Tone stack issues

[duffy878]

Hey guys

so I recently finished a little single ended build and it sounds KILLER. I've got three controls, volume, tone, and a bright switch.

The preamp is basically that of the Dr. Z miniZ, the tone is the garnet from here, and the bright switch is typical fender (I don't remember which schem I looked at for it.)

The amp is quiet and sounds great... but the tone control and the bright switch don't do anything! I think it is a stupid mistake... like grounding something I shouldn't have, etc, etc...

sorry for the poor schematic, I'm on my mac.

Does anyone see any major flaws with this? is the bright switch/tone control redundant? Ive checked my wiring and it is identical to the attached schematic.

[Wakculloch]

The bright cap should go between the wiper and top arm of the volume pot, not to earth; and as the tone control is supposed to be in the feedback loop i suspect that your addition of a bypass cap on the cathode resistor has effectively cancelled the feedback, as this point is now at ac ground.

[duffy878]

Ah! thank you Wakculloch, that makes perfect sense to me... I knew it was something simple and dumb like that. When I get home from work, I'll rewire it.

[tubeswell]

sorry for the poor schematic, I'm on my mac.

Here you go then.

JSchem

[d95err]

The bright cap should go between the wiper and top arm of the volume pot, not to earth; and as the tone control is supposed to be in the feedback loop i suspect that your addition of a bypass cap on the cathode resistor has effectively cancelled the feedback, as this point is now at ac ground.

There is no feedback loop in that tone control. Adam has gotten this wrong. The wiper of the tone pot is grounded, so no signal whatsoever can pass from the output of the stage back to the cathode. It is essentially two separate tone circuits that happen to be controlled by the same pot. One lowpass filter on the output of the stage, and a variable cathode bypass cap to controll bass gain.

We don't know the value of the fixed cathode bypass cap, but I think thats the problem. The fixed cap should be small. I'd try something like 0.1uF to 0.47uF. The cap between the cathode and the tone pot should be bigger. I'd make its value at least 10 times bigger than the fixed cap.

Also, the bigger the cathode resistor, the bigger the difference between bypassed and unbypassed gain will be, and therefore the more effective the tone control will be. In any case, the difference between max and min bass will be small, probably no more than 3-6dB.

To make the lowpass filter more effective, I'd connect the tone cap (1nF) after the series 100k resistor instead of directly after the coupling cap.

[Wakculloch]

After posting had a feeling i may have gone off half cocked, but thanks to the good folk on this forum knew that someone would nail it!

[d95err]

Come to think of it, the variable cathode cap in the original schematic (with no fixed cathode cap) may not be intended to affect the bass response. It is only intended to slightly increase gain when the tone pot is turned down, in order to compensate for the signal loss. That would make this a treble-only tone control.

[duffy878]

I've rewired the bright switch correctly to the wiper and thats now working fine.

I can hear the tone control working but it's subtile. The cathode bypass cap in question is a 3.3 uF. d95err, your saying that value should be lower, and I should connect the tone control after the 100K and not before it?


tubeswell, thanks for the link!

[d95err]

I can hear the tone control working but it's subtile. The cathode bypass cap in question is a 3.3 uF. d95err, your saying that value should be lower, and I should connect the tone control after the 100K and not before it?

I think I got a bit confused in my previous posts regarding the cathode bypass caps, so I'll try again.

What happens when the tone control is turned up is that the 1uF cap is put in parallel to the 3u3 cathode cap, forming a 4u3 cathode bypass cap. That's virtually no difference at all.

What I think the tone control is supposed to do is to increase gain on mid and treble frequencies when the tone control is turned up. To do that, you need to skip the fixed bypass cap completely. Thus, when the tone control is turned on, the 1uF cap is gradually coming into play as cathode bypass, focusing the gain to the mid and treble.

What value caps will be useful depends on the value of the cathode resistor. You haven't said what that value is, so I can't recommend any particular values. Test with a few different values from, say 0.1uF and up and see what happens.

The cathode bypass cap probably won't have much effect until the very end of the turn of the tone control, so turn it up all the way to see what the cathode cap does.

The lowpass filter would be more effective if connected after the 100k series resistor. You may have to experiment with the cap value if it gets too effective (sucks too much treble).

Some of the resistors in your schematic are kind of wierd. What is the 100k/1M voltage divider before the volume control for? And why the 100k to ground after the volume control?

[duffy878]

hm... that makes much more sense to me.

the cathode resistor is the typical 1.5K

the volume control set up is an exact copy of the Dr.Z Mini-Z.

[Groover]

Some of the resistors in your schematic are kind of wierd. What is the 100k/1M voltage divider before the volume control for? And why the 100k to ground after the volume control?

The 100k/1M voltage divider is probably to tailor the overall gain a bit. Also the 1M is in parallel with the 1M pot, so it is really a 100k/500k voltage divider. But because the pot and the 1M resistor are in parallel I am guessing that the wiper will see only 1/2 the signal voltage that it would if a 500k pot were used instead of the 1M/1M parallel network. So more gain tailoring. Same thing could be achieved by using only a 1M pot and limiting it's travel, but we know guitar players like to turn knobs all the way up.

The 100k to earth after the Vol control looks like a grid load to me.

This tone control concept looks interesting. I may try it in my 5F2a tweak-form.

[Bruce / Mission Amps]

I don't think you should have any cathode bypass cap with that NFB tone control option.
Any cap over a few hundred nF would send almost all the signal to ground, wouldn't it?

[d95err]

I don't think you should have any cathode bypass cap with that NFB tone control option.
Any cap over a few hundred nF would send almost all the signal to ground, wouldn't it?

How can there be any feedback in that tone control? The wiper of the pot goes to ground. No signal can feed back through the pot. Or can it?

EDIT: (see image)

[Groover]

Makes sense to me so far...

[teemuk]

How can there be any feedback in that tone control? The wiper of the pot goes to ground. No signal can feed back through the pot. Or can it?

EDIT: (see image)

There is no (plato-to-cathode) feedback, correct, but a cathode resistor bypass cap will still render the tone control circuit's portion at the cathode useless. The control would still work but obviously not in its fullest potential since it wouldn't be controlling the gain of the amp stage.