Local feedback on IT secundary (PP amp) - diyAudio
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Old 15th July 2014, 01:40 PM   #1
Jaap is offline Jaap  Netherlands
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Default Local feedback on IT secundary (PP amp)

In the attachment you see an example of local feedback from the plate of the outputtube to the secundary of the IT. This amp has automatic bias. Does anybody have a schematic for the case of fixed bias ? I like to try this local feedback arrangement on a 807 or GU50 PP amp.
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Old 15th July 2014, 02:06 PM   #2
Jaap is offline Jaap  Netherlands
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Default Here is another example

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File Type: gif EL12_siemens_everst2b1_2b2.gif (84.6 KB, 102 views)
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Old 15th July 2014, 05:33 PM   #3
wicked1 is offline wicked1  United States
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Should probably let the experienced people reply, but I'd try disconnecting the 15k resistors in the first schematic, or 50k's in the second, from ground, and apply the negative voltage to those points.

(I'm not an EE, and that's my hobbyist best guess and I may be overlooking some important details, but that is the first thing I'd try)
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Old 16th July 2014, 02:37 PM   #4
disco is offline disco  Netherlands
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Knowing Jack about fixed, I'd run my negative through 100K to the output tubes' grid. Whenever you can come away with it (as referred to in the previous sentence ) I dear not say. Looks like a very loose control, DC-wise.
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Old 16th July 2014, 04:37 PM   #5
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My guess would be to apply bias at the common point of the two 15K resistors for the 1st schematic (two 50K resistors for the second). The two cathode R and C get removed then. The bias's could be applied separately to the two 15K resistors for individual biasing. The applied bias voltage (likely from a pot) should be bypassed well to ground by a cap, since the feedback networks expect a low impedance there for their voltage divider networks to work properly.
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