The Cerberus project

[romberg]

Fired her up for the first time and am happy to report no smoke! And all three "modes" appear to work! I only played a couple of chords on each setting as its late and the neighbors are home. But the bright and dark plexi channels seem bright and dark. The 2204 appears more gainy. And the SIR #39 setting is crazy gainy. I'll make some clips later when I can turn it up a bit more.

Before starting it up, I did find that I had connected the B+ supply of V1 to the wrong rail node. And... I did this under the circuit board. Lifting the board to fix this without rewiring almost everything was not possible. So, I lifted the plate resistors off of the mis-wired turret and Mcgivered a wire under the board to the correct node by drilling a small hole. It is not real noticeable. Lesson learned. If you run wires under the board, triple check 'em!

Quick question. I'm measuring 495 or so volts at the first cap can. Does anyone have any experience with how well these JJ 500V cap cans can deal with this? It is right on the raggedy edge of their max rating. Should I consider wiring the first cap can so the caps are in series (and cut the reservoir by 1/2)? Or can a JJ cap can take this?

Thanks,

Mike

EDIT: Never mind the dumb question about putting the first caps in series. They share a ground so it can't be done. Still curious about running it that close to its max rating.

[David Root]

Try putting it into the PSUD2 simulating tool on duncanamps.com. I run all my PS thru it before I build.

It will catch the temporary voltage surges so you can see how high and how long they last. In my experience it shows that the first cap AFTER the choke frequently surges higher than the reservoir cap. Depends on the choke inductance IIRC.

Typically a can cap will have a voltage surge rating of 110% of nominal, but JJ's specs are sparse and I don't think they indicate it.

As to running the quiescent voltage so close to the rated, the rating is for continuous use so it should be OK. But I would check the surge on PSUD2, it might shorten the cap's life.

[romberg]

Thanks for the PSUD2 tip! I modeled the power supply in it (had to guess at current sinks) and made a match for the steady state voltages I have. You are right of course in that it shows a nasty short 550V or so spike at the cap following the choke right at start up.

I've read that some caps can handle a spike of +!0% or so (JJ is again mute on this subject). So, here too I may be right on the line. I guess I'll keep the caps I have and look for some 600V ones (does anyone even make such a thing) if they blow up.

Mike

[David Root]

I've not seen 600V multi-can caps. Highest I have seen is the big Mallory cans that AES sells, some of those are 525V rated, but they are about three times the price of the JJs and I can't see that.

Looks good Mike! I am looking at doing something similar but just Plexi to 2204, so I have found your thread very helpful!

[romberg]

Thought I'd wrap things up on my winter project. First off here are some sound clips:

https://www.youtube.com/watch?v=JxbR919Bn-4

and some high res pictures of the build:

http://home.comcast.net/~mike.romberg/amps/cerberus/

I'd like to thank sluckey for the suggestion for adding the dark channel with the extra triode. The plexi configuration is one of my favorites. And that dark channel has a very nice clean sound even with the guitar cranked all the way up.

The 2204 sounds nice. It is a bit brite. But I think these amps are suppose to be. And to my ear, it does that 80s metal thing well. I can even do the low gain 2204 input by running just the plexi bright channel (I think that is the same).

The SIR mode is pretty crazy. I thought I'd have to tweak it. But it is close enough to that AFD sound to suit my needs. With this much gain pretty much any pickups (even humbuckers) will search out flourescent lights and amplify their nasty buzz. I'm pretty sure it ain't the amp because the buzz goes completly away with the guitar unplugged or the guitar's volume set to zero. I may look into adding a noise gate to switch in in the SIR mode. Anyone know of a good but simple one?

Also thanks for all the tips folks gave on keeping the pops out of the switching. I'm happy to report that there are none. Even with all controls set to kill.

Below are the final schematics (with PS voltages) and layouts:

Mike