This week my friend Dorian decided that he wanted to get his Hiwatt DR504 (Wired by the legendary Harry Joyce!! I’m a wiring nerd…) looked at to take care of a few small issues. Luckily this was an old school build with lots of room to work. What a difference compared to that Mesa!

The first problem he needed taken care of was he wanted a bias adjustment pot installed. As designed these amps didn’t have an adjustment, but they were designed 30+ years ago when you could get very good tubes from anywhere and the wall voltage was 110VAC-117VAC standard. In this modern era of 124VAC (my walls) and Russian and Chinese tubes, you would generally be idling the tubes way, way too hot. Installed in the amp were a pair of Electro Harmonix 6CA7’s (Russian, New production) that ended up idling at 56ma even while at the correct 115VAC power. Solution? Re-build the bias circuit to allow more negative voltage, and use the empty octal socket holes to mount the bias pots! I could have done just one pot, but Dorian wanted to be able to perfectly match his tubes on the amp, and we also had two empty holes, so why not! The controls are mounted in those holes so nothing was cut on the amp. He can return to stock within an hour and look perfect.

Now he can adjust his tubes from 3ma-106ma current draw. So, very very cold, to very very hot! Not for the timid, but modern tubes draw current all over the place so this large a range will allow him to use and match any tube he comes across. Maybe he’ll try an EL34, 6L6GC combo? Currently this set is idling at 38.5ma each (approx: 70% dissipation with 455v on the plates)

The next issue to remedy was blocking distortion when you max out the normal channel volume on the amp. To remedy this, I used the Mark Huss 4-input mixer mod (http://hiwatt.org/2_mod.html) which replicates the input circuit of the 4 input amplifier on the 2 channel one.  Worked out beautifully, the amp now gets to the edge of the ugly distortion when everything is set to maximum. I tested it with an EMG equipped Stratocaster, a very hot signal, and it sounded pretty awesome. It should be just fine with everything else.

    The last couple things was replacing the post phase inverter capacitors with new just as a maintenance item at his request, as well as replacing the  screen resistors with new ones as well to replace the mis-matched pair currently installed in the amp. They weren’t broken yet, but, it takes little time to remedy potential issues while I’m already inside the amp.

It was a joy working on this amp, and it’s got all that raw British-ness, and immediacy that Hiwatts are known for. Truly a special piece of gear, that should serve it’s owner for many years to come.

If you’ve got an amplifier that you need tweaked, modified, repaired, or rebuilt, drop me a line at RauldelP@ra2amps.com. We’ll discuss what you need/want done, and what it’ll take to get it there.  I work mainly on tube equipment, but I’ll happily work on solid state as well. Guitar, bass, keyboard, accordion, PA, I’ll work on them all!

-Raul