trinityamps.com

Hi guys... just got my cab in the mail thursday and finally got my sIII completed.

Couple questions...

Which if any of the mods listed on the schem will increase gain in the pre-amp?

Is there documention on how to mod for a post PI master? How much will that change the character of the amp? I completely love the way it sounds right now but I think my neighbors don't care for it much.

The only real successful way of reducing the volume without changing the character of the amp too much is attenuation. Even that is an arguable point as of course your guitar reacts to the sounds coming from the amp at high volume and speakers also change as the volume increases.

Good to see it completed. It looks great! Nice pics in the ferns! [Can I use them?]

Do you want to change the gain/crunch on the TMB channel?

If so, try this:

In parallel with the 2.7K cathode resistor on V2, add another 2.7K resistor from the cathode to ground and bypass it with a 1uF cap. Add a switch to switch them in-out of the circuit. You can go as low as 1.2K on the extra resistor. The smaller the cap value, the more the distortion will be at a higher frequency.

You can try these separately if you like. Cap for gain; resistor for crunch.

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

Dr. Phil @ 18watt.com says" You can reduce the value of the 2nd preamp stage's cathode resistor to around 600 ohms, or even as low as 560 ohms, if you really want to increase the crunch. Maybe put two different cathode values on a switch for cleaner or dirtier preamp tone."

Decreasing the value of the resistor increases the "crunch" and bypassing the resistor with a cap increases the gain. I have never gone below 820 ohms.


You can try using different 12Axx, or 5751 tubes with less gain to attenuate but that will change the sound of the amp.

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

You can try the post PI MV from the 15 and see how that works for you. Schematic is posted in the forum - Trinity 15 docs

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

That's really nice!

Thanks James.

And you can definitely use the pics Stephen.

As for the attenuation, I've tried a couple and I didn't like them all that much. I've had better luck with master volume circuits and my old Top Hat AC-30 type amp has a post PI master that works really well IMO. That's why I though maybe I'd give that a shot over attenuation. Of course, trying more attenuators would probably not be a bad idea first but would cost a bit more as well. The BBQ Mass does look interesting...

Have you tried adapting an attenuator from a Rat Shack wall volume control? There are a few people (including Dr Phil) that believe that is still the most transparent and good value way.
http://18watt.com/modules.php?name=Forums&file=viewtopic&t=7588&start=0
Nigel

I built one of those actually but it causes all sorts of squealing in the amp if I turn the volume/mv on the amp more then about 2/3 up. It doesn't sound great to me either. It gets a rather fizzly sound and seems to add some of its own distortion. On top of that I read over at 18watt that they're specifically 8 ohms and that the impedance mis-match isn't good for the amp (my two 8ohm speakers are wired to 16ohms).

The MASS and THD attenuators work very well. The Rat-Shack attenuator is also supposed to be good (haven't tried it myself). But most attenuators are only transparent at -3 to -9 DB settings. After that the sound will change due to a lot of factors, like speaker response.

The problem with the PPIMV is that it also effects power amp drive, so if the amp gets its distortion in the power amp, the sound can change too much. They are very effective in amps that get their distortion in the preamp and PI.

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Richard Johnson
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Playing an insturment doesn't make you a musician, listening does

You can power scale it using a mosfet. drop the power to the whole amp with a pot. I have done this mod to my sIII. It does work well, but I have to say it does change the character of the amp a tiny bit. Its quite stable and not a big variation in tone, but it definately is detectable.

Bob

Bob, can you share this circuit and any build pics that use it? I have been wanting to try some power scaling scheme but didn't want to invest in one that changed the tone too much?

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

The amp looks great in white. I like the black piping. I would also like to see more on power scaling.

Hi Stephen,

I'm out the door on my way to Toronto for a few days again. I didn't see your post. I can get you pics of my sIII with powerscale in it, allthough I disconnected it to loan it to a studio that is doing some Bruno Pelltier stuff. sIII is going to be on Bruno's next release.... cool huh? Anyway I gots to go... Stephen, maybe I'll send you an e-mail, I can drop off the schematic and I would really love to hear that 6v6. I'll be staying at the stay Inn behind the east mall. So its not that far.

Bob

Bumping this super old thread since I'm back onto the master volume mods. I've tried the attenuation route and I don't really care for the tone I get so I'd like to try a PPI MV mod again. It was suggested above to try the PPI MV in the 15 but I've seen the following mod in the 18watt.com forum docs as well.



Is either one preferable and why? The 15 uses a single 250kA but the other a 1MA ganged pot which I assume is the same as a dual like this? It also seems to me that neither of these would be truly bypassed full up unless one used a switch or physically modified the pot to be shorted in the all up position. Taking apart and shorting a ganged pot sounds like a tough proposition. I plopped the latter mod into to the sIII schem below but just wanted to double check I drew it up correctly.

Try the TC-15 PPI MV. I think it's a better design than the dual. Simpler & easier to bypass with a switched pot or toggle.
From my experience wit the 15, it works quite well.


Why not use the pre-PI MV as in the original sIII design.

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

If you mean the pre-PI MV in the amp now (it's a un-modded sIII) it seems it's wonderful for controlling the pre vs. power amp distortion once you get it cooking but it doesn't work as well at being able to turn the amp "down" a bit while keeping some of the gain. I figure worse case scenario I can just put it back. It's not that I don't like the amp how it is, quite the contrary, I just find I can't enjoy it very often due to the volume so I'm looking for some different things to try. I'm not a huge fan of attenuators after trying a couple and this mod is a lot cheaper then trying a couple more at this point.

Ok. Suggest you try the same on as TC15 and make it switchable as well.

Did you ever try the power 'dampening' mod that was discussed here. That reeduced PI drive.

viewtopic.php?t=827&highlight=attenuator

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

I had a chance to install the PPIMV from the 15 and it's exactly what I was looking for. I couldn't be much happier with it. It really sounds good and I can't hear a lick of difference in the tone with it turned way up vs. out of the circuit entirely. I'm really not sure why they get such a bad rap. Anyway, the only negative I can find is that the 1MA pot starts cutting in/out at the most extreme (turned down) settings due to trying to adjust in a minute range of the pots sweep. I'm wondering if I shouldn't try a reverse audio instead... just not sure I've seen a 1M reverse available anywhere.

Excellent!

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