[JN] Tube rattling

Phil toob-man at sbcglobal.net
Mon Dec 7 06:35:43 CST 2009


Well, I haven't actually used any, but LOTS of the big old transmitter
pentodes rattle; at least 1/3 of my 50 or so 803's and 813's rattle,
including about 25 NOS tubes, so I'm 99.9% sure that it is NOT a
problem. Most rattles are either (1) a loose piece of glue between the
metal base and the glass bulb, or (2) a metal or mica "spacer," a piece
meant to hold two parts a certain distance apart, that can move a bit in
the "non-spacing" direction.

In commercial tubes, the ones used by everyday people, the manufacturers
made sure that nothing rattled, because people thought -- understandably
so -- that something was wrong with the tube, but for tubes used in
radio stations and industry, they just did whatever was best for the
tube, and didn't care if it rattled, because they could tell everyone
that it was okay.

Now, if pieces of fine wire are visibly moving around, the filament is
probably broken. How much can you test these tubes? As you probably
know, on the 813, the two "fat" pins are the filament pins. My OLD and
uncalibrated B&K 2810 (but it has a zero knob!) says that RCA, GE, and
Ken-Rad's are 0.24 to 0.25 ohms, while an Amperex is 0.21 ohms (at
74F/23C), but if you simply GET a reading under 20 ohms, the filament is
good (and if it's over 1 ohm, your ohmmeter sucks!). My almost brand new
EXTECH True RMS 430 sucks, which is why I now own the 2810 ;-(.

I am currently (finally!) working on measuring the Miller capacitance; I
confess that it is a bit harder than I thought it would be, mainly the
part about figuring out which pins I need to hook up to what to get the
equivalent of Miller cap. Hopefully "I'll be back" shortly...

Phil


-----Original Message-----
From: sound-bounces at mailman.soundlist.org [mailto:sound-bounces at mailman.soundlist.org]
On Behalf Of B D
Sent: Monday, December 07, 2009 4:50 AM
To: sound at mailman.soundlist.org
Subject: Re: [JN] Tube rattling

I do want to know, because I don't want to waste money on two bad
transmitter tubes.

/Bruno


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