Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dickjonesify on March 11, 2026, 05:28:21 pm
-
So, I added this tube tremolo circuit to my amp, design courtesy of sluckey…
(https://lh3.googleusercontent.com/pw/AP1GczPCk9ZSKgOrtc4WKkt3eIHioaEtPWQ8KIDjDIr-dTksZzwfKAeyV8BzOeqKDps4rX_pW3ckqtf_1reEeBDy5hCDsDpPUkVp5BSkRvOegYnO3e9EKH_emBC5zNs_wxXynQBJBS-oEH-WA7kc_3WceWxQIQ=w1170-h939-s-no?authuser=0)
I want to add an LED as a visual indicator. I attached a red one between the cathode and ground, parallel to the Vactrol. It works but it’s quite dim. There’s only like 1.7VDC there.
Is there an easy way to adjust the voltage at the cathode? Spoiler alert: I already tried changing the cathode resistor and it didn’t seem to do anything (for the LED anyway).
Another thought was putting a neon bulb in series with the plate of the oscillator. It’s around 200VDC. Might be a better aesthetic choice anyway since the amp has neon bulbs for standby and power already.
-
You can also put the LED in the plate circuit. But I also prefer the look of the neon
-
Just put the indicator LED between the plate resistor and plate.
Putting it where you did will impact performance of the opto-coupler/trem sound.
The cathode voltage is determined by the forward voltage of the LED in the opto-coupler.
-
Just put the indicator LED between the plate resistor and plate.
You can also put the LED in the plate circuit
Just in series? And nothing will catch fire? Haha
But I also prefer the look of the neon
I may order one.
-
I have the LED calculator bookmarked but all the schematics and current limiter resistor calculations show it then connecting to ground, which would require a high wattage resistor and a whole lotta heat. I can’t find any clear instruction about putting it in series with high voltage.
-
The voltage drop across the LED is the same regardless of the input voltage. The current limiter is the triode, approx 1mA or so.
Perfectly safe with no extra resistors needed.
See bottom of this page: https://www.valvewizard.co.uk/trem1.html
-
The voltage drop across the LED is the same regardless of the input voltage. The current limiter is the triode, approx 1mA or so.
Perfectly safe with no extra resistors needed.
See bottom of this page: https://www.valvewizard.co.uk/trem1.html
That’s awesome! Thanks.
Valve Wizard is a site I have bookmarked and have read a lot but I don’t think I’ve seen that page.
-
Just checking: Could I add a neon lamp in the exact same way?
-
The neon bulb drops a lot more voltage (like 50V) than an LED. Maybe a 6.3V indicator lamp, but I don't know the current rating for those.
Edit: looks like the indicator lamps need a lot more current than what you're pulling through a 12AX7. You could maybe set up a neon bulb to be driven by the anode, in a similar fashion to the optocoupler in the original Fender circuit, but that's a lot of added complexity for a blinky light. I would just go with the LED in series with the anode. Stick it in an old fashioned jewel bezel if you want the aesthetic.