2. amps-preamps
  3. 1253132946

Dared MC-7P optimal tubes

Have one of these now and it is amazing. Stock 12ax7 tubes were way too soft so I popped a matched pair of Tung-Sol and a pair of Philips in place. Sound became WIDE and more accurate. Swapping the 5u4 with a NOS RCA improved inner detailing and removed muddiness in the bass and midbass. It's getting amazingly accurate for its modest price, esp. with cellos where it almost totally nails the timbre.

It sounds like it oscillates just a smidge (you really got to listen hard for this) and a tidge of sibilance remains.

So before I spring for a real set of good tubes, what combinations have found success in this low-cost wonder?

Matching it to a restored/upgraded McIntosh MC-2100 (toroid transformer, regulator in front end, caps, wiring, etc.). Input Z on the 2100 is now 100k fixed instead of 50k variable.
inline_phil
vetnik
23 posts
 

thank you very much Casouza, very useful info...

Nikos
inline_phil OP
11 posts
 
Sound is everything. Settled now on Mullard 12AT7's in place of the 12AX7's (left hand pair) because of the high gain in this preamp (too high).

I've documentd the tube swap and sonic differences (email me for a PDF at philip@okstatealumni.org for your copy).

After two weeks of break in, the sound is wonderful. More mods to come...in the spring when it warms up in my workshop.
inline_phil OP
11 posts
 
Finally put the preamp on the bench to see what the heck this sibilance is all about. A 1KHz square wave has a bit of ringing with current configuration. Ended up loading the interconnect cables at the amp end with a 1K ohm, 1/8 watt resistor to tame down the beast.

Wish I could figure out which resistor is the feedback. Gain sure needs to be reduced. Anyone know which resistor is the feedback on this unit?
inline_phil OP
11 posts
 
BTW, the remote is acting up. The antenna is a wirewound bundle that is sensitive to orientation (left/right, up/down, front/back) and when I move it, results swing from unresposive to strange orienting remote positions. Any suggestions?
casouza
239 posts
 
Phil, usually the feedback resistor is the one connected between preamp output and the first line stage tube cathode.
Do not go overboard with feedback, it will kill soundstaging and transient response.

A good tweak to reduce gain that also improves sound quality is to remove any cathode bypass caps in the line stage.

To get rid of ringing (actually RF oscillation), install 300 ohms to 1 K ohms (not critical value) carbon composition resistors in series with each tube grid. Allen Bradley brand is one of the best for this application, available from Parts Connexion for the whopping price of 0.30 each!
These are called grid stopper resistors and MUST be installed very close to the tube grid socket pin. Cut the resistor lead to be soldered to the tube socket no longer than 3/16 " (or 4.5 mm for the rest of the world).
This tweak will damp only HF oscillation and will NOT harm audio performance, unless you overdo the resistor value (1K is the safe upper limit, 300 or 470 ohms should do it).
Also, scope the power supply output to make sure it is not ringing and has no rectifier spikes.
Good luck
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