Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kwm488 on September 03, 2014, 10:38:59 am

Title: my friend vintage silverface twin reverb get this hum
Post by: kwm488 on September 03, 2014, 10:38:59 am

hi

my friend vintage silverface twin reverb get this hum. i never heard this hum before. do anyone know it, and give me a trip about what problem?

http://youtu.be/Xdxh6qC9fnw (http://youtu.be/Xdxh6qC9fnw)

thanks

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: tubeswell on September 03, 2014, 01:49:43 pm

That's what we call 'motor-boating'.


Its caused by a failed filter cap.


If its a vintage SFTR, its probably overdue to have the filter caps replaced anyhow.

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: kwm488 on September 08, 2014, 04:57:56 am

(http://i23.photobucket.com/albums/b383/kwm488/2014-09-08-17-44-18_zpsybt8geyx.png) (http://s23.photobucket.com/user/kwm488/media/2014-09-08-17-44-18_zpsybt8geyx.png.html)Hi
I find out the problem. It is the capacitor which mount to the 10m resistor of LDR.
I remove it. No motor noise now. But also no tremolo.

Blackface TW no this cap, just silverface. So i believe tremolo can work without that caps.
I tested that area resistor, replaced tube, still no tremolo.

I guess the LDR. How can i measure it?

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: sluckey on September 08, 2014, 05:53:49 am

Quote

still no tremolo

You must have the footswitch plugged in and vib turned on for the trem circuit to work. If you don't have the footswitch you can put a temporary ground on the VIB footswitch jack to enable the circuit.

The LDR is actually two simple components inside that black heatshrink tubing. There is a NE2 neon lamp in one end. With the house lights off you should see it flashing at the tremolo speed if you peek in the end of the shrink tubing. If not flashing then either the circuit is not working, footswitch is not plugged in and turned on, or the lamp is bad.

In the other end of the shrink tubing is the actual LDR. When no light shines on it, it will measure several MEGΩ. When lots of light shines on it, it will measure much lower, maybe a few KILOΩ.

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: kwm488 on September 08, 2014, 06:08:27 am

Hi
I have footswitch. THe tremolo is work before i remove that capacitor.

Btw, the ldr is flashing when i turn tremolo on. BUt no tremolo function.

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: sluckey on September 08, 2014, 06:14:02 am

Replace it. Hoffman sells them.

Title: Re: my friend vintage silverface twin reverb get this hum
Post by: kwm488 on September 11, 2014, 11:20:15 pm

hi
i replace it. everything is fine now.
thank you for your help