Mesa Boogie Road King II Power Transformer Conversion from 120v to 240v How to do it

How to convert a 110 volt amplifier to 240 volt Mesa Boogie Road King and others with TAD tube amp doctor power transformer.

117v to 240v Conversion Review

Most of us have been there; you buy an amplifier that is operating on 117 volts but where you come from the mains voltage might be 100, 220, 230 or even 240 volts. In the case of Mesa Boogie amps (and many others I could name) the manufacturers deliberately make the power transformer in the amplifier with just ONE primary voltage input in the transformer. So if you do buy a 117 volt amplifier from the USA where they are much cheaper then you cannot plug it in to the mains socket in the UK, Europe, some of the Middle East and importantly Australia.

So what do you do? Typically guys buy one of those giant carbuncle's (you know the one) that weigh an absolute ton and proceed to lug them around with the amp and all the other kit that's required to play modern guitar. And those converters can be very expensive too. The one attached to the amp I had was a 2Kw unit, hardly transportable and absolutely hated by me; and I'm probably just like you in that respect.


Heres the amplifier we're converting to 240 volts operation

Click any image in this review for a larger view

So what IS the answer? Well, you could call up your friendly amp makers distributor, or even the maker directly and simply ask them to sell you the parts to convert the amp from 117 volts to 240 volts right? Actually not quite. You see, more often than not these manufacturers and distributors have a really nice and cozy relationship where they might collude to make that very hard if not impossible to do. Take one amplifier in question and I'll get you up to speed what happened.

The one I had was a USA V30 head (actually made by Crate - who were taken over by Loud Technologies. It was a 117 volt amp, so I opened the chassis to change the wires around (as you really should be able to); and there were none to change. The maker had deliberately put in a power transformer that was USA 117 volts only. So how did I fix that?

I contacted the 'spares distributor in the UK and ordered a replacement transformer. In the case of this little amp it was not that expensive, (but was expensive for what it cost to make no doubt) so I ordered it. At this point I'll ask you to guess how long I had to wait for the replacement? 4 weeks, 8 weeks? Hell no - 11 months!

So now you can see some of what manufacturers and their associates get up to. And the amplifier that I am going to convert, a Mesa Boogie Road King II (top of the line for Mesa) is no exception in my opinion.

Buying a 240 Volt Power Transformer for the Amplifier

If you look around the internet you can see lots of 'manufacturers' that make transformers. So that's exactly what I did. I carefully scanned Google and anywhere else I could, I looked in the 'forums' which had some makers on there, but not in the context that I needed them. So because Mercury Magnetics kept 'popping up' on Google I visited their website; and sure enough they made replacement power transformers. MM are based in the USA and chirp on about how much the sound is 'improved' by fitting their transformers, often because they use more 'metal' in them etc.; we have all heard about that sort of great marketing, but I for one don't actually believe in that hogwash. And for good reason.

Let's get this right; here is a third party transformer maker that says he makes 'better' transformers than the manufacturer, that sound 'better' than the manufacturers transformers and can cost more. I do not believe that ANY manufacturer will produce an amplifier with 'inferior' transformers in their amp; they want the amp to sound awesome or you will not buy it? right? of course they do. So in any case I checked the MM list of power transformers and was frustrated to learn that there was no power transformer listed in their catalogue online. I emailed them and was advised 'we don't make them for that amp' thanks. I can never understand why a USA 'premium' transformer manufacturer leaves out the top end amps from one of the worlds leading guitar amplifier manufacturers; can you? Unless of course there's one of those little unwritten laws I was talking about earlier. Make up your own mind.

So where did I get my Power Transformer from?

I buy tubes (or valves as they are also called) from Tube Amp Doctor. They are not the cheapest around but are probably the best as I have never had a single TAD tube fail. So I knew they sold replacement transformers and I went online to and found the transformers section of their web. And there it was! A road king II power transformer that supported 100, 117, 220, 230 and 240 volts on its input and was a direct replacement for the 561142 original Mesa transformer as fitted to my amp (USA domestic part number). The one TAD made was 561143 which is a direct export version that I needed. TAD also confirmed that the end caps of the old transformer I was removing would fit the new replacement; perfect.

