MC275/KT88

mcintoshaddict

Active Member
I just upgraded the tubes in my MC275 MKVI from the standard McIntosh tubes to Shuguang Treasure KT88-Z black bottle tubes. I notice that the Shuguang tubes run a lot hotter than the McIntosh tubes. Have other people noticed the same?
 
Perhaps you should obtain the McIntosh Service Manual (PM me -- I'll send you the PDF) for the Mk VI, get a digital voltmeter, open up the bottom, and adjust the bias voltage to between -60 and -62 VDC (which is what the service manual recommends). It's possible that the new tubes are running hot because the amp hasn't been biased correctly for them. I had to adjust the bias when I replaced the McIntosh KT-88's with Electro-Harmonix KT-88's a few weeks ago (the Electro-Harmonix tubes sound MUCH better than the McIntosh tubes did)....
 
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I just checked the temperature of every KT88, and it's only one of the tubes is running hot (V9L). I swapped the tube with V8L and it turns out that it stays with the socket. Is this maybe a biasing issue with the tube/socket?
 
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Or perhaps a blown wirewound plate resistor? Do you notice any distortion on the "hot tube" channel? If you open up the bottom of the amp, you can measure the resistance of the four 220 ohm 5 watt plate resistors while they're still in the circuit (they're the big white resistors located right near the KT-88 tube sockets). They should read about 215-245 ohms on each of the four. If the your digital ohm-meter reads 0, then the plate resistor has blown.... The wirewound plate resistors act like a fuse... They will blow in order to protect the output transformers from being destroyed. They're also pretty easy to replace if you find a bad one.
 
Hello
Everybody think that all KT88 are the same . It is wrong . KT88 from Electro-harmonix do not sound much better than other one . You just have different screen current . Tubes with high screen current value may damage output transformers .It iseasy to check . You output transformers should always be cold .Never lukewarm .
To feinstei :
f the your digital ohm-meter reads 0, then the plate resistor has blown.... Sorry but no ! if the resistor has blown the voltmeter show OL (open loop) not zero . Zero is the lowest possible resistance value . When these resistors are bad you get no value , open in fact like a fuse will show !
 
Hello

To feinstei :
f the your digital ohm-meter reads 0, then the plate resistor has blown.... Sorry but no ! if the resistor has blown the voltmeter show OL (open loop) not zero . Zero is the lowest possible resistance value . When these resistors are bad you get no value , open in fact like a fuse will show !
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I've just successfully serviced my own MC-275 Mark VI when a bad McIntosh tube blew my plate resistors. You can read my progress through the problem in another thread on this forum.

When I hooked my digital voltmeter to the non-blown plate resistors the ohm-meter it read about 216 ohms. When I hooked it to the bad ones, it read 0. Let's say you had an analogue ohm-meter with a swinging needle. The swinging needle will be at 0 if there's no connection through a circuit. The swinging needle will also read 0 if connected to a blown plate resistor or to anything else that has no connection.
 
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Some people have reported issues with those tubes in a 275 (myself included). Mine blew the fuse immediately on power up so back they went. The Gold Lions are a safer and better choice imho.

jblnut
 
Upgraded? I love that term when used to replace something that came factory selected and installed. Is it really a Upgrade? or just different...
 
As I stated in my posting on my MC-275 Mk VI Left Channel Distortion, earlier revs of the Russian produced Electro-Harmonix KT88's that were OEM'd by McIntosh for the MC-275 (I don't know which rev) were reported to have been very unreliable.and caused all sorts of problems causing people to send the amps back for warranty service (research it on the web for further information).

However, the Electro-Harmonix KT88's that are currently being produced (and the ones I bought at Amazon.com) are reported by peoplto be extremely reliable and stable tubes with excellent service life and are reported to make amps that they're installed sound fabulous. So far, after biasing (my MC-275 Mk VI had a bias adjustment reading of -53 VDC whereas the Service Manual recommends -60 to -62 VDC -- I adjusted the little potentiometer to read about -61 VDC), they have performed wonderfully in my now-fixed MC-275 Mk VI both service-wise and sound-wise.

