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KT 120 Tubes in Yaqin MC-100B Amp

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Posted on March 25, 2011 at 21:16:29
Braxus
Audiophile

Posts: 926
Location: Fraser Valley
Joined: February 11, 2006






I will be picking this amp up in a weeks time. I won't be replacing any tubes in it for quite some time (at least till late in the year), but I'd like to consider some options. This amp uses KT88 tubes for power tubes. I have heard of the KT120 as an option for some amps. I talked to one guy on here who is actually using the 120 tubes in his 100B for a few months now. He has had no issues with it so far. From what I gather the voltages are the same between the 88 and 120, its just the bias needs to be readjusted for the 120. My concern is if the power transformers are up to the task of using these tubes and if using these tubes in this amp is safe- for the long term. Can anyone look at the parts and spec diagram to see if this thing is safe to use with 120 tubes? First picture is a inside nude picture of the parts. Second is the circuit diagram. Take a look and let me know what you think.

For a better look at circuit diagram, see:

http://www.tapeheads.net/attachment.php?attachmentid=19748&d=1301105909

 

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Do you have photo of trannies?, posted on March 26, 2011 at 02:10:00
FenderLover
 Kinda hard to tell without seeing trannies or knowing the brand/model used in your specific amplifier.

 

RE: Do you have photo of trannies?, posted on March 26, 2011 at 08:22:52
Braxus
Audiophile

Posts: 926
Location: Fraser Valley
Joined: February 11, 2006



Here's a pic of the amp topside.

 

RE: Do you have photo of trannies?, posted on March 26, 2011 at 20:59:43
FenderLover
 Those look pretty hefty. Just be sure to re-bias. And check over overheating.

 

RE: KT 120 Tubes in Yaqin MC-100B Amp, posted on March 28, 2011 at 21:43:41
casouza
Audiophile

Posts: 755
Joined: March 18, 2008
 I suggest that you buy an inexpensive infrared thermometer and measure the power transformer's temperature with the KT88s after warmup. The power transformer is the one near the power switch.
Then install the KT-120s. If the transformer temperature goes up more than 20 degrees F or 10 Centigrade, I suggest that you remove the KT120s.
A 10 degree C rise in external temperature may not look like much, but the internal temperature rise will be much higher, probably 40-50 C.
Assuming a "normal" temperature of 50 C, with KT120s the transformer hot spot may reach 100 degrees C or more.
I copied the following from a paper on transformers:
Modern transformers make use of thermally upgraded paper that has been chemically treated to improve the stability of cellulose structure. The rated hot spot temperature for this kind of paper is 110°C and it can be seen that an increase of 7°C will double the aging acceleration factor.
For older transformer built with normal Kraft paper, the rated hot spot temperature is either 95°C according to IEEE or 97°C according to IEC.
Google "transformer hot spot", there are lots of interesting articles.

In summary, it is a risky idea,

 

RE: Do you have photo of trannies?, posted on March 28, 2011 at 21:53:17
casouza
Audiophile

Posts: 755
Joined: March 18, 2008
 Fender Lover, looks can be misleading. For example, the output transformers for both channels reside inside the right-side can...they are quite small for the amp's power rating.
There is no way to know the actual size of the power transformer, unless one disassembles the amplifier.
This amplifier costs U$ 500-600 on eBay. I doubt that its power transformer is over-specified to handle an extra 800 mA of heater current.
Just my 2 cents...

 

RE: KT 120 Tubes in Yaqin MC-100B Amp, posted on March 31, 2011 at 22:52:41
Mouse ears
Audiophile

Posts: 54
Location: Dallas
Joined: July 16, 2010
 I have a quad of KT120s I run in my Yaqin MC100b with no problems. The MC100b is a bright amp and the KT120’s are pretty clear, but do not have the same bottom end as KT88s. They do have a big sound stage and are smooth. The KT88-98 that come with the amp are clear, but again pretty bright in this amp. JJ’s provide the best bass, but I recommend trying Shuguang Treasures or GL reissues.

