Mystery amp from France (?)

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sluckey
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Re: Mystery amp from France (?)

Post by sluckey »

Lynxtrap wrote: ↑Wed Jun 16, 2021 7:12 am Yes, right! Sorry, I forgot the cap to ground. This is a textbook DC coupled differential pair, with unusual Ra values.
Except in this strange case it ain't DC coupled. The grids have no reliable bias.
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Lynxtrap
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Re: Mystery amp from France (?)

Post by Lynxtrap »

sluckey wrote: ↑Wed Jun 16, 2021 9:38 am
Lynxtrap wrote: ↑Wed Jun 16, 2021 7:12 am Yes, right! Sorry, I forgot the cap to ground. This is a textbook DC coupled differential pair, with unusual Ra values.
Except in this strange case it ain't DC coupled. The grids have no reliable bias.
How do you mean? The grids of a DC coupled pair are grounded for AC through a cap, like it is in the schematic?
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pdf64
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Re: Mystery amp from France (?)

Post by pdf64 »

The VDC level of the grids of this thread’s schematic is not defined. Hence the bias / operating point is left to float
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Re: Mystery amp from France (?)

Post by sluckey »

Lynxtrap wrote: ↑Thu Jun 17, 2021 10:07 am
sluckey wrote: ↑Wed Jun 16, 2021 9:38 am
Lynxtrap wrote: ↑Wed Jun 16, 2021 7:12 am Yes, right! Sorry, I forgot the cap to ground. This is a textbook DC coupled differential pair, with unusual Ra values.
Except in this strange case it ain't DC coupled. The grids have no reliable bias.
How do you mean? The grids of a DC coupled pair are grounded for AC through a cap, like it is in the schematic?
The input for the DC Coupled Long-Tailed Pair cannot be cap coupled from the plate of the previous stage. It must be DC coupled (hence the name dc coupled LTP) to get a big dc voltage on the grids for proper operation. But in this case, there is a coupling cap from the previous stage that blocks that plate voltage from appearing on the grids of the LTP. Perhaps Merlin can explain it better. Take a look...

http://www.valvewizard.co.uk/dcltp.html
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