[R-390] No reception below 8 mhz, transconductance testing

[Les Locklear]

Jeeez, just yank the suspected tube out, replace it and see what happens. That beats the crap out of expensive analyzers, tube testers and wondering......

I do have a tube tester and the first thing I do when unpacking a "new" to me tube receiver is plug it in. I'm not afraid of smoke or sparks.

If they test bad, chuck 'em in the garbage and put a new one in. How expensive can a 6BA6 or something similar be? I guess we like to bandy it about and get a couple hundred opinions of what it is, but.........

Les Locklear

"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison



----- Original Message -----
From: Bill Kirkland <b13kirkland at outlook.com>
To: Alan Victor <amvictor at ncsu.edu>
Cc: R390A <r-390 at mailman.qth.net>, Tisha Hayes <tisha.hayes at gmail.com>
Sent: 2/18/2017 2:01:17 PM
Subject: Re: [R-390] No reception below 8 mhz, transconductance testing
________________________________
I've often had access to high end network analyzers at work but have been way to scared to have plate voltages hit the front end of the analyzer and then have to explain to my boss how I blew it up. I know there are dc blocking caps but hard to get good ones that cover 300vdc. One slip.

I do get by with a sig gen and a 28ghz 2 story r&s spec a for aligning 455khz it's

Damn I hate touch screen phones
Bill (93rd blackberry)

Get Outlook for Android<https://aka.ms/ghei36>


________________________________
From: Alan Victor <amvictor at ncsu.edu>
Sent: Saturday, February 18, 2017 12:42:35 PM
To: Bill Kirkland
Cc: Tisha Hayes; R390A
Subject: Re: [R-390] No reception below 8 mhz, transconductance testing

It sounds a bit far fetched, however, a test circuit could easily be fabricated for triode, tetrode, etc... that would be plug n play with a network analyzer. Application testing could be done at the desired operating frequency.

Alan

On Sat, Feb 18, 2017 at 6:29 AM, Bill Kirkland <b13kirkland at outlook.com<mailto:b13kirkland at outlook.com>> wrote:
Never late for change. You could revolutionize the tube industry, ok the vintage restoration industry. (but what would all those audio guys do)

Bill

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net>] On Behalf Of Tisha Hayes
Sent: February-18-17 1:03 AM
To: R390A <r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net>>
Subject: [R-390] No reception below 8 mhz, transconductance testing

That is part of my pet peeve even about transconductance testing of tubes.
If you look up the standard for transconductance test it uses a very low frequency (somewhere about 1 KHz if I recall correctly). That's great for testing a tube to work in an audio amplifier, it may not be so great for something that is running IF (455 KHz) or even RF (up to 30 MHz).

So while transconductance testing does give us some value under AC conditions it really does not tell us how that tube is going to work at the operating frequency of the circuit.

I had been tempted (only tempted) to modify my TT-1 for IF-like frequencies
(455) but it would also mean re-calibrating everything to that new frequency, pretty much invalidating any comparison to the transconductance values that are on data sheets.

Maybe if it was still 1957 it would be a fine activity, to drive a new standard for testing tubes for RF/IF operation near their operating frequency.

The other thing to do would be to build a circuit to test distortion of the amplifier function of the tube, but then it gets even more complicated when dealing with gain.


*Ms. Tisha Hayes, AA4HA*



*A sweet romance is not for meI need electricityIf you wanna make me flipHit me with a micro chip*


*I'll be a diode, cathode, electrodeOverload, generator, oscillatorMake a circuit with me*
*-- The Polecats*
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