[Boatanchors] Resolved: De-chirping a Globe Chief Deluxe?

[D C *Mac* Macdonald]

It's a long time ago, but I believe that the
Modified Pierce crystal oscillator circuit was
very stable. The "modified" part seems to be
that the screen of a tetrode is used as the
"plate" of the oscillator, thereby minimizing
crystal current and providing isolation between
the oscillating portion and the actual plate's
output function.

My Walter Ashe novice rig used a 6AG7 as the
modified Pierce oscillator with 150V regulated
to the screen and the ca. 300V plate voltage
was switched off during receive. The 6AG7 in
that mode served as a code practice oscillator
with VERY limited range.  Amplifier to ca. 25W
input was a 6L6G.

The rig had no chirp that I could ever determine.


* * * * * * * * * * *
* 73 - Mac, K2GKK/5 *
* (Since 30 Nov 53) *
* Oklahoma City, OK *
* USAF, Ret (61-81) *
* * * * * * * * * * *


> From: 1oldlens1 at ix.netcom.com
> To: ccheney3686 at charter.net
> Date: Tue, 28 Dec 2010 13:26:58 -0800
> CC: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Resolved: De-chirping a Globe Chief Deluxe?
> 
> 
> ----- Original Message ----- 
> From: "Charlie Cheney" <ccheney3686 at charter.net>
> To: <boatanchors at mailman.qth.net>
> Sent: Tuesday, December 28, 2010 11:13 AM
> Subject: [Boatanchors] Resolved: De-chirping a Globe Chief 
> Deluxe?
> 
> 
> > Thanks to all who took the time to respond to my request 
> > for help. After
> > being assured that the GCD should be capable of chirp-free 
> > operation, I
> > checked components in the area of the 6AG7 and found a 47K 
> > resistor that
> > had drifted up to 60K. I got a significant improvement 
> > from replacing
> > that as well as the 6AG7 (the one in there was "mfd for 
> > Globe
> > Electronics" so presumably the original tube), but chirp 
> > was still
> > noticeable especially listening to the low beat frequency 
> > I prefer for
> > CW operation (~500 Hz).
> >
> > I measured the plate and screen voltages on the 6AG7 and 
> > found it was
> > operating well above the max rating for screen 
> > dissipation. (This
> > appears to be as-designed, I'm guessing as an economy 
> > measure to keep
> > down the cost of the resistive divider?). Worse, the 
> > screen voltage was
> > changing from 380V key-up to 240V key-down. Regulating the 
> > screen
> > voltage at 150 with an under-chassis OA2 pretty much 
> > cleaned up the
> > remaining chirp, without using any non-period components 
> > or making any
> > mechanical changes, so we're good to go.
> >
> > Thanks again for the help. I hope to make a lot of 
> > contacts with this
> > old timer on SKN.
> >
> > 73,
> >
> > Charlie K1LDZ
> 
> I should look at the schematic before commenting but my 
> first thought is that running the screen high is more likely 
> to get the output of the tube up rather than to save the 
> cost of a resistor or two.
> Keyed oscillators do tend to chirp. It may be that a 
> different design could be used such as an electron coupled 
> oscillator. The same tube type will work but the output may 
> be too low. When the plate and screen voltage _ratio_ are 
> chosen correctly an ECO is almost immune to voltage 
> variation over quite wide range and the isolation of the 
> output from the frequency determing circuit gives it a good 
> deal of immunity to frequency variation with load change. 
> The cleanest CW comes from letting the oscillator run 
> constantly and keying the following stages but then you may 
> have problems with backwave especially if using true 
> break-in.
> 
> 
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com 
> 
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