deep boost cap values

[black_labb]

hi guys, looking to convert a 200w guitar amp for use as a bass amp. the amp is fairly bright already and has a bright switch which i find is even too bright for guitar (thats using an alnico II neck pickup). whats a good value to swap the bright cap out to to get a bass boost to get a fuller bass sound from the amp. i cant find any amps with that feature to copy it from.

thanks.

edit: i believe the boost is located in the same part of the amp as one is located on a 5f6a bassman or maybe blackface. basically its a volume pot bypass cap

[MWJB]

A cap accross the ungrounded terminals of the volume pot bypasses frequencies around the unused portion of the control, which is dulling down the signal due to series resistance. At full volume the cap is shorted out & has no effect.

You're not going to get a "bass boost" with any value here. Larger values will bypass a wider frequency spread around the volume, smaller values will just pass higher frequencies.

Perhaps switching to a larger cathode bypass cap at the first stage might help with the bass? Have a look at the various "deep switch" circuits on Tolex Bassmans?

Bass will be harder on the speakers so make sure that these are adequately rated.

[black_labb]

thanks for that, would a rf filter with a break frequency around 150-400hz or so possibly with a resistor to minimist the boost be a reasonable idea to get what i am looking to do? might add a pot in there so i can adjust the break frequency and maybe a second pot to adjust the boost level. dont know if i should drill holes into the faceplate or wether i should just drill holes into that. maybe just leave the bright boost in there as well. if i end up doing that.

i understand the idea of having the cap being more effective at lower volumes, which is understandable, but i cant imagine playing a 200w amp anywhere past 2 for guitar, and a bright switch probably wouldnt be too useful for bass.

the amp is a head, so i will be running it into a bass cab. ive just picked up 2 15" bass speakers from warehouse guitar speakers (30$ for 16 ohm). ive been using the 4x12 cab i got with it for my 5e3 build. after rewiring it to get 4 ohms. not going to get much speaker distortion out of it, but they sound great. nice vintage australian made alnico 12's.

[Enzo]

What amp are we talking about?

There is no boost in most guitar amps unles they have active EQ. Any boost is really a lack of cut.

Snip the bright cap off the volume, you'll not get added bottom on that pot.

As said above, larger cathode bypass cap values, and maybe larger value coupling caps will get some added bottom.

[black_labb]

im speaking about the switchable treble bypass which bypasses the volume pot to the output . you can probably replace word boost with bypass to get an idea of what i mean. i guess i was calling it a boost as it is a higher ratio of bass to the present signal coming through at that stage.

the amp is an australian made amp from the 70's, and it isnt still in production. its a holden wasp, which i dont think people in north america have had any exposure to.

so would a lowpass filter be suitable to give a larger emphasis on the lower frequencies? do you think that a low pass filter would create too big of a emphasis? im not sure if it would be particularlyuseful.

im pretty sure the gain stages use lower value caps, so ill change those to 25uf 25v or something similar if they are anything lower.

thanks

[Bruce / Mission Amps]

im speaking about the switchable treble bypass which bypasses the volume pot to the output . you can probably replace word boost with bypass to get an idea of what i mean. i guess i was calling it a boost as it is a higher ratio of bass to the present signal coming through at that stage.

the amp is an australian made amp from the 70's, and it isnt still in production. its a holden wasp, which i dont think people in north america have had any exposure to.

so would a lowpass filter be suitable to give a larger emphasis on the lower frequencies? do you think that a low pass filter would create too big of a emphasis? im not sure if it would be particularlyuseful.

im pretty sure the gain stages use lower value caps, so ill change those to 25uf 25v or something similar if they are anything lower.

thanks

Yes many of us here are familiar with the Holden Wasp amps.
However, I have a feeling you still don't fully understand what the bypass treble bypass cap is doing. Enzo said it... you can not really boost anything without amplifying it.
Boosting with out an extra gain stage and mixer is really cutting what you don't want.
All passive RC circuits are loss circuits.. you tailor them so there is less loss at the frequencies you want to sound louder.
Not really boosting... just less signal loss at some given bandpass frequencies.

In other words, a 100pF cap might pass more highs around a pot, say a few more db's from 2KHz-3KHz on up because the cap has less impedance (attenuation) then the part of the pot you are trying to bypass but a 2200pF cap will pass those exact same 2KHz on up frequencies too....but a bunch of lower frequencies will see the small cap as a road block with too much attenuation.

The impedance ("resistance" or attenuation) (at given frequencies) the bypass cap exhibits is less then the pot it is bypassing... so in effect, the frequencies, at which that cap is capable of passing easily, will bypass the pot on to the next amplifier stage with less attenuation then those below those frequencies. Those signals reach the next gain stage at a slightly higher level the lower frequencies because the lower ones were attenuated by something or sent to ground somewhere else.
Truthfully, it isn't really much of a filter as it's bandpass curve is very broad, but that's why the R part of the RC tone controls are variable.

Also, any cap in an RC "filter", of any value, will pass all the frequencies above it's 3dB point.
Your 25uF cathode bypass caps forms a type of RC filter too.
A 25uF cap will pass all the same frequencies as a .68uF cap will, but a .68uF will not pass all the same frequencies a 25uF will... specifically, the lower ones will be attenuated.
The net result with the 25uF cap is the amp will sound like it has more bass (to a degree) but it will still have the same treble.

Down below is a .doc file I made up showing what the lower -dB points are of a cathode bypass cap across a biasing resistor of a 12AX7 triode.

[Steve Conner]

Don't some 2 channel amps have an undersized coupling capacitor in the "Normal" channel? The Selmer T'n'B 50 I'm working on has 500pF in the normal channel and something like 22n in the same place in the "bass" channel. I think they intended to have a bright, twangy sound on one channel for guitar, and a full, fat sound on the other for bass.

Holden might have had to limit the bass response of their beast in a similar way to stop it from shredding 12" speakers like confetti. :-o Or maybe they were going for the twangy, chimey treble boosted sound, but to me this works best in low-powered amps like the AC30. A 200W amp with top boost might be like hammering nails in your ears.

Anyway, you'll get the biggest "bass boost" by removing the bright cap altogether, which is maybe what the front panel switch does already. But it's not actually a bass boost at all for the reasons that other posters gave. It's an absence of treble boost that makes the bass seem bigger. If you want yet more bass (you're still not happy even with the bass tone knob up to 10, and the mid and treble at 0) you need to go hunting for undersized coupling caps and swap them out.

[black_labb]

in reality i havent tried it through the bass speakers, ill get those going first. as making a cab will be a bit more work than switching around a few caps. ill have a look at the coupling caps while in there as well.

sorry for comfusing people by using the term bass boost, i guess its not the technical word.

ill see how the circuit fares with some bypass cap changes before i do anything more than that.

thanks luke.