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 Post subject: Tweed Royal Mods
PostPosted: Sat Dec 08, 2012 10:09 pm 
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Thought it was about time to get a "Tweed Royal mods" thread going, for others like myself who can't leave well enough alone! :twisted:

==================

V1 Cathode Resistor and Bypass Capacitor Mods

BACKGROUND:

It seemed to me that a good place to experiment with the TR circuit was the cathodes of the V1 preamp tube. The two cathodes are tied together and then connected to ground through the parallel combination of a resistor and a bypass capacitor. To save some typing (yeah, I'm lazy), I'll refer to these as CR (the cathode resistor) and BC (the bypass capacitor). Both components affect the gain of this first amplification stage (one half of V1 amplifies the incoming Bright channel signal, and the other half amplifies the Normal channel), though in quite different ways. Other things being equal, the value of CR (shown as R7 on the TR schematic) directly affects the V1 gain--lower resistance gives more, higher resistance gives less. The Tweed Royal instructions recommend using a 2.2K ohm resistor at R7, to moderate the gain and give a bit more headroom to the amp. The original Champ circuit used a 1.5K resistor on one triode of a 12AX7 for it's single channel, while the Deluxe dropped this to 820R, but serving two channels with both sides of a 12AY7 tube. Though a lower gain tube (nominally 45% of the gain of a 12AX7), the Deluxe made up for this by using no negative feedback to increase the gain in its second preamp gain stage.

Rather than attempt to explain the function of the BC myself, I'll just quote directly from Ampbook's Cathode Bypass Capacitor Calculator webpage:

"The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as "cathode degeneration." This substantially increases gain. When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but as an open circuit for DC, thereby maintaining DC grid bias. We can introduce treble boost by using a lower capacitor value, one that acts as a short circuit for high frequencies but allows negative feedback to attenuate bass. This is often done for the preamp's bright channel. If the additional gain is unwanted, based on the amplifier's overall gain from the input jack to the power amp, the capacitor can be eliminated entirely."

The size of the BC determines the frequency where the gain increase effect rolls off. As it happens, the 25uf cap used in the TR is large enough to bypass essentially ALL of the useful frequency range of a guitar, so it's what is referred to as a "total bypass cap". For the TR stock circuit, this cap adds nearly 8 dB of gain across the guitar's frequency spectrum, a very substantial effect.

Besides the effect of these two components, there's one more good reason for modifying the V1 cathodes--it's tough to get in any trouble! There's less than 2 VDC on these cathodes in normal operation, so you really don't need to worry about voltage/wattage ratings on the components. And if you happen to make a bad solder joint, nothing's going to smoke or blow up. My kinda mod!

MODIFICATION PLAN

For the V1 cathode modifications to my Tweed Royal, I decided on the following:

  1. Installation of an on-off-on DPDT mini-toggle switch across the stock 2.2K CR. Across one end-pair of lugs on the switch is soldered a 4.7K 1/4W resistor; across the other end-pair are 2.2K and 3.3K 1/4W resistors in parallel. If you do the parallel resistor math, that works out to three selectable resistance values for the toggle switch: 2.2K, 1.5K and 825R. So you've got the recommended value for the CR per the TR instructions, plus the values used in the vintage 5F1 Champ and 5E3 Deluxe circuits.
  2. Installation of a second on-off-on DPDT mini-toggle switch across the CR, this one with a 22uf radial electrolytic cap across one side and a 1uf electrolytic across the other. The 22uf cap is essentially equal to the 25uf cap that comes with the kit (which I clipped out of the circuit for this mod), and the middle "off" position takes the bypass cap out of the circuit completely. I chose a 1uf cap for the third value since it shifts the gain roll-off midpoint frequency up around 100 - 200 Hz; the bass and lower mids remain unboosted while the upper mids and highs get the BC gain boost. In effect, this serves to boost the highs relative to the lows, and helps get rid of the flab in the bass that the circuit can tend to have, especially with humbucker guitars. Choosing a smaller cap value here would shift the roll-off frequency higher, resulting in relatively less bass, while a larger value would leave more bass within the boosted range.

Physically, the installation is quite simple. I mounted the two toggle switches through the front panel below the input jacks and first volume pot. From one of the middle lugs of each switch, I ran a wire over to the tube side of the CR and soldered it to the resistor lead. From the other middle lug, I ran a wire up to the sleeve lug of the bottom Normal channel input jack (i.e. ground). For the capacitor toggle switch, just be sure that the caps are mounted on the switch in the same polarity orientation, and that middle lug for the negative side is the one connecting to ground. For the resistor toggle, it doesn't matter which switch lug goes to which connection point. See photos below for a look at the physical installation.

