ST120 click/flash in one tube

grosen wrote:Hi all,

My ST120 has been trouble free since I built it, but just now I had a click and a flash in one tube (KT120), and now that socket won't bias.  (I get a reading of 20V or so.)  The problem doesn't follow the tube.  Can someone give me a clue about what to check first?  

Thanks,

Gideon

Chad Spackman wrote:A question related to the flashover. In my case it was a 6550. New tubes. Installed, biased, and idled for about 30 minutes. Started listening and while turning up the volume, one tube flashed. My question: are 6550s more susceptible flashover this than kt88s? With a static plate voltage of mid 500 volts, there is going to be a positive voltage during AC swing on the non-driving tube to well over 600v, which appears to be the limit on 6550s.  

corndog71 wrote:This can happen to any tube.  Most tubes are not made with white gloves.  Debris happens.

Bob Latino wrote:I consulted with Roy Mottram just to be sure before answering here ... On an EL34 (and also on 6L6GC) pin 1 is tied to the screen grid (pentode) and should NOT be left unconnected. These tubes are almost always connected to cathode on 90% of tube amps out there. On all KT88, 6550 and KT120's pin #1 has no internal connection to any other pin although some brands of these three tubes do connect pin 1 to the metal base of the tube. It is better to connect pin #1 to pin #8 so that both are near 0 volts rather than having them floating above chassis ground. If you connect pin 1 and 8 together, you can then use virtually any type of octal output tube in a Dynaco or VTA amp.

Bob

Jim McShane wrote:

Bob Latino wrote:I consulted with Roy Mottram just to be sure before answering here ... On an EL34 (and also on 6L6GC) pin 1 is tied to the screen grid (pentode) and should NOT be left unconnected. These tubes are almost always connected to cathode on 90% of tube amps out there. On all KT88, 6550 and KT120's pin #1 has no internal connection to any other pin although some brands of these three tubes do connect pin 1 to the metal base of the tube. It is better to connect pin #1 to pin #8 so that both are near 0 volts rather than having them floating above chassis ground. If you connect pin 1 and 8 together, you can then use virtually any type of octal output tube in a Dynaco or VTA amp.

Bob

Bob,

I think you meant pin 1 is connected to the SUPPRESSOR grid, not the screen grid.

Also, you can use an ohmmeter to check if pin 1 is connected to the metal base - just connect the meter from pin 1 to the base and see if you have continuity.