Now I had the transformer I was ready to wield a soldering iron.... not.

Inside the Mesa Road King II and checking out how to convert it to 240v

So I unplugged everything and removed the chassis.

You MUST ground the capacitors so that there is no high voltages left inside that could kill you! While I am on this subject and before we proceed if you continue past this point I accept no liability whatsoever if you are injured or killed or other damage occurs. Failure to work on a Tube Amplifier and not knowing exactly what you are doing can be fatal.

If in doubt - ALWAYS get the professional to carry out work on your TUBE amplifier - you have been warned.

Moving on, I flipped over the amp on to the bench and took a look inside. There were lots of wires around the power supply area with no markings; I immediately thought that this might be a manufacturers 'ploy' to stop unauthorised work, but later discovered that where I wanted the information, it was printed on the underside of the board.


Chassis and Power Transformer click for larger view

For this conversion you will need a schematic diagram (no that's not someone scheming against you ;-) and it is a critical document for ALL conversions. They are readily available on the internet, but the schematic of the Road King II can be downloaded Schematic-HERE in pdf format. Examining the schematic shows the USA transformer and its wiring; and on a separate page the 'export' version which is really the one we want to work from.


Domestic Schematic (USA 117 volts) click for larger view


Export Schematic (100,117,220,230 and 240 volts)

So looking at the schematic for the EXPORT version I need a few other things too. I did change fitting two MOV's to one as you will see later its simpler; here's the list of other parts:

  • MOV 275 Volts X 1
  • 0.047 Capacitors X 2 as shown later
  • 2nF Capacitors X 2
  • 2A 240 Volt Slow Blow Fuse X 1
  • TAD Power Transformer 561143 (Mesa Part Number)

Examining the Road King II


Rectifier board top view before work click for larger view


Underside of rectifier board showing values of the connections

Taking a look at the top board shows the MOV fitted which was probably 120 volts or thereabouts;  Note also the yellow (two) wires which are 5 Volts and the red (two) wires which are typically 350 volts! These KILL - be very careful. These all come from the new transformer. Looking at the bottom picture shows the values for each wiring connector that we will connect the new wires to from the new power transformer. You can also see where the capacitors are fitted shown as .002 and as .047 on the board.

Other Wiring Details


The brown wiring on the main board is 12 volts consisting of two wires


Main board near the transformer bias and zero voltage wiring; red/blue is bias, green/yellow is 0 volts and red/yellow is 0 volts


Main board 3 volts wiring - green wires

The above pictures show all of the original wiring of the Road King II before the modifications.

Fitting the Power Transformer

Simple really. DE solder all the wires from the old power transformer. Now undo the four mounting bolts (one on each corner of the transformer and the transformer will easily be able to be removed from the chassis. At this point I had a cup of tea. Clean out the chassis to make sure that there is no debris of any kind in there so there are no problems later (remember those high volts as they like bits of wires floating around on a live chassis - NOT!)

Now you should have the old power transformer on the bench and the new one at its side. You will immediately notice that there are far more input wires (as shown on the box of the new power transformer from TAD. There also is a variance in colours between the two transformers; more of that later. The next task is to undo the bolts from the old transformer and fit them to the new one. This is where you probably will have your first problem; just as I did. Those bolts in the old PT will have nylon spacers on their shaft at the top and bottom of each bolt. The bolt is held in place with a nut. Undo the nut and gently tap out each bolt without damaging the nylon components. The ones I had were VERY tight and took some removing. I did not want to damage the original transformer (but I still can't think why) so I was really careful. In fact I had to eventually saw off the heads of two of those bolts to get the nylon spacers out which I needed. When I did that I just decided to use the bolts that came with the new PT from TAD. It all worked out great.


The replacement TAD transformer is on the left; note far more wires; click for larger view


The PT ready to fit with the original end caps fitted

As you can see from the last picture above the power transformer really looks exactly the same as the original except there are more wires for the different voltages. What's more is that once this is fitted to the amplifier you could change it to any of the voltages that the PT supports reasonably quickly with a reconfiguration of the rectifier board. Now the power transformer is in this condition refit it to the chassis ensuring that it is fitted in the same way that the original was and make sure that the ground is connected properly to the bottom left mounting pin. Tighten all nuts and bolts firmly and if you have no self locking nuts then coat the threads with paint or use Loctite.

