• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Bleeding resistor + led

Jump to Latest
S

spoontex

Member
Joined 2016
#1
#2
100K across 350V is 1.2 watts, within rating but it will get hot. A little less if the 33K is in series. 33K for the LED at 350V is 3.7W, on the high side for a 5W resistor. You can try 47K or higher, won't be much dimmer. Much more efficient to run it from the heater supply.

You're going to need additional filtering for a phono preamp - several additional R-C sections.
 
#3
You only need a couple of mA for the LED, so 350V/.002A = 175k

and dissipating power = 350V x .002A = 0.7W (use 2W).

Resistors have limited voltage ratings, so it's best to use two equal ones
of half the value, in series. But, unless this is intended to monitor the HV,
just use the filament supply voltage instead.

The switch may have a resistor built-in, so check the data sheet.
 
Last edited:
#6
You asked exact same at MEF, please read answer there.

EDIT:
Feed the switch LED from 6.3V heater winding.

Your "12V LED" is most probably a regular Red Led (1.9V forward drop) and a series resistor to limit current, and will most probably work with 6.3VAC, which you may even rectify with a single 1N4002 and a 100uF cap to get about 8-9V DC.

Remember this voltage is **floating** , neither positive nor negative connect to ground, except through filament balancing 470 ohm pot or dual 100 ohm resistors.

I find it TERRIBLE they do not supply pinout and Led connections on the page you linked, also dangerous that both Mains and Led contacts live so close to each other and they do no suggest any safety measures.

Be certain to heat shrink any and all terminals there.

EDIT: Blue Leds drop between 3 and 3.3V , all other considerations apply.
Also valvewizard calls AC filament terminals "+" and "-" ..... he should correct that.
Click to expand...
 
Last edited:
#7
Tom Bavis said:
100K across 350V is 1.2 watts, within rating but it will get hot. A little less if the 33K is in series. 33K for the LED at 350V is 3.7W, on the high side for a 5W resistor. You can try 47K or higher, won't be much dimmer. Much more efficient to run it from the heater supply.

You're going to need additional filtering for a phono preamp - several additional R-C sections.
Click to expand...


This is better?


supply.jpg




But I think 350VDC HT, it's a bit much for a phono preamp. How can I reduce the voltage a little? 300VDC for example.


Thanks.
 
#13
A small diode bridge, a 47 µF cap and the resistor and LED on the low voltage side of the transformer would be more clever than burning a few Watts for a mW range LED. Safer as well. Where the LED is drawn in the schematic one can use a bridge wire. I would up the 100 kOhm to 2 x 68 ...82 kOhm in series.

That Valve Wizard uses strange transformers that output + and - 🙂
 
Last edited:
#15
LEDs below tubes?!? For such important functionality you best use a bridge rectifier, an electrolytic cap large enough to reduce flickering and then all LEDs in series again in series with a current limiting resistor.

About the HV: for a phono amplifier ... a CLC followed by a regulator is what I would do.
 
Last edited:
#19
spoontex said:
So,


350V - 300V = 50V


50V/0.05A = 1000R


I can use 2x500R between two caps?
Click to expand...

Math is fine, but: does your Phono Preamp "eat" 50mA? 😱

Screenshot-20210828-132943.png


Looks it actually draws some 8.7 mA
Yes, that looks fine, and in any case, measuring beats imagination any day of the week [tm] 🙂

As a side note, under such low current, using a choke in a CLC is not much justified.
Chokes give you very low drop (low DCR) *and* high impedance; but since you need that voltage drop anyway, it can be replaced by resistors.
Say C-2k2-C-2k2-C which would yield about 300V DC needed.

FWIW I am into Guitar amplifiers, Classic designs (Fender-Marshall-Vox) from the 50s and 60s all used chokes for screen supplies, capcitnce was *expensive* and some used values as low as 8uF 😱

Now almost nobody uses them any more, except "boutique" makers who recreate 70 y.o. Technology, often for "perceived" Mojo, but for decades now they have been replaced by a plain 1K 5W resistor or so.

Of course, modern cap values are 32-50-100uF typically.

Nice regulated supply and if it floats your boat ..... 🙂

Personally a minimalist , would happily use a CRCRC filter with the proper/needed voltage drop and not worry about hum anyway, but hey, it´s your choice of course.
 
#20
JMFahey said:
Math is fine, but: does your Phono Preamp "eat" 50mA? 😱



Looks it actually draws some 8.7 mA
Yes, that looks fine, and in any case, measuring beats imagination any day of the week [tm] 🙂

As a side note, under such low current, using a choke in a CLC is not much justified.
Chokes give you very low drop (low DCR) *and* high impedance; but since you need that voltage drop anyway, it can be replaced by resistors.
Say C-2k2-C-2k2-C which would yield about 300V DC needed.

FWIW I am into Guitar amplifiers, Classic designs (Fender-Marshall-Vox) from the 50s and 60s all used chokes for screen supplies, capcitnce was *expensive* and some used values as low as 8uF 😱

Now almost nobody uses them any more, except "boutique" makers who recreate 70 y.o. Technology, often for "perceived" Mojo, but for decades now they have been replaced by a plain 1K 5W resistor or so.

Of course, modern cap values are 32-50-100uF typically.

Nice regulated supply and if it floats your boat ..... 🙂

Personally a minimalist , would happily use a CRCRC filter with the proper/needed voltage drop and not worry about hum anyway, but hey, it´s your choice of course.
Click to expand...




Many thanks! I will try different options. What do you think about HT voltage regulator?
 
You must log in or register to reply here.
Top Bottom