Mod Suggestions for 75' Plexi clone.

[Pochie45566]

Hey! I know I just finished a thread fixing some low volume issues. they are all resolved now and I am going to transition into the modding phase. This is a clone of a plexi build in 75' so I do not care about originality. Tell me some mods I should do to make it kick ass! I want the typical smooth screaming plexi tone. Here is what ive done so far :) It is based off a jtm45 I think due to the schematic. I already made a few changes to make it more plexi like. What should I do next! Heres a schematic.
 
Changed the .5 cathode caps to .68

Converted the power section to Pentode operation

changed coupling caps to .022

installed el34's and 12ax7 tubes

[shooter]

What should I do next!

http://el34world.com/charts/Schematics/files/marshall/Marshall_Schematics.htm

search this forum, many of us have build "their version".

I would suggest ONE at a time, more play time, less fix time

[Pochie45566]

What should I do next!

http://el34world.com/charts/Schematics/files/marshall/Marshall_Schematics.htm

search this forum, many of us have build "their version".

I would suggest ONE at a time, more play time, less fix time

Thanks!

maybe you guys can answer this also, Why does this master volume look like a post phase inverter master volume just slapped before the phase inverter? Dual gang pot and all.

wait, Im just stupid. after looking at a marshall 2204, a ppimv amp, I can see it is wired in the same way. Weird. Youd think the master volume would be after the phase inverter in schematic

[Ed_Chambley]

It seems to be a plexiglass now, so all you gotta do to, make it scream is jumper the channels and turn the bright to about 7 and the normal to 5.


No magic to the plexi tone. It consists of overdriving EL34 tubes. Nothing else sounds like this.


What are you hoping for?

[shooter]

bright to about 7 and the normal to 5

Noooo... put bright on 3, normal on 6, T&B about 4 and use mid from 3-7

[sluckey]

I don't have any numbers. I just turn the knobs until I like the sound.   

[PRR]

> Why does this master volume look like

I've been staring at it for a week.

Full-up, the power stage clips first. Turned-down, the tonestack driver clips first. The power stage is not a large gain, so needs a large signal to clip. The tonestack driver feeds tonestack which has large loss; plus whatever loss is dialed-in by the master. At some point the tonestack driver will be the limit.

One oddity is that they use a dual pot to also cut gain before the tonestack driver (F.B,I). I assume they were trying to keep overall gain near the same while limiting the maximum output. I have a feeling modern knob-twiddlers would like two knobs, even if most of the happy settings turn out to be what the dual-gang scheme does.

> I want the typical smooth screaming plexi tone.

So find plans for the typical smooth screaming plexi and copy it.

[Pochie45566]

> Why does this master volume look like

I've been staring at it for a week.

Full-up, the power stage clips first. Turned-down, the tonestack driver clips first. The power stage is not a large gain, so needs a large signal to clip. The tonestack driver feeds tonestack which has large loss; plus whatever loss is dialed-in by the master. At some point the tonestack driver will be the limit.

One oddity is that they use a dual pot to also cut gain before the tonestack driver (F.B,I). I assume they were trying to keep overall gain near the same while limiting the maximum output. I have a feeling modern knob-twiddlers would like two knobs, even if most of the happy settings turn out to be what the dual-gang scheme does.

> I want the typical smooth screaming plexi tone.

So find plans for the typical smooth screaming plexi and copy it.

So it is a pre phase inverter? I am kinda confused. That sounds very weird.

[shooter]

So it is a pre phase inverter?

In my world I call that knob "drive",  the oddity for me is the nfb that incorporates the TS in the loop, I don't believe that's typically done, but if it works, roll with it

[Pochie45566]

So it is a pre phase inverter?

In my world I call that knob "drive",  the oddity for me is the nfb that incorporates the TS in the loop, I don't believe that's typically done, but if it works, roll with it

weird. the preamp volumes are acting as my gain knob right now. I may change it over to a ppimv anyway to make it a bit more standard. Do I just continue the tone stack onto the phase inverter through the .02 cap? Then install a ppivm?

