Re: Tube test adaptor for Tektroinx 575 was: Re: [TekScopes] FS: miscellaneous Tektronix manuals


 

Another thing to consider (if you are feeding tube filaments via AC
power) and want to minimize hum is to use twisted pair wiring to
distribute AC power to the tube filament socket connections (rather than
individual single conductor wires which are haphazardly separated from
each other). Twisted pair tends to minimize inductive (magnetic)
radiation within the equipment/chassis that could otherwise occur with
separated wires supplying AC to the tube socket filament pins; the
magnetic field (at 60 hz) can lead to audio hum. Of course, it's also
OK to use single conductor wires which are twisted together in "paired"
wiring for the filament power distriubtion.

Mike Dinolfo N4MWP

On 12/5/18 10:06 PM, Jim Ford wrote:
Yep, a colleague several decades ago told me if you ever work with audio tubes, try D.C. for the heater instead of A.C., you'll like it.  I never forgot that, although I have not worked with tubes much.
Jim F


Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Harvey White <madyn@dragonworks.info> Date: 12/5/18 5:23 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: Tube test adaptor for Tektroinx 575 was: Re: [TekScopes] FS: miscellaneous Tektronix manuals
On Wed, 5 Dec 2018 22:11:12 +0000 (UTC), you wrote:

Brad,
Thank you!
My main interest is audio tubes. I have >1000 different tubes, mostly audio output and small signal/preamp tubes. I also own a Hickok 580A tube tester and a Tektronix 575 Curve Tracer. Thus my interest in using a Tek 575 to match tubes for use in DIY preamps and amps I am planning to make.
I was a electronics tech in the military a long time ago, in the time when tubes were going away and being replaced with solid state, but before the internet and PCs. I don't write code, although I did take a course in Fortran back in the early 1980s.
This group has been a tremendous resource. I truly appreciate the responses that I have humbly received from you and others in the group. I say humbly received because I'm just a audio hobbyist and ME who is humbled because I realize that I know so little about audio electronics who is trying to learn.
For example, I didn't know until recently that tubes usually use AC voltage for the heater. I'm also learning the different terms, such as the anode is usually called the plate. It's humbling that this old technology was invented well before I was born and is still useful today.
Again, thanks to all who responded with the lessons and great information.
Dan
Here's another one, and you may want to think of this in a high end
application.

The original heaters (called filaments) were the same as light bulbs,
simply tungsten wires that were heated to boil off electrons.

These were the first heaters, and needed 'A' batteries to run them.
(the "B" batteries provided plate voltages, and the "C" batteries
provided bias voltages, which were difficult to do without a negative
supply since the filament was both the cathode and the electron
supply.  You simply couldn't have the cathodes at any different
potential since they were supplied by the same battery)

Now another problem was that because the heaters had relatively small
thermal inertia (they heated up and cooled down quickly), the use of
AC for the heaters induced hum on the signal ... think of it, your
electron supply was increasing and decreasing with the AC supply
voltage.

So the solution was to put the heater inside a tube, but electrically
isolated from it.  So the heater could run off AC, and the thermal
inertia of the tube (called the cathode), kept the electron flow more
even.

You could even self bias the tubes by elevating the cathode above
ground.

Lead to a lot of nice designs.

Now, there's still a bit of hum induced by the heater in the cathode
sleve.  (cathode sleve has materials on it that release electrons when
heated). In your amplifier/preamp/whatever, you *may* get a reduction
in hum by running the heater from DC.

Just a thought, you may want to look it up.

Harvey





    On Wednesday, December 5, 2018, 1:28:41 PM PST, Brad Thompson <brad.thompson@valley.net> wrote:

On 12/4/2018 4:52 PM, Dan Cordova via Groups.Io wrote:
  Hi Brad,
Do you have or know anyone selling an adapter/fixture to match tubes on a Tektronix 575?
Or, do you know if there is a schematic I can buy?
Glydeck has a blog about this, but there is no schematic or info to contact him.
Hello, Dan and the group--

Thank you for your inquiry. Here's a column I wrote for Test &
Measurement World
magazine (subsequently absorbed by EDN) which describes my version of
a tube-test adaptor for the Tek 575:

https://www.edn.com/design/test-and-measurement/4389419/Old-dog-newer-trick

You can view a schematic and photos here...

https://www.flickr.com/photos/aa1ip_pix/

I used an external tube socket, small PC board and some two-piece
surplus connectors to build an adaptor, which suffices for small tubes
that don't need plate voltages in excess of what the 575 can deliver.
Heater or filament and screen voltages get supplied by "wall warts" or
other power supplies

In the schematic, J1 and J2 form a "crossover network" to match a
particular tube's pinout. This approach requires wiring or rewiring
the J1-J2 adaptor to test each tube type and thus is best suited to
testing a batch of identically-pinned tubes at once.

Newcomers to tube technology may be surprised to learn that many tubes
with different part numbers can share a common pinout. For example,
JEDEC base code 8BD
defines an octal (8-pin) base which applies to the 6SN7-GT, the 6SL7-GT
and their
heater-voltage cognates along with 25 or so other tube-part numbers.
Providing 12 pins
accommodates "modern" Compactron tubes.

While I noted no spurious oscillations while testing a batch of 6SN7s, you
may need to slip ferrite beads onto plate and grid leads if needed for
parasitic-oscillation suppression.

One point: use care in selecting the grid/base voltage you apply to the
tube under test. If the tube manual states "maximum positive grid
voltage = 0 volts", believe it! Running the grid voltage positive causes
large amounts of grid current to flow can melt the grid.

Questions welcomed-- I don't have any spare adaptor boards left over
but would investigate making more if there's interest.

73--

Brad  aA1IP









Join TekScopes@groups.io to automatically receive all group messages.