Valves

Previously there have been bits of information about valve faults, testing and replacement on various pages. I have now gathered it all together here, where it can be found. So far most if it is based on comments and suggestions from visitors.


Removing and fitting

If you need to replace any valves, remove them by pulling them straight out if possible; do not wiggle then from side to side as this may damage the base or holder.

If the valves have plastic base sections (such as octals), remove them by gripping the bases and not the glass. It is common for the glass bulb to become detached from the base, and be held only by the wires - and if you were to pull on the glass, the two parts could separate. Once the valve has been carefully removed, the two parts can be fixed back together by with a little superglue.


Testing in the set

Nigel Hughes made the following comments:

The first thing you should do with a radio is to obtain the relevant data from valve data sheets. Once you have the HT supply up and running and have replaced dodgy capacitors and resistors, you can do a great deal to check the health of the valves by measuring the voltages on the electrodes. When autobias is provided by a cathode resistor, a voltage measurement across that resistor will enable you to calculate the total cathode current. Cathode current in milliamps equals cathode voltage divided by cathode bias resistor value in K Ohms. Provided that the valve is being used for the function intended by the manufacturer, you can compare this with the total cathode current derived from the data sheets. Get the cathode current by adding the anode and screen currents. In the case of frequency changers you will need to add more than one screen current or anode current. Provided that the total cathode current is more than about half that given in the manufacturer's data sheet, it is likely that the valve will work well enough for the radio at least to run. However, there are cases where valves are used quite differently from the original intention. For example, an RF pentode may be used as an AF amplifier, often with a very high anode load resistor and very low cathode current. However, this should be fairly obvious from the circuit diagram. If you are lucky enough to find your valve in the Mullard data sheets, values are given for the normal cathode bias resistor. If the resistor used in your circuit is similar, it is a fair bet that the valve is being used similarly to the manufacturers intentions.

When you have completed restoration of the radio, note down the actual voltages for each valve. This will give you a baseline for later comparison if something goes astray.

John Perata added:

Test tubes (valves) by measuring cathode or plate currents under actual operating conditions in the radio after replacing defective support components. With some experience you will recognise the correct values. More accurate than the best tube testers as done at actual operating frequency. I use a set of adapter sockets with open plate and cathode pins.


Appearance

Nigel Hughes made the following suggestions:

There is not much that she can do about the internal workings of a valve. However some repairs can usefully be made to the outside. I usually start by cleaning the envelope with a mild detergent. If this does not work, my next step is to use white spirit applied with a soft cloth. I generally avoid going over the valve markings as some of these are already quite faint. Metallisation is particularly difficult to clean. It is worth starting with the soap and water treatment just to get the dust off, following up with white spirit. The markings on metallisation seem to be particularly robust and are not easily removed by cleaning. In severe cases I have successfully even resorted to cleaning with cellulose thinners applied with a soft rag. So far, I have only used this on valves with silver or gold metallisation, which appears completely resistant to attack from cellulose thinners. Silver metallisation can be cleaned very well with Jenolite rust remover, washed off with water after a minute or so. This treatment leaves gold metallisation looking rather bronze.


Repairs

Nigel Hughes made the following suggestions:

If valves have come loose from their bases, it is quite likely that the contact between the metallisation and what is usually the cathode pin of the valve will have been broken. This contact is usually made by a loop of wire round the envelope at the top of the valve base. After the valve envelope has been glued back on to the base, using super glue, epoxy or whatever, the connection can be restored by using conductive paint of the type sold for repairing car rear window heaters. The colour will not match the colour of the metallisation, but once the conductive paint is thoroughly dry it can be over-sprayed with metallic gold, silver or red to match the existing valve metallisation. When doing this, it is a good idea to mask off the valve number markings. If the metallisation is mostly intact, but has a few holes these can simply be touched in with ordinary metallic paint, using a small brush. Note that ordinary metallic paint is non-conductive, presumably because the metal content is low and the metallic particles are insulated from one another by the lacquer in which they are suspended.

John Perata added:

British Rimlock (41 series) tubes, particularly the Mullards have a habit of developing internal leakage between pins at the base. Visual as a black deposit between pins. Check this before suspecting a leaky socket. Can often be cleared with a burst of energy from a fully charged 600 volt electrolytic.

Brian Symons said:

I have "repaired" several valves to save them in the past. Reglueing the top cap on or the base if loose is simple. If the top cap wire or a base wire has broken then you can empty the solder from the pins or top cap, extend the wire with a bit of fuse wire or similar, pull them through and resolder. A wire that has broken off just at the glass seal can SOMETIMES still be fixed. A gentle filing to remove a bare minimum of the glass can allow you to solder on. The easiest way is to form a loop that slides over the wire sticking out and to solder that - sort of like a lead coming through a pcb pad. You must also be careful of too much heat. You can also "adapt" a valve of one base type to another of similar ratings but different pinouts or base type by extending wires as requires and insulating them where they need to be. IT IS VITAL to note on the equipment that such mods have been done or to ensure that all valve markings are rubbed off to prevent confusion.


Replacements

Leon Crampin said:

The UF41 (used for IF amplification in many radios including the Bush DAC90A) is getting a little difficult to obtain. Connect together pins 3,4 and 7 and you can fit a UAF42 or the UF41 without further changes. New Philips UAF42s are available from Colomor for 2. The reference to Internal Connections on pins 3 and 4 on the UF41 was a ploy by Philips to prevent the arch-rival Mazda 10F9 from being used. On every UF41 I have ever seen (quite a few) pins 3,4 and 7 are connected internally.




This website, including all text and images not otherwise credited, is copyright © 1997 - 2006 Paul Stenning.
No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.