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Old 12th October 2010, 11:47 AM   #1
Brit01 is offline Brit01  United Kingdom
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Default Tube data sheets - inaccuracy

Hi guys,


Maybe some of you have seen my OTL circlotron thread but I'd like to start one in regards to tube data sheets and real measurements.

Looking at the 6080 data sheets I set

-40 bias
@ 100 volts

should give me about 60 mA.

I had about 100 volts +- 10 volts
-40 grid bias +- 5 volts


I have 3.9R cathode resistors to avoid current hogging and to measure current.

I measured 140mA and 115mA across the 2 cathodes of 1 triode!!!!!
0.55 volts/3.9 * 1000 = 140
0.45/3.9* 1000 = 115

So definitely an adjustment of the power supply voltage and grid bias needs a large adjustment to get it down to 80-100mA max.

Have other people had similar experiences with data sheet info?
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Old 12th October 2010, 01:06 PM   #2
DF96 is online now DF96  England
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Valves can easily be 20% different from the typical values given in a data sheet. Your bias voltages could be up to 10% out too, so together that adds another 21%. So your result could be out by 45%. NOS valves from good manufacturers often measure high.

Nothing very surprising in your results. People brought up on Spice might find this shocking.
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Old 12th October 2010, 01:09 PM   #3
SY is offline SY  United States
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Wait until you play with FETs and find 5-fold spreads in gm and idss...
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Old 12th October 2010, 02:24 PM   #4
Brit01 is offline Brit01  United Kingdom
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Quote:
Wait until you play with FETs and find 5-fold spreads in gm and idss.


I've 2 FETs in this design also as source followers. These are out also.

I need to match the current through them better.

Maybe with a SS sink on them instead of a resistor?
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Old 12th October 2010, 02:26 PM   #5
DF96 is online now DF96  England
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Design circuits so accurate matching is not so much of an issue?
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Old 12th October 2010, 02:37 PM   #6
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by Brit01 View Post
I've 2 FETs in this design also as source followers. These are out also.
How can source follower present a problem ? Unity gain followers are about the only component that works reliably as anticipated without applying loop feedback (well, the intrinsic feedback may have something to do with this ). Others will deviate from published specs, more or less. Take JFETs for example, it is common for datasheets to contain identical diagrams for same component, showing vastly different parameters.
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Old 12th October 2010, 03:05 PM   #7
Brit01 is offline Brit01  United Kingdom
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Quote:
How can source follower present a problem ?
Wasn't sure.

So I can rule out the FET's as causing a problem in differences in grid bias needed to cancel out DC offset?
The current flowing through each FET on each rail doesn't need to be equal?

Got a 10-12 volts difference in grid bias on each rail to balance the tubes and cancel out DC.
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Old 12th October 2010, 03:53 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Brit01 View Post
<snip>

Got a 10-12 volts difference in grid bias on each rail to balance the tubes and cancel out DC.
Remember the information in the datasheet is typically the mean (in this case data set average) of a large number of samples, so no individual device is necessarily going to have performance that is reflective of that mean, but any sufficiently large sample should..

This is completely normal both for devices within an envelope, and for device to device.
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Old 12th October 2010, 03:56 PM   #9
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I bet your PSRR is about -3, too.

/Something about designing circuits actually worth using... >

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Old 12th October 2010, 05:54 PM   #10
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by Brit01 View Post
So I can rule out the FET's as causing a problem in differences in grid bias needed to cancel out DC offset?
The current flowing through each FET on each rail doesn't need to be equal?

Got a 10-12 volts difference in grid bias on each rail to balance the tubes and cancel out DC.
Adjust the driver tubes' operating points (or whatever it is that is driving your source followers) accordingly or do it the SS way (loads of feedback to take care of sloppy design).
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