Fitting the Capacitors and MOV


Change the wires on this side of the board

  1. Move the white wire (shown in 117v on the right to the 245 Volts connector (2nd point)
  2.  Leave the grey wire (0 Volts) where it is
  3. Move the black wire from T1 across to the left to T2 (2nd point)

Now it's time to remove the old MOV and to fit the capacitors as shown, but fit them all to the other side of the rectifier board as in the picture below. 


All components fitted; note larger caps for the .047 ones and the 270 Volt MOV; click for larger view

Connecting the Wires from the NEW Power Transformer

The above picture shows the power transformer input wires fitted to the top side of the rectifier board in the order that you need to fit them and the right colours of the new transformer wires; as you see I already fitted the components after removing the old MOV.

The next thing to do is to fit the two RED transformer output wires on the rectifier board (to the left of the brown shown in the above picture and in the same place as the original 350 Volt red ones. Get these right it really could be a life and death point. Next fit the two YELLOW transformer output wires to the left of the two red wires you just fitted. The yellow wires are 5 Volts and also fit on the rectifier board.

Fit the RED and YELLOW wires as shown below.


Fitting the 5 Volt and the 350 Volt HT wires to the Rectifier Board

I then fitted the two BROWN transformer output wires on the main board (near the middle of the board where you removed the two originals from as shown below.


Fitting the 12 Volt wiring back on to the main board

Then I fitted the two GREEN transformer output wires on the main board (middle of the board at the top) these are the 3 Volt wires and must be fitted to where you took the old ones off.


Fitting the 3 Volt wiring back on to the main board

Now we get to where we fit the three transformer output wires on the right of the main board which could be confusing. In fact I was scratching my head for a while; and although I did make the amp work originally I moved one wire to where it should be later.


Bias and 0 Volts wiring on the main board.

Examine the main board carefully in the above image. You will see that the circuit board (from left to right in the picture) says:


NOTE: these wires are a different colour on the replacement transformer and you should use the following transformer output wires in the order indicated below.

  1. GREEN/YELLOW (0 Volts)
  2. GREEN/YELLOW (0 Volts)
  3. BLUE (55 Volts bias voltage)

So fit the wires as above from left to right as in the above picture. I originally put one of those 0 Volt wires to chassis ground but that would be incorrect. Fit them exactly as described here. 

Lastly fit the replacement fuse to the mains fuse on the outside of the amplifier. The replacement for 240 Volts should be a 2A 240 Volt Slow Blow Fuse. Now all that is left is to change the power transformers part number on the end cap from 561142 to 561143 so there cannot be any confusion; and replace the mains fuse and voltage markings on the back of the amp top reflect 240 Volt operation with a 2 Amp slow blow fuse. 

Checking the Amplifier for Operation and Use

This is one scary time; and it can be much more scary if you do not have the right equipment or if you do not have a clue of what you are doing. Check and DOUBLE CHECK your wiring, components, soldering and anything else that you have been responsible for.

What you SHOULD have available to you to be safe.

I strongly suggest that if you really don't have any of this kit or do not know how to use it then seek help from a professional engineer as he will just charge you for the testing (much lower cost than the changing of a transformer).

  1. Speaker cab and cable plugged in to the correct amp connector on the back of the amplifier
  2. Volt meter that you know how to use that is capable of handling the voltages
  3. Either a Variac from 0 to 240 Volts plus operation between the mains and the amp voltage input so you can bring up the voltage slowly and look for problems - its usually used in conjunction with an Ammeter OR
  4. You can make a two way socket that incorporates a mains light bulb in series with the power. (check out the Weber books... he shows you how to make one. Basically, if the amp is operating good then the lamp will glow dim, but if the amp is drawing high current (usually a result of major trouble) then the lamp will glow very bright. It might give you enough time to turn off the mains if there is a problem.

If you don't have any of this equipment or do not know how to use it then seek help from a professional engineer as he will just charge you for the testing.

