Question Re: TV-7 tube tester & 6L6GC tubes

[DRHamp]

Thought some knowledgable TV-7 person might have some insight:
I have a Dan Nelson calibrated TV-7/U and I use 6L6GC output tubes in my Raven amp.
My question relates to the TV-7 documentation vs. 6L6GC data sheet values for gm.
The data sheet states that gm in pentode mode is 5200.
TV-7 documentation (TB 11-6625-274-12/1 C3) which I think is the most current states that the MIN gm is a meter reading of 40 which translates to a gm of 5000.

Surely, one of those is wrong - there should be considerably more variance between the datasheet value and the minimum good value.

I know in one of the TV-7 documentation updates, the bias setting was changed from 25(I think) to 40.
Did the Min value accidently get changed to 40 also?

Any help clearing this up would be appreciated.

 

[Palustris]

I really don't understand what you are stating. Below is the data for the 6L6 as found in document C 3, TB 11-6625-274-12/1. Bias is 23 and minimum meter value is 25.

 

[DRHamp]

Thank you for your response - pictured below is a photo of my manual - TB 11-6625-274-12/1 C3. There is definitely a difference in your document and mine -- Thats the confusion - any thoughts?

 

[DRHamp]

The note in my doc says: "Bias was 23, changed per TO 33AA21-5-31M

The TO is dated 1 May 1973 and can be found here:
https://db.tubetest.net/instruments/Military_us/TV7/TO-33AA21-5-31M.pdf

 

[Palustris]

I checked my copy of C 3, TB 11-6625-274-12/1 and see that it is dated January 1962. It is very curious that they would change the test data in 1973. Even 1962 would have included the 6L6GC. I would be curious to know what the difference in testing a new 6L6GC at the two settings shows. Is there a big difference?

 

[DRHamp]

I will have to test again - I stopped when I came across the two settings and was confused as to which I should use - hence my initial question. Bias at 23 = 3.4V and bias at 40 = 9V so I would expect significant difference.
Since the datasheet gm is ~5200, it seems that a minimum gm of 5000(TV-7 doc MIN value) is significantly high
If the bias setting of 40 is correct then I think the minimum value of 40(5000) is incorrect - my assumption only.
I will retest at both setting and post the results.
I just thought someone had run into this before and could clear it up -- I just want to use the correct settings for the correct results.
Thanks again for participating in this dialog
Don

 

[DRHamp]

So, here's my conclusion: The new MIN value of 40 (5000) in my document is a print mistake.
Using datasheet info, AVG NOS is ~5000
Using the "generally accepted" rule-of-thumb, MIN should be ~3250 or a reading of 26.
All values above are +/- 10%

I will still retest at both values and post results(when I get a round to it)

 

[DRHamp]

I re-tested 4 quads this morning at the original bias setting of 23(3.4V) and at the updated setting of 40(9V) and as expected, the resulting gm readings were significantly different. The lower bias setting resulted in a higher gm reading by (on average) 20% - 35%.