Custom 50 Details
Build notes specific to this amp from Mitch.

This amp is a reproduction of a Hiwatt DR504, circa 1972.

The power amp uses a matched pair of EL34 tubes made by SED (Winged C) in Russia. The preamp uses Tung Sol tubes also from Russia, one 12AX7 for the third gain stage and one 12AT7 for the phase inverter. The first gain stages use a couple of real Mullard ECC83's (12AX7) made in England back in the 1960's.

The transformers are made by Heyboer to the same specifications as the Partridge transformers used in the originals. The capacitors are made by Sozo, ARS and F&T. It also uses Cliff jacks and Alpha potentiometers.

The chassis is 11 gauge brushed aluminum, complete with a heat shield as used on the originals to protect the filter capacitors from the heat of the output tubes. The cabinet is made from 3/4" Baltic birch plywood and covered with black Tolex with a pattern similar to the originals.

Chassis assembly was done first. I installed the tube sockets, transformers, capacitors, the front panel, the pots and the switches.

Custom 50

Custom 50

I built the preamp turret board next. The board and layout are similar to the originals.

The originals were built in England by a fellow named Harry Joyce. He also did work for the British Navy. He used many of the same military-spec wiring techniques on the amps he built for Hiwatt as he used on his naval equipment. That's why Hiwatts were built so neatly and ruggedly.

I went with Harry Joyce's method for attaching the flyoff wires to the board. This puts them on the bottom instead of the top. It looks neater, but it meant that I had to roughly measure the wires and attach them at this point of the assembly instead of later.

Custom 50

Custom 50

Wiring the input jacks was next. The space inside the chassis is fairly cramped in the input jack area. I made a little jig to hold them in the proper position so I could pre-wire them outside the chassis. This let me rotate the jack assembly into what ever position I needed to install the resistors and wires. It was a lot easier than trying to wire them inside a heavy chassis.

Custom 50

Custom 50

Custom 50

The next step was wiring up the heaters. I did it Hiwatt style where the wires run straight as they pass a tube socket and are only twisted on runs between the tubes. These twisted runs are short so I just twisted them by hand instead of using the power drill technique. Making a long length of twisted wire with a drill would mean having to untwist and straighten the wires at each tube socket.

I built the small turret board for the power supply and the power amp next. Again this is similar to the way the originals were built. I also did the AC wiring and wired up the power transformer and the output transformer.

Custom 50

Custom 50

Custom 50

Custom 50

Once I had the power supply wired, I was able to test the amp for the first time. I checked all my wiring several times and made sure that the polarity of all the diodes and capacators was right, then I applied power. The pilot light came on and I watched for smoke. There wasn't any, so I measured the voltages. The high DC was a healthy 481 volts. I checked the bias supply and the heaters and found them to be good, too.

Custom 50

Next I finished the power amp wiring by installing the remaining resistors and capacitors.

Custom 50

Custom 50

Custom 50

I installed the preamp board and wired it to its tubes and the power board.

Custom 50

Then I wired the preamp board to the pots and put in the last of the resistors and capacators. The wiring was all laid out similar to the way Harry Joyce did it on the originals. Maybe even a bit neater!

Custom 50

Custom 50

The wiring was now finished and it looks very much like a Hiwatt!

Custom 50

Now it was time to test the amp so I started going through the final checks. I checked my wiring very carefully and all was good, so I applied power. It passed the smoke test. It also passed all the voltage checks. I powered it down and waited for the capacitors to discharge, then I plugged in the preamp tubes. I powered it up again and they all glowed normally, there was still no smoke and all the voltages were still good.

I powered it down and waited for the capacitors to discharge, then I plugged in the power tubes and a dummy load. I applied power again in the Standby mode. All the tubes glowed normally. I flipped it out of Standby, there was still no smoke, no signs of red plating on the EL34's and all the voltages were good.

I plugged in a speaker and a guitar for the first time and it worked like a charm!

The wiring was finished, but there were still a few things to do to complete the amp. The bias had to be checked next. For this, I used a couple of "bias probes" I made using the 8-pin plugs from a couple of old relays, a couple of 8-pin tube sockets and a couple of 1 ohm precision resistors. These aren't as fancy as the commercially available bias probes, but they let me measure the cathode current and the plate voltage fairly easily.

Cathode current can be measured by reading the voltage across a resistor placed in series with the cathode. A 1 ohm resistor will have no effect on the amp's operation and it makes the current calculation easy. Ohm's Law says current = voltage / resistance. If the resistance is 1 ohm, the current simply equals the voltage.

Custom 50

I found that the cathode current was 72 mA with the bias trimmer set to minimum resistance. There was no red plating, but this was way to high. EL34's should idle at around 17.5 watts in a push/pull amp. To get 17.5 watts with these tubes and 454 volts on the plates, Ohm's Law says the cathode current should be 39 mA. I adjusted the bias trimmer until I read 39 mV across the 1 ohm resistor on my bias probe. I checked it for both tubes and found the current to be the same so the tubes I used are indeed a matched pair.

Custom 50

Custom 50

The next thing I did was finish dressing the power wiring in the amp. I used real cable lacing just like Harry Joyce did. No zip ties for this amp!

Custom 50

Custom 50

Custom 50

Custom 50

I did a "soak test" next. This is where the amp runs for a few hours to make sure everything is stable and continues to work properly. Hiwatt did it, so I do it too.

Custom 50

It passed the soak test. The bias didn't drift, the voltages were still good and there was never any smoke or red plating. The last thing was to put the amp in its cabinet. I almost hated to do this, though, because it hides the beauty of the chassis and its wiring!

Custom 50

Custom 50

The amp looks and sounds just like a real Hiwatt and I couldn't be happier with the way it turned out. It was a lot of fun to build and a real honor to try to emulate the ultra neat military-spec wiring that Harry Joyce did on the originals. For me getting the wiring neat on a Hiwatt amp is almost as important as how it sounds.

For more information please contact: message@markinamps.com

© 2011 - 2017              Valid CSS!     Valid XHTML 1.0 Strict              Back