Adding some bottom end to an AX84 High Octane

[JPK]

I found the below 2014 thread where PRR mentioned changing out the first three stages cathode bypass caps to increase bass (post #13). But he said to start with 10uF not 22uF. This got me wanting to experiment on my 10 year old AX84.com High Octane. I bought some caps from Doug, 3 x 10uf and 3 x 22uF. I know the 22uF is total overkill but I wanted to go overkill and back off from there. Also the 5E3 has huge cathode bypass caps. Well I soldered the 22uF in (parallel with existing) and it sure brought out the bass. And as he mentioned it's too loose on the bottom now. And on full gain it sounds horrible as this is a high gain amp. On low gain clean it sounds pretty good. But if you get into it volume wise, the bottom end loosens up. I'll probably try the 10's out tomorrow. Note: the schematic I posted doesn't show the parallel octal socket which has a KT66 in it right now. If anyone has any other suggestions let me know.


https://el34world.com/Forum/index.php?topic=14220.0

[JPK]

I ended up removing the experimental 22uf in the first three stages and putting a 10uF in the first stage. It has more bass but retains the tightness. If anyone else has any other ideas let me know.

[jordan86]

I have an AX84 SEL that I modded more into a jtm45 preamp.

I hate to say it but there’s a thing to single ended. It will never have the punch and authority of a push pull power section. So because of that it feels like it needs “more bass”.  As you are finding, larger bypass caps sort of help but not exactly. I think experimenting with anything between 2uf to 10uf is worthwhile. Find what you like and what works for you. Note: not all the stages need to have the same value caps. You could also leave them smaller and look at using larger coupling caps as well.

In the end it will still be a single ended amp.

[shooter]

It will never have the punch and authority of a push pull power section.

...until you put away those cute 'lil bottles, move to KT88 or 120 runnin hot, add a 4X12 quality cab, then your hair face will feel the wow 

[John G]

Hi, This may be of help. The downside is the MV will need to be moved. Another way would be to put a frequency network from the O/P of the transformer's secondary(cap) back to the cathode of the driver stage. Just thoughts.John

[JPK]

Hi, This may be of help. The downside is the MV will need to be moved. Another way would be to put a frequency network from the O/P of the transformer's secondary(cap) back to the cathode of the driver stage. Just thoughts.John

Hey John thanks for the ideas. Where did you come up with that circuit you sketched up? Does it go right after the tone stack in the schematic I posted? What do you mean move the MV? Where to? And if you have any sketches of that frequency network I'd be interested.

[John G]

Hi, I found the scheme posted used in a cheap low-power guitar amp. I suspect that the O/P transformer was tiny and cheap, and nasty.
By including the plate-to-plate feedback 470 K, the 22nF and the 150K to ground,)allowing the gain to be reduced from the low mid to high frequencies while letting the low-end pass unimpeded.
There are many brilliant minds here that can offer you better design support in this area.
A series cap and resistor back to the driver tube unbypassed cathode from the transformer's secondary may also work. Correct phasing of the transformer is important.
I think you would need the extra stage after the tone stack for any of this to work. As I said, there are better minds here than mine. John
 

[jordan86]

...until you put away those cute 'lil bottles, move to KT88 or 120 runnin hot, add a 4X12 quality cab, then your hair face will feel the wow 

I have used a KT88. It’s still not the same.

For clarification, yea, you can reproduce plenty of low frequencies with the right bypass caps and a good SE OT. Was just playing my SE amp today and it does put out a lot of lows. But it’s just doesn’t feel as good as a push pull amp. :icon_biggrin

[JPK]

I just built this 1 watt push pull that sounds way better than my High Octane. That's what made me want to improve it. I was thinking of duplicating the pre amp resistor and cap values to see if it would sound as good. Then thought after all that maybe it won't sound any better. I never heard that single ended amps have bottom end issues. Could be true. A 1 watt PP blows it away tone wise. I play the thing every day.

[jordan86]

I’ve tinkered with mine quite a bit. Bright caps and dull caps, different values here or there, conjunctive filters and filter chokes, different output tubes, etc. In the end, I have an amp I enjoy but it’s at its best above the fifth fret. Not very toneful as a clean open chord amp or a chugging palm mute thing. But single note blues and rock stuff up the neck, it sings.

[JPK]

I’ve tinkered with mine quite a bit. Bright caps and dull caps, different values here or there, conjunctive filters and filter chokes, different output tubes, etc. In the end, I have an amp I enjoy but it’s at its best above the fifth fret. Not very toneful as a clean open chord amp or a chugging palm mute thing. But single note blues and rock stuff up the neck, it sings.

Do have an as built schematic or even a marked up one? I'd be curious what you came up with. Did you try out any other preamp tubes? Right now I just have a parallel octal socket (running a KT66), a conjunctive filter on a switch (it's always off, tone sucker), and a 10uF cathode cap on the first stage.