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Old 29th September 2019, 10:59 PM   #51
paczeltf is offline paczeltf  Hungary
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I know all this, but this amp was designed and tested without control grid stoppers next to the power tubes just like all other Elekit amps that I have seen. The PCB layout and the high frequency compensation in the GNFB is at least as important and no one has yet complained about oscillations. The only measurement that I saw does not show any trace, all it tells is that the OPT iron core is a bit smallish.
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Old 30th September 2019, 11:46 AM   #52
Disque is offline Disque  Netherlands
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Thank you for your quick reply. I think I saw the measurements too, but I forgot where exactly.
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Old 1st October 2019, 01:21 PM   #53
paczeltf is offline paczeltf  Hungary
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I am not sure if URLs can be posted according to the forum rules. If not, I can send you in PM.

I suspect that the generic recommendation on the control grid stoppers came from the experience with average DIY builders, who use unknown cables and point-to-point wiring with an unpredictable layout, and have no tools to control the results. This all can lead to hum and parasitic oscillations.
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Old 14th October 2019, 09:28 PM   #54
Disque is offline Disque  Netherlands
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I think I found the source of my problems with harshness and sibilance.
The coupling capacitors I used are way too bright in a high voltage application like a tube amp. I used Obbligato premium film capacitors 220nF, which type of capacitors I liked very much in a speaker crossover where they are not harsh and show fine bass.
By changing them for Mundorf Supreme 220nF in the TU8200 I solved the problem. ( I also changed the cathode FB resistor back to 18 Ohms for the E80CC)
Experimenting with the Obbligatos in other tube amps confirmed my suspicions.
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Old 15th October 2019, 05:42 PM   #55
paczeltf is offline paczeltf  Hungary
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Good to hear your problems are solved.
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Old 16th October 2019, 12:04 AM   #56
Disque is offline Disque  Netherlands
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Adding to my last post.

I had some free time and tried a few of my best power tubes with wich I have also experience in other amps.
So over some hours I tried a few 6l6GC strong UOS : Sylvania, GE and Chinese modern production.
EL34 I do not like.
The best result in my humble opinion I got with modern Genalex Gold Lion KT77. Very analytical, lots of microdetail and natural from high to low frequencies. What ever kind of music I throw at it, comes out fine: Jazz, Pop, Classical voices, orchestra or chamber music.
The extra .8 A for the heaters, in addition to the extra .6 A for the E80CC, is no problem: the power transformer stays around 57 degrees Celsius over hours. The casing gets heated up extra by the tubes.
Taste is personal, but I recommend the experiment.

Thanks again for all the feedback on this nice build.
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