Moooo's and 12a's

[ALL212]

I think it's in the Crack manual that you should only use 12AU7's.  What about the other Bottlehead products?  Can you use other 12a's?

Something to do with Mu's and current and I've got this table built but I'm not sure about accuracy.  The lower the mu the higher the current?

12AU7    20 mu

12AV7    40 mu
12AY7    42 + mu

12AZ7    a bit less than 60 mu
12AT7    a bit more than 60 mu

12AX7    100+ mu

 

[Paul Birkeland]

I love the title of your post!

The Mu of a tube is also referred to as the voltage amplification factor (this is more descriptive).  For instance, the 12AU7 in the Crack lists a Mu of 17.  So under ideal conditions, if you put 1V of signal into the input of the 12AU7, you'll get 17V out.  This is 24dB of gain.

If you adjusted the circuit to use a 12AX7 (it won't plug and play correctly), then you will get 100V out from 1V, which is 40dB.  This leads to the situation where the first 5-10 degrees of volume control rotation control the listenable range of the amplifier. 

You also mentioned current and its relationship to Mu.  There's a little bit of  missing piece in here, which is Transconductance.    Transconductance is like Mu, but in terms of current.  The three of these are related by the formula Mu=Gm*Rp.  So to get gain of ~50, you can have pretty low transconductance and high plate resistance (12AY7) or high transconductance and low plate resistance (6C45PI).

So, in summary, the selection of the driver tube is primarily motivated to keep the gain under control.  Consequently, the alternate tube types used are along the lines of 6SN7's, ECC99's, 12BH7's, and 5687's. 

If you picked up something like the Paramounts or Stereomour, selection of the driver tube will become a bit more dependent on the Rp variable as well. 

-PB

 

[ALL212]

Thanks PB!

Would it be safe to say that those with close mu would be compatible?  Could I run 12Az7's in my Stereomour?  How high in the mu pile - how low in the mu pile can I go within the tolerances of the Stereomour amp?

There's just so many alternate tubes that some of them appear to cross the classic 12A lines I listed and the magic smoke was really hard to pack into my Stereomour.

 

[Paul Birkeland]

The Stereomour adds the complication of needing to drive the input capacitance of the 2A3 (call it 100pF).  In the Stereomour, the plate resistance is effectively the output impedance of the driver stage, and this needs to drive the Miller capacitance of the 2A3 well enough to not sacrifice treble.  With the 12AT7 having roughly 15K of plate impedance, the corner frequency will be at or a bit past 100K.

The 12AZ7 has similar specifications, and would probably plug right in for the AT7.

If you tried the 12AX7 (by lowering the current), the plate impedance would pop up to 62K, and your corner frequency will drop to ~25K, which is potentially audible.