tweak fusing valves.
One of the EL34 output valves had blown, and the normally silver-colored getter had turned completely white. Upon opening the amplifier, remnants of a trimpot, including some melted solder, were found on the bottom cover. The trimpot's value was barely readable: 1k. The official schematic diagram is rather rudimentary, offering no clues regarding the origin of the trimpot. Inspection of the circuitry revealed burnt remains near the heater wiring to the EL34s. In the Jadis Orchestra, the heaters of the four output valves are wired in series and supplied with approximately 24 V, making comparison between the affected channel and the seemingly intact channel unhelpful. Further inspection indicated that the 1k trimpot must have been soldered across the heater wires, with the wiper contact connected to ground, a classic method to minimize hum. However, the melting of the trimpot was unprecedented, raising questions. At 24 V, the dissipation through 1k is about 0.5 W, which a standard plastic trimpot should manage, albeit barely. There was no apparent reason to assume that the heater voltage had spontaneously increased. After considerable analysis, the sequence of events leading to this situation was reconstructed. The designer, Mr. Jadis, decided to protect the power valves with a fuse in series with the combined cathodes of each output channel. While this appears sensible, it may not be effective. If a valve loses its vacuum, it will become fully conductive, causing the fuse to blow. The cathode voltage will rise to a level close to +500 V from the high-voltage supply at the anode. The insulation between the cathode and filament may not withstand this condition for long, as the maximum Ufk of an EL34 is only 100 V. This situation would result in a short circuit between the cathode and filament, creating a new current path for high voltage. Fortunately, the current found the 1k trimpot, preventing potential damage to the high voltage supply, output transformer, and/or mains transformer. Once this explanation was established, it became clear that the Jadis Orchestra is not the only amplifier vulnerable to such issues. For example, the widely sold Audio Innovations 500 amplifier lacks visible fuses in its cathode circuitry. However, in most schematic diagrams of the AI 500, resistor R1 is not depicted, yet it is physically present in all but the earliest versions of the amplifier. Resistor R2 is a robust 7 or 10 W wirewound type, while R1 is deliberately a 0.25 W carbon film resistor. Under normal operation, the current through the 15 ohm R1 is about 130 mA, resulting in a 1.95 V voltage drop and a dissipation of 0.25 W. If an EL34 fails, R1 overheats and blows, serving as a fuse. A similar 'fuse resistor' is also found in the Audio Innovations 200 and 800 models. While familiarity with AI models is noted, this protection scheme is present in various amplifiers from other manufacturers as well. In the AI 500, the EL34 heaters are not grounded via a trimpot as in the Orchestra but directly connected to a center tap on the heater winding of the mains transformer. Consequently, if a similar valve failure occurs in the 500, an alternative component will need to burn. It is important to note that before the cathode can reach approximately +350 V, the 63 V rated electrolytic capacitor C1 must fail, which has been observed multiple times. Once that capacitor is compromised, and a short develops between the heater and cathode, it is generally the 47-ohm resistor that will burn.
The Jadis Orchestra amplifier employs EL34 output valves, which are known for their robust performance in audio amplification applications. The design of the amplifier incorporates a series heater wiring configuration, allowing for a stable voltage supply of approximately 24 V to the output valves. This series configuration is critical for maintaining uniform heating across the valves, which is essential for optimal performance and longevity.
The trimpot, identified as a 1k resistor, was likely integrated into the circuit to serve as a hum reduction mechanism, connecting the heater supply to ground. This method is a common practice in tube amplifier designs, as it helps to minimize noise that can be introduced into the audio signal. However, the melting of the trimpot suggests that an unusual condition occurred, leading to excessive current flow through the component, which exceeded its power rating.
The protective measure of incorporating a fuse in series with the cathodes of the output valves is a standard practice aimed at safeguarding the amplifier's components from excessive current due to valve failure. However, the potential failure mode described highlights a critical design flaw. When a valve loses its vacuum, it can become fully conductive, resulting in a dramatic increase in current. This condition can elevate the cathode voltage to levels that exceed the insulation rating between the cathode and filament, ultimately leading to catastrophic failure.
In contrast, the Audio Innovations 500 amplifier employs a different approach to ensure protection against valve failure. The use of a 0.25 W carbon film resistor as a fuse (R1) in conjunction with a larger wirewound resistor (R2) provides a means of current limiting. This design allows for the safe dissipation of power under normal operating conditions while providing a fail-safe mechanism in the event of an EL34 failure. The direct connection of the heater supply to a center tap on the transformer also changes the dynamics of the failure mode, as the components will experience different stress conditions compared to the Jadis Orchestra.
