LED bias for 12ax7 oscillator

[jjman]

I was checking out the articles on the Valve Wizard site and spotted the LED bias option for tremolo oscillators in a guitar amp at the bottom.

http://www.freewebs.com/valvewizard1/trem1.html

I’ve been reading about the advantages of LED bias here but in this case it would be implemented merely so I could have an indicator light on the amp showing the speed and state of the tremolo / oscillation effect. (I wouldn’t mind a smoother oscillation too.)

I’m using a 12ax7 in a similar manner as on the attached schematic. I believe I have about the same voltage as indicated on the cathode of the oscillator (1.7v) and I know I have very similar voltage and resistor values on it’s plate. I doubt that anyone here has implemented this on a guitar amp the tremolo / oscillation effect but I’m wondering if there are any obvious concerns with this.

The article says “…because the oscillations extend to cut-off, the LED will flash in time…” Per my scoping, I don’t think my oscillations currently extend to cutoff using the attached schematic. Or perhaps the cutoff is so short that I don’t see it. Does this LED implementation change the circuit in a manner that increases the chance (or ensures) that cutoff will be reached? Might the LED clip the bottom of the wave? Is it possible that the light will still blink (perhaps only somewhat) even if the triode does not reach cutoff?

Should I use a “low current” LED since this is a low current situation? The below link is the series I’m considering (via Mouser.)

http://www.everlightinternational.co...t/ds300012.pdf

I know it’s cheap and easy enough to answer most of these questions with experimentation but if some of the answers are obvious it would be great to get feedback on them.

Thanx!

[Merlinb]

You oscillator certainly does extend to cut-off, that is the nature of the oscillator circuit. The output will increase until it can't increase any more (cut-off), then it starts to decrease until it can't decrease any more... and so on.

You can use pretty much any LED you like, although low-current red ones are most suitable for the ECC83 / 12AX7 because it doesn't need much bias. Other colours have larger voltage drops, which would bias the triode cooler and so reducing its gain, maybe enough to prevent it from oscillating.

Just replace the cathode resistor and cap with a red LED and it will flash! Simple.

[jjman]

Cool. Thanx.