McIntosh MC-30's, MX110 tube discussion. Perfect 6U8, 12AX7, tube? See post #43 for Steve's input.

Here's some thoughts on the 6U8 - looks like Mr. DeWick chimed in: http://www.audiokarma.org/forums/archive/index.php/t-177944.html

6GH8As are still inexpensive - I got a sleeve of NOS RCAs for $25 - they can be used in place of the constantly escalating in price 7199s with a wiring change to 3 spots on the socket. I picked up some new boards for my Dyna MK IV amps, and they're set up for 6GH8As.

 

Dr. D,

I still love ya.

Best

Bob

 

Anyone try a 6GH8 in lieu of the 6U8? I can attest to the 6GH8's kissin' cousin 7199 (with different pinout and much higher price) as being a short lived tube. Dyna amps need a steady diet of those pesky and expensive 7199s, which was part of the motivation for me to try some different driver boards in my MK-IV amps. Simple unsolder/resolder job that can be easily undone to go back to stock, and 6GH8s are cheap and easy to track down. Perhaps it would last longer in the MX-110 and save folks some $$$ as well.

 

Steve (aka Dr. D),

Sent an email thru back channels into the depths of McIntosh stating the issue & your theory with the 6U8. I'm confident I'll hear something back from them and will post a response.

As emailed under seperate correspondance, you are welcomed on the 6U8 tubes I sent. No further comment necessary at this time my friend.

Best

Bob

 

Heard back from Roger (Russell) & he reports:

"Bob:

I have never owned an MX110 so am not aware of this problem. However, I recall other equipment that used 6U8 and that I had several spares for some reason. Now I wonder whether this is a tube problem or the way it is used. Sorry I can't be of more help.

Roger"

You are getting warm Steve........

Don't expect any further replys until Monday.

Best,

Bob

 

Terry Dewick might have some ideas about the use of the 6U8 in the MX110 as well.

 

"I have a report from a reliable source that 6U8s and 6GH8s can exhibit a strange interaction with output tube grids."

"The 6U8/6EA8/6GH8 family is well known for issues between stages. The 6GH8's are heavily used in color TV circuitry, and are known for failures."

http://audiokarma.org/forums/showthread.php?p=4213830


"have heard from two veterans of McIntosh that 6U8 tube was notorious for failing."

http://www.audiokarma.org/forums/showthread.php?t=87157


"The reliability of 6GH8 tubes is somewhat variable. Some have a tendency to degrade rather quickly. It was nearly *impossible* to keep old RCA Color Televisions (where they were VERY common) supplied with good 6GH8s."

http://www.audioasylum.com/cgi/t.mpl?f=dynaco&m=12352


Bashing the 6GH8 here

http://www.videokarma.org/showthread.php?t=250003


Other little tidbits

http://www.audiokarma.org/forums/showthread.php?t=427477


Interesting 6U8 discussion

http://groups.yahoo.com/group/regenrx/messages/5487?threaded=1&m=e&var=1&tidx=1

 

Cool Beans Steve -- are you going after RCA 6U8's only or 6U8A's as well?? Just curious for filing in my tiny brain.

Thanks my friend,

Bob

 

Cool ...do you have an infra-red temp gauge? That would be the best tool to use on the transformer.

I have one -- but, you're a little more than a 15 minute drive away. (could send it to you, if you wish).

Best

Bob

 

Okay, USPS priority tomorrow to the address on file.

Do it the Steve way -- the right way!!!!!!!

Best

Bob

 

Makes me want to use a variac all the time, or a cooling fan, maybe.

 

Thanks Steve, my sleeve of RCA 6U8s arrived on Monday.

 

I like those towels.

 

Received the following reply from Ron Evans, VP, Engineering at McIntosh & the designer of the MC2301 300 watt tube mono amps and co-engineer of the famous C2300 preamp and other fine amps/preamps in the McIntosh stables.

He is one smart cookie -- I've spoken to him on the phone several times and after 10 minutes, I can't begin to comprehend, let alone jot down on a notepad his minute details in explaining a design decision, etc. At least with emails, I can re-read his responses to my questions multiple times.

Bob,

Wherever possible, McIntosh designs derate operating conditions of each component enough to assure lifetime that exceeds the normal for that type of component. Tubes are no exception. First, the operating conditions are typically only a small fraction of the tube’s maximum rating. Second, the design is conservative so that performance specifications will be met with tubes that test significantly lower than “good” in a tester.

MX110:

The MX110 was out of production long before I came to McIntosh. However, I have had several recent occasions to study the design. The application of the 6U8 in V16 and V17definitely follows the guidelines above. The cathode current is low (the primary parameter that ages tubes). The unit should meet specifications with a weak 6U8.

The conditions are a little different in the tuner, V11. The pentode section is operated at low cathode current, so it should have a long life. It may affect FM MPX separation as it ages. The triode section is pushed a little harder, about 50% of the cathode current design rating. However, the tube gain is not relevant to tuner performance. The conduction must only be high enough to keep the FM MPX light from lighting when no FM Stereo signal is being received.

I almost never use a tube tester for tube evaluation. It only indicates the relative gain at one operating condition that was determined by the tube tester engineer. Any pass-fail criteria have also been determined by the tube tester engineer. It is not necessarily indicative of whether the tube will perform satisfactory in any given circuit application. I have never found any tube manufacturer published data that shows the minimum acceptable parameters for any given tube number.

I hope that this information is helpful, though it is probably not what you expected to see.

Regards,


Ron Evans
V P Engineering
McIntosh Laboratory, Inc.
Ph 607-352-2413
www.mcintoshlabs.com
www.facebook.com/mcintoshlabs

Kudos to Ron for taking up his valuable time to read my email and provide an honest opinion.

That's what has kept McIntosh a household name amoungst audiophiles since the late '40's.

Let the 6U8 discussions begin.

Best

Bob

 

I searched eBay and found the guy, thanks Steve! I need to stock up on these for my Stereo 70!

 

Interesting, other than heat and startup surges I suppose.

Wonder if this "cathode current" over time becomes higher or unstable, weakening the tube quicker?

 

Interesting statement.