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

[Alan Victor]

Yes Bob, that is very true and actually were still wrestling with same
issue today except on the opposite side of HIGH Z, very LOW Z. It's a tough
nut to crack no matter what side of the tech bridge your on. However, there
are impedance meters which can handle reasonable values both low and high Z
over a limited freq range. I have used the hp 4193A for both solid and
hollow state device measurements.

On Sat, Feb 18, 2017 at 1:39 PM, Bob Camp <kb8tq at n1k.org> wrote:

> 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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