[R-390] 12be6

[Dan Merz]

Hi,  I did a study of 6BE6 blocking characteristics because that tube as a
converter is prone to grid blocking (acquiring a large positive voltage on
the grid) in some old radio circuits where the grid return resistor is too
high or the screen voltage drops too low.  Generally speaking if the grid
return resistor  becomes large the tube will self block and must be "reset"
by turning it off and then back on again.  This tendency is enhanced when
the screen  G2/G4 grid voltage is low, for instance if the the resistor to
the screen becomes larger than it should be for normal operation.  Pulses to
the grid can cause blocking to occur and then the tube must be reset when
these conditions exist.  I haven't checked any of these observations in my
SP-600 and haven't observed it happening in that radio but if the grid
resistor or screen grid resistor have drifted higher,  this may be one
possible reason for the finicky behavior in your case.  When I was exploring
this phenomenon, I tested 5 different 6BE6's and all but one were
susceptible to blocking.  All tested as new tubes on a tube tester.  The one
tube that didn't block was a NIB GE tube that had about 25% higher gm than
the other tubes.  Another NIB tube exhibited blocking.  If anyone is
interested in more details,  I'd be glad to send a copy of the manuscript
that was printed in the AWA/OTB,  but I think the summary here includes the
information that might be relevant to what is happening in your SP 600 and
showed that seemingly good tubes can behave quite differently in this
circuit.  The 6BE6's in the SP 600 are mixers with separate tubes for the
oscillator whereas I was looking at the 6BE6 as a converter.  However  pin 7
G3 (the grid that is responsible for blocking) has the same function in the
converter and mixer.     Dan  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Steve Hobensack
Sent: Wednesday, June 27, 2007 4:48 PM
To: r-390 at mailman.qth.net
Subject: [R-390] 12be6


    My SP-600 jx17 is finicky about the 6be6 mixer. Some seemingly good 6be6
will not work. I wonder what passive component in the associated circuit
might be getting out of spec ?
  ...Steve...N8YE

Message: 5
Date: Tue, 26 Jun 2007 15:19:02 -0400
From: "Tim  Shoppa" <tshoppa at wmata.com>
Subject: Re: [R-390] Tube testers
To: <r-390 at mailman.qth.net>, "Joel Richey" <richey2 at mindspring.com>
Message-ID: <46812E660200003700008CFE at gwiavs.nservices.wmata.com>
Content-Type: text/plain; charset=US-ASCII

W2DBO wrote:
>of em, they all tested good, took the rx outa the cabanit and grabed 
>the voltmeter and check all plate and screen voltage everything looked 
>good, worked on that radio for 3 days, the 4th day my Pop came up to me 
>and handed me a box with a 12BE6 and said put this in and I said I had 
>checked all the tubes and their are all good, I put the tube in and 45 
>seconds later it was playing

Seeing how the 12BE6 is a heptode and in a AA5 used as both an oscillator
and a mixer, not too surprising that the tester got stumped by it :-).

Most likely it wasn't oscillating and the tester used a grid not for
oscillation for its test!

I was recently working on SPICE models for heptodes and no tube book gives
useful information for modeling.
Had to do some 3-dimensional curve tracing!

Tim.

_________________________________________________________________
Make every IM count. Download Messenger and join the i'm Initiative now. 
It's free. http://im.live.com/messenger/im/home/?source=TAGHM_June07

_____________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/faq.htm
Post: mailto:R-390 at mailman.qth.net
Unsubscribe: http://mailman.qth.net/mailman/options/r-390