trinityamps.com

and I just noticed the error in the layout & updated it. thanks.

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

6V6 18W modification to increase drive:

Done to the PI in order to drive 6V6 tubes harder. Basically, the stock 18W PI has a reduction in both gain and drive level, compared to a regular Marshall or Fender LTP PI, to compensate for the high gain of EL84s relative to octal power tubes. You need to regain this for 6V6 tubes, by reducing the PI tail resistor to around 20k (from 56k) and the PI cathode resistor down to 470 ohms (like in the Plexi). These two small changes can get the 6V6s "singing" nicely.

You can use two 56K // 56K in parallel to lower the tail resistor and a 470 + 330 in series (820 // 820 in parallel) to get close to the cathode resistor. One DPDT switch can do it.

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

Correction. Totally spaced on my speaker above.

I liked the idea of a Greenback, but had totally forgotten when I made the last post that the speaker I had out in the garage was a brand new Eminence Red Coat (Private Jack, IIRC). Anyway, it sounds good.

Based on another current thread, I suppose I'm about ready to pursue some chances to my sIII v6
1) Want to look into adding a VRM. I checked the VRM thread on the Resource page, but I'm a bit lost as to what actually needs to be done to add a VRM to my amp.
2) Looking into heating up the 6V6 setting to be a little gainier and more in line with the EL84 setting.

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Do you plan to use the 6V6 exclusively, or switch between them ?

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

Definitely switch between them (obviously not on the fly).

I like the EL84 the way it is, so I don't want to mess that up. But if there's a way of successfully "protecting" the EL84 while heating up the 6V6, I'm interested.

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Check the recent update in this post: viewtopic.php?f=12&t=763&p=25572

Let me know how it works for you if you try it.

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

Thanks. Need to study that and figure out how that schematic translates to my layout. I probably won't be attempting this change until I figure out the VRM thing too. That way, I'll open her up once and kill two birds.

Changing R12 and R15 seem pretty straightforward, but some of the rest looks like I need to do some figuring.

Now, R18 (470k) C9 (.01uF), R30 (8K2) and R33 (1K5) are really conveniently wired together given the layout and the current solder lugs. Not sure how I'd wire it with a new R18 and R18b wired differently, especially with R18 in a new position. Looks like a mess!

Also a bit unsure about the best way of introducing R33.

Yikes!

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1) Anybody have a layout or diagram on how best to do the above?


2) From futher up, regarding the VRM, how does one get started on that? The VRM thread in the Resource page seems to talk about various different methodologies of installing it. Are there actual instructions somewhere?

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Some of this is covered in the VRM builders guide, but there are many ways to install a VRM.
1. Replace Power Switch with switched Pot;
2. Replace Stand-by switch with switched Pot; or
3. Install seperate control on rear panel without need for switch on switched Pot.

All VRMs include a PCB to solder the control Pot to.

1. You can mount the MOSFET to the PCB and then drill a hole in the chassis to bolt the MOSFET down; or
2. You can mount the MOSFET to the chassis with a hole in the chassis and run leads back to the board.

Once Mounted, you need to choose how to control the amp.
Implementation #1. All tubes; or
Implementation #2. Power tubes only

We provide parts to scale the whole amp [Implementation #1]
We only provide an 18 watt schematic to scale the power amp [Implementation #2]
For #1 Builders need to:

1. remove the power lead from the stand-by switch to turret/eyelet board and connect it to VRM B+ OUT terminal;
2. add new wire from standby switch to VRM B+ IN terminal; and
3. connect the VRM PCB GND terminal to the POWER Ground lug on the chassis.

Since we chose Implementation #1, We also provide parts and layout to modify the amp inputs to Block DC on the input jacks at low B+ levels. This involves:
1. adding .1uF coupling caps on the grids of V1
2. reocating the 68K grid resistors; and
3. relocating the 1M ground resistor.


