Valve Amplifiers – A Forgotten X-RAY Hazard

A friend of mine asked me about valves a while ago, and mentioned that he had a memory from his youth of a valve that “glowed blue”. I told him at the time that there were a few of those, but it was not until recently that I remembered the reason for it.

Inside every valve, a metal target (the anode or plate) is bombarded with a stream of highly energetic electrons. When they impact on that target, X-rays are given off.

Whereas the glass envelope is generally enough to prevent the X rays from harming people in a closed cabinet, I am not so sure about the modern practice of “displaying” the valves mounted on the chassis with regard to safety.

In days of yore, people were more conscious of this hazard, and were well aware that the paint covering many old valves was there for the reason of stopping X rays. Those patches of blue on a valve were there to help stop X rays emanating from particular hot spots of a valve’s structure, and to identify the valve as a potential X ray hazard.

Frankly, I am unsure what the old regulations about X rays from valves said, and whether they have been updated in any way. I’m just going to make sure that I don’t use any unboxed valve-powered equipment, unless absolutely necessary for fault diagnosis.

You have been warned …



Football, Soccer Reporting – Pathetic


I don’t read newspapers much these days, but I always read the football pages of the tabloids when I visit my local greasy spoon café.  And I’m fed up of the coverage being almost entirely restricted to the premier league.

Last Saturday I bought a Daily Mail, the sports pages of which were heavily devoted to international rugby  – fair enough, it was happening that weekend.  OTOH there were pages devoted to the Premier League = no real comment about anything happening in the Championship (or the tennis, or football north of Watford).  I see the same effect in the Sun, Star and Mirror – I’ve taken to buying the Sunday Post if I want a more balanced view of things.

Today, 13th November 2014, there was an Under-21 international match played between England and Portugal at Turf Moor, Burnley. The score was England 3, Portugal 1, and you will be lucky to find a report in any of the major daily papers.

I got the info from Sky Sport – which is the only place that I can find that has a report of the match.

Strange that although the English claim to have invented football, they don’t say much about it when they win …

Good To Middling, Middling To Fair – grading raw cotton.

I have head these phrases all my life, but only recently found out what they originally pertained to – they are used in describing grades of raw cotton.

There are three main grades –

GOOD is the lowest grade that is useful,

MIDDLING is a better quality,

FAIR is the best quality of raw cotton.

So Good To Middling is between good and middling, and Middling To Fair is nearly the best, but not quite top grade.   And something that is not fit to use is described as Less Than Good.

Now you know, too …

A Couple of Bargain Valves / Tubes

Here’s a couple of tube / valve bargains for you. What keeps the prices down on these types is that the cathode is internally connected to one of the heater pins.

Given that the average amp builder’s skillset does not encompass electronics beyond following a layout, these are being avoided by a lot of people until someone shows how to use them in an application.

The 5R-K16, usually written 5RK16 in dealers’ lists, is a double diode full wave rectifier in a 9 pin miniature package. It has a 5V heater drawing 1.2A, and is very suited to applications where a EZ81 / 6CA4 would normally be used, despite the pin-out differences. A Japanese design for small hifi amps, you might be surprised to find that a lot of them were US branded. At $3 each, they are a lot cheaper than a $25 6CA4. And so many transformers come with a 5V rectifier tap as standard …

What is a 6V5GT ? It is very like a 6V6GT with a different pin-out, and it has that internal connection between the heater and the cathode. At the moment a 6V5GT costs $6, a 6V6GT costs $17.

It is often worth learning a little more than you know already!



I got a message when I tried to start my computer:

“The Group Policy Client service failed the logon.
Access is denied.”

I used my tablet to search for solutions. All the answers I found were incomplete, or not suited to my situation. I did find out that the reason for the problem was that my account files were corrupted. In my case, a lot of them had gone missing!

My quick fix:

Start up or re-start your computer, then immediately hit the F8 key until the recovery screen comes up. Start the computer in SAFE MODE.

Open the Control Panel and go into the User Accounts section – when you get to
Control Panel\User Accounts and Family Safety\User Accounts\Manage Accounts

you will find at the bottom of the box showing the existing accounts a link marked “Create a new account”.
So create a new account with administrator privileges and a password, different name – if your old account is John, call it John1 or Johnny.

Now you can use the new account to get into your computer.

So re-boot the computer, and use your new account!

First thing you should do is run the chkdsk utility because there is obviously something wrong on your hard disk. That may start automatically during the reboot – let it run!

Next, use your administrator privileges to access any files you want in your old account, and move them to the new account.
Then you can go back into the Control Panel and delete your old non-functioning account. After that, you can re-name the new account to whatever your old account was called.

I know, that’s hours of innocent amusement.

WTH find a better fix if you can. I already looked, and that was more hours of innocent amusement and wasted effort before I did it this way.

Using Automotive Valves To Build Guitar Amps

Amp builders across the world are suffering from their mains voltage being higher than nominal. This is very common in both the USA and UK, although I write from a UK point of view. Some days my mains suppply is 240V, sometimes you can watch the metered value slowly wobbling between 230V and 235V at a frequency similar to a wind turbine blade’s rotation.