Overall, the analysis of the Jadis Orchestra and Audio Innovations amplifiers reveals critical insights into the design considerations for tube amplifiers, particularly regarding the management of high voltages and the implementation of protective measures to prevent damage during component failure. Understanding these dynamics is essential for engineers and technicians working with tube amplifiers to ensure reliability and performance in high-fidelity audio applications.One of the EL34 output valves had blown, the normally silver coloured getter had turned white completely. When I opened the amp, I noticed the remains of a trimpot, including some melted solder, lying on the bottom cover of the amp.
Its value was only just readable: 1k. The official schematic diagram is rather rudimentary, so it gave me no clue as to where this trimpot had come from. Inspection of the circuitry showed some burnt remains near the heater wiring to the EL34s. In the Jadis Orchestra the heaters of the 4 output valves are wired in series and fed with about 24 V, so comparison between the affected channel and the channel that seemed still intact wouldn`t be of any help here. Further inspection revealed that it the 1k trimpot must have been soldered across the heater wires, with the wiper contact of the trimpot connected to ground: the classical way to minimize hum.
But I had never seen such a trimpot melt before, so I was greatly puzzled. At 24 V the dissipation into 1k is about 0. 5 W, which a standard plastic trimpot should be able to handle (if only just). There is no conceivable reason to suppose that the heater voltage had gone up spontaneously for a while. So what on earth had happened It took me quite a while before I could reconstruct the chain of events which led to this result.
Mr. Jadis decided to protect the power valves with a fuse in series with the combined cathodes of each output channel. This seems sensible but it now appears it is not. Think of what happens if a valve looses its vacuum. The valve will develop full conductivity. Now the fuse blows. The cathode will be lifted to a voltage near the +500 Volt of the HV supply present at the anode. The insulation between cathode and filament will not survive this attack for very long because the maximum Ufk of an EL34 is just 100 Volt.
A short-circuit will result there (between cathode and filament) and a new current path is created for the High Voltage. Happily the current, on its way to ground, found the 1 k trimpot, otherwise the high voltage supply, the output transformer and/or the mains transformer could have blown.
Once I had established this explanation, it dawned on me that the Jadis Orchestra is not the only amplifier exposed to these dangers. Take the widely sold Audio Innovations 500 amplifier for instance. There are no visible fuses in the cathode circuitry here, but look at figure 1. In most schematic diagrams of the AI 500 resistor R1 is not drawn, but it is physically there in all but the very early versions of this amp.
R2 is a hefty 7 or 10 W wirewound resistor but for R1 deliberately a 0. 25 W carbon film type was chosen. In normal operation the current through the 15 ohms of R1 is about 130 mA, which means a 1. 95 V voltage drop and results in precisely 0. 25 W dissipation. Should an EL34 fail, R1 gets overheated and blows, thus acting as a fuse. A similar `fuse resistor` is also found in the Audio Innovations 200 and 800. I`m naming AI models here because I`m very familiar with them, but I have seen this protection scheme in several amps of other manufacturers as well. The EL34 heaters in the 500 are not connected to ground via a trimpot like in the Orchestra, but directly at a centre tap on the heater winding of the mains transformer.
So, if the same valve failure as described above with the Orchestra would happen in the 500, something else will have to burn. Note that before the cathode can reach something like +350 V, the 63 V rated electrolytic capacitor C1 will have to blow; and it does, I`ve seen it several times.
Once that it out of the way, and a short has developed between heater and cathode, in my experience what will burn is the 47 ohm 🔗 External reference
The Jadis Orchestra amplifier employs EL34 output valves, which are known for their robust performance in audio amplification applications. The design of the amplifier incorporates a series heater wiring configuration, allowing for a stable voltage supply of approximately 24 V to the output valves. This series configuration is critical for maintaining uniform heating across the valves, which is essential for optimal performance and longevity.
The trimpot, identified as a 1k resistor, was likely integrated into the circuit to serve as a hum reduction mechanism, connecting the heater supply to ground. This method is a common practice in tube amplifier designs, as it helps to minimize noise that can be introduced into the audio signal. However, the melting of the trimpot suggests that an unusual condition occurred, leading to excessive current flow through the component, which exceeded its power rating.
