tremolo "roaches"

[J M Fahey]

Glad you solved it.

In this particular case, we need 2 resistors.

* a lowish value one, to make oscillator think itīs loaded with a filament lamp.

Maybe somebody finds and posts the specs for **real old** optocouplers, with a real filament lamp inside (Gibson used similar ones)

In principle, and considering we want low filament currents to make them easier to drive, I guess they used the famous (in my youth) "grain of wheat" lamps or their smaller oriental cousin, grain of rice:

but until you get one (search around, they are still availble), a 12V/30mA resistor will be a good impersonator.
Meaning 12V/.03A=400 ohm resistor.
You used 1200 ohm, clearly oscillator is happy with some 10mA ... so ... who am I to disagree?

* a highish one, in series with LED

You used a modern high efficiency Led, I am surprised at the very low current, but these are very very efficient.

I bet a cheap "special mixed bag of the week" LED will require way higher current, but feel free to experiment.

* that said, it would be nice to get a real grain of **** bulb someday and make vintage perfect roaches.

[johnhoef]

JM,
I used an 1800 ohm resistor, not 1200....and I agree, this is awful high to simulate a light bulb, however, this is the resistance value that produced the nicest sounding tremolo. So, perhaps we are NOT simulating a light bulb, but rather, we are finding a "sweet spot" that the oscillator likes better. I have a bunch of these tiny bulbs in my bulb stock, but none of them worked properly, producing a desirable tremolo sound. The original bulb is super tiny, so perhaps your "grain of rice" variety. My local electronics distributor does stock what they like to call "grain of wheat" bulbs, but I have no idea what to ask for. Now, in hind sight, perhaps the ones with the lowest current rating might work. I do like the idea of using an LED for a more dependable and longer lasting design. I agree, it would be really nice if someone could come up with the specs for the original roach used in these amps. But in all my years of working on amps, this is the first time I have seen a non-neon roach. Making vintage roaches? Cool idea, but not much of a market, I fear....if you do this, keep your day job!

[Mick Bailey]

I finally did find the schematic.......This amp, contrary to the Fenders, uses an incandescent bulb instead of a neon bulb.

That's why I posted the original link - it shows an incandescent bulb.