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Old 25th May 2021, 12:02 pm   #6
Robert Gribnau
Join Date: May 2017
Location: The Hague, The Netherlands
Posts: 231
Default Re: Advice on this linestage build

I am not sure what you mean by the first schematic. If it is the schematic of the tweaked Brimar tone control, than the 12AX7 section has its grid returned to ground through the 470K and 2.2 M resistors. The grid of the 12AU7 section following the 12AX7 is directly coupled to the anode of the 12AX7 and therefore does not need a grid resistor.

Like I wrote, I think that the gain of an EF86 in triode mode is not high enough to cover the range from - 15 dB to + 15 dB. An EF184 in triode mode (mu is about 60 so the gain can be about 40) would be somewhat better. I have never used the EF184 before but I have read that the EF184 can be microphonic.

I hope some other forum members join in because the precise workings of tone controls is a little over my head (except in guitar amplifiers, I never build tone controls in my projects).
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