Testing Times

For me the amp was tested (off video) with the lamp method as I have successfully used that many times and it is low cost to make the inline lamp circuit. Set the amp up on channel 1 diode rectification with main and solo turned off no reverb etc. Now turn on the amp; is the light dim? mine was; and then I proceeded to measure voltages around the board connectors and the rectifier board to ensure nothing was way off. Everything seemed good so I flipped the rectification to tubes on 'bold' (which feeds the maximum voltage from the windings through the amp.


The Road King II under test following conversion from 117 Volts to 240 Volts; click for larger view

I checked the HT (350 Volts); the 12 Volts; the 5 Volts; the 3 Volts and other pertinent voltages and was happy that the power transformer was doing its job just fine. Lastly I flicked the standby switch to on and the amp continued to draw little current so I was happy that the installation was sound. Other tests showed that the amp switched channels OK, that the 'spongy' switch worked and so on. I located a guitar lead and plugged it in, turned up the master and solo a little and hey presto I had output!

Conclusions of the Tube Amp Doctor Power Transformer for Mesa Road King II and Overall View of the Upgrade of the Power Transformer

In fact there were a few issues with the Power Transformer from TAD; but in honesty frankly they were very minor. Problems included:

  1. the brown 12v wires were too short to reach the location on the PCB
  2. The green 3v wires were also too short to rech the proper location on the PCB
  3. The provided screws were NOT high tensile and would have been much better if they had bearing in mind that the original screws were, and that they are really hard to remove from the original power transformer.
  4. There were no nylon inserts provided for the screws which would have helped tremendously with the changing over of the transformer for the new one.

I did report some of this to TAD who advised me that they would be resolving the 'short' wire issue. To be fair to TAD I have not yet reported the other two points but I will be doing and they may well have some resolution for those points in due course. I think that this change over could be accomplished by a lesser skilled person if they don't plug the amp in after their work and get it checked by a pro engineer which overall would save money compared to a full blown pro engineer fitting the power transformer from scratch. REMEMBER: I do not condone this in any way and I recommend that you either get a pro to do the job or at the very least get an engineer to check your work and let him test the amp.

Overall Score for the Power Transformer

I have to give it to Tube Amp Doctor on this one; give credit where it is due. And remember the TAD engineer Stephan who helped tremendously with information so this project could safely be completed; he has got to be taken in to account because often from some companies the support is bad. Stephan was awesome!

So I rate the TAD Power Transformer at 9.5 out of 10. 

I have to say that Stephan added to this score and is a really great engineer always willing to help. He is a credit to TAD; and Tube Amp Doctor are to be commended on employing staff of his quality to assist customers with great technical support where necessary. While this review has focus on a Mesa Boogie Road King II the Power Transformer featured here is suitable for some of the other Mesa Boogie products and the principle for changing them is probably similar throughout many brands of amps. I am not affiliated in ANY way with Tube Amp Doctor of Germany and I bought all of the components personally at full price except for a few capacitors which Stephan kindly sent me.

I would like to thank you all for reading this review of the TAD Power Transformer and how to install it reasonably safely. But also in some ways this review has shown that contrary to where manufacturers do not want amplifiers to work in other countries (by using power transformers that do not support multiple input voltages) that there IS a way other than those big fat and heavy external transformers. If you weigh up the pro's and con's of this upgrade the cost difference is not really that great and by doing things this way you will have an amp capable of being used almost universally with a few mods on that rectifier board. Now that's got to be worth it.

And to those manufacturers who send out amplifiers (and other equipment) with just one plain old input voltage that cannot be simply changed by moving a few wires shame on you. I particularly dislike that sort of money making effort and anyone who agrees should be calling their amp maker and telling them that you don't agree either. As for those distributors? give me a break, most of them will screw you to the floor for a genuine transformer replacement part and laugh all the way to the bank while doing it. They are only outstripped in that sense by Bankers.

Please note that this website or this page has no association whatsoever with any manufacturer in any way. Brand names are used as descriptors only. The views on this page are personal views from using the products which were bought at full price from Tube Amp Doctor directly.

The schematics are (c) Mesa Boogie. All Trade Marks Acknowledged.

© A B Mckenzie 1997-2017. All Rights Reserved