OPERATIONAL IMPACT

Once installed, you have your pick of 9 different combinations between the two 3-position switches. However, they are not all useful. If you plug the various values into the Ampbooks Cathode Bypass Calculator linked above, you'll quickly see that when the cap switch is on the 22uf "total bypass" position, there is no significant difference between the three positions of the resistor toggle. That big bypass cap just swamps the resistor's effect, and the maximum V1 gain is the same in all three cases, about 35.5 dB for a 12AX7 across the guitar's entire frequency range.

With the cap switch in the middle "no bypass" position, the pure effect of the three resistor values becomes clear. Overall gain is reduced--for the 2.2K, 1.5K and 825R resistance setting, max. V1 gains are 27.7, 29.5 and 31.7 dB, respectively. So if you were looking for maximum clean headroom, selecting no bypass cap and the 2.2K CR value would be a logical way to go.

Choosing the 1uf bypass cap gives an intermediate result. In the frequency range where the bypass cap dominates, you end up with the same max. gain as for the 22uf cap setting, about 35.5 dB. At the low end, you see the same gain behavior that the no-bypass setting gave, so the lower the resistor value, the greater the gain spread between the two frequency extremes. Also, the midpoint of the gain transition range moves with the CR value, shifting from about 120 Hz at 2.2K up to about 250 Hz at 820R. So the CR toggle settings with this BC value give a way to take a bit more or less bass off the signal.

A GIF image file showing the gain vs. frequency curves (courtesy of the Ampbooks calculator) for all nine combinations of these two switch settings is posted at http://duhvoodooman.com/miscimages/musical/TR/12AX7_cap-resistor_combos.gif.

...AND ONE OTHER "MOD"

In conjunction with the various CR and BC switch setting combos, you can also change something else, namely the V1 tube itself. Give the original 5E3 V1 tube type, the 12AY7, a try. Another good intermediate choice would be the 5751, whose nominal gain factor (70) falls right about in the middle between the 12AX7 (100) and the 12AY7 (45). These would be especially good choices if you find the overall gain of the Tweed Royal to be a little higher than you'd like. Analogous 9-curve gain vs. frequency GIF files for the 12AY7 and 5751 are posted at these links:

http://duhvoodooman.com/miscimages/musical/TR/12Ay7_cap-resistor_combos.gif

http://duhvoodooman.com/miscimages/musical/TR/5751_cap-resistor_combos.gif

Mod installation photos:

Image

Image

Image

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Dec 08, 2012 11:20 pm 
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That is some nice work voodoo. You can also individualise the catodes by giving each stage it's own value of resistor and cap and make them switchable too. I have a 5e3 at home and I left the "normal" channel stock (820r and 22uf) and made the "bright" channel "brighter" by upping the capacitance with a 1uf and the coupling cap with a 22n cap. The problem with giving each triode stage it's own RC is that you'd need to mod the layout.
I also used just one input and paraleled them together - but I probably shouldn't go into that without pictures. :shock:


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 Post subject: Re: Tweed Royal Mods
PostPosted: Tue Feb 19, 2013 1:11 pm 
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Mini-Watt Voltage Regulation Module Installation

Voltage regulation modules (VRM's) offer an attractive way to reduce the output volume of an amplifier while maintaining its tone. These units scale back the high DC voltage in an amp and can be applied to the entire DC voltage chain or just the power stage, depending upon how they are installed. Though VRM's are available for higher powered, fixed-bias amps, they are particularly well suited to cathode-biased amps, where the installation is much simpler. Once installed, a VRM will allow the user to get a "cranked" amplifier tone at a much lower output volume level, and are much less prone to "tone-sucking" than attenuators, which are often used for this same purpose. They're also much more convenient to use, since the VRM is installed right inside the amp chassis--no extra piece of gear & cables to hassle with.

Though several VRM's could be used with the Tweed Royal, this modification employs the Mini-Watt from SkipzCircuits.com for $35, including US shipping. This unit offers the additional feature of an integrated SPST switch in its power control pot so that the unit can be installed in place of the TR's power toggle switch, thus avoiding the need to drill another hole in the TR chassis for it. Also very popular is the VVR from Hall Amplification, although it's not available with an integrated on-off power switch. Trinity Amps also makes a good VRM (I installed one in their excellent 18W clone amp kit), but I can't find a link on their website, so you'll have to contact them at stephen@trinityamps.com and inquire how to purchase.