The protective measure of incorporating a fuse in series with the cathodes of the output valves is a standard practice aimed at safeguarding the amplifier's components from excessive current due to valve failure. However, the potential failure mode described highlights a critical design flaw. When a valve loses its vacuum, it can become fully conductive, resulting in a dramatic increase in current. This condition can elevate the cathode voltage to levels that exceed the insulation rating between the cathode and filament, ultimately leading to catastrophic failure.
In contrast, the Audio Innovations 500 amplifier employs a different approach to ensure protection against valve failure. The use of a 0.25 W carbon film resistor as a fuse (R1) in conjunction with a larger wirewound resistor (R2) provides a means of current limiting. This design allows for the safe dissipation of power under normal operating conditions while providing a fail-safe mechanism in the event of an EL34 failure. The direct connection of the heater supply to a center tap on the transformer also changes the dynamics of the failure mode, as the components will experience different stress conditions compared to the Jadis Orchestra.
Overall, the analysis of the Jadis Orchestra and Audio Innovations amplifiers reveals critical insights into the design considerations for tube amplifiers, particularly regarding the management of high voltages and the implementation of protective measures to prevent damage during component failure. Understanding these dynamics is essential for engineers and technicians working with tube amplifiers to ensure reliability and performance in high-fidelity audio applications.One of the EL34 output valves had blown, the normally silver coloured getter had turned white completely. When I opened the amp, I noticed the remains of a trimpot, including some melted solder, lying on the bottom cover of the amp.
Its value was only just readable: 1k. The official schematic diagram is rather rudimentary, so it gave me no clue as to where this trimpot had come from. Inspection of the circuitry showed some burnt remains near the heater wiring to the EL34s. In the Jadis Orchestra the heaters of the 4 output valves are wired in series and fed with about 24 V, so comparison between the affected channel and the channel that seemed still intact wouldn`t be of any help here. Further inspection revealed that it the 1k trimpot must have been soldered across the heater wires, with the wiper contact of the trimpot connected to ground: the classical way to minimize hum.
But I had never seen such a trimpot melt before, so I was greatly puzzled. At 24 V the dissipation into 1k is about 0. 5 W, which a standard plastic trimpot should be able to handle (if only just). There is no conceivable reason to suppose that the heater voltage had gone up spontaneously for a while. So what on earth had happened It took me quite a while before I could reconstruct the chain of events which led to this result.
Mr. Jadis decided to protect the power valves with a fuse in series with the combined cathodes of each output channel. This seems sensible but it now appears it is not. Think of what happens if a valve looses its vacuum. The valve will develop full conductivity. Now the fuse blows. The cathode will be lifted to a voltage near the +500 Volt of the HV supply present at the anode. The insulation between cathode and filament will not survive this attack for very long because the maximum Ufk of an EL34 is just 100 Volt.
A short-circuit will result there (between cathode and filament) and a new current path is created for the High Voltage. Happily the current, on its way to ground, found the 1 k trimpot, otherwise the high voltage supply, the output transformer and/or the mains transformer could have blown.
Once I had established this explanation, it dawned on me that the Jadis Orchestra is not the only amplifier exposed to these dangers. Take the widely sold Audio Innovations 500 amplifier for instance. There are no visible fuses in the cathode circuitry here, but look at figure 1. In most schematic diagrams of the AI 500 resistor R1 is not drawn, but it is physically there in all but the very early versions of this amp.
R2 is a hefty 7 or 10 W wirewound resistor but for R1 deliberately a 0. 25 W carbon film type was chosen. In normal operation the current through the 15 ohms of R1 is about 130 mA, which means a 1. 95 V voltage drop and results in precisely 0. 25 W dissipation. Should an EL34 fail, R1 gets overheated and blows, thus acting as a fuse. A similar `fuse resistor` is also found in the Audio Innovations 200 and 800. I`m naming AI models here because I`m very familiar with them, but I have seen this protection scheme in several amps of other manufacturers as well. The EL34 heaters in the 500 are not connected to ground via a trimpot like in the Orchestra, but directly at a centre tap on the heater winding of the mains transformer.
So, if the same valve failure as described above with the Orchestra would happen in the 500, something else will have to burn. Note that before the cathode can reach something like +350 V, the 63 V rated electrolytic capacitor C1 will have to blow; and it does, I`ve seen it several times.
Once that it out of the way, and a short has developed between heater and cathode, in my experience what will burn is the 47 ohm 🔗 External reference