Converting a Fender Hot Rod Deluxe from 6L6 to EL84 using TAD Tone Bones tube adaptors

I love taking things apart. I have been like that since I can remember. While everyone else in my family was playing with their newly unwrapped Xmas presents, I was taking mine apart to see how they worked. Sometimes it’s a curse and other times it a blessing.

Although this has stood me in good stead in my career and my hobbies, it can also get a little tiresome as people come to me all the time to get everything repaired. If I was to charge everyone a buck for every time I fixed something, I would be fairly wealthy. But I don’t, because I’m a nice guy!

Being a fiddler I decided it was time to see if I could convert my amp from using 6L6 power tubes to EL84. For those who don’t know, the Fender Hot Rod Deluxe uses two 6L6 power tubes to produce around 40 watts of sound.

6L6 tubes in Hot Rod Deluxe

6L6 tubes in Hot Rod Deluxe


At the gigs we do, a 40 Watt Tube amp is loud (even with a six foot two Canadian madman at the skins, I can still be louder). Most guitarists seek the tone produced by a tube amp when it is turned right up, the brown sound. But, when turned right up, the Hot Rod is deafening. So I decided to search for a way of reducing the power of the amp so I could turn it right up and not kill anyone.

Enter the simplest/cheapest solution – Power tube adaptors.

These are ‘things’ that plug into your existing valve sockets and then allow you to plug in the new valves into them. This then reduces the power of the amp by a lot and allows you to turn it up. Great!

TAD Tone Bone Power Tube Adapters

TAD Tone Bone Power Tube Adapters

My search brought me to the following place, Amptone, where I found these links:

THD Yellow Jackets

TAD Tone Bones

Before heading off to these sites, I soaked up the info so I knew exactly what they did and what to look for. After a bit more searching I found a couple of sites to purchase them from, these were:

Watford Valves


Basically there are two main types that appear to do the same thing. Having had difficulty in trying to get any THD Yellows because they were not in stock, I opted for the TAD Tone Bones from Watford Valves. I had also heard that the Yellow Jackets had problems with heat and were warping at the sides. As well as that, the Yellow jackets were taller than the Tone Bones and this would cause a problem with space in the Hot Rod. I later found out you get reduced sized ones.

TAD Tone Bones it was then, so I bought them. Went for the Pentode version. The Triode version reduced the power even further, but I was concerned about clean headroom.

That was about 1.5 to 2 years ago I think and they have been running well until now. I have to say, I thought the tubes would have gone first, but the amp started hissing, popping and squealing just before the Bride and Groom arrived. In traditional style I gave it a thump and it started working. Lasted all night, but I couldn’t wait to get it home to find out what the problem was.

The next day I opened up the back and started gently tapping the amps chassis to see where the problem was. This is a typical fault finder’s method. Switch the amp on, then start at one end and work your way to the other tapping until you find the problem. Well I certainly did. The Tone Bones. Something inside them was either broken or causing a poor connection.

So, powered down, I took them out. My first thought was can these be repaired? Well, I say if it was built by man, then it can be fixed by man. I slowly twisted the red casing and it came right off in my hand without breaking anything, much to my surprise. Two curved plastic supports were siliconed to either side and they were easily removed. The silicon was a red colour which I later discovered was an indication it could withstand high temperatures and not the normal 75oC of most silicon.

Removing these I could easily see that there were nine components connecting the two sockets and that was all. All the components were in good condition, however, there were three heavy duty diodes connected in series, but instead of being soldered, they were crimped. My first thought was that this wasn’t very clever, but there must be a reason?

I quickly sketched down the layout, just in-case anything came loose while I was fixing it. Further inspection revealed that the component wires that were inserted into the valve base pins, were also poorly soldered.

TAD Tone Bone Schematic

TAD Tone Bone Schematic

So I set to work re-soldering all the joints. The crimped diodes joins needed a lot of heat to get the solder to flow into the crimping, but I eventually got there. The only thing I needed now was to find some high temperature silicon that would work with the tube high heat. Didn’t buy this but found some other stuff that works up to 200oC and was clear instead of red. I only applied enough to hold it all together, but not too much so I could get it apart again if it screws up.

So that’s it. let them cure for a couple of days and plugged them back in. Superb, back to the compression and crunch of the EL84s.

TAD Tone Bones back in amp and after repair

TAD Tone Bones back in amp and after repair

Disclaimer: I take no responsibility for anything if you use the above schematic to build your own

About Neil Ferguson

Neil provides the vocals (mainly) and much of the lead guitar work. One of the three founders of the band Neil also delights in the bands technical responsibilities.
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