We dont provide full instructions for Implementation #2 as it is more complicated and not as easy to retrofit. But to scale just the power amp, builders essentially need to:
1. cut board jumpers to seperate B+ feeding a.) Power Amp filter caps and b.) PI / Preamp filter caps.
2. add isolating 1N4001 diodes ahead of the VRM from standby switch. One connects to the VRM B+ IN, the other feeds to the Preamp filter cap sections.
3. connect PI/Preamp filter cap sections to the stable B+ feed diode;
4. install VRM board as in #1; and
5. connect power amp filter caps / power lead to the VRM B+ OUT
NOTE: DC Blocking modifications not required for Implementation #2

Hope that Helps!

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

Thanks. Absolutely does clarify a few things, but I do have some further questions.




Great, all of this is pretty clear. From what I've seen online here so far, it seems these are simple choices easy to deal with one way or another.




Before getting into any detailed questions on the "hows" about either of these, I'm curious to know any input with regard to the "why". What are the pros and cons of either approach once the change is made? In other words, without any regard to which is easier or harder, in their operation, how is onw better or different than another? What do most people here do? What does Trinity do if someone orders a completed amp with a VRM already installed?




Not sure I understand this. I gathered from above that Implementation #2 was the easier of the two choices. It's just the power tubes vs. all the tubes. But here, you say it's more complicated. Am I missing something?

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Controlling B+ to the whole amp means a fairly steady drop in volume with the same "Tone". It does run the risk of DC on the input jacks, hence the need for the .1 uf coupling caps. This operates more like a traditional attenuator.

Controlling the power tubes only, reduces power, but typically requires you to adjust the drive / master to regain the same "tone".



#2 is more difficult to retrofit due to having to break the connections to the pre-amp and add additional parts. Probably if you started and designed this way from the get-go, it would be a wash. Not having to add the blocking caps etc. would be a bonus though.

We currently control B+ to the entire amp and include the DC blocking parts.

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

It actually makes the preamp lose some of its crispness and articulation, making the amp sound "browner". Depending on one's point of view that's not necessarily be a bad thing and may be considered an additional bonus.

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Great tones come in small glass jars!

True, no complaints so far.

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

RE; controlling only the o/p tubes

Here's Here's a thread with a schematic so you get the idea.\
viewtopic.php?f=7&t=3140&p=24974#p24974
Here they are controlling the o/p tubes and the PI but you can see it's a matter of where you take the plate feeds from.

Stew

Based on all of the above, it looks like Implementation #1 is for me. In all honesty, the notion of it being an easier mod is very appealing. And any concerns over changes in tone with the power scaled down aren't really a big deal to me. The entire point is to be able to get dirtier tones at lower volumes. If I want the pristine tone of the amp at full power, I'll just play it at full power.

The only actual concern I have is highlighted in the red text of Coco's post. Schematic for #2 but not #1 sounds slightly problematic. But since it's a simpler mod and there are resources here, I'm not too concerned.

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Kind of sounds like an advantage to me.

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It's all a matter of what you want. Some people want the sound to get browner as they turn the VRM down, others want to keep the tone pretty much the same (whether overdriven or clean), but just have less volume. So I'm not going to take sides here. FYI a lot of investigation and discussion has gone into this topic on 18watt.com. Generally option #2 got dropped in favour of varying both the PI+preamp together - let's call that option #3. Just varying the PA on its own, requires an additional post-PI MV control to be added in as well, which really does make things unnecessarily complicated. So IMHO option #1 and #3 make the most sense and are easiest to do.

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Great tones come in small glass jars!

To be 100% honest, the easy bit is most attractive to me.

I see the sIIIv6 as a great amp already without any VRM. I like how it sounds as is. Being able to basically add a new feature to it while still preserving it's original character (with the VRM "off") is attractive. Thinking that I'd only really be wanting to do this when I'm trying to get dirtier sounds at lower volume, a little browning it up isn't terrible.

And even if the tonal change with the VRM dialled way down was very drastic to the point it sounded like a different amp, fine. So be it. It's like getting almost another free amp.

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With the Tramp we in fact deliberately use the VRM to change the tone of the amp and make it browner.

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Great tones come in small glass jars!

Interesting! I know this thread is on the 18 watt board and it was about my 18 watt amp, but I can't help it!

I'm beginning to look at the next project. Trying to decide between two totally different amps: Tramp and Triwatt. I love the idea of being able to dial in distortion tones at reasonable volumes on top of a good base clean tone. Both seem to have that ability, but in completely different ways.

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