using an inductor and capacitor as a low pass filter at the 12ax7 cathode

[daz]

Yes !!! why i didn't mention this i have no idea. try this....put a .0033 cap in series with the NFB loop. I'm not sure whether to put it before or after the resistor thats right after the tap, but i've been using it right after. i think i had it after the 100k that comes off the tap a while back tho. been meaning to try it in both places and see what bthe difference is. In any case, try this because it will give you a low end thats no obtainable any other way. it's just strong as he||. this is whats commonly referred to as "resonance". usually with a pot to take it in and out of circuit, but just try the cap alone and see what you think. If you find you want it adjustable add a pot. But it will give you a really strong low end. More power in the lows than *amount* of lows, but it may be just the ticket. of course you must have NFB or this is all a moot point.

[Joey Voltage]

Yes !!! why i didn't mention this i have no idea. try this....put a .0033 cap in series with the NFB loop. I'm not sure whether to put it before or after the resistor thats right after the tap, but i've been using it right after. i think i had it after the 100k that comes off the tap a while back tho. been meaning to try it in both places and see what bthe difference is. In any case, try this because it will give you a low end thats no obtainable any other way. it's just strong as he||. this is whats commonly referred to as "resonance". usually with a pot to take it in and out of circuit, but just try the cap alone and see what you think. If you find you want it adjustable add a pot. But it will give you a really strong low end. More power in the lows than *amount* of lows, but it may be just the ticket. of course you must have NFB or this is all a moot point.

It will work either way, the only problem is that you have no control over it. the lower the cap value, the more the mids will be effected as well. The common value is .0047uf, most of the time bypassing a 1M pot .0033uf is an old Mark Cameron thing

[Dave H]

What i really wanted to accomplish with my amps was to kill the bass early on so that it overdrives nicely and doesn't become a swamp of mud, and then boost it in subsequent stages so that it still has balls in the end. But I wanted the bass boost to be done in a shelving way, so that the highs are still there but just reduced in volume. I didn't want to just use a low-pass filter because by the time the bass frequencies I want are effectively boosted, the sparkly highs are all cut off.

To kill the bass early on you could use a small value coupling cap from the plate of the first stage and/or use a small value for the first stage cathode bypass cap. The coupling cap will have more effect because there is only about 6dB of gain to play with by bypassing the cathode resistor. To filter out the mud I like to use a either a 2n2 coupling cap driving a 1M load or a 4n7 cap driving a 470k load.

To get the shelving bass boost you want try putting a series resistor and capacitor in parallel with the plate load of a later stage. If the plate load is 100k try a 47k in series with 10n connected across the 100k. If you just put a capacitor across the plate load without a series resistor it won't have the shelving effect. It will just be a low pass filter rolling the highs off at 6dB/octave. If that's too much bass boost change the series resistor from 47k to 100k. To boost only lower bass frequencies increase the value of the 10n cap. To boost higher bass frequencies reduce the value of the 10n cap.

Dave H.

[jrfrond]

Too much bass response up-front creates IM distortion, sometimes referred to as "rolling", and sacrifices the power-bandwidth of the amp. Bass response in guitar amps, to me, is overrated, and more often than not, excessive, plus a waste of RMS power. It seems to me that it is often employed to counteract poor speaker cabinet or speaker model choice. Players who want thumpin', kick-ass bass response really should be using infinite-baffle (closed-back) cabinets. For instance, vintage Marshall stacks or even half-stacks have that trouser-wavin' bass response because of tight filtering and the 4 x 12 (or 8 x 12) wavefront, with no phase cancellation from wraparound bass frequencies as happens with open-back cabs, and Marshall cut the bass right up-front with a .68uF cap across the 2.7K cathode resistor.

The NFB series cap or "resonance" control can simulate thump, but only if the amp has a NFB loop.