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What's the best way to cut the brightness on the Plexi V3 layout? I have first channel preamp cathode at 4.7uf, 820, the TMB channel .68uf 820. Other than changing the 4.7uf to 220uf, any other changes to cut the brightness?

I use a strat with single coil vintage p/u's, and a BBQ 30 watt Blue alnico well broken in.

Thanks, Al

I don’t have the version 3 schematic in front of me … however I can comment on the TMB channel from playing around with my plexi. The TMB channel can be very bright with a Strat, in particular the high E string can sound really ice picky and the low E can fart out a bit when pushed. At the same time if you plug in a typical Gibson humbucking arrangement it can sound a lot darker. So the tone stack will require quite different settings for both guitar types.

I will let others comment on changing component values to suite a Strat. However, before you get into changing components don’t be shy to try out some radical settings on the tone stack. I have found that by dropping the bass a bit and treble way down while maintaining mids I get a much better driven tone with a Strat. The notes are much better defined across the board when pushed. There is still room for improvement (the high E is still just a tad bright) but overall not a bad approach to begin with.

This probably isn't the answer you are after but it's worth giving it a try.

Sorry if this thread is old, but adding a 100pF or 120 pF cap across the plates of v3 (phase inverter) can reduce the brittleness of the amp.

Like our Plexi MkII

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

I've just finished a Plexi Mkii build last week and am finding the same thing - it's too bright to my tastes. To be fair, this is my first 18 watt build so perhaps it's just an 18 watt thing.

I've converted the Normal channel of the Mkii to a regular Plexi Normal channel this week and I do like that channel a lot better for clean sounds now but there's still a hint of harshness (hard to describe but it's just there) with both humbuckers and single coils that I haven't been able to completely dial out with the single Tone knob. The Mkii TMB channel I'm struggling with a little more - it's too bright (even with the Treble rolled all of the way off) and doesn't push enough bass. I've been playing around with changing out components on the TMB channel as suggested in the Mkii Build Manual but haven't found the combination to get this channel to warm up, for lack of a better description.

In any case, one thing I'm curious about is that my Mkii was built with Sozo caps which (as I understand it) have a break-in period and will mellow as they settle in. I only have 7 or so hours play time so far with the Mkii - should I expect the tone to warm as the caps break in?

Also (and apologies if I should be asking this in a new thread regarding Plexi Mkii specifically), I haven't been able to determine the effect of optional cap C20 in the Mkii schematic - would adding this cap help in my efforts?

I've also noted the recent thread regarding the resonance control - I think I'm going to try this.

The Sozo caps will mellow a bit. Try this : Increase the cap across the PI plates and imcfease the power tubes cathode bypass cap to 200if or so.

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

Adding C20 (330pF) is not going to help you. It will pass more high frequencies, bypassing R42. I tried it and ended up removing it because it made the amp way too bright when the gain wasn't cranked up. It adds an interesting flavor the the dirt when the gain is cranked, but didn't work for me with clean tones.

For me, the trick with EQing the 18W amp was balancing the cathode bypass caps on V1 and coupling caps from V1 to V2 with the distortion tone. Caps that are too large in those spots will muddy up the distortion. But having a nice distortion leaves the amp with a bland clean tone. I really like the stock values in the Plexi Mk II design for distortion tone (sans C20). For me, the answer was the NFB loop, with resonance and presence controls. Those controls add highs and lows back in the power amp section without disturbing the nice distortion tone of the gain stages. In order to implement this, however, I had to move the output of the normal channel to the ground side of R8 (removing the ground connection), to free up the second input of the PI. This leaves you with stacked tone controls, and since I'm not a fan of the normal channel single tone control, I just removed it. As Stephen pointed out in another thread, someone else posted a schematic with NFB, and "jumped" channels here: http://www.unimind.us/images/Trinity18_ ... ematic.jpg

Thanks for the clarification regarding C20.

Going with Coco's suggestion, I've upped the value of C21 from 100pf to 250pf which has really helped in taming the overall brightness to my ears - I think I'll leave it at 250pf for the time being. I have the V6 option and have been running it on the 6V6's so the power tube bypass cathode cap was already 220uf - I will be upping the EL84 side to something similar though.

While I was into it, I changed C3 from 250pf to 500pf and C4 from .01uf to .022uf. Basically, I've started heading in the direction of changing the Mkii TMB channel from Mkii specs to regular Plexi specs with the idea that I'll start tweaking for more gain from the TMB channel from there, taking cues from the Trinity "TMB" TMB channel schematic. To that end, I've put R8 on a switch to take it in and out of the circuit - with it out of the circuit, there's a little more sizzle (really only just a slight bump in gain with it out of the circuit).

One thing I've noticed with these changes is that the sweep of the Treble control doesn't seem to affect as much of a change in overall tone.

I have implemented the cascade mod on a DPDT switch and although it's nice to have as an option, I like the TMB channel on it's own with the changes referenced above. I have previously installed an NFB resonance control in another 20w amp and it went a long way to warming up that amp which was also overly bright (it is only a single channel amp, so not as involved as having to work with 2 channels).

Glad you are finding a sweet spot. One of the noce things about the design is easy mods.
The treble should not be affected but maybe the brightness is not accentuating the treble control any longer and it is harder to hear the change.

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