2. amps-preamps
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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

Showing 2 responses by casouza

casouza
239 posts
 
Nikos, I suggest that you call the distributor and make sure that the Dared preamp has a regulated heater supply for the 12ax7 tubes.
A plain, unregulated DC supply (as found in 95% of tube preamps) will shoot up 20% if you remove two tubes. The line stage tubes will not last long.
It is safer to plug two inexpensive tubes in the phono stage, if you have no use for it.

Phil, try Phillips Holland or Amperex 12ax7 / ecc83 to get rid of the sibilance.
Also, Herbie's labs dampers may help reduce microphonics, which subjectively increases treble distortion.
Have fun!
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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