Ameritron grid overload protection Kit.....Anygood?
Hello: I have a used 811H amp that runs good, but see on Ameritrons website that they have for sale a GOP-100 " KIT" that provides for grid overload protection and is "easy to install" . Has anyone bought and installed it in a 811H? Is it a kit that you solder in the componets? How did you install it..any issues? Is it worth buying and installing it? Also they have a In rush current protector ( ICP-120), that stops " a terrible in rush of current to your amp when you first turn it on that will eventually damage your amp" for $80.00. Is it worth it? any issues? I read one account that said the unit went faulty with a sticking relay and someone else said that Harbach electronics has the same thing for $25.00 just looing for any advice thanks, Joe B
You really don't need the GOP unit. You have a grid current meter, use it. Only amps with tubes like 3CX800 and 8877 need this. Although the GOP won't do anything bad I just feel it really is not needed if you load properly and use the grid meter. The SS unit will stop the thud at start up. It too is not a bad thing either. Harbach does not make a SS unit for this amp to my knowledge.
I have never autopsied a kaput tube with a literally burned-out grid. However, I have autopsied many tubes where the filament is bent as the result of having delivered too much grid-I and is shorting against the grid.
Originally Posted by KC4LNX
. . One KISS way to protect a tube's filament from excessive control grid-I is :
1. put a fusible element in series with the DC path of grid current. This can be either a 3ag fuse or a smallish carbon-film resistor of 20 to 51 ohms. However to keep the fusible element from burning out on 10m the grid must be adequately bypassed for RF with C. For an 811A or a 572B, 1000pF seems to be enough.
2. Since 100% of the grid-I comes from the cathode/filament, it is important to limit the peak-current than can be drawn from the HV filter caps during a major glitch such as an intermittent VHF parasitic osc. Limiting I-pk to <200A seems to be enough, so for a 2000v supply, a 10-ohm minimum glitch-R in the +HV will do the job
• QST Magazine photo of a filament to grid short:
Last edited by AG6K; 08-02-2012 at 12:27 PM.
I recall reading a post by a well known ham who knows more about the AL-811H than just about anybody. He commented that he has never seen an 811A tube with a damaged grid in this amp.
When the Chinese 811's short its filament to grid; Id call that damaged. Ive seen AL-811's with the grid glowing for a very short time, dont know if that causes damage.
Molybdenum (Mo) wire is often used to make grids. Since Mo's melting point is 2,617°C it can run orange hot in a vacuum without damage.
Originally Posted by KM1H
I have Never autopsied a tube with melted grid wires or grid wires that were out of place. When there's a filament to grid short the element that's out of place is the filament - which apparently gets bent by a large pulse of current.