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[K9AXN]

The GDO will produce good results with any coil without regard for other external influences.  It has been used to measure the SRF's for a capacitor with 1/4" leads. 

HH75: 
Your comments seem to suggest that the GDO measurement always yields the physical length of the line. Does this mean GDO measurements show no change in frequency if the choke is moved about?

K9AXN:
Not at all.  Earlier recommended was using a small piece of dielectric placed near the center of the choke to move the SRF down rather than using the grounded wire.  Changes the turn to turn C to change the velocity factor (Distributed properties).  The grounded wire can change the lumped properties and distributed.  The first SRF detected by the GDO or VNA will represent a shorted 1/2wl wire.  Best indication can be viewed using the scope probe with the tip shorted (Current probe) as in your photo.  Moving the wire along the choke will reveal a current null/voltage peak at the center and current max/Voltage null at the ends.  You will find this property near the frequency where the length of the wire would be 1/2wl long. 

The problem encountered when using a choke that has multiple SRF's is placement becomes critical.  Whereas, if you use a choke wound with a length of wire that represents a frequency higher than the highest frequency used, and permeability enhancement, there will be no SRF activity.  The first choke in this url is such a choke. 
http://k9axn.com/_mgxroot/page_10843.html

These chokes are still available.  One entry which I cannot find at the moment, stated that they were currently available and provided the name of the vendor --- still looking.

Gene Senti, designer of the 30L1, was the first to use a low value, Marginal as G3RZP correctly states,) plate choke to enable the use of all frequencies from 3.2 to 30MHz.  Heathkit and most other GG designers followed including Bill Orr.  This is a long story for another thread.   

Try the shorted scope probe to visualize the distributed properties of the choke. 

Regards Jim K9AXN 

[HAMHOCK75]

The GDO will produce good results with any coil without regard for other external influences.

In the photo from web site showing coils, those coils are all lying on the bench. Have you ever raised them off the bench to see if the GDO can detect any difference?

[KM1H]

Gene Senti, designer of the 30L1, was the first to use a low value, Marginal as G3RZP correctly states,) plate choke to enable the use of all frequencies from 3.2 to 30MHz.  Heathkit and most other GG designers followed including Bill Orr.  This is a long story for another thread. 

As I understand that type of choke design a portion of the amps output RF traveled a bit down the choke which reduced values in the tank circuit.

Carl

[HAMHOCK75]

Carl,

Since you own and know how to use a VNA and a GDO, perhaps you can start a thread that reconciles measurements between these two types of instruments. I think that would be more interesting then to have the subject buried in a restoration thread.

I used to have an Eico 710 but can't find it. We used to have an amateur radio lunch weekly here on Wednesdays before Covid where hams would bring items to be given away in a drawing. I might have done that with the Eico.

Eico 710's on eBay cost far more than nanoVNA's and many don't even come with a coil set and are untested so I don't think I will be buying another.

I feel the VNA found a problem and a solution and it's time to move on.

The 30L1 and its followers are also an interesting amplifier design but I would never call it a grounded grid design. As you might recall, on another thread, I measured the actual signal level on the grids of my SB200 from 80-10M and the voltage on the grids is 30-41% of that on the cathodes. That type of amplifier is really unique but not what I would discuss here. It too deserves it's own thread.

Not sure I will be doing though. I am starting to examine the problems with sold state amplifiers. One interesting problem involves the filters between the amplifier and the load. Some filters apparently have a 2:1 swr at their input even when terminated with good 50 ohm load. This causes amplifier shut down  with a modest swr load like 1.2:1 because at the input to the filter, which is what the amplifier sees, the swr goes over 2:1.

[KM1H]

Carl,

Since you own and know how to use a VNA and a GDO, perhaps you can start a thread that reconciles measurements between these two types of instruments. I think that would be more interesting then to have the subject buried in a restoration thread.

The VNA is a complete instrument that can stand on its own and a GDO is just a simple accessory that can be used with a VNA or stand on its own.

The GDO can also be used as a tuneable indicating wavemeter when you shut the oscillator off and use it to sniff RF from something such as a TX, amp, oscillator etc. In fact the GDO got its start as that absorbtion wavemeter in the 1920's when it was a requirement to be able to indicate the frequency of those free running uncalibrated drifting oscillators that hams were using and often not on a ham band!

Old QST's had lots of articles on that and many published station photos showed them.

I built my GDO in 1956 from a 1949 CQ article and still have it but hasnt been used since I picked up the Measurements Model 59 at the final National IRS auction in 92.

[K9AXN]

The GDO will produce good results with any coil without regard for other external influences.

In the photo from web site showing coils, those coils are all lying on the bench. Have you ever raised them off the bench to see if the GDO can detect any difference?

They are lying on the bench for a photo shoot.  Yes, they are measured lying on a folded cotton cloth and finally in place because the chokes that bracket the ham frequencies are vulnerable to placement and SRF shift.

My thanks for making the thread interesting and revealing some of the capabilities of the VNA instrument.  You took the time to explain the intricacies of the charting.  Your interpretation of the charts suggests your solid grasp of the underlying theory and power of the instrument when used by a qualified person.

Good luck with the amplifier and a great day to you.

Regards Jim K9AXN 

[HAMHOCK75]

The AL-84 has no bias adjustment so the bias current drawn is dependent upon the condition of the tubes. The highest bias current drawn by the good 6MJ6’s I have is 20 mA with the typical around 15 mA. For four 6MJ6’s this would be a total of 60 mA. By comparison, the two 6JS6A sweep tube finals in the Yaesu FT101EE are biased to the same total of 60 mA. Below is plotted the efficiency of the AL-84 compared to the Icom IC-2KL. The 6MJ6’s used in this test draw a total bias current of about 20 mA.

This plot shows that weaker 6MJ6’s with lower bias currents shift the operating point close to class B. The green line is the theoretical efficiency of a class B amplifier. The blue line is the theoretical efficiency of a class A amplifier. The orange line is the efficiency of the Icom IC-2KL amplifier which has a bias adjustment for each of it's two amplifiers which are combined for 500 watts output.

[HAMHOCK75]

I learned the reason that the AL-84 and the 2KL amplifiers have different curves above. Tube amplifiers like the AL-84 can be class AB₁ or class AB₂. The AL-84 from the manual is AB₂. Solid state amplifiers can only be class AB. Class AB₂ places the AL-84 closer to the class B curve. The result is slightly higher output power. 

It is explained at the link below,

https://rec.radio.amateur.homebrew.narkive.com/g2hryDLq/what-is-the-difference-between-class-ab1-and-ab2-in-an-amplifier