Jim candela jcandela at prodigy.net
Tue Oct 19 21:08:09 EDT 2004

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Don,

You said, "paralleled 800MFD, 450VDC caps". I think you mean series. I
wonder how clean the plate voltage needs to be for a tube tester? If you use
the mega-capacitor bank, consider a suitable series resistor to limit the
peak current in the event of a short circuit to something manageable for the
components you use. Does each capacitor come with a shunting equalizing
resistor? I would make sure your bleed down time is under 1 minute or so.
Big capacitor banks scare the hell out of me... I once took a 1000uf 450 vdc
capacitor charged at about 300 volts arm-to-arm. That was my 2nd worst
shock, and this one knocked me out for a while.

Below I am pasting something from an old post that is relevant towards your
tube tester:

Wow! I have been inundated with stuff from higher minds than mine! Besides
your excellent replies, I got a somewhat similar, yet different perspective
from Patrick Turner from the R.A.T. group, (newsgroup: rec.audio.tubes) that
I paste below. The guy wants me to calculate Mu, and plate resistance as
well. After all, Gm = U / Ra.

> but why concentrate on the Gm?
>
> If I want to find out how to set up a tube I find out what
> all three important parameters are, the Ra, U, and Gm.
>
> You can set up a tube with a choke load, and strap various values
> of RL across the choke.
>
> Then after the bias is set apply a fixed signal input voltage to get about
> half the maximum anode voltage swing before clipping, so thd is
negligible.
> Then measure the voltage gain, and do so for at least two different RLs,
> say 5k, and say 10k.
>
> Voltage Gain, A, = U x RL / ( RL + Ra ), for all tubes.
>
> So since we know what A is, and RL is, we have two equations for the
> two different loads, and from these we can work out
> Ra, U and thus Gm, because for all tubes,
> Gm = U / Ra.
>
> The most constant parameter for all tubes is the U,
> because its determined mainly by electrode distances,
> but Ra and Gm vary widely.
>
> The other way to measure Gm is to simply have no choke load,
> or large value RL, and just have a 10 ohm R to the B+,
> and measure the undistorted current change with grid voltage changes.
> This don't tell you about Ra or U, which change dramatically
> when G2 is connected to the anode.
>
> The first method of mine involves algebra.
> But from one of with two equations, U can be expressed as a factor x Ra,
> and this can be substituted where U appears in the second equation so that
> this then has only numbers and Ra present, so Ra can be found, and its
value
> inserted into either equation, and the value of U found,
> and thus Gm.
>
> The idle conditions for the tubes can be changed, and the 3
> parameters will change.
>
> The tests and measurements can be done using a class AB PP pair,
> which will indicate what the 3 design values are when combined.
> In AB circuits, the Gm usually drops a bit after one tube cuts off,
> resulting in a flattening of the amplified sign wave,
> which is compression, or lot of 3H present.
> But when biased to near class B, the crossover distortion indicates
> that Gm is initially low, then becomes greater as more tube current flows,
> and the phase of the resulting 3H is opposite that of compression.
> Some tubes, like the 6DQ6, display increasing Gm
> even in class AB circuits at the extreme of the amplitude,
> so that the wave peaks up before clipping, and again the 3H
> is opposite phase to a compressed wave, so Gm is increasing lots,
> even up to just before clipping.
>
> In triode signal tubes, such as 12AT7, 6DJ8,
> quite large variations in Gm and Ra occur for different
> idle currents, and some have deliberately engineered
> variable parameters by means of variably spaced grid wires.
> Many pentodes have also thus been made to have variable U and Gm,
> to make the gain vary a heck of a lot, which is handy for
> constant sound level from microphones, regardless of the distance away
from
> the person using it becomes.
>
> Once you have established that a given load is ideal for
> a pair of say 6L6GC tubes, then quite a few others might be substituted.
> Speaker loads vary between say 4 and 40 ohms, so absolutely strict
> It pays to err on the conservative side, IMHO,
> ie, allow for a high value RL than a low one,
> as the best fidelity is then possible, and tube substitutions
> are more successful.
>
> Patrick Turner.

Regards,
Jim

-----Original Message-----
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Merz Donald S
Sent: Tuesday, October 19, 2004 11:49 AM
Subject: [AMRadio] UPDATE: Testing Transmitting Tubes

Just thought I'd throw out an update on this. I am building an emission
tester for older, mostly 800-series transmitting triodes. It will be able to
measure emission current at zero bias or adjustable bias (using an external
bias supply) at any plate voltage up to about 2500V. The HV is full-wave,
lightly-filtered DC. Filament voltage is adjustable 0-10VAC. I have an old
metered, rackmount regulated supply that uses dual 6L6's for regulation. It
will do 100V--so he becomes the bias supply.

I gutted an old Beckman, tube-type frequency counter to use the cabinet and
chassis. I just happened to have this unit laying around here. It has a
unique cabinet design with sides that come off and modular chassis
construction--even though, underneath the covers, it's a conventional 8.75
inch high, 19" rackmount unit. This design allowed me to remove sections of
the chassis to accommodate the 2 big Variacs and the big HV iron. The top of
the cabinet pops off with just 2 Dzus fasteners. I am going to put holes in
the top with the tube sockets underneath, mounted on an inverted chassis
bolted to the top cover. This way the whole unit won't have any protrusions
or non-internal wiring and none of the tube socket connections will be
exposed or accidentally accessible.