The problem is simple – if you have a PT set at, say, 235V, and the hole in the wall supplies 250V, the 6.3V heater voltage is more than 6% higher than it should be. You have +/- 5% as the default spec on most 6.3V heaters, so 250V turns the amp into a valve-eater, as Bronsboi is my witness.

Automotive valve types can operate on a wide range of heater currents, because the heaters are designed to run in a car radio . A car battery at peak charge gives about 14.4V, whereas in the cold of winter it may only give about 10V. The heaters of the valves can allow for this variation, although shortened life may be the result if they are run below 12V for sustained periods.

So if you run them at 12V on the heater, they can handle the variations in mains current easily. The design optimum is usually 13.4V, but 12V or more is acceptable.

A lot of the automotive types can be run on AC or DC heaters, although some are specified as DC only.

There are problems finding rectifiers that run on 12V systems – and never forget that you want a separate transformer tap for the rectifier because the amp’s HT/B+ supply draws from the rectifier cathode.
The most useful one I have found is the 12BW4, which is the automotive version of the 6BW4. These are very similar to the EZ81, but slightly less poky – you can run a pair of 6V6 types, but EL84 are just a bit too much.

Overall, the most useful types for amp building are
7058 – derived from 12AX7
7059 – derived from 6U8
7060 – derived from 6AU8
7061 – noval version of 6V6, derived from (DC only) 12AB5.
6679 – 12AT7

All these can be used with AC on the heaters.

You can’t use the 6V heaters on a 7058, but otherwise it is a RCA 12AX7, and the prices are shooting up to reflect that. They are currently at about $20 from TubeDepot.

The rest of the valves I have mentioned are $3 – $4 in a lot of places.

The most obvious possibility with these is a Marshall 18W style amp with output resistor values adjusted to suit 6V6, rather than the standard EL84 output valves . Use 7058 instead of 12AX7, 7061 instead of 6V6, 12BW4 instead of EZ81.

I keep running across other 12V types with possibilities. 12CA5, 12CU5, 12ED5 are all cheap and could be used to build small amps.

Lots of possibilities!

Laptop Cooler Tray Cools Amp Head!

Having created my innovative independent laptop cooler, it is not much use with my laptop – it is too small and the laptop rocks if I use it. I can use it for other purposes, but it needs a small modification.

See looky here –


Underneath a laptop, there is no need to protect the back of the fan, but in other applications the fan is wide open to poking fingers and so forth. I need some kind of ARRID (Anti-RugRat-Intervention-Device). Back to the pound shop for one of these –


Oven Mesh – lets you cook on the oven shelf, and also  can be used to block unsightly holes in the back of an amp head while permitting adequate ventilation. That’s another £1 spent on this project –

So I cut about two-thirds of the sheet and used it to prepare my ARRID, tying the corners with that plastic coated twist wire used to secure cables in new appliances –


That shields the fan from poking fingers. It can be attached to the fan with a couple of strips of gaffer tape, but before you do that, you need to pass a bungee rope (another £1) through the gaps between the ARRID and the frame, so that the assembly can be easily attached to an amp head. Just strap it over the back vent with the bungee, and it will pump air into your amp for forced cooling purposes.

For the purposes of demonstration, imagine that the cardboard box is an amp head, and the fan is over the back vent –


So that is how to make a cheap and independently powered cooler that fits most amps for less than £5. Not very pretty, but it works, and you don’t need to make any modifications to the amp to use it.  The last one I made was neater, which is probably why someone gave me £10 for it.

Cool Laptop Cooler!

Using a laptop cooler stand is a good thing for your laptop. Unfortunately, most laptops don’t have enough USB sockets for the number of things you want to plug in, and mine is no exception.

So I bought a laptop cooler stand – from the pound shop – plugs into the USB –

LaptopCoolerBox   Unfortunately my £1 only got me this tray with a fan, not the nice laptop illustrated on the box –


But if I plug this in to my USB, I have to disconnect something, or use a USB hub to give me more sockets.

If you don’t already have one hanging around the house, for another £1 your local charity shop / thrift shop can probably provide you with a “wall wart”  mobile / cell phone charger that puts out a half-wave rectified 4.5V to 5V DC. Read the small pint specification on the charger.


Before you start, plug the cooler into the USB so that you know which way the fan is supposed to blow. If you connect it up briefly the wrong way round, you will probably not damage anything, but you need to know which way round is correct.

Get some high technology –


Use the side cutters to cut two terminals off the connecting block, then chop the USB plug off the laptop cooler and the connector off the phone charger. Bare the wires and connect them using the piece of connecting block.


It can be a pain to strip back those fine wires. I find that making a nick in the insulation and just pulling a chunk off can be more effective than tying to strip them conventionally, where you can easily cut into the wire itself.  And when you make the connection, fold the wire back over and trap the insulation with the screw as well, because the wire on its own is often  not thick enough to make a good connection!

Of course, soldering the wires together and potting the joint up in epoxy is a more reliable solution, Glen, but this is a £2 apparatus when all is said and done.

Here it is working – you can tell it is working because it is fitted with unnecessary blue LEDs to tell you –