tweak fusing valves.

Posted on Feb 6, 2014

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

Click here to download the full size of the above Circuit.

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...

Visit page

Leave Comment

characters left:

Related Circuits

12V Battery Checker Circuit

This is a 12V battery checker circuit that uses 3 LEDs that light up at their respective voltages. The red LED lights up when the battery voltage is between 8 to 10V, the orange one at voltages...

230V Flasher Circuit diagram

This circuit operates with 230v. you can use this circuit to decorate your parties. I think this will be a wonderful circuit to you all. Here DIAC ER 900 and Triac BTW 11-400.

Automatic light activated switch circuit

The circuit can be used for switching OFF a particular lamp or group of lamps in response to the varying ambient light levels. The unit once built can be used for switching OFF a lamp when dawn...

BC548 Heat Sensor Diagram Circuit

Heat sensor circuit can be used to control any device using heat sensor. In this circuit a thermistor and a resistance is connected in series. This arrangement makes a potential divider circuit....

GSM CELL PHONE JAMMER SCHEMATIC

A wonderful diy gsm jammer or cellular mobile phone jammer schematic diagram for use only in GSM1900 with frequency from 1930 MHz to 1990 MHz. The GSM1900 cellular cell phone network is made use of...

Biosensor circuit

This week was spent completing a number of different objectives. Firstly, the software was downloaded to design the schematic for the circuit to be translated onto a PCB. Much of the week was spent...

Instrumentation circuit

The mechanical chassis prototype, as can be seen from the image on the left, was made using two materials: foam and aluminium. Foam was chosen due to its easy manipulation and aluminium due to its...

Hum Circuit

Although our kit utilizes an off-the-shelf microcontroller (AtMega328P-PU based Arduino), we additionally designed a simplified version. This allows other collaborators and students to potentially...

New Circuits

Zero consumption Anti-theft High power Code Switch for E-Bike Batteries

This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
Ultrasonic distance finder circuit

The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Simple Metal detector circuit with CD4030

Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Fully automatic watering circuit for flower pots

Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
Simple deep searching metal detector circuit

The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
Sub-Harmonic Bass Converter for Electric Guitars

The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
Echo Depth Sounding Sonar for Boats

In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
100W HiFi Audio Amplifier Circuit

This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
Plant-Pot Water Level Indicator Circuit

A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....

Create NOW your QR MENU for your Restaurant / Hotel / Bar / Caffe,

Comprehensive ordering/paying features. fully personalized to suit your Business.

Update Anytime! Easy and Simple!

Popular Circuits

USB Single Cell Charger

Lithium Polymer Batteries are a very common source of power today. Many electronics gadgets have...

Diode tester using PIC 16F84

This is a simple use of the PIC 16F84 about a diode tester. Test procedure : We set «1» to PB0...

Power Amplifier 6

This simple circuit was a presented, it can give power output about 1 watt, when connected to a...

How to implement a soft power switch controllable by microcontroller

A circuit such that the microcontroller can toggle a GPIO pin and shut the whole system...

LED ramping circuit

In this circuit the intensity of LED will vary in a ramping fashion. The circuit consists of...

Circuit Project: Wireless Doorbell

It is a wireless doorbell with a cost of about $10. 00. So, we have to change our attitude to...

Negative Auxiliary Voltage

Some circuits need a negative supply voltage that only has to supply a small current. Providing a...