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Been a long time lurker. Now I'm ready to pull the trigger on a plexi brownish design thanks to Unimind and all those that helped him being a tweaking mad man. Question it seems that most of the tones I liked from the brown mod seems to be in uncascade mode. but i was still thinking of leaving it in there but how much effect would there be by building a lower gain normal and then cascading it into the plexi brown (could really use a nice clean). In the start I'm think I will just start with the standard plexi build and order the extra parts required to mod ta a plexi brown.

sazafraz, welcome to the forum.
I too really like the straight Plexi tones (non-cascade) and I struggled to reduce the gain on that channel as I was getting blocking distortion(Fizz). I think I now have that under control and it sounds great. One of the biggest changes to tighten up the amp and give it more of the Plexi vibe is to add negative feedback and presence control. However that completely changes the circuit as both channels get mixed and sent through the tone stack.

I was never really happy with the cascaded tones I was getting so I have now made my amp switchable between the Plexi Lead and JMC 800 circuits. There is only one channel. In Plexi mode you get 2 gain stages before the tone stack (like the standard plexi channel). In JMC 800 mode you get 3 gain stages (cascaded). The nice thing is if you plug in to the low input when in JMC 800 mode it bypasses the first gain stage and you get a super clean tone.
That's what I did .

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Les

http://www.soundclick.com/unimind

I get to pick up my kit tomorrow !

Looking forward to meeting you and a local builder too!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hey Les

your original "brown plexi" clips for the most part were non cascaded, tmb channel, dimed with no negative feedback as I recall. You state here that the presence and negative feedback give "plexi vibe"

Are your latest clips with presence and negative feedback?

What are your thoughts with and without? I have got my Trinity amp sounding fantastic and it has this really rich, insane distortion that I am SO happy with that I can honestly say that I have now built an amp that I like better than any amp I have bought, at least in that regard. That said, amps that I own like my Marshall Plexi reissue, 2203, 2204, and even my BBQ Java build are a bit tighter on the bottom end. I like how my 18 watt distorts, and the amazing richness, but it does not have the tight bottom end that those amps do.

just curious an your take about presence/feedback..

OK so I think I had it backwards. True Plexi vibe would be a softer/squishy feel which would be less negative feedback. More negative feedback dampens the output and gives a tighter feel. You can read more detail here: http://www.aikenamps.com/NegativeFeedback.htm . I perfer the tighter feel you get with more NFB.
Yes. All my latest clips are with NFB on. I leave it on all the time.
It is really a subtle thing. I can hear a diference in the slight reduction of output gain when I flip the switch from off to on. But at moderate to high volume levels on my attenuator, I can feel the difference in the way the amp responds to my picking. It has a sharper, quicker attack that I think of as more touch sesitive (others may think of it a less touch sensitive, I don't know) but it is definitly a tighter feel. Without the NFB I think the amp feels more soft and dynamic in that I can go from cleaner to dirtier tones depending on how hard I pick.

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Les

http://www.soundclick.com/unimind

I have actually been thinking about adding a variable negative feedback control so that I can dial the amp from soft to stiff.

There is a video by Pete Thorn demoing his amp that has this type of variable negative feedback control. Check it out here. http://www.youtube.com/watch?v=QGErfbbJ ... age#t=200s

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Les

http://www.soundclick.com/unimind

I think your original brown plexi schem was switchable NFB like the some of the 18watt.com builds but you now mention a presence control - do you now have a presence pot? What resistor value are you using? 47k?

You might dig the "presonance" thread at ampgarage.com - interesting stuff from some of the wreck tweakers

Also, if you have not, perhaps try a choke - I have a 4h on one of my amps and like it and thought I might try even higher value to tighten it up even more. Originally it was part of my effort to try and quiet hum down on my amp but turns out the power in my condo is really noisy and I was trying to fix my amp instead of the noisy power. However after installing it I think that I like the subtle difference and have ordered a few different higher value chokes to try. MM sells at 10h for the 1974x amps so I thought I might try something like that just for kicks.

Always had the presence pot. Yes 47K.
I have thought about adding a choke and/or upping the value of the first cap can to 50/50uf or 100/100uf.

