Switchable plexi and cascade mode help

[jack2911guitarblues]

hi to all!  Am enjoying the slow learning curve of amp construction. Built a plexi clone and modded with a switchable Hotswitch. Worked fine, but missed the "real" plexi Magic, so now I changed to a circuit similar to a RR MOD. Wired up back to plexi and the cascade mod feeds the Normal channel into the Treble channel. Works well. I left the bright cap on Bright vol, and swapped coupling caps around, so Normal(dark channel) gets the .0022uf cap and Bright channel gets the .022uf cap. That seems to tame the woofiness of normal channel a bit, and I now have 2 gain controls to play besides a master volume as well. Can anybody explain the best way to use the 2 gain controls ?
And also, if I change the Normal channel Cathode bypass cap from 330uf to say 100 or smaller, will that affect the Plexi sound a lot?  I read that a lot of modders change to a much smaller bypass cap on V1a, to get rid of that boomy bass. By the way, if anybody's interested I have a schematic of the 1987/1959 plexi with a switchable cascade mod that retains the mixing capabilities of Normal and Bright channels in the Plexi mode.

Any suggestions please???????? Thanks heaps🎸🍺🍻🍻🎸

[jojokeo]

Jack, there's a lot of info here to discuss all at once with abbreviations and such also thrown in. It would be easier for most to slow down a bit and tackle one or two things at a time. This is also the best approach when making adjustments or mods to things because then you'll understand which thing you did either helped or hurt before moving forward. Another is that your interpretation of what "real" plexi magic is may be different to someone else? So these general descriptions can be misleading and not the same to others?


I "think" I understand what you're getting to regarding the "woofiness" and cathode bypass cap. The higher values provide full bypassing of the stage it's used in, meaning that ALL frequencies are increased in gain - which includes the all of the lows too. This is something that you may not always want especially early in the amp stages for a variety of reasons. In the first stage lowering this down to a typical Marshall value of 0.68uF will remove a lot/most low end content but also will tighten up & provide some punch to the lower end. Sometimes it's too much? So try something like 2.2uF or 2.7uF. These lower values provide what's called a "shelving" affect to the frequencies that are being boosted by the bypassing with the capacitor. So as an example all frequencies above 820 hertz will be boosted but those below will not. Experiment by trying anywhere from 1uF to 10uF and see how much lower end is removed when doing so until you find what you like.

On your gain control use - The earlier gain controls the most preamp "fizz" type of gain (overly compressed saturated signal) which can mask the later stage's harmonic content and gain up to and including power tube saturation - which is what many ultimately really prefer over preamp gain. BUT to each his own here, and it can also depend on overall amp volume too? No hard rules for this, just play around and on one day you like this and on another you may like it later on? Variety is the spice of life and it's always nice to have options. 

[jack2911guitarblues]

Hi jojokeo,

Thanks for your response. Yeah looking at my post you are right, I guess I was rambling on a bit!
So essentially what I was trying to say was that I wanted to incorporate the Cascade mod, AND still keep the Plexi ability to blend Normal and Bright channels. I have achieved that, and my question is: if I change V1a bypass cap to a lot smaller (to a 2.2uf or 2.7. From 330uf) like you suggested to take out some low end in cascade mode,  will the Plexi sounds be a lot different when "jumpering" channels?
I guess like you said just experiment, so I will definitely try that.
 Thanks. Jack

[rdh001]

"if anybody's interested I have a schematic of the 1987/1959 plexi with a switchable cascade mod that retains the mixing capabilities of Normal and Bright channels in the Plexi mode."


-- Jack would you be kind enough to post this?


Thanks

[printer2]

I am a little behind in my amp building, want to build this yet. Ignoring the Trainwreck part (bottom input), the two inputs are Plexi when S2 is one position, cascaded 2203 in the other.

[DummyLoad]

S2 under input jacks and Rk-Ck of normal channel is not right. in parallel mode the 910R/22uF should be grounded. you have 10K Rk under normal channel when in parallel.

suggestion: a third pole that switches between Rk-Ck values with current second pole that toggles input jacks of normal in-out of ckt.

--pete

[printer2]

Should have moved the switch position of one of the poles in the schematic. Say the pole on the input jack should be on the 910R when the other pole is on the second stage. If you have it in plexi mode and do not have a jack plugged into the normal channel the input is open open circuit and you will pick up noise. With the two pole switch in cascade you also need to know not to plug into the normal mode with another guitar or the channels jumpered. Was hoping to do it with a two pole switch, a three pole will work. Just came up with the arrangement for my own use, need to think of how others may use it also.