2. amps-preamps
  3. 183518-179778

Tubes for a pre stage... matched NEW or unmatched identical NOS?

I have this question for so long:

On a integrated amplifier that uses a pair of 12au7 tubes (or alternatives) on the preamp section (Rogue Sphinx, for example) what is best option:
a) to have an unmached unbalanced pair of identical NOS tubes
b) to have of matched balanced pair of modern tubes
c) to have balanced 12au7 tubes no matter if they are matching

Once asked JJ why they didn’t offer matched 12au7 type of tubes and the answer was the it is irrelevant for the kind of use they are intended in audio circuitry. Is this right?

My question only makes sense when forced to keep tube-rolling costs down (or tube substitution when the stock ones fail) because the obvious and better choice is matched NOS, like those from Amperex or Mullard.

audiofilo123

Showing 5 responses by ieales

ieales's avatar
ieales
1,201 posts
 

I'm curious if the clear tops were a cost saving measure? Definitely simpler construction.

Perhaps a thinner glass.

Anyone run them for 24 hours on heater only before listening?

ieales's avatar
ieales
1,201 posts
 

The Sphinx likely uses tubes for the phono section, so low noise is paramount.

For maximum RIAA fidelity and channel balance, the g1/g1 and g2/g2 sections need to be well matched, assuming the same topology for each channel.

The big fly in the matching ointment is that tubes seldom match across the entire operating range and matched tubes are matched in a matcher, NOT your amplifier and thus operate under totally different conditions. Tubes matched on one matcher may have very different numbers on another matcher.

Tubes matched today aren't matched tomorrow as they age at different rates.

See ieLogical Rolling

ieales's avatar
ieales
1,201 posts
 

12AU7-based mu-follower preamplifier stage driving a 100Wpc (into 8 ohm), class-D, solid-state power amplifier

A mu-follower has plenty of gain which has to be padded down to drive the Hypex UcD-180 Class D. Weird.

ieales's avatar
ieales
1,201 posts
 

What are your favorite 12au7 type NEW production?

What amp? What purpose? What circuit? What music? What room? What system?

ALL sound different in different places. The validity of most opinions is ZERO.

And after how many hours as they change from first turn on...

ieales's avatar
ieales
1,201 posts
 

12au7’s in usa or european old stock form are plentiful and relatively inexpensive, even for well matched sets 

The problem is what is 'well matched'? At the datasheet params? NONE of my tubes operate anywhere near them.

I've bought plenty of NOS 'matched' 12AU7 & commercial variants. NOT one came close to the claimed Gm when operated at the amplifier parameters. NOR were the Gm matched at datasheet parameters. 

NOS tubes often have significant gas after ½c. on the shelf and need a while for the getter to clear it. Some advocate baking, but others say never. Induction heating may be the answer.

Expect any NOS tube to change significantly over the first number of hours.

Methinks Confirmation Bias accounts for a fair portion of NOS enthusiasm.

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