How important is it for the 83 in the Hickok testers , to be ballanced from side to side , and would two UF4007 diodes and a voltage regulator be beter , to keep the voltage steady and constant .

My testers started droping off testing 6550's and EL34's , they would start reading good and slowly drop off . I also replace the 81 lamp with a NOS Tung Sol instead of the forien made one in it and it has stopped glowing untill a goodly load is place on it , also put a weaker but ballanced 83 tube in it .

Do not replace the 83 with anything else.

On large heater current tubes like the 6550, 6AS7 and EL 34 you should monitor the heater voltage.

In answer to your first question, the sections in the 83 have to be balanced to a reasonable degree. If they're not, the ripple will be excessive, and this will result in erroneous readings. Most Hickok calibration procedures mention how to check this.

The jury is out on replacing the 83 with a pair of silicon diodes. Again, this is all available on the web. Nobody is using voltage regulators, but it is necessary to add a pair of 10-ohm, 1-watt resistors in series across the filament pins to balance the diodes against the filament winding. Some people like the diodes better (inexpensive, less heat, faster warm-up), but evidently Hickok designed the tube chart settings with the voltage drop of an 83 tube in mind, so calibration may suffer somewhat. It's easy enough to salvage the base from a dud four-prong tube and make up a solid state replacement so you can try it without making any modifications to the tester. That way, if you don't like the diodes, it is a simple matter to put the 83 back.

The first thing to check when readings fall off is the heater voltage at base of the tube. Very often a tube which draws a lot of heater current will cause the line voltage rheostat and/or power transformer windings to heat up, reducing the voltage to the tube. Re-adjusting the line voltage control should take care of it.

The extra plate-current load for the Gm test is going to drop the heater voltage somewhat. It can't be helped, because the whole tester is fed with a series dropping resistor.

Emission in a good tube is fairly independent of heater voltage, but one with marginal emission will show a drop as the heater voltage goes down. It's a good test of remaining life.

As mentioned, depending on the type of Hickok tester you have, a SS replacement can work, albeit the plate voltage will run a bit higher (about 7-10V). This in itself should have a minimal effect on the GM reading, put will increase the plate current if measured (if not compensated for), and might pull down the plate transformer if at it's max output. If you have a Hickok tester with an external jumper for the plate, I would check plate current otherwise use a dropping resistor in the plate or inline ma meter to measure current. You may be having a problem drawing more current then the transformer can handle. On some of my Hickok tube testers, the readings tend to sag around a plate current of 70mA, so I have seen the same thing with these tubes. Try a slightly higher bias voltage (lower plate current) and see if the sagging GM diminishes.

I also have had problems with oscillation and had the same dropping GM with these tube types (on some Hickok tube testers) until I put in a 0.5-1.0nF capacitor (do not use anything bigger as it effects the GM) from the plate to ground (or cathode). Also check that your bias voltage (as well as other voltages) are correct at the tube during testing, and if anything changes when the GM drops. If you do not have a bias meter voltage, and/or your bias dial is scaled as opposed to indicating actual voltage there are conversion charts posted for Hickok dial reading and the correct bias voltage. Use an extended octal test socket for the voltage/current and to see if a small capacitor makes any difference. If your rheostat is getting very hot (or you have a weak/marginal transformer(s)), try to use a variac to set the line voltage instead of the rheostat. This can give a bit more power to the transformer. On the 539B/C, I also experienced a poor connection at the heater jumper terminals which caused increased heater voltage drop, but you could also have a bad connection elsewhere if your voltages are off.

Haven't seen a problem with the filament voltage dropping over time when testing these power tubes, thus effecting GM. Easy enough to check, but as mentioned, I do not feel this is causing the dropping GM.

The 83 tube balance is pretty important to have it reasonably balanced plate to plate, as you can only compensate for so much when calibrating these testers. But switching out the 83 tube without recalibrating the tester will cause a larger GM error and does not fix (nor do I believe cause) the problem you are experiencing. If you do use a SS replacement and you want the voltage to match the calibration procedures, then you can compensate for the tube voltage drop by using a zener diode (I use a 5.6 or 6.2V 500mW) back to back with each diode (2 1N400X, 2 Zeners X to 2 10 ohm resistors; diagram in the Hickok 539 Calibration post). Might post which tester you have any what your findings are.

Pentode characteristics are fairly independent of plate voltage.

Agree, context was for general effect in plate voltage on plate ma when using a SS replacement, effect on pentodes is minimal compared to triodes. The overall impact of a few extra plate volts is still negligible given all the other variables in these testers. Although, some pentodes will test fine in my 580A at plate/screen 200V and at 250V I see the GM/plate ma sag as described after 5-10 seconds of testing. The problem with oscillation common to the 580 was independent of voltage, ant at least in my unit eliminated with the plate to ground cap.

I think that's pretty common on the smaller Hickoks. I think they intended you to press P4 just long enough to get a reading. I have a TV-2 which has separate filament and plate transformers which helps eliminate that problem. The final adjustments are done under load which also helps but it's not a quick set up if you are in a hurry.

Found out the big problem with my 539C was the the 81 lamp , it had been replaced with a Chinese made one and it had keep geting brighter with age , it glows 5 times brighter than the Tun Sol one that i just put in .