[R-390] Weak BFO Cured

[David Wise]

?Yes.  All the high-gm tubes in the 7BK pinout (6AH6, 6AS6) use extra heater current to light their big fat cathodes.  I didn't want to rewire the socket for 7CM, or go to 12V and change the PTO tube.?  My mod is the least invasive, in the ways I care about.


Dave

________________________________
From: Alan Victor <amvictor at ncsu.edu>
Sent: Monday, May 06, 2019 10:54 AM
To: David Wise
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] Weak BFO Cured

Hi Dave. I think its all about gm, transconductance. The higher the value the better and the transistor is typically 40,000 micro-mho/mA. Most small signal tubes will not be nearly as large. Large FETs may also do well. The J310 or a pair in parallel.

73' Alan

On Mon, May 6, 2019 at 9:06 AM David Wise <David_Wise at phoenix.com<mailto:David_Wise at phoenix.com>> wrote:
I did not try other tube types.
It's not gain in the sense we're used to thinking about, it's output impedance when operating as a cathode follower.  A tube with lower impedance is likely to work, for the same reason as my mod.
This doesn't come out clearly in the spec sheets.
Unfortunately, 6BZ6, 6CB6, 6DC6, and 6DE6 are disqualified because pins 2 and 7 are reversed.  (6JK6 and 6EW6 are also disqualified by heater current.)

Dave Wise
________________________________________
From: r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net> <r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net>> on behalf of Perry Sandeen via R-390 <r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net>>
Sent: Sunday, May 05, 2019 3:42 PM
To: r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net>
Subject: Re: [R-390] Weak BFO Cured

Yo Bubba Dudes!.
Dave Wise wrote:   Some years ago my BFO gradually weakened and died. Replacing the tubedid not help.
OK, question.  Did you try one with higher gain i.e. 6BZ6, 6JK6 etc?

If not, do you think that it would have solved the problem?
Regards,
Perrier



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