It's also reported around the web that "Gold Lion" brand Russian tubes are produced at the same plant and when "dissected" (in other words taken apart) have exactly the same parts in them (grids, plates, connectors, glass etc) as the standard Electro-Harmonix tubes. However, they are packaged in a more aesthetically pleasing way than the standard cardboard-boxed Electro-Harmonix tubes. Thus, I decided to not spend the extra $105 or so for the "Gold Lion" branded tubes. I don't know, nor do I claim to know whether there's some "magic" in using the "Gold Lion" branded tubes that'll allow them to out-perform the standard Electro-Harmonix tubes.

Link to quad matched Electro-Harmonix tubes at Amazon:

https://www.amazon.com/Electro-Harm...TF8&qid=1519050144&sr=8-1&keywords=kt88+tubes

Link to quad matched Gold Lion tubes at Amazon:

https://www.amazon.com/Genalex-Gold...TF8&qid=1519050144&sr=8-2&keywords=kt88+tubes
 
I buy all my tubes now from someone who owns a tube tester (Jim McShane). This takes the risk of getting a bad tube out of the equation and you can also get a more closely matched set. Costs a few dollars more but well worth it.

jblnut
 
feinstei,

If you were responding to my post, my query was related to electro-harmonix KT90's and whether anyone had tried. I had read your post on the KT88's. I have a quad set of KT90's that I've been lent by a dealer who had used them in an MC275 with good results. So, I've been given an opportunity to do the same. I'm going to try them with: 1) a set of Mullard input tubes, and, 2) a set of McIntosh original input tubes.
 
Please report your results with the KT90's. I was not responding to your query since I didn't even know that the MC-275 could accommodate KT-90 tubes (what is different from KT-88's?). My posting was simply to share my findings from research on the Internet and my personal experiences with the KT88 tubes that have gone bad on me and what I replaced them with. Again, I look forward to your reports on the KT-90's versus the KT88's...
 
I buy all my tubes now from someone who owns a tube tester (Jim McShane). This takes the risk of getting a bad tube out of the equation and you can also get a more closely matched set. Costs a few dollars more but well worth it.

jblnut
Can you give us contact info?
 
Hello
Everybody think that all KT88 are the same . It is wrong . KT88 from Electro-harmonix do not sound much better than other one . You just have different screen current . Tubes with high screen current value may damage output transformers .It iseasy to check . You output transformers should always be cold .Never lukewarm .
To feinstei :
f the your digital ohm-meter reads 0, then the plate resistor has blown.... Sorry but no ! if the resistor has blown the voltmeter show OL (open loop) not zero . Zero is the lowest possible resistance value . When these resistors are bad you get no value , open in fact like a fuse will show !

I had McIntosh tubes in there originally, the transformers we warm. The output transformers are about 90 degrees each and the power transformer is about 105 degrees. With the new tubes (Shuguang KT88-Z), they run close to the same temps. I don't understand how the transformers are supposed to be cold. Were you talking about the unit being on or off?
 
I had McIntosh tubes in there originally, the transformers we warm. The output transformers are about 90 degrees each and the power transformer is about 105 degrees. With the new tubes (Shuguang KT88-Z), they run close to the same temps. I don't understand how the transformers are supposed to be cold. Were you talking about the unit being on or off?
I didn't understand that message either. Made no sense whatsoever in light of my experiences fixing the MC-275 Mk VI that had the blown 220 ohm screen resistors..
 
The output transformers are about 90 degrees each and the power transformer is about 105 degrees.

You are explaining that output transformers temp is closer to power transformer temp ? Fahrenheit I suppose
don't forget that your output tubes warm the output transformers . If youwant to check the temp measure them from the opposite side of the output tubes .
Cold output transformers is a good sign .

I didn't understand that message either. Made no sense whatsoever in light of my experiences fixing the MC-275 Mk VI that had the blown 220 ohm screen resistors..
Sorry if you don't understand but once you repair the same quantity of MC275 that I did , you will probably understand what I mean

42 years working on MCINTOSH
 
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