[R-390] Megacycle detents, nominal B+, and bad tube brands

[Roy Morgan]

At 06:39 PM 6/6/2005, Tim Shoppa wrote:
>The megacycle detent on my 390A seems not as tight as it should be.

Tim,

The detent spring, and to a lesser extent, the detent disk (whatever the 
right name is) are the culprits.  Bending the spring is a start. Do check 
that it is not worn out and making little shavings from the disk 
(unlikely). You may be able to bend it sideways a bit to bring a new 
portion of the spring to bear on the detent disk.

>Or any ideas for a more permanent solution?

New spring and or disk.  Also, check lubrication of the gears/clutch etc. 
between the KC actions and the MC actions.  Just a bit of oil or grease in 
the right spot may reduce the friction enough to to the trick.

>... The B+ I measure (audio B+) is 227V in AGC or MGC and
>252V in Stand-By.

We hear advice to avoid standby for any but brief periods. The tubes that 
have no cathode current can develop a dreaded condition called "cathode 
interface" were in the thing acts as if a resistor has been magically 
inserted in the cathode lead.  The magic is chemistry of the cathode 
emitter coating, of course.

>... To lower the non-regulated B+, is the answer more dropping
>resistance (200? 250? Ohms?), a 40V 10W Zener, a bucking transformer, ???

The bucking transformer is a very good idea, in my opinion.  Measure the 
filament voltages in a number of places accurately to see if they are mush 
above 6.3.  If so, add a current inrush limiter and then a bucking transformer.

>And on bad tube brands: "JJ" (Eastern European manuf) ECC82's/12AU7's
>are not too good in the 5814A spots.  I've had two develop filament-to-
>other-element leakage that gave intermittent hum, just in the past week.

Thanks for the experience report on these tubes. If I remember right, the 
JJ brand is sold into the hi fi and musician amplifier markets.  It's 
possible that those folks don't fuss as much as we do about tube life and 
the cost of replacements. Also, it's possible that JJ simply got a bad 
batch of tubes. (I suggest you send them back to them to see what their 
response might be.  Of course the minimum correct response is a set of 
replacement tubes by return mail.)  The reputation of eastern European 
tubes of recent manufacture is generally quite good with a few exceptions 
(e.g. the "7199" tubes that turned out to be another type altogether, 
enveloped with altered pinout and having remote cutoff in the tetrode 
instead of the linear one in the real 7199.)

>Luckily I have a good supply of JAN 5814A's.

Shhh!  Don't let on.  Your security contractor will scold you!

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --