Bias Rite measurement discrepancy

[jetwolf]

Can anyone think of an explanation for this?

I just installed a quad of the GTEL34Ms in my Marshall 1959 homebuild last night. This amp has installed quality 1-ohm resistors between pins 1 and 8 and then the extra lead through pin 1 continued to ground for cathode current conversion. I first checked the amp with my Bias Rite, and thought my tubes looked quite a bit out of spec for matched tubes. But then I decided to doublecheck by taking the pin 8 (cathode) readings and they were pretty much right on.

BIAS RITE (measured twice)
Tube
A 36.6 A 38.0
B 31.3 B 32.4
C 40.4 C 42.2
D 35.3 D 36.9

FLUKE- PIN 8
Tube (measured twice)
A 35.9 A 37.4
B 35.8 B 37.2
C 36.1 C 37.5
D 35.9 D 37.4


As you can see, the Fluke measurements on Pin 8 are right on. The Bias Rite B and C tubes are off a bit more than I would like. One is about +5ma and one is -5ma.

Next, I picked Tube A to compare the Bias Rite measurements using the same tube and tube slot Slot A but different Bias Rite heads. The measurements were pretty much dead on.


SAME TUBE A/SAME TUBE SOCKET A (far left) (I had already raised the bias to bring it closer to the 30-31ma (489v at plate) range from my first readings, so ignore the fact that these are lower)

Bias Rite A 30.6
Bias Rite B 30.8
Bias Rite C 30.8
Bias Rite D 30.8
Fluke MM on Pin 8 30.6

All right dead on. Any ideas on what is causing the 5ma discrepancy on the two right tubes when measured by the Bias Rite? Which readings should I trust?

[Blue Strat]

I'd trust your Fluke readings. I've heard several similar reports on the Bias Rite in recent months.

[unklmickey]

Originally Posted by Blue Strat

I'd trust your Fluke readings. I've heard several similar reports on the Bias Rite in recent months.

same here, Mike.

i find it perplexing, though, how Jetwolf was able to get readings from all 4 heads, that were so tightly grouped in his last test.

are the problems with the bias-rite heads intermittent?

does it have something to do with using more than one head, at a time?

a real head-scratcher, eh?

[jetwolf]

That's what really bothered me. I really expected to see that one or two of the heads were off by 5ma when measuring that same tube/ same socket, but they weren't. That's what I was thinking, too: could it have to do with all the heads being in use at once? Is it a clue that the two that are off are a mirror of each other (one plus 5, one minus 5)?

[UfoPilot]

I'll bet the resistors are off in the bias rite. Have you measured them? are they really 1meg?

[Jon C]

I've seen Bias Rites off by 13-20v measuring plate current (395 v. 408; 367 v. 393 are 2 examples I recall), so I only use my meter now...

[fullerplast]

Bias Rite readings will drift with the battery voltage....although it should be a uniform drift across the heads. Plate voltages readings in particular can go way off if the battery is low.

[jetwolf]

I did check the 9 volt battery, and it showed 9.2 volts.

[jetwolf]

Originally Posted by UfoPilot

I'll bet the resistors are off in the bias rite. Have you measured them? are they really 1meg?

Not directly. But that's why I did the second test. Measuring the same tube, same socket, with each of the four bias rite heads, all reading matched the Fluke measurement of same tube/socket. If the resistors were off, these wouldn't have matched, would they?

[Blue Strat]

Originally Posted by jetwolf

Not directly. But that's why I did the second test. Measuring the same tube, same socket, with each of the four bias rite heads, all reading matched the Fluke measurement of same tube/socket. If the resistors were off, these wouldn't have matched, would they?

Couldn't be. Sounds more like an intermittent problem. I would contact Ted. He's the only one who can do anything about it. Hopefully enough people have tipped him off that he's investigating.

[unklmickey]

Originally Posted by jetwolf

That's what really bothered me. I really expected to see that one or two of the heads were off by 5ma when measuring that same tube/ same socket, but they weren't. That's what I was thinking, too: could it have to do with all the heads being in use at once? Is it a clue that the two that are off are a mirror of each other (one plus 5, one minus 5)?

i don't know for sure. but i think YES!

the fact that one reading was 5 high. and the other 5 low, could be a coincidence. but my gut says, that fact will end up being the rosetta stone in sorting this problem.

[Johnny Z]

Remember the tolerance of a resistor has alot to do with the actual value of a measurement, some resistors are off as much as 20%. If you want a better reading you need higher tolerance resistors such as 2%. So in your case a reading of +/- 5ma is most likley in the ballpark. I'm not sure what level of tolerance they match tubes at for curent draw and or how they are rated, I don't think there is an industry standard. So when you consider that not all the tubes will draw the exact same current you should expect some variation within the normal operating range. I think would trust the Fluke measurements.

I usually measure direct to the socket contacts in the amp without a probe, the more things you put in the way of a test the more chances for a bad reading.

[Gary Brennan]

Would this bias rite problem relate to the ones where you use your own meter?

gb

[Blue Strat]

Originally Posted by Gary Brennan

Would this bias rite problem relate to the ones where you use your own meter?

gb

Not from what I've heard. In fact, it might be the opposite.

[suhr_rodney]

Originally Posted by Johnny Z

Remember the tolerance of a resistor has alot to do with the actual value of a measurement, some resistors are off as much as 20%.

According to Ted the BiasRite uses 1% tolerance resistors.