3stage + CF options

[Smokebreak]

I'm planning another hotrod, and have some questions about preamp staging.
I'm looking at 3 options, all of which will be simple switchable between "plexi" mode and then the added stage. All 2k7/.68 cathodes

1. That Jose preamp that's around : It adds a stage at the front of the amp, and has 68k split at 1st vol wiper > grid of stage 2(plexi stage 1) : I've got one built like this, but you really have to crank mids and kill the treble to get balls. [img646]http://img19.imageshack.us/img19/6981/arredondo1.jpg[/img]

2. Explexador setup: stage 1 is plexi stage w/ 470k split at volume wiper, stage 2 is the added , switchable stage : [img:4170:2895]http://elektrotanya.com/PREVIEWS/634632 ... .pdf_1.png[/img]

3. JCM800 setup: stage 2 would be the switchable stage, but there's no split between stage 1/2, the attenuation is 470K before the volume on stage 1 .. ...so stage 1 isn't really a true 1987 thing. You could make stage 1 the added stage here, but then you're back to 68K splits: [img1600]http://mhuss.com/MyJCM/JCM800_2204.gif[/img]


The dillemna I see with adding the switchable extra stage up front is that you are forced to have the 68k split, so that when in plexi mode, stage 2(stage 1 in plexi mode, has a 68k grid resistor, and not a 470K).

I guess my simple question is what can I expect the difference to be between a 68k split and a classic 470k split, and, just looking at these preamps, is there really going to be that much difference between them?

[Reeltarded]

I normally make the second or third stage switchable after having been through the same questions. If you are dividing equally the resonse is different from a 68k split vs 470k but equal division is still same voltage. 50:50.. so.

It's so easy to add the stage just previous to the stage driving the CF. That's what I do.

I dink with the mixers, but generally split each stage 470k/470k except the added stage gets mixed short on the signal side depending on how much signal I am looking for. I have run the ground sides as high as 1M and varied signal legs for response. The larger resistance is angrier.

Dave Friedman uses 68k splitter on lots of amps first stage, I think.

Many things work. Everything is a little different.

[Smokebreak]

Thanks, that's exactly what I was looking for.
Concerning 2nd stage split where you suggested upping the ground leg for mean, that could just be another vol pot, eh? Now I just need to figure out how many vol pots is too many. I'm up to 5 if I count that one:

1st stage, 2nd stage(maybe), CF clipping master, post TS normal master, PPIMV...yikes

Concerning the Jose type CF and normal MV, is it plausible to use a dual gang pot, and wire DPDT to each gang, to cut down on space?

[Reeltarded]

The original mod amps based on 4 holers didn't have masters, just the cascades I and II and a volume knob for the added stage. This is very effing loud. It's like standing in Niagra Falls.

I have found it's better to use dividers for everything but that first stage then use the CF master and the normal master like on an 800. You are better to put the control of such a thing after the first stage or never hope for a clean sound.

As far as the CF and simple on the same dual pot, yeah you can make it all switchable.. BUT if you turn either up full they act as if they are bypassed for the most part. I never max the CF master. I always leave at least 5-8% rotation of headroom up there. Just a touch.

My gain staging happens on 3 pots. 4 is getting nutty. 5 is out of hand!! The cool thing is once you find normal operating ranges on the stages you can measure the difference and sub in dividers or not.

I dunno. It's all good.

[ampgeek]

I went the "out of hand" route with 5 total volume controls on my currently in-process build.

However, I bought some front panel space by putting the tone controls on a 1.75" X 1.75" PCB with thumbwheels and an on-front-panel slope control tone knob. I am betting that the end user will be able to find a "fixed" group of settings with the thumbwheels and tweak-to-taste with the slope control. We shall see!

My layout is reasonably spaceous in a 17" X 10" X 3" chassis.

I etched up all of my boards last night (regulated DC system for pre-amp filaments and relays, presonance, tone and clippers) and will be drilling (ughhh!) and loading them up this weekend.

Let me know if I can help you out with some of my ongoing work.

Cheers,
Dave O.

[Smokebreak]

Thanks Dave, I've been following your thread and your layouts are killer! I'm scribbling out a schematic as we speak

I think I gotta go 4 gain control. I think I can live w/o the PPIMV but if I can't it always goes on the back for me anyways. That second stage is where I'm stuck. I was just playing a 50w I have w/ a pot there, and at noon its just amazing rock tone(PPIMV max, just simple master at noon), and cranked its just nuts, which I dig, so I gotta have that variability. I imagine that'll make the treble peaker go a little wacko at differnt settings if its on half the pot, but then I could use Miles' idea of the peakers on a switch...which means...another hole...argghh.

Here's what I'm planning:

2x6V6
Edcor 300-0-300(loaded) PT
Deluxe OT 6.6K
5H 50ma choke

I'm sheepish about my PT. Should I go higher? I figure I need about 430-440 plates?

