Vox AC4 clone - taming the bass

[Yazoo]

I built a Vox AC4 clone with an EF86 pre-amp tube. It works but the bass tends to "bottom rumble" more than it should. Rob Robinette's excellent series of Micro amp builds recommend changing the 25uf cathode bypass capacitor on the pre-amp tube/valve to 1uf to tame bass response. I wanted to check before I did this to the AC4. Can you foresee any problems with an EF86 or should I just go ahead?

Thanks in advance.

[GibsonGM]

Can you post a schematic?  Generally, changing the cathode bypass to 1u is fine and will lower bass conent, but there is likely also a screen bypass cap that offers opportunity for some treble boost.   Cathode cap may get you where you want without further 'tinkering'.    Pop the schematic up so we can be sure what you have....I see no issues

[Yazoo]

This is the schematic I used thanks.

https://www.premierguitar.com/diy/amp-diy/how-tube-amps-work

Just scroll through the article to find it. The official original schematic omits a couple of connections but this shows them all.

[bartimaeus]

No idea about the cathode cap mod. But you could swap out C6 (coupling cap after the EF86) for a smaller value. The stock 47nf is full bass, also used in the AC30 Normal channel. The AC30 Top Boost channel uses 560pf, which is a heavy bass cut. Maybe try some intermediate values.

[amptramp]

The cathode resistor/capacitor time constant is 4.2 Hz as drawn and with a 1 µF cap, it would be 106.2 Hz.  This will have some effect.

BTW pins 2 and 7 are the internal shield on an EF86, so they should be grounded.

C6 at 47 nF is a bit too high to do any bass limiting.  The turnover is under 3 Hz so you could afford 1800 pF down to 1000pF.  The suggested 560 pF would force the turnover to around 235 Hz, more than an octave above the 81 Hz lowest guitar note and it will definitely be noticeable.  It all depends on how much bass you want to cut.

[Yazoo]

Thanks for all the advice. I swapped out the cathode capacitor with a 1uf cap and the bass has been tamed. I wanted a nice bright amp for that jingle jangle sound and that did the trick.