Bass Issue?

tube-a-lou

Addicted Member
Hi all,

I'm restoring a 60's Thomas Vox Pacemaker guitar amp, and for me it has a touch
to much bass, so I have no schematic. In between the treble and bass (pots) controls there
is a 1uf cap, it was a electrolytic which I changed to a poly and it has bass now (more) plus on the
bass pot on one outer and middle tab is a 100ohm resistor. If I change the cap would it change the
deepness of the bass and which value should I attempt?
 
hello. we need a schematic to advice on that subject. Increasing a capacitor can increase the bass or tame it, depending of the application.

you have usually coupling capacitors between the amplifier stages that act as hi pass filters, decreasing the value will cut the bass. Using some on-line calculator like this one you can check the cut freq. http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php NO AFFILIATION

Is it a solid state amp or a tube amp?

I'd stay with the original design and turn down the bass pot.
 
It's a solid state, I found this it's from one of there other models but they seem
to use the same circuit in all the models just with the reverb, which mine does not
have. This has the preamp section with the two tone controls.


fullsizeoutput_6177.jpeg
 
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I see. C1 is a hi-pass filter, reducing C1 will drop the bass getting into the amp.

Then C11 cuts the bass out of the volume control to the power amp.

C5 cuts the bass going to the tone control

If you just want to cut the bass without any other mod, replacing any of those caps is the easiest way. I think , to really hear a difference, you could start with 1/10 of the value and listen, to adjust later to your taste.

If you want full bass, but changing the way the bass control works, you need to modify the tone control circuit. Remember, capacitors let high freqs to pass, and block or offer high resistance to lower freqs. So any time you increase a capacitor value, you let more low freqs to go that way. And, reducing a capacitor, less low freqs can go that way.
 
I started at C1 first dropping it from .1uf to .047 it's a coupling cap so that's easy. C11 on mine is
a .2uf don't know if I could find a lower cap than that but will try, C5 is a electrolytic and I think
that was replaced to a 2.2uf by someone else. My main problem is the the bass control does not
have much control over the bass I could go 0-3 and it's fine after that 4-10 just boomy.

Thanks for the help it really great!
 
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That's right on the bass pot lug's too. it's the 1uf and 100 ohm resistor, would that effect the tracking of the bass control. Here's a picture of the inside of the amp.


P1070043.JPG
 
I took it out last night and put a 1uf poly cap in and woof!. What I really want is a
smoother taper when I increase the bass control.
 
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Okay tonight I removed the 1uf cap, I put in a .0047 and that made a big difference the
bass is more adjustable now. I'm still going to try a .01,.022 and .047 the .047 is about
halfway so it should make some difference. If anyone has any ideas feel free to add
something.
 
Check out this site. Vox and Fender tone controls explained, I think all your answers are there. Read the basics and scroll down to the "Tone stack bass". https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm

Seems that the bass is cut by C5 and R10-11-13 making a RC high pass filter, being R11 variable, the cutoff freq changes. The resistance will change from 1.5Kohm to 11.6Kohm http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

There are ways to parallel a resistor to a pot to make it smoother.

I see your bass pot is 10K , but the "output" is in parallel with 100 ohm, so one half is always less than 100ohm. You are by-passing C9 with the bass pot.

Imagine you have the pot at 1/2, you have 5K at one side, and less than 100 ohm on the other side (the other 5K in parallel with 100ohm). That's 50:1. With the pot at 10%, you have 1K at one side , and <100 ohm at the other side. that's 10:1, and you said at that level things are OK.

Seeing this, I think a resistor between the wiper and the end connected to C9 will make it smoother. Imagine you have a 5K resistor in parallel with that half of the pot, when the pot is at 50% (5K), you have 2.5K at that half (5K pot, parallel with 5K resistor), and <100 ohm at the other half, that's 25:1, a bit better than the 50:1 ratio without the resistor. A smaller resistor will decrease that ratio. For example, a 2.2K, when parallel to the pot at 50% (5K) will give you 1.52K, that's 15:1 ratio against the 100 ohm resistor. The problem is that the max resistance will be always lower than the resistor value, so it will never reach 10K, so the bass will never be at max.

Doing this will have unpredicted results in the tone control slope. But I'd try a couple of resistors to see if you find a good balance. I think 5K is a good starting point.

Also, reducing C5 should change the range of the control. I'd try less than 1uF (0.68 perhaps). This shouldn't affect the treble, since high freqs should pass that capacitor.
 
I did change C9 to a .0047uf and heard a big difference, if this route doesn't work I'll start trying
the resistor change. I was surprised tonight when I changed it the sound opened more and
the bass control doesn't get deeper till it starts to hit 6 on the knob. Before the switch I could
not go past 4 on the dial it would get to boomy. I'll read the info thank you
 
Check what's happening at midrange freqs when you change C9. From what I see, reducing C9 offers more resistance to low freqs, so they go through the potentiometer, which is in parallel. At a point, the pot completely bypass the capacitor, so perhaps this is the easier solution without cutting the bass response.

Before your change, more low freqs could bypass the pot, perhaps that gave less control.

Taking a second look at it, I see the effect is similar to what I suggested, a resistor in parallel with C9, so signals find less resistance at the pot.
 
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Okay, It's really hard to change anything on the board, there's to many wires to remove and
it's a big job, so it's better to change things outside. I alway's wait till I have to do a major job
on it like replace the rest of the caps and such these amps aren't known for easy access. So
I'm hoping this plan of action works.
 
Well I took out the 2uf cap and C5 at put in a 68uf but that had no effect still bassy I even tried
raising the resistor on the bass pot to 5K nope nothing works there.
 
Hello. I think you were on the right track decreasing C9. Or, what I suggested was to connect a 5K resistor in parallel with C9. Replacing the 100 ohm resistor with 5k should increase the bass.
 
Further update, I replaced C11 from a .22uf to .068uf the bass got weaker but the noise
level increased.

I goofed! the reason why C11 was noisy was because I put in a .0068uf not a .068uf,
this is what caused the noise I guess it was to weak. The C11 is for the driver section
and when I changed it the bass did get weaker so I might try a .1uf 100v.
 
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Making headway this morning, changed the .22uf C11 cap to a .1uf and the bass is a bit less will have
to try a .047uf down from the .1uf and see what it does.
 
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