You say you would like to implement something Real Simple ...
Well, the description might not be, the final set up is....
With reference to the "Power Meter" schematic as linked in your post :
1. Replace the 220K pot with a CDS-cell or LDR (Light Dependent Resistor.
Light : Licht, nicht leicht, dennoch nicht schwer :-)
2. Remove the 0.1µF/400V cap. and relocate it to the new-projects box.
3. Replace the 390-470K resistor with the a.m. 220K pot.(P1)¹.
4. Don't connect P1 to tube anode but to OPT-secondary, hot side², instead.
4. Connect the P1/LDR node to one terminal of an additional 0.1µF (C1) cap.
5. Connect the other C1 terminal to the 1N4148/1N4148 node.
6. Solder two Leds³ in antiparallel and ground one terminal.
7. Connect other Led wire via a series resistor(Rs)¹ to OPT-sec., hot side².
8. Leave rest of circuit as is.
¹. see text below.
². assuming the OPT secondary, neutral terminal, is grounded.
³. chose 3mm red leds for easy mounting and lowest Vfwd drop.
For convenience I nominated parts, left to right, LDR, C1, D1, D2, C2, R1.
Both Leds and LDR must be mounted face-to-face in a light-tight enclosure.
I assume while looking to the "modified-schematic" you already got the grasp
of it's principle. While an amp's power level increases, the Leds light up
and at the P1/LDR/C1 node the voltage decreases in relation to the P1/OPT
node because of decreased resistance of the lit LDR. Absolute voltage
increases however, but not in a linear relation to the input.
Typical LDR values are 10Mohm in complete darkness to 100ohm in bright
daylight. The "transfer-ratio" of the Leds/LDR combination is such that
only very little Led current is required to drive the LDR's resistance down
to a value appropriate for the application. Rs chosen depends on the desired
"compression-ratio", Led efficiency and of course the LDR sensitivity. In my
experiment Rs = 10K ...47K, corresponding to only 2.1...0.45mA Led current
at full blast, is appropriate.
In my case the application was an EM84 (Vg = 0...-22V) as "power indicator"
in a 70W amp with 4xEL34 in ppp used as a guitar amp. I say "was", as the
amp. parted house a long time ago but probably is still on duty somewhere on
the globe. Yet found the schematic and the homebrew Led/LDR "optocoupler"
prototype in the junkbox. Some modifications were made in relation to the
values as mentioned above. F.e. C2 is changed to 1µF, for sake of test &
measurement at 50Hz, by means of a 24V PST and Variac, to get rid of ripple.
For P1 a decade box is used instead.
The C1/D1 clamp and D2/C2 combination, a doubler in essence, will output a
Vdc at the D2/C2/R1 node of around 2.8 x Vrms. In power amps of ~30W into
8ohm the Vout of ~15Vrms, or ~44Vpp, is sufficient to drive an EM80 (Ug
= -1...-14V) from the OPT without the need to connect to a source with
higher ac voltage like the anode of the output tube. If the doubler's Vout
is not sufficient, I think a tripler or a quadrupler could be used instead.
As an alternative, and if present, the 16Ohm OPT tap comes in handy
with ~1.4 times the 8Ohm output.
A refinement can be made by connecting the cathode of D1 to a -1.6V
(typical) source, obtained via the output tube bias circuit or rectification
of the filament supply etc. In this way Vg will be at -1V at zero signal,
thus providing deflection at the EM80's indicator (what's the name of the
green "curtain" anyway?) at the smallest signal, or without threshold.
Results of the EM84 "compressor" derivative:
Pout¹ Vrms¹ Vpp Vg¹ Vg² Vg³
0.00 0.00 0.00 -1.0 -1.0 -1.0
0.25 1.41 4.00 -3.6 -3.1 -2.6
0.50 2.00 5.66 -4.6 -4.2 -3.4
1.00 2.83 8.00 -5.9 -5.4 -4.5
2.00 4.00 11.3 -7.3 -6.9 -5.9
4.00 5.66 16.0 -8.8 -8.5 -7.6
8.00 8.00 22.6 -10.2 -10.1 -9.4
16.0 11.3 32.0 -11.5 -11.6 -11.1
32.0 16.0 45.3 -12.8 -12.8 -12.7
64.0* 22.6 64.0 -14.0 -14.0 -14.0
¹. Pout Output in Watts into 8Ohm.
¹. Vrms as adjusted at PST-out via Variac.
¹. Vg @ Rs = 10K and P1 = 20K3
². Vg @ Rs = 22K and P1 = 42K0
³. Vg @ Rs = 47K and P1 = 89K4
At Pout = 0.25W, Rs is chosen such that 30MR-LDR10M.
For R-LDR = ~10M, Rs = 10K @ P = 0.25W
For R-LDR = ~20M, Rs = 22K @ P = 0.25W
For R-LDR = ~30M, Rs = 47K @ P = 0.25W
* For a given Rs, at Pout = 64W, P1 is adjusted to gain Vg = -14V.
You may notice that at low listening levels there already is quite some
deflection. For 0.25W(!) this is 20%, 16% and 12% deflection respectively,
assuming Vg = -1V = 0% and Vg = -14V = 100% deflection. At a medium level of
4W it is 60%, 58% and 51% respectively. As stated before Rs and P1 values
depend on the specific Leds and LDR as used and have to be re-established
for any other combination, and for other output levels of course, but I
don't expect immense deviation from the values given.
Happy experimenting,
Rgds, Jan.
"Tom Schlangen" schreef in bericht
...
Hi RATs,
both projects I have on the bench at the moment
have a larger dynamic (read power) range than the
first amp I utilized magic eyes (for merely the
nice look of them when flickering).
You can see the circuit I used in the little
amp half way down the page at:
http://www.ndh.net/home/schlangen/ro...aten/em80.html
Useful reading range voltage at the grid of EM80 is
between 0 volts (shows thin line) up to ~ -18 volts
(full reading).
It is nearly impossible get a reading at normal
listening levels whatsoever from the magic eyes
when the max. reading (-18 volts at grid) is
adjusted for max. reading at max. output power
for a stronger amp.
A simple switch and a voltage divider gives a
"sensitive" range for low amp output levels
and a range for higher output levels, but
having to switch the ranges manually is not
practical.
Adding a zener so that at least the EM80 isn't
overdriven isn't optically nice and practical
either, since there is no change in readout up
from a certain amp output level.
Does some have an idea how to implement a
Real Simple (!) sort of dynamic range compression
or AGC, without making things too complicated?
At most I would dare to add one double triode for
_both_ EM80 present (one additional triode system
per channel).
Any ideas appreciated,
Tom
--
Live is too short to be taken seriously.
- Oscar Wilde