Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GuitarPilot65 on July 24, 2025, 06:02:23 pm

Title: DRRI tremolo Intensity issue
Post by: GuitarPilot65 on July 24, 2025, 06:02:23 pm

I’m in the “Vibrato” channel. I need to turn the Intensity knob nearly to 10 to hear anything noticeably pulsating. No footswitch issues. Where should I start troubleshooting?

Title: Re: DRRI tremolo Intensity issue
Post by: stratomaster on July 24, 2025, 06:16:17 pm

Pull up the DRRI schematic and read the notes. Test the tremolo per those notes and annotate any deviations.

Title: Re: DRRI tremolo Intensity issue
Post by: tubeswell on July 24, 2025, 07:57:21 pm

Could be bad roach. Could be bad LFO tube. Could be aged caps in the LFO circuit.


Try tube rolling first

Title: Re: DRRI tremolo Intensity issue
Post by: GuitarPilot65 on July 25, 2025, 11:49:24 am

Thanks for the replies. I don’t have an oscilloscope so I’ll try “tube-rolling” first. They may be original.


Is there a way to test the opto/ “roach”?

Title: Re: DRRI tremolo Intensity issue
Post by: tubeswell on July 25, 2025, 12:33:44 pm

Quote from: GuitarPilot65 on July 25, 2025, 11:49:24 am

Is there a way to test the opto/ “roach”?

If there is no continuity where there should be, or if the resistance is significantly different from what is expected, the roach may be faulty.

Title: Re: DRRI tremolo Intensity issue
Post by: stratomaster on July 25, 2025, 02:40:07 pm

Quote from: GuitarPilot65 on July 25, 2025, 11:49:24 am

Thanks for the replies. I don’t have an oscilloscope so I’ll try “tube-rolling” first. They may be original.


Is there a way to test the opto/ “roach”?

You can get by without a scope, though it is the best easy. Set your meter for AC for the voltages in the oval and DC for the ones in the rectangles. Test points can be found clearly labeled on the layout page for easy identification and cross referencing.

To test the roach you can often peek into the open end of the heat shrink to confirm whether or not the bulb is on, blinking, and responsive to changes in speed. Sometimes just dimming the lights will enable you to see the flashes without the need to peek much.

On the other end of the roach you can test for resistance. As the light comes on the resistance will drop. All the Intensity pot is doing is adding this changing resistance in parallel with the pot to ground.  It's entirely passive, and basic resistance testing will test for basic function.

For finding ticks or odd artifacts in the trem you'll need the scope. Or you can spray and pray with the cap trick, moving lead dress, reassigning ground points, or just straight replacing the roach.