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Old 25th May 2021, 11:33 am   #5
nzoomed
Pentode
 
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 118
Default Re: Advice on this linestage build

Quote:
Originally Posted by Robert Gribnau View Post
The first valve does have a grid resistor (470K).

With the EF86 in pentode mode the gain will be about 110.

To be honest: I am not sure about the effect of the high output impedance of the second valve in your circuit (which will be even higher with the EF86 in pentode mode) on the way the controls will work (will the frequency curves be the same?). In the schematic you linked to, the second valve is followed by a cathode-follower, which has low output impedance (and a 'gain' of a little under 1).
Sorry, I was referring to the first schematic, neither of the last two stages have any grid resistors, on either the 12au7 or 12ax7.

I was hoping I could have reduced the tube count by using an EF86 in this way, plus i have a ton of the things so thought it would be good to make use of them.

I have been reading this here and it shows it in a pentode configuration.
It says the EF86 has alot of gain so has used an EF184. I have a few of these too.
https://www.sciencedirect.com/topics...ring/baxandall

Would I be better off keeping the EF86 in triode mode and adding a 12AU7 cathode follower at the end?
From what i understand, a 12AU7 can take a high impedance load and give a low impedance output which is useful in a line stage.
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