The modification shown below was done to scale the entire amp (i.e. both the preamp and power stages) based upon reports that this method seemed to work best for a 5E3 type circuit. However, the Mini-Watt can be installed for just power stage scaling with just a couple of additional changes, and the parts to do this are included in the kit. The Mini-Watt kit comes with a number of documents covering the technical specs and installation directions. The include two "marked up" amp schematics showing how the main module and supporting components are installed in the amp circuits--one for a 5E3 Deluxe circuit and one for a Marshall 18W clone. You might think that you could just follow the 5E3 schematic for the Tweed Royal....but you would be wrong. This is because of the TR's single-ended and push-pull operational modes selected by the 3-position rotary switch, with the standby position in the middle. This switch configuration makes it impractical to install the module on the output side of the standby switch, since you'd end up with the VRM serving only one output mode or the other. So the installation has to be altered to put the VRM on the input side of the standby switch, between the rectifier output and the rotary. I checked out this modified installation with Skip Menicos, the honcho at SkipzCircuits, and the following installation scheme is based upon his recommendation. I've included a marked up version of the Tweed Royal schematic showing where all of the connections are made, along with a couple of photos of the finished installation.

The basic installation sequence I used is as follows. Keep in mind that I did this on a previously built TR--the sequence would be a bit different (and the overall installation easier) if this modification is incorporated into the initial build. BTW, if you're doing the Mini-Watt installation on a finished Tweed Royal, I'd strongly suggest that you disconnect the jewel light, remove it, and tuck the wires out of the way, to give more room to install the Mini-Watt. Re-install it at the end:

  1. Disconnect & remove the on-off power toggle switch and mount the Mini-Watt module in its place. Don't re-connect the AC leads yet.
  2. Disconnect the output from the rectifier at the rotary switch, and connect it to the "B+" eyelet on the M-W.
  3. Also connect the + side of the 20uf electrolytic cap provided with the M-W to the B+ eyelet and connect the - side to ground.
  4. Drill a hole in the chassis between the PT and the first OT and mount the MOSFET and ceramic insulating pad.
  5. Run wires from the G-D-S legs of the MOSFET to the corresponding eyelets on the M-W module.
  6. Connect the "A" eyelet on the M-W module to the rotary switch contact where the rectifier output was connected.
  7. Connect the "G" eyelet (next to the "A" eyelet) of the M-W module to ground.
  8. Connect the two diodes between the #3 pins of the 6V6 sockets and ground. The cathode (banded end) of the diode connects to pin 3. Morgan made the excellent suggestion of connecting the anode end of the diodes to the unused #1 pin on their respective 6V6 sockets. Then it's a simple matter of connecting the two #1 pins with a wire, and running another wire from one the #1 pins to ground. I used the ground buss on the negative side of the amp's filter caps for this ground connection.
  9. Connect the 330K bleeder resistor and the 0.047uf/630V film cap across the + and - legs of the TR's first filter cap (C10 on the schematic).
  10. Connect the AC leads to the "AC" eyelets on the M-W module. Doesn't matter which goes to which.

NOTE: The Mini-Watt kit comes with a couple of metal oxide resistors--a 22K (for a 5E3 circuit) and a 10K (for an 18W circuit)--one or the other of which is installed on the Mini-Watt PCB when only the power stage is being voltage scaled. In such cases, the 22K voltage dropping resistor between the 2nd and 3rd filter caps (C11 and C12) is removed, and a wire is run from the "PRE" eyelet of the Mini-Watt PCB to the + side of C12. Do NOT make these modifications when scaling the whole amp! Leave the "Rx" resistor spot empty on the Mini-Watt board and don't connect anything to the "PRE" eyelet.

Here are the marked up TR schematic and some labeled photos of the installation. Some of the detail is pretty fuzzy on the last photo--sorry, best I could do.

Image

Here's a close-up of the Mini-Watt module in the above schematic. A full-size version (2640 x 1540) of the schematic is available HERE.

