in the V4 position and balanced input. Great! One less tube to worry about.
 
Dear Ron, i have 275 VI.
And i have "only" pair 12ax7 gold lion.

Where is it more convenient to replace? V3 and V5 (phase inverter) or only one 12ax7 in V4 ?

12ax7 : V3 and V5 phase inv.
12ax7 : V4 input preamp.
 
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