no bass

[sting7777]

I am troubleshooting a severe lack of bass on an 18 watt clone.

I am going to try swapping out the .01uF coupling caps to .022uF and then maybe the cathode bypass cap from a 50uF to 200uF or so (some take it as high as 1000uF).

These are the two reccomendations at the 18watt forums.

Any thoughts on other areas to look at? Volume is good, mids are good, high end is there - just no oomph at all.

[NTBluesGuitar]

The cathode bypass cap is a good start...if you notice that the 5F6A has a 250uF first stage cathode bypass cap. However, when I build this circuit for folks I bring it back to 25uF to cut back on the bass for guitar applications.

I think you've got a good start with the two ideas you have now. Give it a try and see if you need to do more later.

[steve25]

Does it sound power wise like its breaking up too soon ?

[sting7777]

Does it sound power wise like its breaking up too soon ?

No - it gets real loud and breaks up fairly late. Overall - it sounds great but its almost like there is low shelf EQ applied.

[curtisstetka]

Have you tried different speakers and cabinets?

[sting7777]

The cathode bypass cap is a good start...if you notice that the 5F6A has a 250uF first stage cathode bypass cap. However, when I build this circuit for folks I bring it back to 25uF to cut back on the bass for guitar applications.

I think you've got a good start with the two ideas you have now. Give it a try and see if you need to do more later.

So should I play with the bypass cap in the pre or power stage or both?

[sting7777]

Have you tried different speakers and cabinets?

Yes - this is definitely not a speaker/ cab thing - I have a Weber 12 in a closed back that could be a bass rig its got so much bottom end - but does nothing with this amp. The speaker is an Eminence Governor and has plenty of wallop with any other amp.

[NTBluesGuitar]

So should I play with the bypass cap in the pre or power stage or both?

Start with that first stage cap. The power stage isn't as noticeable in this case.

[sting7777]

OK - so using this as a reference: http://www.turretboards.com/layouts_...e2_attilio.bmp

I replaced the .01 caps (10nf on the referenced layout) with .022 and I am now using the .1uF cap as shown (originally was a .01 on my build).

There is a bit more mid-bass but nothing on the low end (low E doesnt vibrate anything at all).

I had an extra 25uF that I added in parallel with the existing 50uf on the first stage and noticed no difference. I have some higher values coming in the mail in the next few days that I can try as well.

Anyone else see any obvious issues in the referenced circuit that would lead to no bass? Any other values or componenents to swap out or just go for larger bypass caps once I have them in hand?

thanks for taking a look...

[steve25]

Start high , with .47s coupling the PI to the outputs and then work your way down if that gives you the missing punch.

[sting7777]

So I think I have the problem sort of fixed in that with the amp cranked up - there is now some good low end.

Anything I should be looking at in terms of lack of bass when at the lower end of the volume scale?

I am comparing to my PR which has lots of bottom end even when used at lower volume levels.

I tried jumpering in additional .1uF caps in most signal cap paths to see if there was any noticeable difference and there was not.

I am still going to significantly up the bypass caps to see what effect that has. I dont want the bass to be farty when cranked - its not now - defintely more of a mid-pass punch.

[steve25]

Do you have a way to test the amps true RMS output?
I have heard some OTs that when not matched to the proper impeadance lost a good amount of low volume kick.

[sting7777]

Do you have a way to test the amps true RMS output?
I have heard some OTs that when not matched to the proper impeadance lost a good amount of low volume kick.

I have a Fluke DMM. Is there a specific procedure or measurement I can take?

This was a Pro Jr. conversion - so it may just be part and parcel of the way everything work together. The good news is that it now thumps when its cranked - so it definitely does the Marshall thing now... I still need to tame the high end a bit - but thats another kettle of fish.

[sting7777]

OK. I am happy with the amp now. I ended up removing the bright cap. HUGE difference. There was so much high end sizzle with it in place that the bass was lost. So it sounded OK cranked up - but terrible at low volumes.

It now sounds great at all volumes and the tone is somewhat usable (it never had a huge swing). I went to a .022 orange drop for the tone cap and I can completely tame the high end now if needed (helpful with a tele). Still not as much variation as a tone control on a guitar- but usable. I have never heard a really useful single knob tone control but the amps I have played with one in general dont really need it and I just leave it centered or where its least noticeable.

Best part is that there is plenty of bass and I can shake the house with a low E now.

Thanks for all the help and suggestions as always.