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Old 10th December 2012, 10:10 PM   #11
rrrs is offline rrrs  United States
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The fact is that with my OPT measured 7400pF primary to secondary capacitance you effectively get at 15Khz a 1.4K resistance from nominal 20K primary to ground; in case secondary is grounded!
I expect this to influence response below 20Khz as the load on the plate of the output tube would be reduced a lot; at 20Khz it would fall below 1K compared to nominal 20K!
Anyway, will give it a try and report if I find a difference.
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Old 11th December 2012, 01:07 AM   #12
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I wonder what the primary self-capacitance is -- probably pretty big then.

Did you snoop around higher to look for the resonant spike and such? The overall dropoff might not change much with the secondary, but the spike ought to.

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Old 11th December 2012, 01:20 AM   #13
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I'll be very interested to see what the acoustic measurement shows. That big capacitance between the windings should really drain off the HF, but measuring should tell you.

Also the trick of a large resistance from one side of the secondary to ground and measure the current thru it should tell you a lot. Let us know what you find.
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Old 11th December 2012, 06:43 PM   #14
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Hi, will do acoustic measurements once I get new soundcard; managed to fry input on my one when trying to measure frequency response of the Amp and was lazy to build a input protection circuit....
Might take me few weeks to get it done, but will report what I find out!

I did measure the frequency response of my OPT, just using my soundcard output/input, but obviously was not able to do a floating measurement as secondary gets grounded that way through the soundcard; you will see a small peak at 40+K.
You will see 3 different measurements using 3.2K, 4.7K and 10K as a primary load, all into 6.5 ohm on secondary.
4.7K is close to plate resistance of 211 tubes I am using.
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Old 11th December 2012, 11:29 PM   #15
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Hi, Pano's suggestion of measuring AC voltage drop/current trough resistor connected between secondary and ground was really interesting; I used 27K from secondary to ground and got:
100Hz – 3.5Vpp
500Hz – 15.2Vpp
1KHz – 22.5Vpp
10KHz – 23.9Vpp
15KHz – 19.7Vpp
20KHz – 15.7Vpp
So, there is definitely capacitive coupling dependant on frequency; developing approximately 0.3mA current across 27K resistor at 1KHz, while there is hardly any current al low frequency.
Not sure how to interpret this data, but will do also a acoustical measurement once my sound card is fixed and report if I can find any measurable frequency response difference between grounded OPT, floating OPT and OPT connected to ground trough large resistor.
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Old 11th December 2012, 11:44 PM   #16
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Secondary with a load resistor, then the 27k to ground from one side of the secondary??


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Old 12th December 2012, 03:04 AM   #17
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At what output voltage???
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Old 12th December 2012, 03:53 AM   #18
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Yes, secondary with 5.7ohm dummy load across the speaker terminals and the 27K from one terminal to ground.
Did the measurement at approximately 1W (had about 8Vpp across the speaker terminals).
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Old 12th December 2012, 04:22 AM   #19
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Interesting. Definitely some coupling there, but the 27K resistor really pulls down the voltage from the ~200Vpp you were seeing before. Pulls it down into a safer area.
Am I calculating right that the leakage impedance is about 200K?

As the 27K to ground is pulling less than 0.02W, you could likely load it harder.
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Old 4th January 2013, 09:51 PM   #20
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So, I did finally get around measuring acoustic response and there is a difference; small but clear to see. At 20kHZ there is approximately 1.5db more roll-off with OPT secondary grounded vs. not grounded. I did re-measure few times and kept getting same result; so would say it is reliable; unfortunately forgot to measure with large resistor to ground, but would expect to be similar response as not grounded.
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