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Tube matchin

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Posted on November 20, 2015 at 22:10:13
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 I am cleaning up a tube preamp (RM5) and am trying to evaluate the various 6922's that I have. They were tested and certified by Roger Modjeski when I bought them but they have become separated from their boxes with the specs.
What I want is to get the best match of gain between the sections so the output of the channels are as close as possible. The preamp consists of a single stage line and a two stage phono (3 tubes). I do not own a tube tester but do have a VOM and an oscilloscope (Tektronic 2336YA).
I measured the voltages at the plate, grid and cathode with the mute control on. The grid (0.00v) and cathode (1.00v) voltages of all the tubes were pretty much the same but the plate voltages can vary by about 2-3 volts (average 65v in the phono circuits and 89v in the LS). I also fed a 500 Hz sine wave from the computer and looked at the oscilloscope tracings of the left and right channels (XY alt mode) placing the various tubes in the line stage position. I have chosen the tubes that seem to have the best concordance between sections.
All this seems logical to me but I really have no idea what I am doing although I know enough not to discharge the caps through my body. I seem to have a consistently higher gain in the right channel (about 1 db). I believe the stereo input signals are the same (I need to check that or get get a mono splitter) and the scope is properly calibrated.

Any advise or suggestions would be appreciated. Also is there a simple way to measure the noise of a tube.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

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RE: Tube matchin, posted on November 20, 2015 at 22:39:59
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Your signal generator and 'scope method is sound. To check symmetry of calibrations, first reverse the connectors at the scope to confirm the scope is calibrated equal on both channels. Then repeat with the inputs to confirm the difference is really in the tube under test, and not in the signal source.

 

"I seem to have a consistently higher gain in the right channel (about 1 db)", posted on November 21, 2015 at 06:55:11
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 The best tube tester is your preamp.


Here is the ultimate scientific test: Swap tubes between channels.

If the 1dB imbalance follows it will be easy to find the culprit, moving the tubes back between channels one at a time. If the imbalance does not follow, the problem is not related to the tubes, probably could be your phono cartridge or some issues with the other components.

There is an unbuffered Balance, then Volume, then DIP switch Volume block in series before the output tube. Any of those components can be unbalanced. Why don't you just use your balance control to fix the problem?

 

RE: Tube matchin, posted on November 21, 2015 at 06:55:42
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 Great . I did as you suggested and things looked good. I then rigged up a mono y-splitter and hooked the signal from the single channel of computer function generator to both channels. I then measured the output on the both channels using X-Y alt and voila, the signals superimposed exactly.
.
The results for the line stage and phono stage look the same :).

Any suggestions on how to evaluate tube noise. The pre-amp is dead silent when hooked up so audible noise is not a problem. I just like to know how to do the evaluation, if possible. Is a computer function generator (I am using REW) adequate for this purpose?



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: "I seem to have a consistently higher gain in the right channel (about 1 db)", posted on November 21, 2015 at 08:14:13
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 Thanks for the reply. As you can see below, the problem is resolved. I have a copy of that schematic and I can tell you it is not correct with several omissions. I have re-created a more accurate schematic but have not had time to trace out the power supply circuit which also has errors (for example, look at the wiring of the GLED and RLED's).
BTW I was using the balance control to fix the problem (I guess that is why it is there) but my anal compulsiveness wanted that detent dead center.
Here is my corrected schematic:





"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: "I seem to have a consistently higher gain in the right channel (about 1 db)", posted on November 21, 2015 at 08:53:12
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 You may just have some imbalance in the level control. Run 60Hz signal into the preamp and see what's at the grids.

Also, if those dip switches are different between channels, gain will be different. (This is just a lump of adjustable global negative feedback)

 

RE: Tube matchin, posted on November 21, 2015 at 09:36:29
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Noise is always difficult to measure accurately, because there are so many possible sources which may confound the measurement.

First, the test rig must not contribute significantly if you want the tube noise. That means a quite power supply, and checking the noise from bias and plate load resistances to be sure they are all small relative to the expected noise.

Second, the signal level is small; you need a really quiet amplifier with a gain around 1000 (60dB). This raises the tube noise to a level that can be seen on a measuring device such as an oscilloscope.

Third, you really need a spectrum analyzer, since tube noise has a white noise component plus a "1/f noise" component. The spectrum analyzer is also good for detecting small levels of hum and other interferences, which would otherwise distort or dominate the measurement.

Here's an interesting thread on triode noises:

 

Noise measurement, posted on November 22, 2015 at 12:29:44
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 Paul, just to add to what you've said, the external 60 dB amplifier is really the key to this. It's also relatively simple to construct. If one selects wide bandwidth opamps with very low noise specifications (nV/hz), two can be connected in series, 30dB gain each, in order to produce a very high quality amplifier. In real world terms, it's possible to construct an amplifier like this with a simple regulated supply (or just a bipolar "cleanup" circuit) that will enable viewing a noise floor below 100 nanovolts. Input this to a PC soundcard with FFT (spectrum analysis) software, and your system will be orders of magnitude more sensitive than what's needed to analyze tubes.

 

RE: Noise measurement, posted on November 22, 2015 at 14:22:09
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Agreed. I would use batteries for the preamp, to ensure against power supply noises.

 

RE: Noise measurement, posted on November 22, 2015 at 15:45:56
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 "I would use batteries"

Yes, especially if the preamp will only see occasional use. Also, if one desires to make measurements down to DC, a split voltage source is required. Two 9V batteries are much less expensive than two regulated supplies. BTW, if two opamps with +30dB gain each are chained, only the first needs to be low noise. The second (and whatever follows) will have virtually no impact on the noise floor measurement.






 

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