Hello & welcome back to our blog. We recently carried out service & repairs to a 1978 Hiwatt [UK] DR103 amplifier which obviously has had a long & hard working career in rock & roll bands. This DR103 is the 2-input model with active mixing of the normal & brilliant channels, as distinct from the late 1960’s/early 1970’s 4-input model with passive mixing of the two channels. We suspect that both versions of this model have their fair share of admirers.
This amplifier was working but had issues with microphony and very scratchy pots – some pots had dead areas on their sweep, so were barely useable. The amp had been modded by persons unknown further back in its history. The high frequency response of the brilliant channel had been boosted to ear-drum shredding levels, and was to all intents & purposes unuseable. Apparently the previous owner had only ever played on the neck position humbucking pickup of his 335 style guitar, hence the need for extreme high frequency pre-emphasis.
In addition to this problem, the amplifier had high levels of hum & noise. We started out by giving the chassis a quick clean up and tightened up a loose transformer. We checked the fuses, and as is so often the case in older amps, they were not offering any protection at all as they were the wrong values ! We installed new fuses, carried out an electrical safety check [PAT test], then moved on with the main part of the service.
All pots [excepting bass & treble] were in very poor or damaged condition, so we replaced them with those excellent CTS pots. One source of unwanted noise came from the input jacks – they weren’t shorting to earth [ground] when the guitar lead jack plug was removed. We cleaned all input & output jacks, plus the 9-pin valve sockets with DeOxit, which corrected the remainder of the intermittent problems.
Two only of the preamp valves were tested as being faulty and/or microphonic, so we replaced those accordingly. All other valves, including the quartet of EL34’s tested OK. This amp has obviously been in regular use since manufacture in 1978, and the various electrolytic capacitors in the power supply still appeared to be serviceable. As the customer’s budget did not allow for complete replacement of electrolytics, we left them alone.
There was still a remaining source of hum within the amp, and this turned out to be a lack of earth [ground] reference for the 6.3V AC heater filament supply. The pair of 100 ohm resistors that provided a virtual centre-tap for the 6.3V AC looked perfect but measured O/C. We fixed this problem with the installation of a pair of 100 ohm 1 watt resistors. The final issue to be resolved was removing the MODs to the front end of the amp.
This DR103 already possesses a very bright voice, especially when plugged into the brilliant channel. The amp had been modded with a double layer of additional brightness, so the brilliant channel was unbearable with a Strat or a Tele. We restored the front-end circuitry to original spec, removing both MODs. How does this amp sound now ? Just fantastic, actually, although at 100 watts I would get thrown out of every gig we do these days, not to mention making my existing tinnitus problem even worse ! It’s just so British, with a warm & fat midrange, sweet & crisp top-end, and a tight but powerful bottom end. I love the balance of tone across the 6 strings of the guitar, but unfortunately for this amp to sound at its best, you have to play bloody loud.
So, we heartily recommend vintage Hiwatt amps as an investment for all you amp collectors out there – they were so well made in the first place, and are a pleasure to work on from the techo’s perspective. We are currently building several custom amps from 15 watts to 60 watts [to a customer order], that while not a carbon copy of the Hiwatt, are very strongly inspired by the Hiwatt and will deliver the classic British tonality at more sensible volumes. These amps are offered with a choice of EL34, KT66 or KT88 output valves. We also offer a single-ended KT88 model at 15 watts output.
Thanks for checking in again, and there’s plenty more amp talk to come. IR.