[Reeltarded]

You can drop it on down in the sub 400 range if you want. Makes for otherwise impossible 6v6 swaps. I like using a lower B+ on the 50s (420v) but I use JTM45 style dropping string with all stages on that one node.

6k6 OT haha teeny tubes

OMG dave used the logan's run font

[Smokebreak]

Wow I just realized that in the Jose scheme stage 1 and 2 both have pot AND dividers. Seems most others' added stage(when it comes 2nd) either has one or the other.

[Reeltarded]

Yeah. I don't have one happy instance of dumping a stage right on the next grid resistor. Even dividing off the pot with something, but never nothing. Fuzzomania.

The last thing i want is a honkin futzface.

[Smokebreak]

Yeah. I don't have one happy instance of dumping a stage right on the next grid resistor. Even dividing off the pot with something, but never nothing. Fuzzomania.

The last thing i want is a honkin futzface.

Hmm . I'm still a little unclear here. So either divider, or pot and divider is good, but pot to grid is bad? Excuse my caveman speak.

[Reeltarded]

I am a cookie cutter. Every stage gets dividers. I normally split 50:50 to the next grid. I have never found a reason to do otherwise as far as values, using the coupling and cathode selection for the shaping there.

The gain control comes from the junction of a splitter, and then if i am hitting it with lots of stages that pot might actually get anything from 180k-470k to gnd as well.

Depends! I always split. I have never had it sound good to dump a raw stage on the next. I am not that tweaky awesome.

I spent a week on the phone with Colassal once where we both built the same amp with a different power section but there was something missing in the schematic and that was a divider. The amps sounded outrageous until the gain hit 7 and then sounded like Barney Frank reciting the constitution from memory as fast as he could. Fthhhhhhhhh 8 actually sounded hyperawesomistic but anything higher was shite. I wish i could make it sound like it did on 8 but everything i did to make the values all work out the same failed. Blocking without splitting.

Split stage into raw stage into 470k/470p into 1M into 68k grid on CF driving stage.

OMG that shit was great. If anyone has an idea I'm all ears.

[dave g]

3 gain stage + CF preamps are tricky to get right. There are a few basic reasons why.

The 3rd gain stage + CF pair clips in a really unique, great sounding way. See here: http://www.valvewizard.co.uk/dccf.html for more info. In the standard arrangement, it will soft clip the negative side of the waveform seen on the grid of the 3rd stage.

Often, the 2nd stage (which may or may not be switchable, like it is shown in the emplexador schematic) is cold biased. This means that it will clip on cutoff rather than grid conduction - clipping the negative half of the waveform seen on its grid.

The problem here is that in addition to clipping one side of the waveform, the 2nd stage also inverts the signal. This means that the 2nd stage and 3rd stage + CF clip opposite sides of the waveform. Too-symmetrical clipping cancels out even order harmonics.

My favorite arrangement biases the switchable 2nd stage hot (around 730 ohms for the cathode resistor). This means that the 2nd stage will clip via grid conduction instead of cutoff (be sure to add a 100k-ish grid resistor to the 2nd stage), clipping the positive half of the waveform seen on its grid. After inversion, this already-clipped side of the waveform becomes the negative side of the waveform seen by the 3rd stage's grid, so the same side of the waveform actually gets clipped twice. In other words, MEGA even order harmonics. You will need a divider in between the 2nd and 3rd stages to do a significant amount of attenuation, which you should tune by ear.

Using a pre-PI MV and running the power section clean can get you a dumble-ish lead tone. Running the pre-PI MV high and then using a post-PI MV brings the PI clipping into the mix. The combination of the highly asymmetrically clipping preamp plus the symmetrically clipping PI gives you the super fat and crunchy cranked marshall sound.

By the way, this is exactly the same strategy used in the trainwreck express, except the express does it slightly differently. The 2nd stage in the express is really nothing more than a recovery stage to make up for the loss from the tone stack. The third stage is super cold biased, which gives the asymmetrical clipping. The power section is pretty much straight up 50 watt plexi.

See my post here which includes spectrum analysis plots justifying why an asymmetrically clipping preamp plus a symmetrically clipping power stage sounds awesome:

https://tubeamparchive.com/viewtopic.ph ... ht=#271577

[Reeltarded]

How does your mixer work out after that stage? I was messing with 680R there with a 2.7k or 1.8k before it with the first stage coupler simply turned around onto the second stage grid from the board.

toad's wild ride..

[stiltamp]

Who knows which switch type the first switch is?
Which diode type is applied in parallel to the 1 Megohm potentiometer?

[Reeltarded]

That is simple a pair of transistors with the emitters aimed at each other.

I think the switch is a simple DPST my eyes are worse than that file though.