JTM45 curious modification
- Thread starter Rick 330
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jbzoso2002
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Wow, never seen that before.
I will be interested to know what that is.
Jimmy:2cool
I will be interested to know what that is.
Jimmy:2cool
demundo
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I think the bulb acts as a diode to tame voltage spikes
I have heard of people connecting a low voltage incandescent bulb (12v - 24v) across the output transformer secondary, this apparently does 2 things:-
1. when the amp is idling there is not enough current flowing to light the bulb so it has a very low resistance and effectively shorts the output preventing hum and background noise at idle.
2. when a signal is flowing the bulb will light up, it's resistance will increase and it will act as a variable resistance in parallel with the speaker load. This apparently has an expander type of effect on the sound.
I can't tell from the photo if this it what it was for sure but can't really think of anything else that it is likely to be. I've never tried this myself (and wouldn't recommend it as a sensible modification) so can't comment on what the effectiveness of such a modification would be.
How could a bulb act as a diode? A diode allows current to pass in one direction and not the other, a bulb is a filament that will pass current in either direction.
1. when the amp is idling there is not enough current flowing to light the bulb so it has a very low resistance and effectively shorts the output preventing hum and background noise at idle.
2. when a signal is flowing the bulb will light up, it's resistance will increase and it will act as a variable resistance in parallel with the speaker load. This apparently has an expander type of effect on the sound.
I can't tell from the photo if this it what it was for sure but can't really think of anything else that it is likely to be. I've never tried this myself (and wouldn't recommend it as a sensible modification) so can't comment on what the effectiveness of such a modification would be.
How could a bulb act as a diode? A diode allows current to pass in one direction and not the other, a bulb is a filament that will pass current in either direction.
oldog
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My memory is vague on this but I remember seeing a vintage amp that used a small wattage incandescent light bulb in a circuit as a built in compressor. I think it was a Standell.
oldog
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Here I found it https://youtu.be/4pZXVB6ObG4
Strictly speaking it's not really working as a compressor. A compressor is an active effect that boosts the lower level part of the signal and attenuates the higher level parts of the signal, the light bulb is simply acting as a variable resistance in parallel with the speaker, at low signal levels the bulb filament will have a very low resistance and will tend to attenuate the signal, at higher signal levels as the filament heats up it's resistance will increase and it will have less of an effect on the signal. As I said I have never actually tried it myself so can't really comment on how this effects the sound but I have heard it described as more of an expander type of effect than a compressor, YMMV!
This is very interesting, I really appreciate the input. I love gear that tells a story and seeing period mods still in place is cooler than a cover-up, to my mind. The amp has an extra speaker out, as can be seen; the other addition is, slightly scarily, a Bulgin mains outlet. I can't imagine daisy-chaining 240 V between amps would be considered advisable today…
All I can recall is that the bulb-with-pot arrangement was apparently designed to "turn down gain as the volume was turned up". That was Phil Wells' opinion, though that was 20+ years ago.
The added pot obviously lies between the bulb and the original speaker out, but how that pot functioned and where the bulb got its juice from, I do not know. Was there originally perhaps some form of light-sensor involved, or am I way off the mark...? I could post a whole circuit pic if that would help.
All I can recall is that the bulb-with-pot arrangement was apparently designed to "turn down gain as the volume was turned up". That was Phil Wells' opinion, though that was 20+ years ago.
The added pot obviously lies between the bulb and the original speaker out, but how that pot functioned and where the bulb got its juice from, I do not know. Was there originally perhaps some form of light-sensor involved, or am I way off the mark...? I could post a whole circuit pic if that would help.
The bulb and the pot appear to be wired across the output jack connectors so they would be in parallel with the speaker, the bulb would be powered by the output signal from the amp, no additional power supply or light sensor would be involved. At low output levels there would not be enough power to light the bulb and the filament would have a very low resistance, as the output level increases the bulb would light up (the higher the level, the brighter the bulb would glow) and as the filament heats up it's resistance would increase (that's how bulbs work!). The pot was probably included to add some additional level of control to the circuit.
TM1
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Yeah, looks like a current limiter..
Don
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Like the bulbs in a THD Hot Plate.
I've never seen a schematic for a THD Hot Plate so don't know for sure but I seem to remember reading somewhere that the bulb is used as part of the noise reduction circuitry and not for attenuation. A suitable powered light bulb wired in series with a speaker will work as an attenuator though.