Wednesday, July 8, 2009

Are tubes with broken bases still good?

Here's a pair of EL34's that came out of a very unhealthy Traynor YVM-1. Take a close look and compare the bases:

broken guide post or key on a vacuum tube base

The black cylinder in the center of one of the tube bases is broken off and the bottom of the glass envelope is poking through the bass. This central post is commonly called the key, guidepost or locator pin. It's a bit easier to see here:

broken aligning key on a vacuum tube base

So is there anything wrong with using a tube with that black center piece broken off?

Well, yes and no. It doesn't have any electrical function, but you do need to be careful. To explain why I'll start with the bottom of the unbroken tube:

vacuum tube base from the bottom

You may be able to make out the numbers labeling the pins, You can also see that the center piece is not a perfect cylinder, it has a small protrusion between pins 1 and 8. I've diagramed both in red here in case it's not completely clear from the photo:

vaccume tube base key and pin numbering
That little protrusion is meant to lock into the groove in the tube socket. With it in place the tube can only plug in in one orientation.

vacuum tube valve octal socket

Someone must have tried to force a tube into this socket in the wrong orientation. You can see a chip in the socket made by the tubes key as it was forced in:

vacuum tube valve improper insertion

My guess is that that is what broke the key off of this tube:

vacuum tube valve with broken base key or guidepost
With the key broken off you don't have to fiddle around and align the tube. I can fit it in 8 different ways. That certainly makes things easier. Let's look at the connections made when the tube is inserted. Here's the same socket rotated 180 degrees so the key slot point down:

proper output octal output tube valve alignment

Now I've overlayed the schematic diagram for an EL34 to show how it's internal elements are connected to those pins.* The black block at the bottom of the diagram indicates the key. I've lined that up with the groove in the socket to show a properly inserted tube. I've also labeled the socket connection with ballpark voltages you would expect to find on those sockets.

vacuum tube valve pin out voltages from the bottom
Now imagine that the tube is inserted so that the key in the diagram lines up with the chip made in the socket when the tube was forced in. The voltages present on the sockets stay the same the tube elements that those voltages are connected to would be different. I've rotated the tube diagram below to illustrate the point:

improper output octal output tube valve base key alignment

The most obvious problem here is that the 445 volts on socket #4 is now connected to pin #7 on the tube.

Inside the tube pin #7 is pin # 2 by the heater element. That effectively connects the 445 volts on socket #4 directly to socket #2. In the amplifier circuit socket #2 is connected to ground through the heater transformer.

This ends up giving the 445 volts a very low resistance path to ground, which translates into a huge amount of current:

improper output octal output tube valve guidepost alignment
Grounding the screen voltage should have blown the fuse but apparently it didn't. It could be that the chip in the socket was just the first attempt and once the key was snapped off the tube was plugged in again in yet a another orientation (once that key is there's only a 1 in 8 chance you'll get it right by guessing). If you do this and you're real lucky the fuse will blow before you cause major damage. This amp wasn't lucky. One side of the output transformer blew. The open transformer winding caused the arcing between pins 2 and 3 on the socket on the other side:

In total the output transformer, the output tubes, screen resitors and tube sockets all needed to be replaced. Not cheap!

So can you still use a tube with the key broken off? Yes, sure. It'll still work if it's aligned right. Just keep in mind the steep price you could pay if you are off by a pin or two, you may not get a second chance.

* You may have noticed that the numbers on the bottom of the tube run clockwise while the numbers on the base and the diagram run counterclockwise. Tube diagrams assume that you are looking at the top of the tube socket. Usually the tube diagram will match the tube base but since we're looking at the bottom of the tube socket in this I've flipped the tube diagram to match the view. Doing that causes the numbers to progress in the reverse direction.


drjjpdc said...

I did the same thing to a KT88 tube on my Quicksilver V4's today. Thank you for your excellent explanation. One other thing if the tube socket is not visisbly cracked or damaged, do I have to fish out the broken tip that fell through the opening into my amp?

akavalve said...

The key that broke off isn't conductive so it's not going to short circuit anything. I'd generally get it out just for good measure, but in a stereo amp that really doesn't move around much you should be fine leaving it in there (and if you're not used to working inside a tube amp it's much safer to just leave it).

Anonymous said...

Broke off the key on a tube, but have the piece that broke off, any recommendations on what glue to use to glue it back on? Anyone done that here?


drjjpdc said...

Bruce, just look at the 2 comments above you. The important thing is that you put the tube back without the key. And make sure the holes are lined up with the right numbers. You can still do it without the key but you just to look at where the break was carefully, otherwise you will blow a fuse.

For safety sake go back to the original post for the whole explanation.


Anonymous said...

You are not "looking at the bottom of the tube socket". You are looking at the TOP of the tube socket, since the bottom of the tube is a mirror image of the top of the socket you need to invert the tube pin-out to match the socket. Another way to say it is that you are looking at the bottom of the TUBE from ABOVE, not from below, so the image must be reversed.

The bottom of the socket matches the tube base diagram, something important to remember when working under the chassis!