[kchapdaily]

hey guys
 
just finished my starving student, and i thought it would be cool to wire up an led to let me know when the tubes are conducting. 
 
my first thought was to just use a microcontroller to turn the led on after 30 seconds, but i was wondering if there was a way to use one of the tubes outputs to tell the microcontroller when its conducting. i was afraid this would degrade the sound quality or mess up voltages going to the mosfet. can anyone give me some insight?

 

[nikongod]

I would just wait 30 seconds.
 
However, I thought about it a bit more, and it should not be particularly difficult to build an indicator. 
 
Right click and open in new tab to see in all its glory. 56Kdeath
 

 
I have not built this circuit so use at your own risk. I'm pretty sure it will work. I would appreciate it if someone else would check my work anyways.
 
Theory of operation:
When the tube is not conducting there is 0V across the 2Kohm cathode resistor. 
The BJT requires a positive bias voltage on its base to turn on. 
When the tube is conducting the cathode voltage turns the BJT on. At this point, the 50-200ohm resistor sets the current like a CCS (it is a CCS) 
 
The formula for the current is:
(Vb-Vbe)/R=I
where:
Vb is the cathode voltage
Vbe is Vbe, from the datasheet - typically 0.6V
R is the resistor between emitter and ground 50-200ohms in my schematic. 
 
NOTES!
Be sure to heat-sink the transistor if its burning off more than 1/2W
I *assumed* 1V as the bias voltage and Vbe=0.6V. Measure what you actually have and do the math as necessary.

 

[kchapdaily]

thanks thats perfect! i was going to try something like this but i was afraid it would degrade sound quality. will this leave sound quality intact?

 

[nikongod]

 
Quote:

thanks thats perfect! i was going to try something like this but i was afraid it would degrade sound quality. will this leave sound quality intact?

Depends who you ask
 
I'd bet a few bucks that (assuming I drew the circuit correctly...) the circuit does absolutely nothing to change the sound in any way. 
 
OTOH, there are HI-Fi MFR's who will not put power indicator lamps on their equipment, and remove it from equipment where it came that way "because they change the sound". You cant make this stuff up. 

 

[kchapdaily]

people these days

 
 
since its not actually on the v+ output im sure it wont do anything. and even if it does i can just take it off