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Les

http://www.soundclick.com/unimind

Where are you placing the choke? If it's in the screen circuit, it will actually make the amp feel softer on the attack and looser. That's in fact one of the key reasons why a typical Hiwatt is so much tighter and stiffer than a Marshall of the same power level. Also why a Vox AC15 is softer and looser than a classic Marshall 18W. However, placed in line with the B+ line, a choke will make an amp somewhat stiffer.

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Great tones come in small glass jars!

Interesting as usual Zaphod...

I had thought the lower "h" value of the Marshalls made them looser than amps like SLO's or others with 10 h or higher chokes - not so much the placement of the choke

I placed the 4h across the first filter cap can in place of the resistor - as per the 18 watt mods in Tube Depot's kit instructions - seemed like an easy way to install though I was uncertain as to the best way to install and could not find any 18 watt specific choke install schems or layouts

My original goal with the choke was to try and temper hum, not so much to stiffen. I have since found that the AC in my condo is quite noisy and my amp was not that bad, though definitely noisier than some of my store bought amps. Adding the choke seemed to slightly improve the noise though I cannot say as much about tightness - I was so focused on hum and noise that I was not paying attention to the tightness as I was making changes but in the end I am happy with the improvement the choke seems to have introduced.

Any suggestions you might have would be greatly appreciated as usual

Hiwatts (and the Triwatt) are very stiff amps, probably more so than a SLO. And they have no chokes at all. The stiffness comes from the lack of choke and also the large filter caps. A SLO would be even stiffer if you took the choke out. One of our forum members here, in fact built a Hiwatt with a switchable choke, and reported on this difference with the choke in and out of circuit. What a choke does is fight against changes in current flow, by changing the voltage across the choke itself. So with rising current a magnetic field in the choke generates a voltage which counteracts that rise in current, and vice-versa for a falling current. So if a choke is feeding the screen supply it will tend to counteract fluctuations in screen current, thus making a power stage feel softer and more compressed, although the effect will be somewhat offset by a large value second filter cap. With a smaller choke and very large 2nd filter cap value, the amp will feel pretty stiff because the cap is dominating over the choke.


So it sounds like you installed the choke between where power is fed to the OT and to the screen grids. So the choke is basically regulating the screen supply. Anyway, what made Tube Depot any kind of authority on Marshall 18W amplifiers?? Personally, I would get rid of it, and go back to the stock 18W circuit.

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Great tones come in small glass jars!

I did not say they were, only that I found some simple instructions to install a choke on an 18 watt in the mods section for their build after searching in vain all over the net and posting for help at a few other forums.

As I said my goal was more to try and reduce hum and had read several places that adding a choke can help quiet down power supply noise in some cases so I thought I would see if it would help in my build. I have since found that my condo has noisy AC upstairs where I work on my amps but think the amp sounds good with the choke so have left it in

I have never had to use a choke to reduce hum. Mostly wiring. But if you like the sound, then it's all good.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Question if you from the way I'm reading the schematic if you plug in both the high and low inputs on one channel they both become low inputs correct. (like in just about and standard amp

Yeah Ima gettin' there just busy at work too

Finally I have my DIY 18 W Plexi ready. Sounds fine I think going through some mods for TBM channel to have some more gain for heavy riffing.

http://i1089.photobucket.com/albums/i34 ... G_0819.jpg

Works same as standard amp. High adds a resistor in parallel to mix so it lowers the input resistance. If you plug in both, you have taken that out of the mix, so yes, Low Input.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Black/Silver - Nice. Any inside shots??

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Ok Im all done for now works very well only made on major blunder cause I'm a tard sometimes. Coco I figured out the 470k resister was me not you and I apologize . But I guess also found out how to make a pre phase inverter master volume. Accidentally put a 470k resister at r15 instead of 470r.

Some photos (sorry for the shiat camera) and yes i don't always follow the rules


By sazafraz at 2012-03-13


By sazafraz at 2012-03-13


By sazafraz at 2012-03-13


By sazafraz at 2012-03-13

Hi

Inside picture of my DIY. I just printed the layout and schematics I found from this forum. It works and sounds fine.

http://i1089.photobucket.com/albums/i34 ... G_0895.jpg


I´d like to modify TBM ch for more crunch, suitable for heavy riffing , channel swithing by ABY pedal.