Curve Tracing Bench Test Setup w Schematic

[Crestone]

Hi All,

As I mentioned in my introductory post, I have access to a pretty good assortment of lab equipment (DMMs, PSUs and oscilloscopes). 

I want to perform some tube matching, curve tracing (by hand) and NOS vendor spec verification.  I looked at some of the testing options out there and wasn't all that impressed for various reasons (price, DIY assembly etc.), so I thought I would make use of some of that bend equipment.

The attached schematic is my initial foray into a bench setup.  As such, I thought I better get some more eyes on it before I dive in.  All comments welcome.  While very simple....ya just never know!

FYI....I'll be building a box to house the tube socket, resistors and (red and black) banana jack test/power points.

[Crestone]

Discovered a major mistake.  Originally intended this to be fixed bias setup.  When I realized that was going to be a problem with my PSU, I added in Rc, but forgot to move all of the PSUs and DMMs from ground reference to cathode reference.  oh boy!

Fixed that and also added in a pot for Rc instead of a fixed resistor.   Although I can already adjust Vgk at the PSU, figured it would give me more flexibility and another knob to turn.  Of course, I just have to know what Rc ends up being, so added a couple more test points/DMM to measure Rc.   

               

[PRR]

Welcome.

I have no idea what the 1k/50W is there for.

Your plan gives only +positive+ grid voltages; in audio grids are usually negative of cathode.

There is NO attempt to limit positive-grid current. Most audio tubes won't take much. (And sure no need for 5A of Ig1 on any tube which fits through the door.) I'd put 10k-100k in series so Ig1 can't be over a mA.

DMM-C is to measure BOTH Vgk and Ic? Since Vgk typically controls Ik, it seems wise to have two meters. Yes, Ik can be inferred from Ip+Ig2. But a mis-set Vgk can melt a tube faster than mental math.

[Crestone]

PRR, thanks for the welcome and for the comments.

The 50W resistor was my attempt to simulate a plate load.  Can easily be and most likely will be eliminated as I'm thinking it is not really necessary.

Grid voltage will be negative of cathode.  Since my PSU is not capable of negative voltage, I should probably move the reference for PSU-2 back to ground and keep Vg less than Vk.  That was my original thought, anyway.

The voltages and currents listed at the PSU's are their ratings and are there to remind me of what their capabilities are.  As I typed them in, I realized that others may see them as test ranges.  (Should have listened to my inner warning.)  That said, I should and will run a 100k in series with the grid supply to limit grid current supply as you suggested.  Thanks for that catch!

DMM-C is a dual measurement, dual display DMM, so you can think of it as two separate meters.  My only limitation is the common needs to be "common" :-) for the voltage and current measurements.

Anything else I missed here??  Lol

[PRR]

Do not re-invent round rolling transportation accessories.

http://www.tubebooks.org/Books/RDH4.pdf  (25MB PDF)
Page 91 (p131 of PDF); really pages 13 to 125

[Crestone]

One of my favorite references.  3.1 was mostly my inspiration to get started on this project.  Unfortunately, (or rather fortunately, depending on your perspective), my equipment and needs require altering the wheel a bit. 

Version 3 below has changed and grown quite a bit after some serious contemplation this weekend about where I wanted to go with this.  Hopefully, this is the final schematic and it is at the point now that a layout is required.  I'll get going on that next.