Repairing Vintage Equipment - AVO 8

Repairing Vintage Equipment – Radio tubes and thermionic valves



The opening… Repairing Vintage Equipment – Radio Tubes and Valves

We recently received a mail from a reader whom asked us what the procedure (self-repair) was if they had received a valve amplifier which was faulty but needed to conduct a repair – what to do, where to go and as a last resort, how to repair.  A local repair depot had quoted nearly 150 British pounds to repair but did not state what parts needed to be replaced.

Unless the repair depot was a service agent it’s quite common practice not to state what parts are going to be replaced in the quote – service agents are usually more expensive and often customers play repair companies against each other to get prices driven down or even attempt a repair themselves. This article is not written for the novice but for someone in the electronics field having not encountered valves before. We expect you to know your way around an oscilloscope and most of all remember that valve or tube equipment operate at lethal voltages – you have been warned.

Repairing Vintage Equipment - AVO 8
AVO Model 8 Mk7 by Megger

We’ll stick to the term ‘valve’ here, one which I am accustomed to – tube is invariably used in American books, both referring to the ‘thermionic tube or valve’.

Valve equipment uses potentially lethal voltages so anyone attempting a repair must be aware of the dangers involved.  High powered equipment, more so. Medium power transmitter anodes can run up to 3KV or more with power supplies delivering up to one Ampere or more. These voltages and current will kill you if you are not careful. You have been warned.

A valve device is a voltage device. They have a very high input impedance. Voltage driven devices usually are. Valves are sold usually as diodes, triodes, tetrodes or pentodes. The diode conists of a heater, cathode and anode. The anode is sometimes referred to as a ‘plate’. The triode has an extra electrode called a control grid. The tetrode has a control grid and screen grid. The pentode a control grid, screen grid and a suppressor grid. Power tetrodes are also very common. See images of the five below with pin out. Valves are still manufacturerd in China and Russia. It is imperative that readers here know the differences between the valve types and where or why they are used. Wiki does have the answer.

Repairing Vintage Equipment - common tube types
Thermionic Tubes – Radio or Electron Valves

Valve equipment invariably uses less components than a semiconductor unit in audio or radio for equivalent power, especially so in the higher power range, so don’t be surprised to see a 5 valve device delivering about 200W or more into a loudspeaker load. I have seen small SSB transmitters with two devices in the output stage running at 3KW input (class AB1).

With modern circuit techniques it is cheaper to build in safety circuitry using semiconductors – this will be seen invariably in high power transmitters. The rectifiers in the PSU can be removed and replaced with far more efficient silicon devices – newer equipment won’t use valves to rectify as they are costly and very inefficient. Semiconductors of course are much cheaper too and run cool.  High power audio equipment may use circuitry to monitor currents, voltages, bias etc and with modern circuit techniques many audiophiles discard the older technique in favour of semiconductors using only the thermionic devices in the amplifying stages and low noise components outside this chain. As this article is intended more for someone not having knowledge of lower power valve equipment we won’t be looking into this circuitry or the modification thereof. Note the warnings and cautions. We won’t be held accountable for your early demise.

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