[sdiy] Tube amplifier question

[René Schmitz]

Hi Jürgen et al.,

> They aren't sensitive - IQ Mini Lady (I think this was the name) in the
> living room and four JBL Control 1's in the studio.

I'guess a EL84 PP will give you more than enough power to drive these.

> Yes, that's exactly what I was thinking about. What do you think about
> the EF86 triode-connected for the voltage gain stage?

I haven't used the EF86 as a triode myself, but generally it is said to
make an excellent triode.

>>Ouch... I wouldn't recommend a 12AX7 with that. 
> 
> 
> Thanks for warning me!

There is a second reason, and that is the dynamic input capacitance.
Cpg multiplies with mu, so that you might end up below the optimum load
capacitance for the phono cardridge.

> Have you calculated the approx. output impedance?

According to the maths given in the tube cad journal
(which was what I had consulted back then) the Zo is 1/2Gm,
which is 1/(2*11mA/V) = 45ohms for the 6N6P.
(From the datasheet: Vp =120V, mu = 20, rp = 1.9k, Rk=68,Ia = 30mA, -Ug=2V)

The optimum Ra and Rk would be 90ohms and 68ohms respectively, but since
there is not 120V but 115V per tube, and you have to subtract the Vg, I
did calculate with 10mA/V. So Zo would be 50ohms. The more complicated 
formula for Zo that is shown on these pages gives 59ohms.

> I guess with a hundred volts of unloaded output swing,
> mismatch won't hurt at all - we don't want to vaporize
> our headphones after all.

Yes, but its not a question of how much unloaded swing, but
of how much power is available.

> I even wonder about the *optimal* output impedance.
> Often when phones are connected to power amps, there's
> just a series resistor ar not-so-low-impedance 
> voltage divider, and while this certainly changes the
> frequency response, I don't know what I'd prefer, now
> that I'm accustomed to the rather high driving impedance.

Thats an interesting question!

Every new speaker and headphone I had and some of the amps had initially
a different feel to my ears, but after a while I think the ear actually
adapts.
(Audiophiles call this "tube burn in", but I think its actually the ears.)

> Is this so? I was surprised to see R4 having a rather
> low value in your circuit. I thought higher anode resistor
> would decrease output impedance (but decrease large signal
> drive capability). So I would have thought your cuircuit
> was optimised for drive capability instead of low small
> signal output impedance. (?)

Yes, obviously, I did the optimisation for the most symmetrical action.

> I also worry about switching the amp on shortly after switching it off.
> I tend to oversize PSU capacitors (avoiding chokes), which will worsen
> the problem. I could use a relay to protect the phones, but can I trust the
> relay? A transformer might cut the worst spikes by saturating (will it?).
> I'm quite anxious about my phones (and even more about my ears).
> And I never plug in the phones after power-on. This must be absolutely
> foolproof - I tend fall asleep when I hear music at 3:00 AM, and imagine
> how a brownout of 20 seconds might wake me, with the phones still on! (;->)

When the heaters are off long enough so that the output cap is
discharged, then they will go into conduction slow again. If the power 
cycle is fast, the output cap isn't really discharged yet. A large PSU 
cap actually helps there, by keeping the supply voltage up for some time.
I can only say, that I have absolutely no problems with transients,
power cycling it fast or slow doesn't produce any disturbing pops,
clicks or the like, even at full volume. And thats entirly due to
the inherent slow-start slow-stop feature of the heating.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159