Image

Photos from the Mini-Watt installation in my own Tweed Royal:

Image

Image

Image

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 Post subject: Re: Tweed Royal Mods
PostPosted: Wed Feb 20, 2013 6:31 am 
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Good grief Bob great info and explanation.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Wed Feb 20, 2013 7:03 pm 
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Great info and very well done- thanks!

sooooo more about how it sounds?

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 Post subject: Re: Tweed Royal Mods
PostPosted: Thu Feb 21, 2013 2:30 pm 
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oldlefty wrote:
....sooooo more about how it sounds?

The Mini-Watt works very well. Does a great job of letting you dial down the output with very little coloration of the sound. I have a Weber MicroMASS 15W speaker-motor type attenuator that also works very well, but both will begin to alter the tone if you crank them down far enough. I think this is just a reflection of the physical reality that at some degree of signal attenuation, tone coloration is inevitable. So if you want to kick out the jams without waking the baby in the next room, I'm not sure anything will let you do that without your tone suffering. But the Mini-Watt lets me crank up the Tweed Royal high enough to get that nice, juicy tweed overdrive and still keep the volume very reasonable (the definition of which is not getting yelled at by my wife & daughter while they're watching TV at the other end of the house with only one door in between).

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 Post subject: Re: Tweed Royal Mods
PostPosted: Thu Feb 21, 2013 5:55 pm 
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Well.....
Since we're on the subject or VVR/VRM's.

I got my Trinity Amps VRM today.

Here's the installation sheet I got with it - also they did supply the pot - it is not "user supplied":

Image

Seems pretty straightforward - the only thing the instructions don't mention is the wire connections to the back of the pot:

Image
Image

Right now my board just got populated - I havent even set it into the chassis yet - so I have plenty of time sort this out and make room to get it mounted.

Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 7:25 am 
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The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 8:31 am 
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ChrisM wrote:
The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.

^^ This.

The switch on the back of the pot has absolutely nothing to do with the voltage regulation function of the VRM. It's just an independent SPST switch that's been mechanically integrated with the pot. It allows you to use the existing hole in the chassis for the power toggle switch for mounting the VRM and so avoid having to drill another one.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 10:40 am 
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Does the VRM come with a switched pot or a standard, non-switched pot?

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 11:07 am 
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I believe Trinity sells their VRM both ways. I interpreted DZ's comment to mean that he had the version with the integrated switch, but maybe I misunderstood.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 6:06 pm 
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ChrisM wrote:
The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.



Yeah........one of the "duh" moments in the middle of the night....than I had a meeting all day and couldn't respond to my own post...derr....

Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Fri Feb 22, 2013 6:07 pm 
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duhvoodooman wrote:
I believe Trinity sells their VRM both ways. I interpreted DZ's comment to mean that he had the version with the integrated switch, but maybe I misunderstood.



Correct - I got the switched pot.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 8:36 am 
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ChrisM wrote:
The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.



Awesome - since you used the Trinity VRM - maybe you can save me some time and questions when it comes to wiring it in :lol: :lol:

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 10:52 am 
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danielzink wrote:
ChrisM wrote:
The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.



Awesome - since you used the Trinity VRM - maybe you can save me some time and questions when it comes to wiring it in :lol: :lol:

Sure :lol:
That Tweed Deluxe is one I built a couple years ago.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 11:07 am 
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ChrisM wrote:
danielzink wrote:
ChrisM wrote:
The connections on the back of the pot are the power switch connections. So just treat those two lugs like the two lugs of the power switch.

In the pics you attached I did exactly that. The VRM replaced the power switch on the chassis.



Awesome - since you used the Trinity VRM - maybe you can save me some time and questions when it comes to wiring it in :lol: :lol:

Sure :lol:
That Tweed Deluxe is one I built a couple years ago.



Dang. oops.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 2:34 pm 
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So.....with regards to the diagram....I have (3) connections to make. (1) to ground and then (1) from B+ on the TR to this board and then back out to where B+ would normally be. From the discussions prior - I'm assuming this is from the standby switch ?

Sorry for being so dim - but which is B+ ?

Thanks, Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 3:05 pm 
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B+ in is the wire coming from pin 8 of the rectifier (that is where your raw B+ voltage comes from). B+ out goes to the lug on the rotary switch where the wire from pin 8 of the rectifier used to connect to.

Like DVM mentioned earlier, you can't install the VRM after the standby switch because of the dual Modes of the TR.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 4:17 pm 
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Morgan wrote:
B+ in is the wire coming from pin 8 of the rectifier (that is where your raw B+ voltage comes from). B+ out goes to the lug on the rotary switch where the wire from pin 8 of the rectifier used to connect to.

