[sdiy] Tube amplifier question

[René Schmitz]

Hi Jürgen et al.,

> Small power amps with 2x EL84 push pull for the speakers (I never hear
> music very loud over speakers, so that should be plenty)

I'm using a tube amp for monitoring my synths, its a single ended one, 
based on a russian EL84-like tube. If your speakers are sensitive, then 
3W can be plenty....

> Separate Headphone amp with EF86 (connected as triode) and 12BH7 as
> cathode follower with no overall negative feedback loop.

Sounds like this might be something for you:

http://www.aleph.co.jp/~takeda/radio/tube/12AU7-muFollow/6N6p-WhitFlw-exp-schema.gif

Also very similar to the audio stage of your compressor.
(The 6N6P is more or less equivalent to the 12BH7...)

> Phono preamp (yes, for my old records) with 12AX7 and passive equalisation

Ouch... I wouldn't recommend a 12AX7 with that. The output impedance is 
too high IMO, better to use a medium mu triode or better yet a pentode, 
this gives you more drive for the network, and also more gain. No need 
to worry about "pentode sound" with the small signals from a vinyl 
pickup. The buffer at the output can be made with a medium mu stage. The 
12AX7 is IMO too whimpy to drive your cables or line stages.

(I have a personal preference for medium mu tubes and circuits that 
allow a larger current to flow through the tubes. The 12AX7 seems to be 
designed to be a compromise, the target was to save a stage in a radio 
or amp or so, without loosing too much sound. The preference in guitar 
amps is just that, more gain with less stages. Also there is a tendency 
that new tube schemos get copied from old ones, there is very little 
innovation.)

Anyway, a good starting point for own experiments would be this:
http://www.sonic.net/~ktstrain/RIAA_eq.htm


> (2) Is an ultralinear transformer the way to go? I have seen some "famous"
>     amps like the Quad II use cathode feedback instead of the ultralinear
>     configuration (other tubes, too, of course ...). What's the general
>     pro and con of ultralinear vs. cathode feedback, or should I even use
>     both ? (I see that the winding is easier in ultralinear, and these
>     transformers are very common, so I'd need a good reason to do it
>     otherwise.)

The QuadII is more on the exotic side. The question of wether UL or 
straight PP is a matter of taste. As well as Triode vs. Pentode vs. Beam 
Power tubes.... Much hogwash can be heared on these topics. I'd say try 
and hear for your self. A UL transformer could give you the room for 
experimentation.

> (3) Is the headphones amp with EF86 and 12BH7 a good idea? Should I use
>     an output transformer, or bring the output impedance down with a WCF
>     and don't use a transformer? (My Sennheiser HD450 phones are 300 Ohm
>     types, and I can get down to ca. 100 ... 150 Ohm with the 12BH7 in
>     WCF configuration. Not much damping then, and I worry about the transients
>     thru th ephones during power-on.

I'm driving even 32ohms headphones with mine, although its actually a 
mismatch. (Note that the values are optimized to give the lowest output 
impedance.) A WCF is a push pull circuit and as such works into an 
optimum load, where the action is most symmetrical, and the second order 
distortion dissapears. (I had the maths for the WCF somewhere, but it 
eludes me right now, but I can dig if necessary.) That means that there 
will be a compromise between matching and damping.

The turn on transients are not very critical, if you provide a discharge 
path for the cap. (In case you plug in the phone after turn on.) The 
soft starting feature of the tubes prevents "plopps" on turn on. 
(Provided you don't use a relay-delay circuit to switch the plate supply.)

Cheers,
  René

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