The Electar Tube 10 Project

One of the main reasons I started up Rock Dog Amp Repairs is because, funnily enough, I just LOVE working on amplifiers. A lot of the repairs I do these days are fairly routine, which is good, but every now and then somebody gives me a project to complete that is a little out of the ordinary. The Electar Tube 10 Project is one such job. Also, I thought it would be perfect to present on my 'Do-it-Yourself page as, if you are interested in electronics and have the time and resources, you may be able to do something similar with you own amp. Please keep in mind though, that even a 10 watt amp contains LETHAL voltages and I wouldn't recommend that anyone attempts a tube amp project of this nature unless you at least know how to safely discharge filter caps and where these high voltages appear in a tube amp! Having said that, check this out!

A very nice gentleman, Chris, called me up a few weeks back and run this project by me. It seemed completely feasible  to me and was the kind of thing I've been doing myself since I was a teenager. Chris wanted the chassis of his Electar Tube 10 amp taken out of its combo enclosure and installed into its own cabinet. He will then use the 'head' with any variety of speaker cabs. There were also a couple of mods he tracked down on the net designed by Riff Ron . The mods are basically  about removing  the  negative feedback loop and changing some parameters  of the first gain stage. There is also an emphasis on shifting the frequency response of the amp to avoid lower frequencies muddying up the sound.  I had a play of the amp today to familiarise myself with its voice before attempting the mods (and also to make sure it still worked after relocating the tube sockets). I tried the amp through a sealed single 12 extension cab fitted with a Celestion G12T100 and as expected, it sounded huge! I did note though there is not a huge amount of headroom in this little amp and I'm hoping the mods wont effect its clean tone to much. The plan  though, at this stage, is to make some of the mods switchable, if possible, once I'm convinced they are worth doing.

(Please see the note at the end of this article)

This is how the chassis looked when it arrived in the post. As you can see the tube sockets are mounted at the back which would make the chassis completely unsuitable for mounting in its own box. The first thing that needs doing is to relocate the tube sockets. Fortunately there is plenty of room to do this.

Its basically just a matter of removing the wiring to the tube sockets, installing the sockets into their new locations and rewiring them. I made a drawing of where all the wires went. Even when I have schematics I still make drawing and notes when removing wiring. It makes it that much easier to re-assemble.

After  the holes for the sockets and the socket's  mounting screws were drilled, all swarth and metal shavings  were carefully removed from the chassis and surrounding bench area.

With the sockets re-installed it was now time to rewire them. I used nuts and bolts to mount the sockets as opposed to the machine screws and threaded holes that were originally used. I also applied just a dab of low strength 'Loctite' to the threads of the mounting screws during re-assembly. I want to be able to remove the screws if I ever have to but I don't want them shaking loose all by themselves.

The pictures below show this part of the job now completed. The tube sockets were rewired, the tubes installed and the amp tested. Also, in the true spirit of mods, these mods (with the exception of the new holes in the chassis of course), are totally reversible.

The next part of the project was to build the cabinet for the chassis. We decided to use pine timber (just like early Fenders) and Bunnings, here in Wollongong sell just what I needed. I started by cutting the width of the pine board down to size. I allowed for the cabinet to be just wide enough to protect the things that stick out like switches and fuse holders, if the amp is laid down on it's face or back.

Once the glue had dried, I filled all the nail holes and joins with pine coloured wood filler. I used a router on the edges to give them a better look. The next step is to install some wood framing inside the cab to assist in fitting the chassis and the back and front panels.

For the front panel I cut out a piece of ply and cut four large hole to allow for air flow through the cabinet. After painting it flat black I covered it with a small piece of vintage Fender Bassman grill cloth I had left over from some speaker cabs I made recently. The panel was crewed in using small battens I had installed earlier. At this stage I had fitted some nickel plated corners that I purchased from Penn Fabrication in Melbourne.

From the back you can see the small battens I fitted to mount the front panel. The horizontal battens are to rest the chassis on.

I used the original chassis mounting screw holes to fasten the chassis into the new cabinet. Not one extra hole (other than the holes for the tubes) needed to be added to the chassis.

The chassis was fitted to the new cab and screwed into place. I fitted the last of the corners and a piece of powder coated steel mesh (left over from a fold back cab I reconditioned recently) and that was about it. Before mounting the chassis I installed a speaker output jack. I used a thin piece of MDF timber painted flat black to cover up the old tube socket holes and provide access to the speaker jack. I was a little undecided about fitting a full sized handle to such a small cabinet but found it was impossible to move the amp with one hand with out one. I used a Marshall style handle (again from Penn) and the job was finished.

Note on Mods: Before mounting the chassis I tried the mods suggested in the Riff Ron's article. To do this I had to remove all the knobs and pot nuts and remove the output transformer so that I could get the main circuit board out of the chassis. I removed the grounded end of R5 as suggested and connected the end of the resistor via a switch to ground. I then did the same with the negative feed back loop so that I could basically switch these mods in and out. With the board still out of the chassis, I powered up the amp and tried the amp with and without the mods. Removing R5, as expected, increased the volume of the amp but this increase was virtually imperceptible, so that didn't really ring my bell. Disconnecting the negative feedback loop made no audible difference as far as I could tell. Just to make sure, I connect the output of the amp to my 'scope and that showed no difference either. I was a little surprised as I expected something should happen when you consider some presence controls on amps (like a Marshall JCM900 for example) simply adjust the amount of negative feed back in the power amp. This said, however, most presence controls are pretty subtle. To be honest, I didn't do all the mods as Ron suggested so this would have made the change in tone much less dramatic. To get the full effects of the mods it would be essential to change the cap values as Ron has described. The value of these components are very important. Since writing this article, Ron has now updated his mods page with a description of the mods and the effect they will have on the amp's sound. It would have been nice to have my customer, the owner of the amp, present whilst doing these mods so that he could hear the difference in sound and decide for him self if he wanted the mods to be permanent. The sound of an amp can be very subjective. Under other circumstances I probably could've persevered with the mods because from Ron's description they  seem to address some of the less desirable sound issues this little amp has, but at this time the amp is still standard. I'm happy to say though it sounds really good, through the new speaker cabinet all the same!

Here is the completed unit. Chris had this speaker cab made for him by a speaker cab builder on the central coast. He did a really good job too. It's made from pine and features dovetail joints. It's open at the back and sounds very good. At Chris's request I installed a DPDT switch that allows the impedance of the cabinet to be switched between 4 and 16 ohms. I fitted the corners to match the corners I had fitted to the head cabinet. The size of the cabinet really improves the amp's bass response and gives this little Electar amp quite a huge sound. All in all, a very successful project!