The definitive "rectifier tube sag" thread

[mhuss]

I wonder how the tube rectifier curves are measured - in real time or at several static points? Does the tube effective resistance really change from (say) 1000 to 200 ohms and back 100 or 120 times a second? Hmm, time to pull out the curve tracer. :-)

[J M Fahey]

I wonder how the tube rectifier curves are measured - in real time or at several static points? Does the tube effective resistance really change from (say) 1000 to 200 ohms and back 100 or 120 times a second? Hmm, time to pull out the curve tracer. :-)

Both.
The printed tube curves we all know from classic datasheets were measured "slowly" , "by hand", adjusting rheostats and watching meter needles, then graphing the points and very probably using curved rulers such as these:

or these:

to join the measured points into a continuous curve.

Curve tracers on the contrary actually go through all the intermediate points many times a second and show a nice curve in a display, but with less precision.

All this comes from pre-computer times where people was anyway able to design very advanced stuff, such as plane wing curves, ship hull shapes, aerodynamic car shapes, turbine blades and even motorcycle gas tanks, all with graphic methods.

[mhuss]

I know, I still have a set of French Curves from my EE drafting class (way back in the day). :-)

I wasn't questioning the accuracy of the tube book curves, but rather I was wondering if the dynamic (real life) response is different than the collected static responses, and if so, in what way, and how much. A curve tracer would help answer this general question.

[dai h.]

It was discussed on MEF as few years ago, someone clever wrote that the voltage drop followed a power law. I think that's what it was, I'm afraid I didn't fully grasp it. I've searched in vain for it.

(searched through my stuff but couldn't find anything) Might be something to do with "Child-Langmuir Law"? (see p.279)

http://link.springer.com/content/pdf...9536-0%2F1.pdf

Electronics 27

kjs crystal radio, diode test

http://diyaudioprojects.com/Forum/vi...=2912&start=20

(sorry if I'm way off here...)

[loudthud]

Here are some plate curves of various rectifier tubes. All are GZ34 except the last two shots of a NOS RCA 5R4GYB. Note the horzontal scale was changed because the 5R4 has such a large Voltage drop. All tubes are new or NOS. All tubes measured on pin 4 only with 5.0VAC on the heater, pin 8 was grounded. The center of the screen is zero Volts zero Current.

[mhuss]

Nicely done, thanks!

The 'curves' look pretty close to linear, and the turn on isn't particularly soft. I'm thinking a SS diode and resistor would be a fairly good first-order approximation.

[loudthud]

Here are the curves from a pair of 5Y3 and 5U4 tubes. The setup was modified slightly with a center tapped heater winding, the center tap was grounded. Curve is for pin 4 only. The RCA 5Y3 looks fairly new but I pulled it out of an amp so it has some hours on it.

[trobbins]

You couldn't ask for a nicer turn on an off characteristic - simple and soft.

:-)

[Gingertube]

Some "Brain storming" (or maybe brain fade).
Attached is a Powerscaling circuit I prepared for another discussion. It has a fixed current limit of around 2 Amps to suppress startup inrush current to the following filter caps and an adjustable current limit control labelled "SAG".
If we used a non linear resistance for the 15R current sense resistor (maybe a light bulb or a tube heater or if they are too slow then a parallel resistor diode combination across the existing 15R) then simulation of rectifier SAG becomes possible (although not necessarily practical).
I used this exact circuit in a 4 x 6V6G Git Amp - I found the SAG control did little sonically till turned well up, by which time the nominal +345V B+ had dropped by 200V. Of-course, in that amp I also had a tracking (of the B+) bias supply regulator which would have influenced the result.

Circuit is a Kevin O'Connor design.

Cheers,
Ian