5879 & EF-86 versus 6SJ7

[W5FH]

Hello-
In a recent octal build using 6SJ7 players commented the amp was a little too bassy even with bass pot all the way down. The amp used a parallel 6SL7 input stage driving Fender tone stack (as used in HRD ML-212) with 6SJ7 as second stage. There is a recent thread here where I described all this.
I learned that the screen bypass capacitor has a pretty noticeable effect on the bass response. Lowering values of preamp cathode bypass caps had a lesser effect. Lowering the value of 6SJ7 screen bypass from 0.1 uFd to 0.047 made a fairly noticeable decrease in bass response and brought everything to where it needed to be.
I am looking at using a pentode like 5879 or EF-86 in a new build and was curious if this same effect occurs? I have not used the 5879 or EF-86 yet in a build and was thinking if this effect is similar to the 6SJ7 I could capitalize on it and use switch selected values to enhance or reduce bass.
Thanks all for help.

[martin manning]

Yes, it will have a similar effect. See fig. 5.13 in Blencowe, Designing Valve Guitar Amps. You have an R-C voltage divider, where the impedance of the lower leg (the C) is frequency dependent.

[W5FH]

Thank you Martin -
I really appreciate all the help you give, tnx!

[W5FH]

Martin-
If you have time for a quick question -
I have been studying the biasing of the 5879 as used in some of the amps on The Hoffman forum where it is the second stage. The loadline calculator I like to use has the EF-86 but not 5879. I have started a build, targeted to be used for bass and pedals, that I plan to have a paralleled 6N2P-EV driving a tone stack followed by a 6N2P-EV triode as 10K cathode cold clipper driving the 5879. Would you have recommendations for biasing the 5879? I am thinking a gain of around 70 or so, fairly clean center biased. However the amp will have a master volume between the 5879 and phase inverter.
One concern of mine is that I plan to have optocoupler tremolo following the 5879. If it is like the 6SJ7 I know that you do not want to heavily load down the output of pentode. I see in some of the 5879 amps there is a DC coupled cathode follower slaved to 5879. The amp is called "Dual Pentode", I will post the schematic. I read in builders comments the C/F smooths the tone some too.
Might I ask for your commentary on this? This will be my first build using 5879.
Sincere thanks my brother.

[W5FH]

I have followed tubenits builds with great interest. Here is the schematic of 5879 with DC coupled C/F.

Dual Pentode.gif

[sluckey]

One concern of mine is that I plan to have optocoupler tremolo following the 5879.

I recently came across a schematic that modulated the screen grid of a pentode preamp tube with a tremolo LFO. I cannot recall where I saw it. I'll keep jogging my memory. Meanwhile, just know that option exists. This method probably won't affect the plate loading.

[W5FH]

Thank you, Steve-
I appreciate you taking the time to help. I use your Tremonator circuit in a lot of my builds. Thank you for that wonderful gift.
Byron

[Cannibal]

One concern of mine is that I plan to have optocoupler tremolo following the 5879.

I recently came across a schematic that modulated the screen grid of a pentode preamp tube with a tremolo LFO. I cannot recall where I saw it. I'll keep jogging my memory. Meanwhile, just know that option exists. This method probably won't affect the plate loading.

It's the 60s pentode input ac15 if I am not mistaken. It has a quirk: if you turn off the LFO the depth pot works as a screen grid compression control, very cool sound can be harnessed