Alembic F2B Tube not light up

[bancika]

I don't think that 6.3V vs 12V would make any noticeable difference in sound

[wui223]

I remover the wire from pin 9 to ground. IT WORKS. But the gain is way too high, i only turn a 10 degree of the volume knob, the unit start to give me overdrive sound ald. I built according to the original schematic. Any remedy ?

[puretube]

log pot.

or parallelling a smaller R to it.

[wui223]

I cant find a log 1M pot for the volume. I just wonder the original Alembic preamp has the same problem. Anyone who has experience with this monster ?

[R.G.]

Okay so that is 6.3v per heater half verses 6v making for a difference of 0.3v. I don't know what kind of difference 0.3v in the heater would make to the conduction of the cathode but I wouldn't count on much. For me I see it as one of those "mojo" things. If you like it that way then by all means go for it.

6.0V versus 6.3V on the filaments will make almost no difference in the sound. It *will* make a difference in the life time of the tube if something else does not kill it first. Lowering filament voltage even a little lengthens tube filament life. See below for the inverse of this.

This info is, by the way, in the Tube Amp FAQ at GEO.

I don't think that 6.3V vs 12V would make any noticeable difference in sound

It will make a big difference in tube life. Light bulb filaments (heater wires in tubes are this) have a life that is inversely proportional to the 13th power of the applied voltage. Go over the rated voltage even a little and life gets cut dramatically. Double the applied voltage and life is cut to minutes or seconds. When the heater burns out, the sound vanishes, which I think is a big differnce in sound.

I cant find a log 1M pot for the volume. I just wonder the original Alembic preamp has the same problem. Anyone who has experience with this monster ?

Read "The Secret Life of Pots" at GEO.

[wui223]

OK. So the conclusion is use 6.3V for heater. What make me confuse is, the original schematic suggests a 1 M pot for volume. I wonder why the gain problem occur. Could it be due to the tube ?

[puretube]

It`s not a gain "problem" - it`s a tubecircuit... (benefit).

assume 100mV @ input, amplified 25 times in the first triode = 2V5;
amplified 50 times in the second triode = 125V.

(any 9V-stompbox would`ve clipped that to 9V or less - not so the tube-circuit).


Since you want something to fit into your low-volt signal chain,
R8 (1M) & R9 (100k) form a voltage-divider, to reduce the max-volume to ~1/10th;

the vol-pot dials something from 0 to over 10V from across R9,
while something around 1V to 2V would maybe normally be expected from a normal stompbox.

That`s why you should use a logarithmic pot: to have only a quite small portion put out
on the first half part of the rotation.


IMHO, you can forget about a 1M or 500k log pot across R9,
but instead make R9 the volume pot itself: 100k log.
(maybe easyer to find for you).

I say: "log" (or: "audio"), as opposed to: "A" or "B" taper,
coz` the latter 2 are defined contrary in different parts of the world...

If it`s still too loud, you can again parallel a resistor...

[wui223]

Thanks for all. I will try to replace R9 with 100k log.