Re: [Amps] post

[R.Measures <r@somis.org>]

On Dec 21, 2004, at 5:36 PM, larry williamson wrote:

> Just wondering if anyone has had this problem with this amp. My amp
> is still under warranty so that is good. Anyhow, after having the
> amp on standby for about 1 hour, a very loud bang and is heard.
> After a call to Willie at Ameritron. We came up with a bad tube and
> a blown 51-ohm grid resister.

Larry --  A bad tube does not make a big bang because an arc in a 
vacuum is not noisy.   For example, when testing a vacuum-C, when the 
high-pot tester potential rises to the breakdown point and the C arcs, 
the sound heard is barely  audible because there is no atmosphere 
around the arc to transmit sound.   Thus, the sound you heard probably 
originated in the atmosphere, not in a vacuum.
-  Big-bangs and blown grid resistors often accompany intermittent VHF 
parasites.  The way to determine whether a parasite or a gassy tube 
blew the grid-fuse R is to measure the anode-grid leakage current of 
the tube with a high-potential tester.  Set the tester to c. 2x the 
anode-V that is present in the amplifier and measure leakage current.  
If the current is under 10uA, the tube does not have a bad vacuum.

> The D117 diode is was ok and did not
> need replaced. Now this has happened two times now on different
> tubes. Willie said it was a tube problem and the quality of the
> china tubes was not that good.

Early Chinese tubes had problems -- especially with the glass recipe.  
Recently produced Chinese tubes seem to be pretty good --  If they are 
in a stable amplifier.   In an amplifier that is marginally stable, the 
better the tube, the greater the chance of regeneration and stentorian 
bangs.

> He also said not to leave the amp in
> standby very long.
> Has anyone got any thought this?

--  The TL-922 is probably the best example of an amplifier that is 
unstable in standby. An HF amplifier should be unconditionally 
VHF-stable in both standby and operate.  The way to achieve this is to 
artificially reduce the VHF gain of the tube by decreasing the VHF-Q of 
the parasitic suppressor.   cheerz

>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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