6BM8 triode plate resistor Q

[tubenit]

I have done a series resistors approach before to attempt to lower gain and increase a cleaner/clearer tone.  And it worked!

My understanding is the triode section of the 6BM8 is more like a 12AX7 or a 12AT7?   I'd like it to be more like a 12AY7,12AV7 or 12AU7 with less gain.  I want to use the 6BM8 triode in the LTPI and NOT substitute a 12A_7 version.

I'm trying to think thru an amp design prior to building it after the first of the 2023 yr (meaning that  I don't have the amp here to experiment with currently).

Would this approach work and also give a cleaner tone in the LTPI in lowering gain?


With respect,  Tubenit

[kagliostro]

Ciao Tubenit

That 220pf capacitor is confusing me

why use it ? To bypass some frequencies and prevent theyr attenuation .... but why only one cap and don't use one by side ??

However using a split load resistor on a LTPI may be the solution of your ... "problem"

May be the read of this page can be of some help to find an answer to your question

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

Franco

[sluckey]

Using split plate load resistors doesn't actually change the gain of the tube. The resistors simply form a voltage divider so you will send less signal to the next stage.

[tubenit]

That 220pf capacitor is confusing me          why use it

https://el34world.com/Forum/index.php?topic=12723.msg153636#msg153636




Sluckey,   thanks for the response!  always helpful


with respect, Tubenit

[kagliostro]

I didn't read all the link, but the question Is why only One ?

[sluckey]

Using the cap as shown is NOT a smoothing cap. It actually boosts the high freq signal by bypassing around the resistor connected to the plate. Putting that cap across the other resistor would be a smoothing cap because it would shunt some high freq signal to B+ which is signal ground.

[kagliostro]

Thanks Steve

That has a sense, now all is fine

Franco

[tubeswell]

My understanding is the triode section of the 6BM8 is more like a ... 12AT7?

Yeah but with A = 70, gm = 2200uS (half the transconductance of an AT, with a smidge more gain)

[PRR]

....half the transconductance of an AT...

The book-specs on 12AT7 are taken at very high current (10mA) for RF applications. We don't run them that way in audio (even as reverb driver). Gm goes down with plate current. For quick estimate, roughly as square-root. Some junior engineer was tasked to plot Gm in a range of conditions, see page 4:
https://frank.pocnet.net/sheets/093/1/12AT7.pdf