• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

„New“ Pentode – measuring resistance between grids G2 and G3 – any idea?

I recently bought some NOS EL3010 pentodes from TFK. Out of bad experience I measured them for shorts between the various grids, anode, and cathode, and the capacitances. One tube shows a resistance of ca. 280 Ohms between grids 2 and 3, so not a dead short but not exactly “open” either.

Does anybody have any experience with this type of fault and can the tube be rescued? If this would just be some getter material that found it’s way onto the mica wafer I could try and “burn” this off? Many thanks in advance.

P.S. One tube is completely dead, the welding between the wire coming from the anode cap and the anode structure appears to be broken. I would not have expected this from a special quality tube that is supposed to be shock resistant but there you go.
 

tubewade

Member
2007-06-03 8:54 pm
Martin,

Yes, 280 ohms is a short when we are talking about valves. Since it is already useless in its present state, there is little to be lost by sending a little current through there to see if you can burn off whatever is causing that short. It may not work or maybe it will. This, of course, unless your supplier has some return policy.

Wade
 
Thank you all for your replies. I was hoping that this might just be a deposit of getter material that I could burn off but upon further testing, the short sometimes "disappears" when I move the valve then comes back, so this looks like a piece of the screen wire that moves around :scratch1:

I am now trying to get a refund as the tubes were described to be new and "in working condition". Unfortunately the EL3010 is a very rare tube that is hardly ever seen on ebay, so I would indeed prefer to have a working tube.