music, guitars & other obsessions

Octal Tremolo Amp Build

Pretty much finished with the Octal Tremolo amp build, at the end, I could not get the tremolo circuit to function properly, I tried many ways to modulate the audio level based various schemes Fender, Gibson, RG Keen, etc., alas nothing worked as described, and the frustration grew to the point I was making careless errors which nearly blew the amp up! it was quite puzzling why these supposedly proven circuits did not work as expected. I doubled checked the voltages, wiring time and time again, but nothing worked. So, I just gave up… Instead I will build a pedal which works on non-lethal voltage…

Since all the tremolo circuit does, is to amplitude modulate the input signal, I just modified the Fender Vibro Champ bias wiggle scheme a bit – instead of connecting to the cathode of the second preamp tube, I fed the tremolo signal to the LED connected to the cathode before it hit ground, i.e., the bias of the stage is set/modulated by the voltage generated from the tremolo stage. Since there are some DC across the pot, it does create a bit of scratching noise when turned, but not too bad and certain something I can live with after struggling to get the darn thing to work for days!

As a side note, RG Keen’s MOSFET tremolo circuit worked nicely (except for the interface to the preamp part, at least for me), I made some minor changes to make it easier to use and eliminated some parts that I thought was not needed – I took out the 200V voltage regulator section completely, and replaced the 33K bias resistor with a 50K trimmer, since the whole idea was to properly bias the tremolo circuit regardless of the incoming B+ voltage, with the trimmer, I can dial it in and forget about it (unless of course the line voltage changes a lot permanently, then it would require a re-bias).

The tremolo using the modified bias wiggle sounds pretty nice, but it does take away some of the gain, so when I switch it off, there is a noticeable increase in gain from the amp, but that’s to be expected.

Here is the gallery of the build:


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s