B&K Precision 700 Tube Tester with 5Z3 Rectifier.

[caesparza]

Hi everybody.

Recently I had the chance to buy a B&K 700 Dynamic Mutual Conductance Tube Tester in very nice shape (a nice addition to my B&K Precision 620 Emission Tester). I've been reading about it a lot, the tester works with an 83 mercury rectifier tube, the one I got came with a 5Z3 instead. I've read in various places that the 5Z3 is not a proper replacement for the 83. The 5Z3 is supposed to be the closest non mercury rectifier to the 83.

I just checked voltages on the tester and all of them are within specs according to the schematic in the manual. I did the whole calibration process without any issue, and performed some test that resulted in consistent values with the 620 tester. I haven't replaced electrolytic caps or other parts, just had to solder the #55 bulbs to their bases to get proper contact.

I would like to understand why the 5Z3 is not suitable for the task, I understand the voltage drop is very different between the two rectifiers, but at the end of the day, is not the voltage fed in the tubes to be tested the important value after all?

I'm planning on doing the SS rectifier upgrade on this one, two 1N4007 diodes and two 10ohm 2w resistors connected with a 250mA Fuse (could not find a 200mA one), seems to be very popular and work pretty well on this testers. I really don't want to deal with the 83 mercury rectifier and the problems associated with its handling.

Attached a picture of both testers for fun.

[Helmholtz]

Schematic?

[Jon Snell]

The 83 is using mercury vapour to enhance the power output and help to keep the voltage drop, over the usuable current range, fairly stable. The 5Z3 does sag where the 83 does not. (Same heater/filament current ratings).
A pair of 1N4007s with the 10R surge limiters work very well.

[caesparza]

Attached the schematic.

[caesparza]

The 83 is using mercury vapour to enhance the power output and help to keep the voltage drop, over the usuable current range, fairly stable. The 5Z3 does sag where the 83 does not. (Same heater/filament current ratings).
A pair of 1N4007s with the 10R surge limiters work very well.

Hi Jon, thanks for the answer. You mean the SAG on the rectifier affects the test? Because voltages seem pretty solid and stable on the unit. I'm not an expert in the matter, but I would think that the rectification process should not affect the testing results as long as the voltages are the proper ones. I don't get the the voltage drop thing. Sorry about that.

[christarak]

the tester works with an 83 mercury rectifier tube I'm planning on doing the SS rectifier upgrade on this one,

I have the same B&K. I just replaced the 83 tube with a plugin SS replacement for an 83. I calibrated it afterwards, and it works like a charm. Never had an issue since. Unfortunately, age prohibits me from remembering where-the-hell I bought it from.

[g1]

I would think that the rectification process should not affect the testing results as long as the voltages are the proper ones. I don't get the the voltage drop thing. Sorry about that.

The voltage drop occurs when the rectifier tube is delivering current. So the voltages may seem correct when the unit is idling, but they will fall to incorrect values when you are actually testing tubes with it. This will cause inaccuracy.

[Jon Snell]

If you take a look at the data sheets, the 5Z3 drops up to 58volts and the 83 a mere 17volts on load. The 5Z3 drops a few volts off load and the 83 drops 15volts.
There is the descrepency of more than 40volts making the results after test different to the data shown.
The 1N4007 mod drops about 5volts on load and about 1volt off load.

[caesparza]

g1, Jon, I get it now!. Those voltages are for the unit at idle, as g1 mentions. When you press the test button with a load (tube for testing) everything is going to change. Thanks a lot.

I'm waiting for a couple of 10ohm 2w metal oxide resistors to complete the device, do you think two 10ohm 2w carbon film would do the job?. I guess I prefer high temperature resistors. And finally is the 250mA fuse a good value for the replacement in case of a diode shorting?

I've checked two designs for this:

1) 2 diodes 2 resistors. Like the one described here: https://tubesound.com/solid-state-re...t-for-83-tube/
2) adding a couple of zenners with the diodes. Like this one: http://www.mark-o-matic.com/diy/ss-83-tube.html

In either case I'll add the fuse between diodes and resistors for extra protection.

I found this information on the antiqueradios.com forum:

"For the 83 replacement:

1. The best circuit is to replace each vacuum diode section with a solid state rectifier (1N4007 or equivalent) in series with a Zener diode of approx. 10 to 15 volts. Anything from no Zener to about a 15 volt Zener will work almost equally well, as the classic Hickok transconductance circuit is VERY insensitive to the exact voltage.

2. Use of a resistor in place of the Zener diode is a mistake, not just because of any voltage drop considerations, but because the resistor will adversely affect the transconductance measurement circuit. This would make the calibration even more inaccurate if the set is calibrated using the "Hickok" method (as in the calibrations values given in the calibration procedure associated with Daniel Schoo) than it already is.

3. Theoretically, you shouldn't need dropping resistors to simulate the filament of the rectifier tube. To omit them might have significant effects on the filament voltages due to lack of the expected amount of voltage "droop" in the power transformer without the additional load. This would especially be the case in sets that don't allow line voltage compensation when actually testing the tube.
But more importantly, two resistors in the filament circuit ARE needed in the SS replacement to make sure that the half sine waves from each diode section are symmetric. Here's why: Surprisingly, depending on the phasing of the filament voltage and the rectified plate voltage, and depending on how the SS replacement circuit is wired, and how it interacts with the way the tester is wired, it is possible to get a situation where one half cycle will have the plate voltage plus one half the filament voltage and the alternate ones have the plate voltage minus one half of the filament voltage. This is VERY BAD for the transconductance measuring circuit, since it requires both half waves to be as symmetric as possible to work correctly.

4. So the best SS circuit configuration is where there are two filament resistors of the same value and the two solid state diodes are both connected to their junction, i.e. all of them have one end connected together. (There are one or two other combinations that are "safe" but this is the easiest.) This circuit looks something like an "X" and is the circuit configuration given in the late Alan Douglas' book, except he omits the Zener diodes. As noted above it makes almost no difference, in fact my 539B works slightly better without any Zener diodes.

5. Except for possible transformer voltage droop considerations, the exact values of the filament resistors is not critical. Anything from 0.83 ohms (which would duplicate the power dissipation of the original tube) to 20 ohms or so will work.

Additional comments:
1. Some people advocate inserting a fuse of some type. This causes no harm but, in my opinion is unnecessary. 1N4007 diodes are rated at 1000 ma. and 1000 volts, giving more than ample safety margin and they are highly reliable.
2. Some people advocate an extra diode in series with each 1N4007 (or equivalent) diode in case one diode fails. This is also unnecessary, but is so cheap and easy that I sometimes do it anyway, and again, it does no harm."


Probably less parts mean less possibilities of failure, so I tend to prefer the simpler design (number 1).

Thank you for your time and help.