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Old 14th June 2012, 08:43 PM   #871
Salas is offline Salas  Greece
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Old 14th June 2012, 08:44 PM   #872
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Anyway, I took some actual measurements (in which I'm certainly not an expert). In my setup the pre is driving a 27 triode, having a grid to cathode capacitance of 3.1 pF. As a load I paralleled a 10K ohm resisor and a 3.3pF capacitor.

In the first setup (with cathode bypass) a 1KHz square wave of 200mV rms gave an output of 990mV rms (5 times amplification). In the second setup (without cathode bypass) a 1 KHz square wave of 200mV rms gave 460mV rms output (2.3 times amplification).

Here you'll find some results. Voltages are derived from a Fluke 87 true RMS multi meter. Don't know what frequency domain it can handle.
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Old 14th June 2012, 08:46 PM   #873
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Old 14th June 2012, 08:50 PM   #874
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You lost half your gain without the cathode cap, expected. Don't load it under 20K btw, it has about 1.5K Zo in common cathode and bypass mode for instance.
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Old 15th June 2012, 10:47 AM   #875
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You lost half your gain without the cathode cap, expected. Don't load it under 20K btw, it has about 1.5K Zo in common cathode and bypass mode for instance.
Agree. I had 3k9 on my R120 and EL34-triode at 240V. The amplification was low of course, and the sound was very 'warm' - second harmonics. But I drove a Hitachi FET output directly with it and that worked like a charm. At the end I thought it too - well - vague.
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Old 15th June 2012, 11:25 AM   #876
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Ran my PSU setup in PSUD last night. According to it, should be ideal. Guess that means i will spend this weekend scoping and rewiring PSU. Salas, ZM, what is your impression, if any, about high quality cap like PEH169 in prefilter vs motor run cap? Got my order queued up, and can add peh or bhc to the list. Not sure if it is woth the coin for an electrolytic, that in theory at least, should not be as good as film/motor run cap.
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Old 15th June 2012, 11:44 AM   #877
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Ran my PSU setup in PSUD last night. According to it, should be ideal. Guess that means i will spend this weekend scoping and rewiring PSU. Salas, ZM, what is your impression, if any, about high quality cap like PEH169 in prefilter vs motor run cap? Got my order queued up, and can add peh or bhc to the list. Not sure if it is woth the coin for an electrolytic, that in theory at least, should not be as good as film/motor run cap.
Try PEG124, 33 or 47 uF on second C in filter and parallell up to a value where it gets silent with the ear in the cone. No need to go as big as PEH169 HV stuff for a start. IMHO.

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Old 15th June 2012, 11:47 AM   #878
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more a question of sound difference, if any. I am audiophool trying to learn secrets of audiophoolery
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Old 15th June 2012, 11:59 AM   #879
stajo is offline stajo  Sweden
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more a question of sound difference, if any. I am audiophool trying to learn secrets of audiophoolery
That might be a bit application dependent but my last little EL84-amp ended up with a combination in second C. 50 uF motorrun and 100-ish PEG124. To much plastic narrowed the sound to my ears. I actually prefer good electrolytics to plastic as base.

Do some listening tests, its fun and it might surprice you.

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Old 15th June 2012, 12:52 PM   #880
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Exactly what i was looking for. THnaks
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