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 Post subject: VRM and 18W Questions
PostPosted: Fri Jul 23, 2021 5:27 pm 
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Joined: Mon Jul 19, 2021 7:36 pm
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Hey all.

Repaired a busted up JCM900 SL-X last year and did a couple of kit pedal builds. I guess I have the bug and want to try my hand at an amp kit. Very happy to support a Canadian company so I am glad I found Trinity amps. Have a couple of questions though.

What exactly does the VRM module do? It isn't clear to me from the description. Is it an attenuator? If so, I wouldn't need one as I have a Rock Crusher. Or is it more like a variac type of thing?

The FX loop. Says tube powered but also true bypass in the description. Is it tube buffered when on, but can be switched off and true bypass?

Having a hard time deciding between Custom Plexi and Plexi MkII. Not too many quality sound demos out there. Any opinions that people would like to share?

Thanks!


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PostPosted: Sat Jul 24, 2021 5:54 pm 
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I'll do my best to answer and I'm sure others will chime in too. A Trinity Amp kit is a really good way to get into amp building. They have great layouts, schematics, instructions, and support. Betting money is that you will end up with a successful build that sounds great.

The VRM is a Voltage Regulation Module. It is basically a pot to act as a voltage divider, a mosfet to sink the unused voltage and some r/c parts. It is like having a variac built into your amp, but a little safer since it can only turn voltage down and not up. You can set it up to scale the whole amp or just a section (like power tubes). The tonal effect it has it to both lower the loudness of the amp and the headroom (in a way). But at really low levels you lose power amp distortion so the headroom thing is a little non-linear.

I didn't build the effects the loop, but yes it has a switch to remove it from the circuit that makes it true bypass. Yes, I believe it is also "tube buffered" so to speak. The send stage is a cathode follower and the return is a normal amplification stage. I am not sure that effects loops play that well with a marshall style amp. But you can give it a try.

Custom Plexi and the MkII are not all that different. I went with the Custom Plexi for more of a classic rock style, MkII is more the van halen type tone. Build either and then tweak it to be whatever suits you most.

-Dan


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PostPosted: Mon Jul 26, 2021 1:59 pm 
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Joined: Sat Sep 25, 2010 3:04 pm
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Hi and welcome to the forum.
My first kit amp was a Trinity Tramp which was a great build with excellent support and documentation. And it sounds great. I've also built a Triwatt which turned out fantastic as well. You really can't go wrong with any of the kits.

As Dan mentioned, the VRM can scale the voltage down for the whole amp or just the power section. I dumps DC voltage from the B+ to a chassis-mounted mosfet to dissipate the heat. I've used them in a few builds with great success, going back to when Dana Hall used to sell his VVR modules. In particular, I used them on 2 18-watt style amps and it works great. You might have to do some tweaking to the input section (caps) to take care of scratchy pots when the VRM is turned way down, but its not much of an issue but inherent with the way it works (to my understanding). I think it preserves to tone and feel of the amp overall, but at the lower ranges it does seem to make a difference.

Regarding the Custom Plexi vs MkII....I can't speak directly to the comparison but I do have a Trinity Custom Plexi and it really nails that classic Plexi sound at a much more manageable volume. The Custom Plexi runs 2 6V6's, whereas the MkII runs EL84's, so that may account for some differences in tone as well as the pre-amp changes. The Custom Plexi can also run a SS rectifier which tightens up the feel similar to a typical Plexi, whereas the MKII will typically only run with a tube rectifier. You can also run the Custom Plexi with a tube rectifier, so that option is nice.

If you are considering an effects loop in a classic 18-watt style of amp, its my understanding you will need to select one of the 2 channels to insert the loop, leaving the other unaffected. The loop is inserted right before the PI, and because the 2 channels in an 18 watt amp hit the PI at different grids, vs. having them mixed prior to going into the PI (like a 1959 or 1987 Plexi does) you can't really have the loop on both channels. I'm sure there is a way to get around this in this style of amp (18 watt) but I am not sure how it is done.


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