Alembic f2-b , how much current?

[bent]

Hello,

Approximatly how much milliamp one channel of the alembic f2-b is using, WITHOUT THE HEATER ! Cause i know that the heater on 12v series need 150mA, but i cant find the answer for the rest of the circuit....

http://www.moosapotamus.net/IDEAS/F2B/alembic.htm
I try the google and search function, but no luck so far !

Thank's

Bent

[gtudoran]

As far as i see... max 10mA for anode voltage... but i recommend  a little more bc you have to filter the anode voltage and if your tranny doesn't support capacitor charge current then "bye bye tranny".

So more like 30-40mA for a secure 47uF / 400v

Best regards,
Gabriel Tudoran
Analog Sound

[PRR]

I get two answers.

It is the standard Fender 12AX7 stage. 1K5 cathode 100K plate. This stage will stand with plate at 69% of supply voltage. If supply is 300V then plate is 207V. 300V supply and 207V plate is 93 volts across the 100K plate resistor. 93V/100K= 1mA near-enough.

That's per stage. Two stages (one 12AX7) would be 2mA.

FWIW: the absolute maximum per stage would be tube dead-short, only the 100K resistor to slow the current. At 300V that is 300V/100K= 3mA per stage. That's worst-case, very worst-case. If tube were dead-short the signal would be dead-zero. In fact we always bias so the tube and the 100K "split" the available voltage. If you assume a half-split of 300V, then 150V/100K is 1.5mA, and you are unlikely to draw more. In fact many triode stages give more voltage to the tube, less to the resistor, and I happen to know the 1K5+100K condition leaves ~~30% across the 100K.


OTOH the power supply shows voltages, and if you believe them the answer seems clear:



The whole thing draws 1mA.

I believe this is incorrect. Maybe wall voltage changed 1% while he moved the meter from C5 to C7.There's also an obvious rounding-error: R2 and R3 are equal, there's no other place the current can go except through both of them, yet R2 shows 1V drop and R3 shows 2V drop.

Perhaps these numbers were taken with ONLY R4 470K bleeder connected. 314V/470K is 0.66mA, which still is not exact. The plan values are for general guidance and fault-finding, not exact specs.



I'm betting on 1mA per stage, 2mA per 2-stage channel, plus 1mA for the bleeder: under 3mA.

Round-up for AC-DC conversion factors. Say 5mA.

You can't buy a power transformer so small, 230V 5mA. That's 1.15VA which is VERY small and VERY fine wire. They can make a 5VA or 10VA for the same price or cheaper than a 1VA.

And low-low-VA parts SAG badly. That heater supply can hardly stand any sag-- as designed it can make a full 12V but wall-variations can dip the DC at ripple frequency. So you best use an ample transformer here. Its un-sag will keep the output healthy even in a dim dive.

Note the schematic calls for 35VA transfomers in both spots.

T1 has has nearly 4VA load (the 12V 0.15A heater plus regulator loss times current facor) plus the large-Watt of the 300V supply. But note that the 4VA heater is on one of two windings: to be ultra-safe T1 must be a 8VA part. T2 can be smaller. But again, PTs much under ~~5VA are not common and not cheap.