Like DVM mentioned earlier, you can't install the VRM after the standby switch because of the dual Modes of the TR.
oh. I guess I missed that.

After re-reading his post I would imagine it's this section that outlines the problem....

Quote:
because of the TR's single-ended and push-pull operational modes selected by the 3-position rotary switch, with the standby position in the middle. This switch configuration makes it impractical to install the module on the output side of the standby switch, since you'd end up with the VRM serving only one output mode or the other. So the installation has to be altered to put the VRM on the input side of the standby switch, between the rectifier output and the rotary.
So....question is.....is there a way to integrate my VRM ?

Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 5:04 pm 
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danielzink wrote:
So....question is.....is there a way to integrate my VRM ?

Absolutely. You just need to follow the basic instuctions I posted above, ignoring the steps that pertain to the extra components that come with the Mini-Watt (the diodes, 20uf electrolytic cap, 0.047uf film cap and 330K bleeder resistor. You'll be making these three connections to the VRM board:

  • Pin 8 of the rectifier to the IN eyelet of the board.
  • OUT eyelet of the board to the pin of the rotary switch formerly connected to the rectifier.
  • Connect the GROUND eyelet to the chassis ground where the PT is grounded.

Obviously, you'll still need to mount the MOSFET on the chassis and connect it to the VRM per the Trinity instructions.

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 5:53 pm 
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duhvoodooman wrote:
danielzink wrote:
So....question is.....is there a way to integrate my VRM ?

Absolutely. You just need to follow the basic instuctions I posted above, ignoring the steps that pertain to the extra components that come with the Mini-Watt (the diodes, 20uf electrolytic cap, 0.047uf film cap and 330K bleeder resistor. You'll be making these three connections to the VRM board:

  • Pin 8 of the rectifier to the IN eyelet of the board.
  • OUT eyelet of the board to the pin of the rotary switch formerly connected to the rectifier.
  • Connect the GROUND eyelet to the chassis ground where the PT is grounded.

Obviously, you'll still need to mount the MOSFET on the chassis and connect it to the VRM per the Trinity instructions.
This is good news :D

But - I'm still slightly confused as to Morgan's comment:
Morgan wrote:
Like DVM mentioned earlier, you can't install the VRM after the standby switch because of the dual Modes of the TR.

As I'm typing - I think it's getting a little clearer.

I can't mount it like in a "normal" amp (that doesn't have dual modes) utilizing the normal B+....but I can still use it as my on and off switch....just making connections differently.

Thanks for all the help guys !!!
Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 7:06 pm 
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That's right Dan, sorry for any confusion. What we're saying is that because the TR has the dual modes, you have to install any voltage regulating gizmo on the hot side of standby mode instead of on the switched side. They usually go after the standby switch. None of the people DVM and I correponded with about this thought it would be a problem. So, just follow our instructions.

In my earlier post, I said that you can't install it after the standby switch; I didn't mean you can't install it at all. Which when I think about it, could be confusing if you don't know what I mean by after the standby switch. Sorry about that.

Make sense? :D

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 Post subject: Re: Tweed Royal Mods
PostPosted: Sat Feb 23, 2013 9:57 pm 
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Morgan wrote:
That's right Dan, sorry for any confusion. What we're saying is that because the TR has the dual modes, you have to install any voltage regulating gizmo on the hot side of standby mode instead of on the switched side. They usually go after the standby switch. None of the people DVM and I correponded with about this thought it would be a problem. So, just follow our instructions.

In my earlier post, I said that you can't install it after the standby switch; I didn't mean you can't install it at all. Which when I think about it, could be confusing if you don't know what I mean by after the standby switch. Sorry about that.

Make sense? :D
Clear as a bell.

Thanks again.

Dan

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 Post subject: Re: Tweed Royal Mods
PostPosted: Thu Feb 28, 2013 4:01 am 
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Great write up for the Mini-Watt, mine on it's way in the post :-)

I have one question about the schematic though, would it not be better to break into the AC between the fuse and the transformer, instead of between the socket and the fuse ?


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 Post subject: Re: Tweed Royal Mods
PostPosted: Thu Feb 28, 2013 7:04 am 
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You could certainly wire it that way. I just connected the Mini-Watt switch the same way the original TR power toggle switch was wired.

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