[sluckey]

In my world I call that knob "drive",  the oddity for me is the nfb that incorporates the TS in the loop, I don't believe that's typically done, but if it works, roll with it

What nfb? That pot simply lowers the signal level applied to the grid of that second tube. I'd just disconnect that F.B,I wire and never think about it again. While doing that, I'd also swap the wires on the wiper and hot terminal of the other half of that pot just to make it look exactly lipe a JCM800 2204.

[shooter]

What nfb?

sometimes I see things nobody else can

thanks for pointing that out

[Pochie45566]

In my world I call that knob "drive",  the oddity for me is the nfb that incorporates the TS in the loop, I don't believe that's typically done, but if it works, roll with it

What nfb? That pot simply lowers the signal level applied to the grid of that second tube. I'd just disconnect that F.B,I wire and never think about it again. While doing that, I'd also swap the wires on the wiper and hot terminal of the other half of that pot just to make it look exactly like a JCM800 2204.

Ill disconnect fb1  and do you mean swapping the wires coming from the phase inverter and the one going to ground or the wire coming from the tone stack and ground? What would that do?

[sluckey]

My mistake. It's already wired correctly. 

[Pochie45566]

My mistake. It's already wired correctly. 

so it is not a ppimv? weird

[sluckey]

so it is not a ppimv? weird

It is a "PRE PHASE INVERTER MASTER VOLUME". Just like the JCM-800 2204 because it occurs prior to the PI.

"ppimv" has generally become accepted to refer to a "POST PHASE INVERTER MASTER VOLUME". I don't get the logic behind that but I accept it. No way to distinguish between pre or post MV.

Another one I have trouble accepting is VVR which stands for "VARIABLE VOLTAGE REGULATOR". Ain't no voltage regulation occurring in that circuit! Sure, it's adjustable but it does not regulate anything. Just another acronym that means something to a guitar pickin' tweaker but has no real meaning in the electronics world.

[shooter]

has no real meaning in the electronics world.

you might need a new thread if we're gonna beat up acronyms

[sluckey]

No way! Probably need an alternate internet to cover that topic. 

Remember, I worked for the federal government for 40 years. Those people are the acronym gods! And you can't even talk to an FAA employee without using acronyms, anything from equipment to the latest employee sensitivity training program. I swear there is a group of employees that are paid to sit around brainstorming catchy acronyms. Then they pass the latest list to another group that will make up a program name to fit a particular acronym. Finally the program name is passed on to yet another group that will match the name to some new concept or program or even a new piece of electronic equipment.

I bet you think radar is a word??? 

[shooter]

And you can't even talk to an FAA employee without using acronyms

Yup, there is a dark n spooky place where ppl with mis-matched socks and weird hair think up acronyms 

The Navy trained me well in them, and the civilian world was just as bad, AND they were NOT standardized!!  GE called their DAS cabinet something different than Phillips, which was different from Technicare, which was .......
and now, musicians and tech are mixing, and God help, probably breading

bet you think radar is a word

nope, but I forgot it and have no desire to even google it

[DummyLoad]

LASER is another acronym that's commonly used as a word nowadays.

1) disconnect wire F.B.I from ganged pot and use it like a 1959 model with added MV. now you have a 2204/1959 hybrid. that's the first mod i usually make to the 1959 ckt: add a MV after tone stack.

2) make .5uF under V2-A switchable or remove altogether - 1959 pre-amp did not have that cap and can make tone "fizzY" sounding of you dive hard. some like it better with it.

3) use 5751 for V1 and/or V2 holes to add slight headroom & reduce gain slightly.

4) try 12AT7 in V3 (phase inverter) hole.

5) replace V1 with a pentode and use the Vol2 pot to vary G2 voltage?

not much more to do to that ckt. except rewire another type... e.g., add / remove stages, etc.. 

--pete