Slope resistor and effects send level?

[Firestorm]

Getting back on topic: the "tightness" of the bass is affected by many things: size of the filters (compare Marshall values over the years); feedback; speakers themselves; cabinet. There's lots to experiment with.

[67Mopar]

I'm thinking that contributors to the bottom-end looseness may be the .033uf couplers. What about the .1uf?

[IMG:626:468]http://i1282.photobucket.com/albums/a536/67Mopar1/Screenshot2013-06-08at12940PM_zpscf977461.png[/img]

[67Mopar]

Speaker cabinet is a 4x12 pine enclosure w/V30's.

[Firestorm]

I'd hold off messing with the cabinet. Try. 02s in the PI. Also look at the feedback circuit. What tap are you taking the feedback signal from? You've got 147k feedback resistance (with high frequencies bypassing the 100k, sort of an anti-presence circuit). If feedback is small near the speakers' resonant frequency, they can get loose sounding. That's what a "resonance" control does.

[67Mopar]

Signal is from the 8 Ohm tap. There's plenty of range at the presence, but the bottom is just a bit too much. Would adding an electrolytic 100uf/63V cap at the end of the 8 Ohm tap help?

When I hit the front-end with a boost pedal, the bass tightens up, but I lose something in the highs.

[Colossal]

I agree with Firestorm's recommendation of lowering the output coupling cap values from 0.033uF to 0.022uF. That cap in conjunction with the bias grid leak sets the recovery time for bias excursion. Bloated low end can be controlled here, you want a faster response for low frequencies.

[Firestorm]

Would adding an electrolytic 100uf/63V cap at the end of the 8 Ohm tap help?

Yikes, no. Yikes.

[martin manning]

What's with all the blocked out parts on the schematic?

[VacuumVoodoo]

I'm thinking that contributors to the bottom-end looseness may be the .033uf couplers. What about the .1uf?

To get control of loose bottom you need to tighten it as early in the signal chain as possible. Garbage in-garbage out.
Limit low end bandwidth after 1st stage to 120-150Hz with a 12db/octave HP filter. A combination of RC constants of 1st gain stage input and its plate coupling to 2nd stage input can be used to approximate this function. It will be somewhat sensitive to guitar source impedance but it'll still be better than nothing.

[67Mopar]

If I need to address it at the front, would I be looking at maybe lowering the 2.2uf cathode bypass cap at V1b input? I don't really want to mess with the plate/cathode values, so I'm hoping there's an easy fix for this. I mentioned the 100uf cap because the MK50 and MK50H II both have this cap in the nfb loop. Those amps have both 'presence' and resonance' controls, so maybe that makes a difference...I don't know? You guys are seasoned pros at this, so I'm just taking it step-by-step...

Can I go as low as .01uf at the PI couplings?

[67Mopar]

What's with all the blocked out parts on the schematic?

That's how it was originally sent to me.

I tried .002 PI couplings, .022 mid, and .033/.022 bass caps. It did lift the bass some, but also had a negative effect on the overall tone/feel. The further I went, the more it started to sound like an SLO100. Not that that's a bad thing, but I'm just not into 'that' sound. I'll just leave it stock and use a mild boost to lift the bottom.

[VacuumVoodoo]

If I need to address it at the front, would I be looking at maybe lowering the 2.2uf cathode bypass cap at V1b input? I don't really want to mess with the plate/cathode values, so I'm hoping there's an easy fix for this. I mentioned the 100uf cap because the MK50 and MK50H II both have this cap in the nfb loop. Those amps have both 'presence' and resonance' controls, so maybe that makes a difference...I don't know? You guys are seasoned pros at this, so I'm just taking it step-by-step...

Can I go as low as .01uf at the PI couplings?

There's not enough data for an advice of any practical value:

1. Complete schematic of the preamp
2. How is the low end "loose"?
2a. Clean & loud or doesn't change with volume?
2b. Distorted/OD, at any volume?
2c. All of the above?
3. Tone controls have any effect on low end ?
4. Have you tested with different speaker cab?
Above assuming it's not something outside the amp altogether, like wobbly wiggly old strings...

[Colossal]

I'm thinking that 100uF on the 8R tap into the NFB loop is arbitrarily large so as to pass all frequencies, but just keeps DC off the pot(s) downstream. Without seeing the whole schematic though...

[Reeltarded]

If a boost pedal is doing what you want to the overall tone I think you should stay stock everywhere but the first stage coupler. Make that up to 10x smaller.

The boost pedal likely acts as a bandpass cutting off higher in the lows and lower in the highs. For instance a lead spec Marshall has .0022u instead of a .022u coupling that first stage. I have even gone smaller and left very large couplers in PI and output sections with excellent results, as eluded to above.

One more thing; nice to have you here. Thanks for hanging.

[dobbhill]

I would try 0.68uf cathode bypass on V1. My favorite bass reduction solution.
YMMV
D