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ROAD KILL AMPLIFIER - GOLDENTONE 8 / 50 Here is another restoration project, also a Goldentone amplifier. This one I've called the Roadkill project, because the amp was found by the side of a road. It had been put out for a hard waste garbage collection, and was spotted by the current owner as he was driving past. This is what the box looked like when he found it:
The amp has six inputs, marked Instrument / Microphone. Two channels, each with Bass, Treble and Volume controls. Channel One also has a tremolo. All of the small signal valves were pentodes (two EF86 as the first stage for both channels, an EF86 for the power amp's volt amp, and a 6AU6 for the trem oscillator). The phase splitter is a 12AX7 (long-tailed pair). For a while I thought it may have been designed as a PA amp for a set of microphones, but why was there a tremolo? A bit of mystery, that one. But there was a nice surprise when we turned the box around:
The original amp was an ultralinear design, using a pair of KT88 power valves, with a 5AR4 valve rectifier. The B+ was 550 volts, with a choke pi filter. These are the original valves, still with the GEC sticker on the bottles. Their measured emission is still 120% their original design spec!! This was one serious amp - I reckon its power output was nearer 60 watts than 50. The build label says Rose and Clark, Melbourne. I think the amp was probably built in the early 1960's. The amp had been used as a bass amp most of its life, but I discovered later that it probably started life as an amp for an electric piano - which explains the tremolo, and the big, clean hi-fi style power stage.
You can see the cathode resistors just to the right of the blue power capacitors. There was a lot of heat damage on the board, and one set of cathode resistors had been repaired at some stage - unfortunately, the replacements were rated too low, and had burnt out. I expect this is what retired the amp. Due to the age and condition of the electronics, the amp needed a complete rebuild. Following an extensive discussion and soul-searching, the new owner and I decided to rebuild one channel to its original specification (pentode, with tremolo), but to convert the second channel to something a bit more useful for lead guitar - it's now a Tweed Bassman pre-amp, which we thought was still in keeping with the age of the original design. We also decided that 60 big clean watts limited the amp's use for lead guitar, so we decided to convert the power section to a more traditional output stage, using a pair of EL34 valves. By changing the power supply to a choke input filter, the B+ is reduced from 550 volts to just under 400 volts, and the amp now has an RMS power rating of 30 watts. The amp can be converted back to its original configuration at any time, by rewiring the choke and putting in higher voltage capacitors.
The chassis was sand-blasted and re-painted with a double pack automotive enamel.
As can be expected from such huge transformers, the amp runs very cool. This transformer set is already 45 years old, but there is no reason why they wouldn't last for another 45 years. You can't beat quality valve amps for a long life.
The pre-amp is connected to the power amp with an umbilical cable that takes the pre-amp B+, the heaters, a standby switch loop, and the signal input cable. A master volume has been fitted to the power amp chassis.
These pictures don't show the face plate in all its glory unfortunately - all of the graphics on the panel are silver on a black background, and the panel has now been covered with a lacquer finish. My thanks to Mark Huxtable for finding this great old amp by the side of the road, and Bob Rundle for giving me the clue to its original purpose. And of course, many, many thanks to the great amp engineer who designed this thing in the first place - by selecting top quality transformers the amp is assured another lifetime or two. By the way, this amp was offered on E-bay in the condition shown in the first picture - the original owner got no offers, so carted it out to the side of the road, just as Mark drove past. Which just goes to show, what is one man's garbage is another man's gold. |
The amp is a Goldentone 8/50. The designation means 8 valves, 50 watts.
The power amp is HUGE.
The amp had spent 20 years in the back of a shed.
The power amp was cathode biased.
The old board was re-used.
The original rectifier valve is still in good condition.
The pre-amp has been made safe with a new mesh cage.
The roadkill amp box has been re-covered with polished black vinyl.|
Neale Valve Amplifiers. pure vacuum, pure tone. |