1978 Hiwatt DR504 Custom 50

Hiwatt amplifiers are known for their loud, mostly clean, and uniquely British take on guitar tone. They boast the neatest wiring/build quality of any amplifier that I know of. The company was officially started by audio engineer Dave Reeves after being laid off by Mullard (the famous vacuum tube manufacturer) in 1968. His first big order came from Ivor Arbiter's Sound City music store in England -- these became the original Sound City amps. These early units were just Hiwatt amps that were re-branded. Sound City amplifiers later on used their own designs after Reeves parted ways, but for the rest of the years they made them they always bore his influence and share a lot of DNA with Hiwatt. After that initial contract with Arbiter expired Hiwatt came into their own and were very successful manufacturing tube amps until Dave Reeves died unexpectedly in an accident in 1981. The original company closed it’s doors in 1984. The circuit design used in most Hiwatt amps that set them apart from Marshall or Vox, are the combination of idiosyncratic tone stack values, a fixed bias phase inverter, and a novel presence control. The use of Partridge Transformers, Mullard vacuum tubes, and Fane speakers, add up to making the beloved “Hiwatt sound”. Harry Joyce and Co. who were the team that helped wire these amps (modeled after Reeves early units) used truly military grade spec and craftsmanship - the techniques they used came straight from the British Navy from which many of the team had been employed by or enlisted with. It is obvious to anyone from just looking at the guts of these amps the care and quality that were put into them. Harry Joyce and his team were so strict with quality control that the original contract they signed for Hiwatt stated they would only make 40 units a month!

This particular model (the DR504) uses Hiwatt’s late 70’s two input preamp schematic married to their 50 Watt power amp design. It is essentially the same in design, make, and sound as the more famous 100 Watt DR103 but with half the output power. In fact almost all Hiwatt amp models within a given year use identical preamp circuits, the differences between them are only the power supply and power amplifier circuits. Most models also use the same chassis, which is drilled out with enough holes for multiple designs. Notable users include: Glenn Cornick of Jethro Tull, David Gilmour of Pink Floyd, and Pete Townshend of The Who.

This amp arrived in mostly original condition with the exception of someone having modded the bias circuit to provide better idle plate dissipation, this would have been fine but the technique used was rather shoddy. Two other spots of wiring needed to be redone because of the same poor quality work. The customer wanted to have a reliable amp for the coming years and to keep this DR504 as original as possible. Overall it needed a full restoration including a cap job, new power/misc. resistors, cleaning, and new fuses.

The amp was given a full restoration. All the axial lead electrolytic capacitors were replaced with upgraded voltage and temperature rated Vishay brand electrolytic’s. The multi-section cap can’s were replaced with new high quality Slovenian made JJ brand 500v type units. The power dropping resistors were upgraded to 2 Watt and 3 watt Metal Film type and the Screen Grid resistors were upgraded to 5 Watt Cement type units respectively. This was done for better noise floor and reliability. The plate resistors in the Phase Inverter were replaced with 2 Watt Reduced Mass Metal Film type units due to heat damage. Two new 3AG Fuses were installed as the ones present were not rated correctly for this circuit.

The tubes were tested and all were good. This was the final tube compliment: V1-V3 = MULLARD ECC83, V4 = WESTINGHOUSE 12AX7, V5-V6 = GT 6CA7 Matched Pair. The non-adjustable Fixed Bias was set to %56 Class AB Plate Dissipation with a B+ of 484 VDC. This was done by altering the resistor values in the bias circuit. The tube sockets were all treated with De-Oxit to remove/prevent corrosion. All pots were cleaned and hardware tightened.