What is a "Grid Clip"? 19 Sep 2008 Mike at MDBVentures.com Visit us at http://www.MDBVentures.com - Great prices on great tubes! A Grid Clip can be made in several forms. It can be a flat metal plate, a "C" clip, or a mica washer. It's purpose is to reduce microphonics in the tube. The primary source of microphonics in a tube is when the elements inside the tube move in relation to each other. You can hear this happen when you thump the tube with your finger. There are a number of aspects that can cause the various elements inside the tube to move. The primary source of microphonics is the grid moving in relation to the cathode and plate. Stability of the grid is important. If the grid moves, it changes the transconductance of the tube, which is what causes the microphonics. The most common cause of grid movement is the grid supports moving. In most tubes, the grid supports are held in place by the mica support washers at the top and the bottom of the tube. For cheap tubes (typically found in old tube based TVs), there is little or no additional support for the grid. They simply rely on the pressure fit through the mica washer to hold the grid in place. For audio tubes, there is usually some attempt to stabalize the grid a bit more. The first step is that the connection from the grid support to the pin on the bottom of the tube is usually heavy and is used to keep one or both grid supports stable. The problem is that it still does not deal with the supports at the top of the tube. They can still vibrate if the holes in the mica are loose. To combat that, the grid clips are used at the top of the tube. The "C" clip is a small "C" shaped piece of metal that is spot welded to each of the grid support posts at the top of the tube. This provides stability to the posts by pushing them against the mica so they can't move as easily. In some later tubes, the "C" clip was replaced by a flat plate welded to the supports. This helps to keep the posts stable, but is not as good as the "C" clip because it still allows the supports to move within the mica holes if the holes are not tight enough. In a few cases there was a misunderstanding of the purpose of the plates, and you will see a small individual piece of metal welded to the top of each support. If anything, this just makes the problem worse by adding more mass to the grid supports. In other tubes, multiple mica supports are added to minimize the chance of the mica holes becoming loose. It is an easy addition with minimal cost since there is no additional welding needed. It is common to see this method used in the "industrial" numbered tubes (such as the 6201). In some tubes, the "C" clip is replaced by a special mica washer that is used to apply pressure to the supports. This works with tubes that have low mass for the grids, but does not work as well for larger tubes. This is more common in low microphonic pentode tubes (such as the 5879) where multiple grids need to be stabilized. In more expensive tubes, additional work is done to try to stabilize the grids, cathode and plate. Bendix Red Bank tubes are particularly prized because they went to great lengths to make the tubes as stable as possible (which also made the tubes very expensive to build). Not all tubes even with the same tube number are constructed the same way. Some may have the grid clips while others may not. The tube number will not tell you about this construction difference. The only way to know for sure is to physically look at the tube. Note: don't confuse the grid stablizers with the screen grid radiators. On large beam power tubes, you will sometimes see large plates attached to supports at the top of the tube these are used to help radiate the heat from the screen grid which in power tubes can build up quite a lot. --- A quick first test on how much microphonics a tube will exhibit is to hold the tube next to your ear with your thumb and middle finger while lightly tapping it with your index finger. Each tube will sound slightly different. The type of sound will not necessarily tell you specifically how much the tube will exhibit in the way of microphonics nor what the characteristics of the microphonics will be. However, it is a quick first test. What you want to hear is a short tight sound, "dink" or "thunk". A longer ringing sound, "ding" is not as good. The louder and longer the sound lasts, the more likely it will exhibit microphonics. --- Even the best most well constructed tube will still have some sensitivty to microphonics. Tubes are physical beasts, so there is no way to completely stop it. One way to help minimize the problem is to isolate the tube socket on rubber mounts. This reduces the transmission of any vibrations that might get to the chassis from getting to the tube. Be sure to use flexible wiring to the tube socket, or you will defeat the purpose of the rubber mounting.