The Cerberus project

[romberg]

After thinking about it for a while, I'm about ready to build my first Marshall. I now have a pile of parts and "most" of a plan. I would welcome any advice others may have about this little venture of mine.

The idea comes from reading that monster thread over at metroamp about SIR #36 and #39. Some fellow (I wish I remembered who) posted a schematic of a pre-amp that is switchable between the marshall 1959, 2204 and SIR #39 setups. Three amps in one box. How cool is that!

Here is my take on the design for it. The pre-amp is mostly the schematic from the metro thread. The power amp is vanilla 50w (with a PPIVMV).

Before I heat up the iron, I am looking at a few things that others who have built a similar circuit may be able to help me with:

1) The two 470k resistors branching off of V1a that feed the two gain controls. Do, I need both of these? I understand that they function as the top 1/2 of a voltage divider for the next stage. But could I not just use one and have it feed both of the pots? It occurs to me that they may act as mixers in a way I'm not getting.

2) The voltage divider coming off of V1b. This uses the stock values of 470k/470k from a 2204. But I've seen various specs for a SIR #39 that use 470k/270k here. To keep the switchable 2204 I'd like to keep those values. But, the 10% or so less signal may be important for that added stage. What do folks who have built something like this think? Could I make a "totem pole" divider and try and get both? I'm not sure if the gain reduction here is important for #39.

3) I have an extra triode. It is bothering me. I'm considering two options. The first is to wire it up as the other input stage from a 1959 and make it switchable with a pot mounted switch. The second is to wire up a switchable matchless parallel first gain stage. Thoughts?

4) I'd kinda like to make the signal switching via a relay controlled footswitch. But I've not bumped into a decent looking kit/board to do something like this. So, may just use panel mounted toggles. Unless someone knows of a nice relay board that I can power from the heater winding.

Anyway, back to planning I go. Appreciate any input.

Mike

[martin manning]

In each of the cases where there is a 470p//470k you have a high-pass shelving filter, so these are more than just voltage dividers. The difference between the 470-470 and 470-270 division is less than 3dB, with a little less treble, so I think pick the one you like rather than make it switchable.

[sluckey]

3) I have an extra triode. It is bothering me. I'm considering two options. The first is to wire it up as the other input stage from a 1959 and make it switchable with a pot mounted switch. The second is to wire up a switchable matchless parallel first gain stage. Thoughts?

I like the 1959 option. I did something similar with the 1987. The two Plexi preamps are hard wired to simulate jumped Plexi channels.

http://home.comcast.net/~seluckey/amps/ ... ual_50.pdf

4) I'd kinda like to make the signal switching via a relay controlled footswitch. But I've not bumped into a decent looking kit/board to do something like this.

Hoffman has one that I like...

[romberg]

Thanks Martin. I sorta figured that the difference between these values could not make that much of a difference. But I was not sure.

Steve, you've talked me into adding the other 1959 triode. Below is my attempt. Unlike your version, I am attempting to "recycle" V1a for first stage duty in all three preamps. I'm guessing that since I now use v1b for the dark 1959 channel, that some signal from it will leak into the signal going into v2a (stage2 in 2204 and SIR 39).

I think this might be ok. But perhaps I should be connecting v2a into the junction of the two 470k resistors after the bright and dark volume pots? Which would make the "jumpered" 1959 channels the frist "stage" always. Again, I'm not real comfortable with my understanding of mixing resistors.

BTW your layout is very nice! What did you use to create it? I may now need to make one of my own.

Mike

[Smokebreak]

4) I'd kinda like to make the signal switching via a relay controlled footswitch. But I've not bumped into a decent looking kit/board to do something like this. So, may just use panel mounted toggles. Unless someone knows of a nice relay board that I can power from the heater winding.

I recently converted a straight up 36 build into a switchable beast . I wound up using 3 relay boards and 1 power supply that I got from Dave(Colossal) that posts here. Boards were excellent, and Dave's customer service was absolutely top notch. Powered from 5V winding,as well.

[romberg]

I recently converted a straight up 36 build into a switchable beast . I wound up using 3 relay boards and 1 power supply that I got from Dave(Colossal) that posts here. Boards were excellent, and Dave's customer service was absolutely top notch. Powered from 5V winding,as well.

Found your thread and Dave's site. Those do look nice. I was looking for something small and compact that would not make the inside of the chassis look like the inside of my laptop .

Mike

[sluckey]

I'm guessing that since I now use v1b for the dark 1959 channel, that some signal from it will leak into the signal going into v2a (stage2 in 2204 and SIR 39).

I think this might be ok. But perhaps I should be connecting v2a into the junction of the two 470k resistors after the bright and dark volume pots? Which would make the "jumpered" 1959 channels the frist "stage" always. Again, I'm not real comfortable with my understanding of mixing resistors.

BTW your layout is very nice! What did you use to create it? I may now need to make one of my own.

