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

Sat Feb 18 16:39:50 EST 2017

Hi

They grappled with this issue ..eer … many decades ago :)

Getting a voltage that you can measure out of a tube at RF involves
either massive drive or a really high impedance. The only practical 
answer is a really high impedance load. That involves tuning the 
whole thing up for each tube (at least for each type). 

That’s going to be a complicated piece of gear. We get away doing
this stuff on modern parts with simple gear because they deliver 
measurable signals in a 50 ohm system ….

Bob

> On Feb 18, 2017, at 4:28 PM, Alan Victor <amvictor at ncsu.edu> wrote:
> 
> Hi Les, Hi Bill. I agree with both your views. However, science is fun! And
> it would be just neat to take modern tools and apply them to older
> technologies and see what you have. P.S. on blowing up analyzers... Hate to
> tell ya... They do it today all the time. SiC and GaN devices running at 50
> V and all they have to do is oscillate for a brief moment. Poof there goes
> another 50 GHz front end receiver.
> 
> On Sat, Feb 18, 2017 at 12:21 PM, Les Locklear <leslocklear at hotmail.com>
> wrote:
> 
>> 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@
>> 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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