It was probably fitted with a selenium plate bridge rectifier originally, which has been replaced by a modern silicon bridge much more recently. There is a space on the board where it was probably mounted.
If memory serves me correctly you could usually tell when a selenium rectifier had burned out because they gave off a smell very much like rotten eggs.
The EL84 came out in 1953 so it could possibly be from 1955.
ppluis0 wrote: ↑03 Jan 2023, 18:07
Quite interesting is the tremolo generator (at the upper part of the diagram)
Consist in a neon bulb connected as a relaxation oscillator and a shaping network to alter the grid polarization of the very first triode stage.
A neon is pretty much a high voltage tunnel diode. It starts to conduct when there is somewhere around 90v across it and stops conducting when the voltage drops to around 60v. (All voltages from memory and probably dependant upon the neon used.)
To make an oscillator you put a capacitor across the neon, which holds charge and discharges into the neon once it starts to conduct. You feed this from a higher voltage through a current limiting resistor.
The charging time (off period) is determined by the applied voltage, and capacitor and the resistor values.
The discharge time (on period) is determined by the capacitor value and the current drawn by the neon.
This is something I found interesting about the neon+LDR module (nicknamed "Trem-Bug") used by some models of vintage Fender amps:
The Secret Life of “Trem-bugs”
Even today, the old-school tremolo tube circuitry within Fender’s ‘Blackface’ and ‘Silverface’ amplifier models still harbours some secrets.
Secrets that have a direct bearing on its function and tone. Secrets that amp gurus and proud owners of these gorgeous guitar amps will undoubtedly find fascinating.
Firstly, the “NE-2” glow lamp used to make the trem-bug. It should be a type “NE-2U”, as opposed to the more common “NE-2H” lamp. The NE-2U is a high brightness indicator lamp and was specially designed for the purpose of controlling photocells.
A small amount of a radioactive additive was added to the neon gas, specifically Krypton-85 gas, which improves the lamps start-up reliability when operating in complete darkness (a.k.a. the “Dark Effect”).
Being radioactive, Krypton-85 emits β-particles (electrons) and these help the neon to ionise and strike more reliably when a high voltage is applied.
Unfortunately, Krypton-85 has a half-life of 11 years so it’s effect gradually diminishes over the years: after a few decades, these lamps still work, but don’t strike as enthusiastically as when they were brand new.
This is one of the rare cases where it’s better to use a new tube rather than NOS one.
Secondly, the photocell in the trem-bug is a TO-8 (approx 11mm ⌀) CdSe (cadmium selenide) type, not CdS (cadmium sulphide). CdSe photocells typically exhibit a significantly shorter time constant than CdS photocells (approximately 10 milliseconds in contrast to 100 milliseconds); in layman’s terms means they’re faster. They’re also more sensitive to red light in the colour spectrum, as shown in the graph above. Ionised (glowing) neon gas emits distinct frequencies of light, with its highest spectral peaks being between 600nm-690nm, which means both types of photocell will exhibit resistance changes in response to light, however the CdSe device will respond more rapidly. Finally, a CdSe photocell can easily be distinguished from a CdS type as they’re black in colour, rather than bright orange.
Finally, LDR's cannot currently be used in devices that have to comply with ROHS directives, so they will not be present in any current design.
I found and old youtube clip -- not the best quality, but it gives some idea of this tremolo... and I think it doesn't sound too bad. If it doesn't sound great over all, I think the speakers are to blame. It seems like most of these Italian amps use radio speakers...