 

RE: Do you have photo of trannies?, posted on April 2, 2011 at 07:22:08
jking67
Audiophile

Posts: 57
Joined: October 22, 2006
 Actually this amp has two power transformers.They are located at the back of the amp and are vary large as well as the output transformers. That is why this amp weighs in at 60 lbs plus. Just thought I would clear that up. Good Day, Joe

 

Thanks, Joe., posted on April 2, 2011 at 07:52:29
casouza
Audiophile

Posts: 755
Joined: March 18, 2008
 I did not know that. That's why a forum such as this one is so useful.
Good day 2 U 2

 

RE: Thanks, Joe., posted on April 2, 2011 at 08:09:49
Gonzalo


 
I concur with Casouza completeloy I had my MC100B for a couple of years now and must tell you that the best value for money on power tubes are the Sophia Electric sourced GEKT-88 a sweetheat of a deal the KT88-98's are not that great, too bass heavy and they shout in the mids. they shione on teh top end but it's 1 of of 3 while the GEK's do all things pretty good.

now if you wan to invest some serious $ into tubes the NOS 6550 TUng-Sols are the best bar none, the comes the Ei KT-90's pre conflict I thik type II's are teh best bass you can get out of this really good amp for the money.

I particularly enjoy it in triode mode, I think UL is only for rock & Roll, definetly triode is a lot more musical, better mids amazing stage end pristine clear top end.

As far as the KT120's I sincerely think the oputput tranny's are warm enough with the KT-88's if you push them harder with KT 120's you are likely to se early death due to metal fatigue or worse.
I'll put in GEKT88 and take teh rest of teh money and put it in some serious 5751 for the input tube This ampl simply loves them especially GE black plates 5751 amazing!!1 and soem good old 6SN7's like tung SOl tall bottles or Ken -Rads 6SN7-GT.
I have a few pairs of the RCA's gray glass, Ken-Rads 1940's, tung-sols 1950's tall bottles and some Sylvania, Raytheon and Tung-SOl black glass I trasure them all as they all bring something different to he table.

Mind you I have a lot more $ on tubes that I have on amp. you can imagine.
Nowadays my preamp has developed a hum that I haven't been able to get rid of so . . not listening to this combo much lately.

But I sincerely would not put KT-120á in it too many good tubes to buy into the hype of the KT120s, especially if you want to hear whatthe best this amp can do is most definitely into triode mode.

Hope this helps;

Gonzalo

 

RE: Thanks, Joe., posted on April 2, 2011 at 09:46:27
casouza
Audiophile

Posts: 755
Joined: March 18, 2008
 Gonzalo, thanks.
I agree 100% with your choice of tubes,
5751 is way better than most 12ax7s: quieter, less microphonic, less grainy, cleaner sound.
Your 6SN7s are some of the best NOS tubes.

I believe you meant to write GEC or Gold Lion re-issue KT88s. These are amazing tubes, probably better that the original 1960s GEC KT88, if you value a very even tonal balance. The original KT-88 is slightly muffled in the treble range, though its sound is very seductive.

I do not believe that the KT120 is hyped, it comes from the same factory that makes the excellent GEC KT-88. It is just too risky to pop them in any tube amp without knowing the safety margins of the transformers. If the output transformers are specified to handle 100 watts and the power transformer can handle the extra heater current with almost no rise in temperature, then it is a GO.

Changing from KT-88s to KT-120s may increase power output 20-30%, which in terms of sound pressure levels adds roughly a decibel to the maximum room sound pressure level (peanuts IMO).
To increase room SPL levels noticeably, say by 3 db, one needs to double the power output.

I plan to try the KT120s in a power amp with beefy transformers, for their sonics, not for the relatively small in-room SPL gain.
Thanks for a great post.
Enjoy the music!

 

RE: KT 120 Tubes in Yaqin MC-100B Amp, posted on January 5, 2017 at 07:58:27
ChopperCharles
Audiophile

Posts: 1
Location: NC
Joined: January 5, 2017
 I know this is an ancient post, but I wanted to reply to get the information out there. I've been running KT120s in my MC-100B for over a year now with no problems. Transformers are warm but not hot to the touch. Tubes are hot as blazes, but no problems with them. I'm biased at 1.0v, anything less than that and the bass starts to break up at higher volumes. The sound of the KT-120s is... well, like someone flipped the loudness switch on. Much more bass available than the JJ Tesla KT88s I was running. Overall I'm quite pleased, but the sound will not be for everyone. With bass-heavy music like Daft Punk, things start to sound muddy, and it's because I'm finding it's easy to drive my Vandersteen 1Bs into over-extension even at lower volume levels. But for rock and blues the 120s shine!

Charles.

 

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