Tube issues, 12at7 affect KT88?

Well you do have to draw the line somewhere. When I do a mod on someones preamp or amp I take a long hard look at what was the intent was of the designer of the circuit. In the case of Dynaco, there were always cost considerations. This was the stereo for people who couldn't afford McIntosh or Marantz, or wanted to build it themselves. Heath kits were also popular. Obviously with the ST-70 or MKIII I would never replace the transformer unless it was damaged. BTW on the MKIII's I had heard that the older gray painted transformers were better then the newer blacks ones but I was unable to prove if that was true or not. There are other sites with other circuit board replacements. I would check them all out. I have absolutely no problem with an all triode tube input and driver section. If it's something like a 12AU7 coupled to a 12BH7 that works just fine.

I agree with you about that line, which is another reason why I haven't bothered with the ST70 yet. No rush, I love the sound of my Aragon 2004, and only want to get a tube amp running well enough to see what all the fuss is about.

Hey Tony, I have the same amp and have had the very same problem you have. The 4 12AT7's provide the bias to each KT88 output tube thereby if one 12AT7 goes bad, then the corresponding KT88 will glow cherry red. The Mac's are built rugged and there is usually no need to investigate further but to replace the bad 12AT7 and get rid of it or at least mark it so it won't be mistakenly reused. I would also replace the KT88 as the Genalex KT88 that went cherry red several months ago on me finally gave up the ghost in a bright display of arcing a few days ago upon power up which blew the mains fuse. Replaced the fuse and the KT88 and all is well.

Cheers

Thanks. I had the tubes tested, and the one that was in the number 2 spot tested poorly, which I think corresponds with the number 2 KT88. I did plan on replacing it because I figured it severly shortened it's life. I'm going to have them all tested just for curiosity to see the differences in numbers.

Tony

Hi Tonydec

What was the outcome of your checks? Were the KT88s damaged by the faulty 12AT7 tube?

Thanks