It's possible that some of the dark channel would leak through into the high gain circuit. But those 470K mixing resistors also serve to isolate the bright and dark channels from each other. You would probably have to run the dark volume really high to be noticeable with your ears. I like the idea of running both plexi channels into V2A. It would be interesting to try it both ways.

I've never ventured into the high gain world other than the 2204 circuit. I did a layout for a guy a while back that was a high gain amp, but I don't know if he ever built it so I can't really say the layout was successful. Here it is....

http://home.comcast.net/~seluckey/Aro_Project.pdf

I'll be following your progress with this build. Good luck.

Oh yeah, I use Visio for all my schematics and layouts. There are a lot more drawings on my website. I just finished a 1960 Vox AC-15. It's a small low gain amp but has the highest parts count of any guitar amp I've ever done. Take a look if you want. You may get some ideas about the layout for your project.

http://home.comcast.net/~seluckey/amps/index.htm

[Smokebreak]

One general idea with high gain stuff is that you want to cut bass early, to keep things tight, then introduce it later or gradually, to keep it massive. That said, I'd be a little worried that always running the parallel inputs(with the dark channel's massive cathode cap) may make things a bit farty in higher gain settings. I'd be curious to see how that works out, though.

[romberg]

One general idea with high gain stuff is that you want to cut bass early, to keep things tight, then introduce it later or gradually, to keep it massive. That said, I'd be a little worried that always running the parallel inputs(with the dark channel's massive cathode cap) may make things a bit farty in higher gain settings. I'd be curious to see how that works out, though.

Your words of caution are well, taken. I'm switching (pun intended) to relays and cutting the dark channel out of the picture when the high gain channels are engaged. With this setup, I think I can audition the jumpered channel first stage with clip leads. It would be simple to then keep it or leave it out.

Now relay control circuits...

Mike

[Smokebreak]

I see you have a 1/2 of an unused relay! You could use that to ground out V1b or V2b when in 2204 mode, to help with noise, though I think grounding grids is more effective. Or you could use it to switch in a series 2n2 with your v1a 22n coupler, to tighten things up further, if needed, or wanted .Just more things to play with

You may want to experiment with some resonance of some sort, on down the line. Be it fixed, depth pot, or Martin's Presenence . It'll put that thump back in.

Another area that's fun to play with is the 10K stage. Switching that down to 2K7/.68 gives that 80's thing, and 5K is nice.

[Homebelly]

Hi Steve..

Have you actually built this?

I have this layout from some where and i have wondered if there would be much of a volume jump when switching from the 1987 to the JCM channels? I have wondered if there needs to be a volume control for the JCM stage?

I also have the layout for the pre-amp alone that i think you put together for some one to build as a rack..???

3) I have an extra triode. It is bothering me. I'm considering two options. The first is to wire it up as the other input stage from a 1959 and make it switchable with a pot mounted switch. The second is to wire up a switchable matchless parallel first gain stage. Thoughts?

I like the 1959 option. I did something similar with the 1987. The two Plexi preamps are hard wired to simulate jumped Plexi channels.

http://home.comcast.net/~seluckey/amps/ ... ual_50.pdf

4) I'd kinda like to make the signal switching via a relay controlled footswitch. But I've not bumped into a decent looking kit/board to do something like this.

Hoffman has one that I like...

[sluckey]

Have you actually built this?

No, but some guys over on Hoffman's board have.

I have this layout from some where and i have wondered if there would be much of a volume jump when switching from the 1987 to the JCM channels? I have wondered if there needs to be a volume control for the JCM stage?

There is a volume control in the 2204 preamp so you should be able to match volume levels with the 1987.

[Homebelly]

Have you actually built this?

No, but some guys over on Hoffman's board have.

I have this layout from some where and i have wondered if there would be much of a volume jump when switching from the 1987 to the JCM channels? I have wondered if there needs to be a volume control for the JCM stage?

There is a volume control in the 2204 preamp so you should be able to match volume levels with the 1987.

If you have the plexi channels set up for a cleanish tone, around 4 or 5 on the dial, and the JCM channel set for lots of gain, about 6 or 7, won't there be a loudness jump when switching from the plexi to the JCM?

[romberg]

If you have the plexi channels set up for a cleanish tone, around 4 or 5 on the dial, and the JCM channel set for lots of gain, about 6 or 7, won't there be a loudness jump when switching from the plexi to the JCM?

I'm expecting that there could be. It all depends on how hard the stage before the cathode follower is being driven by each alternate "driver". The difference in output would be that gap between clipping and how close each "channel" is approaching overdriving the very last gain stage.

Mike

[romberg]

Thanks to the feedback so far I now have the relay situation sorted out. I'll change between the thee options (Plexi, 2204 and SIR #39) via a panel mounted three position rotary switch. There will also be a two button foot switch which will override the panel selector. One switch for "boost" mode and one for "SIR" boost.

Now, I've really changed things from the 2204 kit I'm using as a base. Think I'll need to do my own layout (to make sure I get it all right). Anyone know any good software (preferably for Linux)?

Mike