Coupling Cap question

[TarheelTechinTraining]

I've got an all tube 50 water I'd like to use with a bass. The amp is quite nodded and basically for guitar. So looking at the schematics for guitar and bass it seems one difference is the caps tying the PI to the output. The guitar amp uses .022 and the uses .1
What changes can I expect using .1 versus the .022? Thanks in advance, Greg

[nosaj]

You might want to help yourself by posting a schematic or at the very least a model.

You could play with this.
https://www.ampbooks.com/mobile/ampl...bass-response/

nosaj

[Malcolm Irving]

The coupling capacitor forms part of a high pass filter. For bass, the frequencies that we want to pass through are an octave lower than for guitar. (The frequencies are halved.) Typically the coupling cap for bass wants to be at least twice that for guitar. (Some bass attenuation for guitar is often desirable, especially if overdrive is in play.)

Here is a web-based calculator which is useful:

https://www.ampbooks.com/mobile/ampl...ing-capacitor/

Edit: simultaneous post: nosaj's recommended web-based calculator is more relevant in this case!

[Tom Phillips]

...What changes can I expect using .1 versus the .022? ...

Greg,
The short answer is that there are many additional things that will affect the low end bass response of your complete amp. Especially since you say it is “quite modded.”. Therefore, the best way to find out what the performance change will be with your exact amp is to just try subbing in the 0.1µF caps. The easiest substitution approach is to just clip 0.1µF caps in parallel with the existing 0.022µF caps. That will actually change the part value to a nominal 0.122µF but the difference is irrelevant for the purpose of this test.

Now for the longer answer...
Following are some other considerations:

1. Using the calculator that nosaj posted we see that, with the existing 0.022µF caps, the low end frequency response rolls off from 27.5dB on the flat portion of the curve to 26dB at 40 Hz which is the lowest fundamental frequency of a standard bass guitar. If the coupling cap value is changed to 0.1µF then the response at 40Hz is ~27.9dB. We see that the roll off at 40Hz improves from 1.5dB down to only about 0.1dB down with the larger coupling cap. The 1.4dB improvement is small and I’d say that few people will notice the difference.
2. The above discussion applies only to the frequency response of the phase inverter portion of your amp. The overall bass response will depend upon the whole system from the preamp to the speakers. Whey you try to play loud bass it is likely that the low end power response roll off caused by your output transformer and your speakers will have a much larger effect than the phase inverter coupling caps.

So now we are back to the easy answer and the quick capacitance change experiment to answer your question.

To summarize, the capacitor value will make a difference but it will be small and the sound change may be swamped out by other factors.

Cheers,
Tom

[TarheelTechinTraining]

Thanks Tom. I guess a schematic would be good, wouldn't it. I'll give it a try, subbing that is. The schematic will be a little longer. Thanks for the direction, Greg

[Enzo]

is it something we have heard of? In which case the schematic may already be online or in our files.

[olddawg]

The most important things will be the speaker/cab, the volume you plan to play, and the style you play. There are few rules. People have used some interesting combinations in the studio over the decades for different reasons. I would just plug your bass into it, plug it into whatever BASS cabs you have available and twiddle the knobs to see what you can get out of it before you do anything. DO NOT USE A GUITAR SPEAKER CABINET FOR BASS. You could also take the easy way out and pick up a Fender Rumble 60 off of CL for cheap.

[J M Fahey]

Agree and confirm: the main bass influence is not in the PI, and to boot .022/1M are already more than low enough: around 8Hz.
Even .022/220k means 32Hz cutoff.

But guitar specific preamps often have huge bass cut, specially if meant for overdriving (Marshall bright channel) or being "chimey" no matter what (VOX brilliant channel) , others were actually very good for Bass (Fender Blackface/Silverface) and the guitar preamp was actually better than the specific Bass one (which was muddy).

But we need your data now

At least amp brand/model.

[Justin Thomas]

others were actually very good for Bass (Fender Blackface/Silverface) and the guitar preamp was actually better than the specific Bass one (which was muddy).

Yeah, try it on any vintage BF or SF Bassman head... agree, and add, I get more complaints about the volume of my bass when I play thru my 50W Bassman & 68 Fender 2x15" than any "modern" "bass" amp with a "proper" "bass" cabinet loaded with "bass" speakers. Yes, I use the "Normal" channel... efficiency rules in my world of bass. I hate "bass" cabs. OLD Ampegs are okay.

Justin

[TarheelTechinTraining]

hopefully I did this correct. I started with a busted Jtm60 and tried to create a jcm 800 , kinda. Basically my own, with others help, spin on a tried and true workhorse.

[eschertron]

[ATTACH=CONFIG]35990[/ATTACH] hopefully I did this correct. I started with a busted Jtm60 and tried to create a jcm 800 , kinda. Basically my own, with others help, spin on a tried and true workhorse.

I think none of the coupling caps will be a problem. My only suggestion is that the notch in the tonestack may not go low enough into the mids. But I guess that's a "season to taste" call; just something to think about.

[TarheelTechinTraining]

So maybe drop the 22n cap to 100n, at the mid pot?

[Enzo]

Go over to Duncanamps and download the Tone Stack Calculator. It allows you to change part values in tone stacks and watch their effect in real time.

[eschertron]

+1 to the Tone Stack Calculator. It will help decide what you may want. Again, I'll give the vague answer "to taste", suggesting that knowledge-based tweaking (SWAGs) gives better results than the alternatives (WAGs and doing nothing). Happy hunting!

[TarheelTechinTraining]

Got it. Thanks again