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Ampdog, What you say is good news, and I appreciate your experience regarding your extensive tube testing. Also interesting that you mention the Tungsol 6L6GC's, my limited experience with a quad of those was also favourable (as I mentioned in...

True, except that (I assume) valves remain largely human assembled, and this will count for variability. Also, internal tube structures are 'jigged' into place with tools ,spacers, supports etc. that will wear out with time. Anodes, cathodes, grids and heaters are either pressed into shape...

Some information regarding electronic equipment ?burn in?: ?Burn in? generally refers to the removal of potentially unreliable product from a manufacturing process. The concept is associated with the so-called ?bathtub curve? that suggests that in early stages of a product life, the...

Thank you for your comment. That is correct. Testers will do a small signal measurement at one operating point (although some will allow the user to choose the signal excursion) However power tube matching is not a small signal problem, it's a large signal one. So I am questioning the value...

hello fredeb, thank you for this conversation, it is giving me some further ideas. You are correct in what you say, it could be a manufacturer issue (although where are these 'common' brand tubes manufactured?) If the Ia curves match 100%, the gm will of course also match. But if this is not...

Well, I'm no expert in this business of tube matching, but I have bought a few 'matched pairs' (& paid extra) from Tube Depot and generally the match is very poor. I'm not sure if TD do their own matching, or purchase them 'matched' from the OEM or possibly from a company that specialises in...

or.... what to do when in quarantine... during lock down 2.0 'Just for fun' I have used my elCheapo Tube curve tracer data to write a small program that will analyse tubes and suggest matched pairs. This GUI shows 16 6CA7 tubes test result files with a corresponding tube no 1-16. By clicking...

I forgot to add: The tube base is one of Pete Millett's (www.pmillett.com) tube bases. These can be detached & others screwed in. Alternatively the banana plug connections can be used to connect to a tube socket of your choice. The on board filament supply provides 2.5, 5, 6.3 and 12.6V dc @...

Here is an update on my DIY tube curve tracer. I have used an Al toolbox as the enclosure. It's bigger than it needs to be but it does the job. Final voltage and current ranges are as follows: Anode sweep voltage: 0-150, 0-200, 0-300, 0-400V; Iamax = 300mA Screen fixed voltage: 100-300V in...

I guess that if the feed forward gain is high enough the main contributor is the fed back signal ~ 1/91 x output signal (approx). If Vout = 20V peak the fedback signal = 220mV which would be the difference between what the screen and cathode 'see'

This signal is -(i x R4) where: i  the small signal current through R4 = the sum of the cathode current plus the current fedback from R12 and C7  R4 is the unbypassed resistor. ??

And, it may also be interesting to read some of the discussion from 2015: https://www.diyaudio.com/forums/tubes-valves/282487-pentode-screen-bypass-capacitor.html

Another look at the pentode stage in isolation: Inject a signal into g1 gives a gain of 147x; Inject the same signal into g2 gives a gain of 3.7x

Oops, I forgot to connect a decoupling cap on the EF86 side of R11. If I do it reduces the pentode stage gain and brings the loop gain down to 28dB (rather than 31.2dB) which is closer to Ampdog's 26dB.

Apologies for coming into this a bit late, I have been outside of reliable internet communications. Firstly, I know little or nothing about classic (or other) amplifier configurations; my interest is in the measurement and modelling side of tubes, and even that is only superficial. Having said...

And here is the 6CA7 in ultralinear mode  :D

No genius I'm afraid! I remain a hacker when it comes to this type of thing. Other people write programs in C++, whilst I write them in C--  ;D The tester is self contained, it does not need to be operated from another computer. The RPi runs Raspbian, a version of Linux so all the data files...

Here is a 6CA7 anode and screen current in 3D...

Sorry, I just realised that the breadboard picture does not show the small screen, encoder wheel and BIG RED button. I think all tube projects should have a big red button.

After thinking a bit about the comments made here and elsewhere about tube testers I decided to knock together my version of a relatively cheap but versatile tester. My go-to tester has been the uTracer, mainly because it can provide the LTSpice models, but also it was relatively inexpensive...