"ZeeTso" wrote in message
...
hi there!
first to state, i got no real expirience with vacuum tubes, but i
understand basic electric/electronic principles, and have some soldering
skills 
that said, i have a question regarding voltage and current for 12AU7
(ECC82) heater in cheap and simple headphone amp. tube is in input stage,
driving an op amp.
now, design says that amp needs 12V DC power supply. there is an LM317
current regulator in series with heaters, and it has 5ohm resistor giving
max 250mA load (calculated value).
with 12V DC supply voltage there is only ~10,5V between pins 4 and 5
(heaters in series).
what is confusing me is the fact that every 12AU7 datasheet says it needs
12,6V for heaters (in series).
is this value absolutely necessary, or it might vary?
how is that affecting tube performance and sound?
i connected the amp to regulated power supply and noticed that it sounds
better with higher than 12V supply voltage.
to get 12,6V for heaters i need ~15V DC power supply, so i guess that
would be best option.
what says you?
p.s. original schematic is in pdf file he
http://www.fredsamplifiers.com.au/do..._Valve_KIT.pdf
p.p.s. sorry for gramatics, english is not my native language...
Tubes are less noisy at lower heater voltage. So if a tube is to be used in
micropower mode, it might be good to reduce heater voltage.
But why not use a simple dropping resistor?
But in general the circuit looks ridiculous.
1) The tube works with grid current bias which adds to distortion when the
source impedance is high.
2) All power supply noise get fed into the audio path.
3) Distortion is determined by the op-amp which is not great at low-ohm load
(head phones). Looking at the op-amp circuit, it will be losing loop gain
quickly when the load resistance goes below 2K or so. Op-amp works in class
B for reasonably loud sound.
Generally it is not clear what is the purpose of the tube at all. Perhaps to
INTRODUCE some distortion and colour the sound in some pleasant fuzzy way?