I drilled and painted the panel Sunday. I then immediately discovered that
the 0-300ma meter I was using did not have enough range at upwards of 2500V.
I needed at least 6-800ma. So I replaced that meter with a 0-1000ma Simpson.
But of course, the hole size is different. So now I have to cut a larger
meter hole on my already-painted aluminum front panel. I took comfort from
the fact that the black wrinkle didn't turn out that great anyway...

Most of the parts for this thing came from a supply I bought at a hamfest in
2003. For \$40, it had a 7.5 amp 120V Variac, 2700V transformer (amps rating
unknown but probably in the 500ma range), 4 K2AN silicon HV rectifiers and a
bodacious capacitor bank of paralleled 800MFD, 450VDC caps. The meters I am
using are from the junkbox (note to self: Don't buy any more meters at
hamfests...), as are the sockets and miscellany. But anyone seeking to build
one of these with new parts would have a big bill on their hands. Check the
prices on new 7.5 amp Variacs these days...

I expect to be able to test to following tubes in this unit: 203, 211, 805,
808, 810, 838, 852, 25T(G), 35T(G), 100TH, 250TH and RK-65. The test is far
from definitive. But it will tell me whether the spares I am saving are
worth saving.

The top cover is big enough that I will still have some space for tetrode
and pentode sockets. Screen and suppressor supplies would have to be
external. But that is phase II--or something I may never do at all.

73, Don Merz, N3RHT

-----Original Message-----
From: owner-glowbugs at piobaire.mines.uidaho.edu
[mailto:owner-glowbugs at piobaire.mines.uidaho.edu]On Behalf Of Merz
Donald S
Sent: Tuesday, September 21, 2004 3:27 PM
Subject: GB> UPDATE: Testing Transmitting Tubes

Well, here's where I am with this...

-- The guy who built the tester that AES is using says that he has some
information on that unit that he will pass along.

-- Another guy says that he has built an adapter for the Tektronix 577 curve
tracer for transmitting tubes. He says that he will write this up for

-- Two guys said that they built custom testers for the 4CX250 tube. One of
them was expanded to test other tubes and was demo'ed at Dayton. The other
was used in the guy's job. No details seem to be available for either of
these.

-- I checked the AWA index and found only one reference to testing
transmitting tubes. It's a 50 word mention on page 21 of Volume 22, number
1. There is one diagram. The test the guy is proposing is a minimal
emissions test using a known good tube as a reference. He does this for
triodes only using AC both on the filament and on the plate & grid (which
are connected together). It has the virtue of being simple. But it hasn't
many other virtues.

-- Finally, there is the e-mail below from the AWA's Ed Gable. This seems to
be closest to the lines along which I am thinking--a scaled down emission
tester.

The GE Ham News Hartley oscillator tester is more complicated than I want to
get into. And the Ham News static tester is very similar to the AWA tester
described below.

So this is where I am headed, though still open to better ideas. Keeping it
simple, so triodes only at this point.

73, Don Merz, N3RHT

-----Original Message-----
From: Edward Gable [mailto:EGABLE at rochester.rr.com]
Sent: Monday, September 20, 2004 2:11 PM
To: Merz Donald S
Subject: Re: Testing Transmitting Tubes

Hi Don:  We built a tube tester for Big TX tubes at the AWA Museum.

1.  Use big filament Xfmr and variac to accomodate any FIL voltage.  Use FIL
voltmeter.
2.  Use variace on HV supply for 0 to about 1500 volts.  Need about 300 Ma,
but not too much filtering.
3.  Put voltmeter and ammeter in HV supply.
4.  Parallel wire a bunch of convenient tube sockets and allow for clip
leads for socket-less tubes like 833.
5.  To test, get tube data for tube.  Look at chart and see what the tube
current should be for a
tube with zero bias.  For example, a type 810 might draw 200 Ma @ 1500 volts
with zero bias.
6.  Connect the grid to the filament (zero bias condition)
7.  Turn the FIL on and set proper voltage for the tube under test.
8.  Turn up the HV variac and watch the current & voltmeters.  See if the
current vs voltage is close
to the spec.  It varies a lot from tube to tube, but low emission,
non-useable tubes really show up
this way.

SAFETY FIRST !!!

73,

Ed Gable  k2mp
Curator, AWA Museum

----- Original Message -----
From: "Merz Donald S" <merz.ds at mellon.com>
<baswaplist at foothill.net>; "'Glowbugs (E-mail)"
<glowbugs at piobaire.mines.uidaho.edu>
Sent: Monday, September 20, 2004 10:19 AM
Subject: Testing Transmitting Tubes

> Has anyone built a transmitting tube tester? Or seen any plans on the web?
The only thing I have seen on this subject is the issue of GE Ham News that
covers the subject (Vol 6, #3, May/June, 1951). I have some power supplies I
can use to run the static test. But I need to build something to hold the
tube sockets, filament transformers and plate current meter. I hate to
reinvent the wheel and would be happy to follow a pre-existing design if I
could find one...
>
> Any ideas?
>
> 73, Don Merz, N3RHT
>
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