McIntosh MHA200 tube rolling issue

[mlundy57]

I finally received my MHA200 a week ago. It worked and sounded just fine with all my headphones except my favorite ones which are low impedance (23 ohm), high sensitivity (110-120dB), and very detailed. With these some tube noise is audible with the volume at listening levels and nothing playing, so not a black background.

Based on recommendations in other threads on this amp I ordered some Mullard CV4024 KQDD/K (12AT7s) matched for high gain, low noise & microphonics, and balanced triodes and some super low noise (phono stage), NOS RCA 12BH7As. All the tubes have arrived and I started by replacing the stock 12AT7s with the Mullards.

After making the switch, turning the amp back on, and letting it warm up for a half hour I started to listen. There was almost no output except for way more noise than before on the left channel compared to the right. I turned the amp off, let it cool down, and switched positions for the Mullards. After powering back on and warming up, the issue remained, almost no output except for noise on the left channel compared to the right.

So, not an issue with the new tubes, maybe. I swap the stock tubes back in and the same issue persists. After a few minutes while turning the volume control, all of a sudden the left channel kicks in and is now working properly. I aging put the Mullards back in and the same problem return. Swap back to the stock 12AT7s and the issue persists. Turning the volume control doesn't have any affect. After a half hour of playing, still no amount of volume knob turning gets the left channem to kick back in.

Any thoughts, ideas of what might be going on and how to fix it?

Thanks,

Mike

 

[mlundy57]

I figured it out. Turns out it helps if you pull the amp far enough out of the rack to see what is written on the top plate or at least pay closer attention to the owners manual. For some reason, the amp works better when the tubes are put in the correct socket. When it wasn't working I had put a 12AT7 in a 12BH7 socket. Got the tubes sorted out with the Mullards in the correct 12AT7 sockets and the stock 12BH7s back weher they belong. Now to let the Mullards burn in for awhile then see what the noise level is like.

In the Army we referred to this as an operator headspace and timing issue. Granted that reference will likely only make sense to folks who have experience with the Browning M2 50cal machine gun, but maybe not.

Mike

 

[Monsterzero]

You're lucky you didnt fry the tubes. I put new tubes in the wrong sockets on a La Figaro 339. Tubes died a smelly horrible death.

High sensitivity headphones are hard to enjoy with tubes as you're finding out. Good luck with the Mullards.

 

[mlundy57]

Yep, got lucky. Once I figured out which tubes went where, swapped the stock tubes for the NOS Mullard 12AT7s and RCA 12BH7s, and let them run a few hours the MHA 200 was quiet even with my most sensitive headphones. The sound was also fuller, richer, and with no loss of detail. Right now the new tubes only have about 5 hours on them so I expect things to improve even more as the accumulate hours.

 

[u2u2]

Yep, got lucky. Once I figured out which tubes went where, swapped the stock tubes for the NOS Mullard 12AT7s and RCA 12BH7s, and let them run a few hours the MHA 200 was quiet even with my most sensitive headphones. The sound was also fuller, richer, and with no loss of detail. Right now the new tubes only have about 5 hours on them so I expect things to improve even more as the accumulate hours.

Those Millard tubes will serve you well. The change out of the original power tubes was a good move as some of us had problems with them. The stock 12AT7 are good sounding tubes. Per chance did you try the new power tubes with the old drivers?

 

[mlundy57]

No I didn't. My initial concern was eliminating the noise. After swapping the 12AT7s, the noise was reduced but not eliminated. The next step was to swap the 12BH7s to see if that eliminated the rest of the noise. Since it did I concluded that some of the audible noise was being generated by both types of tubes. Since I was unwilling to accept any amount of noise, I didn't see the point of putting the stock driver tubes back in. Therefore, the sonic improvements I heard were based on the combination of the Mullards, and RCAs but with only a few hours on them.