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Old 30th January 2012, 01:10 PM   #511
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I will do a test tomorrow and see if light impinging upon the cathode LED's can modulate the output of the 6SN7.
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Old 30th January 2012, 02:00 PM   #512
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I did that with a 12AT7 a few years ago and posted it. No effect, which is totally unsurprising.
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Old 31st January 2012, 12:15 AM   #513
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I think I have found my problem. I need more time to confirm...

Related Question: How do you determine if you have overloaded an isolation transformer? Is it total VA in each transformer divided by line voltage (perhaps with a fudge factor for losses and or headroom)?

My problem might be related to this.
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Old 31st January 2012, 12:24 AM   #514
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One way to tell is when saturation causes the transformer to throw out an excessive 60Hz field!
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Old 31st January 2012, 02:56 AM   #515
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^or it is too hot to touch.....
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Old 31st January 2012, 05:38 PM   #516
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Quote:
Originally Posted by luvdunhill View Post
isolation transformer?
Max continous load of a smaller transformer is usually around 90% +/- of the VA rating divided by secondary voltage. Larger transformers ( 1kW and upwards) usually have somewhat higher efficiency. The real ( as in complex phase angles) wattage is always lower than the VA rating -There's always both core and winding losses......
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Last edited by AuroraB; 31st January 2012 at 05:41 PM.
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Old 5th February 2012, 07:30 PM   #517
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I am planning on trying out my Sowter 3575 and am looking to tune the RC zobel to this transformer. First, it's not even close to the Cinemag values, much more capacitance is needed (note that this is a 10K:10K transformer, where the Cinemag is 15K:15K. I just realized that, and that fact might invalidate these tests?). So, what value would you choose (or what other values would you try). Datasheet:

http://www.sowter.co.uk/pdf/3575.pdf

Setup is Tek CFG280 (50 ohm output impedance, square wave of the Pk-Pk value show at 1kHz) into two wires of transformer primary, the sheild is connected to a secondary ground. There is a center tap, that I have just left loose (good idea? bad idea?).

Image #27 shows just 100K across the secondary, which represents my stepped attenuator (the last picture, sorry cannot reorder them).

Image #28 shows just 100K || 10k across the secondary, which represents my stepped attenuator in parallel with just a 10K resistor

Image #29 shows (6.8nF + 10K) || 100K

Image #30 shows (10nF + 10K) || 100K

Image #31 shows (16.8nF + 10K) || 100K
Attached Images
File Type: png TEK00028.PNG (13.1 KB, 393 views)
File Type: png TEK00029.PNG (13.0 KB, 382 views)
File Type: png TEK00030.PNG (13.0 KB, 380 views)
File Type: png TEK00031.PNG (13.3 KB, 378 views)
File Type: png TEK00027.PNG (13.0 KB, 289 views)

Last edited by luvdunhill; 5th February 2012 at 07:34 PM.
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Old 5th February 2012, 07:46 PM   #518
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While it's nice to eliminate the ringing, you have to, at the same time, be mindful of what compensation capacitance is doing to the bandwidth of the amplifier -- were the oscillographs you show done with the preamp "live" ? -- as you have to deal with the tube capacitances which affect BW. I tuned my pre with a 2x365pF variable capacitor -- and found that it was find without any compensation.
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Old 5th February 2012, 07:58 PM   #519
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Nope, not live. I assume you still put a resistor across the volume pot in magnitude to result in the secondary inpedance of the transformer? I could definitely do it live I guess, but measuring bandwidth would be tricky as I don't have all the bits and pieces ready to do that. Good points of course, I appreciate that.
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Old 6th February 2012, 01:35 PM   #520
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Then try using 10kHz as the fundamental, rather than 1kHz.

You can use one of the freeware FFT/soundcard programs -- or get the little FFT plug in module for your scope. The new line comes with FFT built in , but the older ones (I have a TDS3012B) need the TDS3FFT.
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