how do you know a bad tube?

Depending on what kind of tube failure you are experiencing, you can sometimes get a good idea by simply looking at the tubes closely.

Newbee
Well I sort of got that bit all by my lonesome, but it's good you pointed it out. ...and if you have but one tube and no twin? (several tubes, two singles, different types: and one type is pairs)??

Montytx
Thanks. perhaps you missed my handle, and refference to an oscilator (audible indicator), but thanks anyway. Although, unless one is familiar with the tube archetecture beforehand, past the obvious, what could one hope to glean?
Maybe I missed your point.

if there are same/same tubes on either channel, certinaly one can swap back and forth. Sure, if a tube won't light up.. yep, it's dead. There are tube components that do not duplicate the tubes within it throughout its design.

Then what?

In that case there is no other tube present to swap about within the unit. That scenario happens to be the case in an upcoming unit for me, so I thought it may well be also the way with the possible amp (s), therefore I thought to avail myself of some greater experience, or unbeknownest to me, device or method, before the fact.

it would seem at this point only having exact same spares or a tester, is the ticket... and/or at least an inexpensive integrated with pre outs and main ins, or a couple of real budget pre & amp pieces about just in case.

You should have at least one extra set of tubes. That extra set will provide the tubes to swap and test for the bad unit. It also means your equipment is never out of commission just because a tube is going bad.

Jim,
when I owned the VK5i I found switching tubes side to side could usually isolate or even make go away the problems I was having with noise....like other posters said, if the problem switches to the other channel then you have identified the culprit.
regards,
LarryK

Larryken
Got it. Thanks. i never did notice much of a diff in the VK5i, from swapping from side to side... as the thing biases itself each time... but then I must never have had a severely poor tube. the biggest/best thing in that unit I found was having all the same tubes in the signal path, and play with something in the shunts. ultimately, i found that the simplest and best way to outfit the unit.

Larryi
it sure does seem the way. Another set entirely. Geezzz Louise.

See I was thinking there is some really neat gizmo out there that one can plug a tube into for just strength or short testing. Like a wiggins voltmeter.... and I'd not have to spend hundreds for a tester i probably cant read well enough anyways... and need to periodically recalibrate.

Oil well. Thanks. Thanks very much... of course if there is someone out there who knows of a simple tester which does a good job is sturdy, with B-I-G meters, and seldom requires calibration...I would like to hear about it, and where it is available.

A couple of the tube amps I am thinking about have quite a number of tubes in them... and some singles in there as well... an affordable tester I could use does seem relevant to have if you're going to pursue an all tube system, doesn't it?.