Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on March 26, 2012, 05:05:36 am

Title: Fender vibrochamp, modified with a cascode (schematic)
Post by: bruno on March 26, 2012, 05:05:36 am

Me and a friend of mine, we're currently adjusting a modified vibrochamp clone to work with a cascode input (using a 12AU7) and cathode follower feeding a typical fender tonestack.

We have drawn the following schematic, and tried the cascode in the amp already, and it seems to work nicely. Next we're going to start with the rest of the changes.

Our main issues here are making sure the tremolo still works properly and the negative feedback loop, which is now feeding before the tonestack (We're not sure if this is ok).

If you guys could double check the schematic it would be great, I'll post some photos and clips of the finished amp as soon as possible.

EDIT: Schematic has been corrected.

Title: Re: Fender vibrochamp, modified with a cascode (schematic)
Post by: sluckey on March 26, 2012, 05:24:56 am

After a quick look I can see three errors on the schematic...

1. The 100K plate resistor for the first tube should be connected directly to pin 1.

2. Connect the wiper of the volume pot directly to the grid of the second triode. That .02 cap will prevent the second triode from biasing correctly.

3. The plate of the second triode should connect directly to the grid of the third triode. That .02 cap will prevent the third triode from biasing correctly.

Title: Re: Fender vibrochamp, modified with a cascode (schematic)
Post by: bruno on March 26, 2012, 06:23:54 am

Thanks sluckey, I'll change it, and upload the new version today.

What do you think about the NFB feeding back into the second gain stage before the tonestack and cathode follower? Will there be any problem with this?

thanks

Title: Re: Fender vibrochamp, modified with a cascode (schematic)
Post by: sluckey on March 26, 2012, 06:32:39 am

Quote

What do you think about the NFB feeding back into the second gain stage before the tonestack and cathode follower? Will there be any problem with this?

It's unusual. I don't know how it will react. Just try and see. I also don't know how important the NFB is to the 'Champ' sound.

Title: Re: Fender vibrochamp, modified with a cascode (schematic)
Post by: bruno on March 26, 2012, 06:44:04 am

Probably not very important... would there be any other place after the tonestack where I could feed it?

thanks

Title: Re: Fender vibrochamp, modified with a cascode (schematic)
Post by: sluckey on March 26, 2012, 08:10:57 am

You could use a very large resistor and connect to pin 5 of the 6V6.