Denco 512-Plus 5 valve amplifier 1956

Lovely surviving piece of audio history I found in a loft in Ruby this Month. It was sold in a box of old radio parts with 2 smashed valves and lots of useful spares. The EZ81 was one of the broken valves that soon lost favor when the By100 silicon diode arrived a few years later. Because the cost of a replacement is very high because of the rarity of this power supply double rectifier diode I fitted two more modern silicon types I had to hand in the way shown in the diagram. All connections fit to the old valve base and I removed the heater wire from the broken valve, so all looks nice cosmetically. Even the valve power guitar amps of the 1960's were fitting BY100's  as the EZ81 became hard to find.

V1= EF86 (pentode), V2=12AT7 (double triode), V3+4 EL84's (pentode) & V5 EZ81 (diodes)

Because of the high efficiency of silicon diodes a 2,2K ohm resistor is the popular value to be added in series with the supply, it will get hot so 5-10 watt resistor will be needed.

Extra parts fitted under EZ81 base 2.2K and two By100s or better (I used BY223 types) Anodes going to red wired from TX (painted end of a diode is Cathode)

Circuit found on line used by guitar amp companies in replacement for the EZ81 valve

Remove the heater wire from it's valve base to stop valve working if one fitted while new parts are wired in. The tone switches and input switches are easy to clean, skeletal type and quite large. The signal inputs are aerial sockets, seen above, if you fit the amp in a cabinet best to hard wire external plugs and leave original in place. The other cracked valve was an EL84, these are still about but around £15-£60 depending on where you buy them from, I found a new one in Hong Kong for £10 with free postage.

The chassis before I wired new diodes, all smoothers were fine (Radio Spares type) In fact all parts were in remarkably good condition considering nearly 65-years old. The chassis was an early version and all the parts had been hand cut and bolted, even the 5 side of the aluminium chassis, all the scribe marks for the drilling of holes clearly showing. Amp manufacturers never really went into mass production mode, they were made mainly for the hi-fi enthusiasts in small quantities. This design was inspired by a pamphlet produced by G.E.C for 40 pence

Above the original inpiration publication from the late 1940s. Below, nice original chicken head control knobs with nice tone adjustments (78 or LP) with a brushed gold finish on the front panel 

Also the two black sockets at the bottom are for tape in and tape out, everything for the lover of high quality monoral sound. If you have one of these units please let me know, Id' love to hear from you.