tone circuit issue

[EKDENTON]

I'm working on a little 3watt practice amp that has a one pot tone control that can be bypassed with a switch. Everything seems to be working good except that when i turn the tone pot CCW with the tone circuit active the signal volume is being cut as the resistance of the pot decreases. It's not too bad until the pot is almost full CCW.  Would adding a resistor between the .0047 and ground help in this case? Or if someone is familliar with this tone circuit they may know if it is a design flaw or possibly a missing component?

[FranciscoPerez]

Shouldn't that 100K resistor be placed between the tone pot wiper and gain pot?

[sluckey]

I believe that's normal for that tone circuit. Be sure you are using a .0047 and not a .047.

[EKDENTON]

It is the .0047uf

[kagliostro]

If you want to experiment other one knob tone controls

http://chasingtone.com/yourguitaramp/guide-to-single-knob-tone-controls/

Ciao

Franco

[EKDENTON]

Francisco, the tone knob has no effect on the tone other than it is losing signal when you turn it ccw.  Maybe the coupling cap and the 100K should both be on the other side of the switch where you have the red line drawn to? that would put the 500pf in series rather than parallel. What do you think?


This little thing has way too much gain but I want to get the tone control working properly first then i will fix the gain.




Hey Kagliostro, Iv'e seen that website, and used some of those same circuits. Good reference site for tone stacks, thanks. I will try to get this one to work right first before i give up on it. I like that the tone circuit can be bypassed to get more gain and once i get my tone and gain adjusted right it should be a really cool little practice amp.

[EKDENTON]

Okay i found a tone control that looks like the one this amp uses, except that the switch has been  added to the wiper and the coupling cap is mixed in on mine.


The resistor value is different, and a 1M pot on this bitmo.

[sluckey]

The Marshall 18W uses a very similar tone control on the Tremolo channel. Works well. Take a look...

     http://sluckeyamps.com/18w/18w.pdf

[EKDENTON]

It seems like the switch should not be on the wiper but between the pot and ground to lift the ground and take the tone circuit entirely out. I looked at the 18watt Steve put the link to and  redrew this tone circuit so that it should work similar. I also moved the switch and the coupling cap. I don't have the 100k drawn in now. If that is needed I can add it back in.

Does this look like it would work okay?

[sluckey]

The switch only takes out the treble bleed (bass cap). The tone pot will still operate as a variable "bright" cap on the treble.

Missing a coupling cap on the plate. 6K6 grid needs a dc path to ground.

[EKDENTON]

If I remove the LED I can wire it like this and I think this should take every bit of the tone circuit out. I moved the coupling cap back where it originally was so it would not block the dc path to ground for the 6k6. Does this look closer?

[kagliostro]

What about something like this ?




Franco

[EKDENTON]

I can see how that would also take the tone out of the circuit. I can't see the rest of the circuit and I haven't seen that tone circuit before though so I better stick to one we know works good. The one steve has is know to work good so I was trying to follow one that we know works well and then figure a way to lift the circuit. It is not as simple as I thought (just lifting the ground) because the wiper of the tone pot still keeps the 500pf connected in my first drawing so I changed it and used both sides of the dpdt switch to lift the ground and disconnect the wiper of the tone pot. means I don't get to use the pretty LED to indicate the tone is bypassed :-(

[kagliostro]

The previous Tone Control is in Sluckey's 18W schematic (the link he posted http://sluckeyamps.com/18w/18w.pdf)

Here a version for your Tone Control



--

...... I don't get to use the pretty LED to indicate the tone is bypassed :-(

Simply use a 3PDT instead of a 2PDT Switch



Franco

p.s.: Remember you need coupling capacitors


EDIT: if the .01 cap and 100K resistor have to be present on the circuit when the Tone Control is OFF this is one way to do it

[EKDENTON]

The bypass cap is the .01uf. it's there.  Do you know what amp has that tone circuit so I can see how it fits in the schematic? It looks like if you throw the switch you lose the entire preamp. If I could see the signal path from the preamp to the power tube it might help me get my mind around it. Thanks.

[sluckey]

Look at the normal channel TS in the schematic I posted above. I didn't point to it earlier because the TS in the trem channel is closer to your original TS.

[kagliostro]

...... the TS in the trem channel is closer to your original TS

From the Tone Control of that trem channel the difference with your Tone Control is the position of the decoupling cap and the presence of the 100K resistor

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Personally I would like to give a try to the normal channel Tone Control of the 18W

Franco

[EKDENTON]

Okay I will copy the normal channel, lift the ground with the switch and keep the .01 coupling cap on the preamp side of the gain so the 6k6 grid is not blocked from ground. Then I can still use the DPST switch and use the LED?
Hey Franko the LED's I have are color changing some fade colors then flash colors, the others gently change colors.  They dont have three terminals. These are awesome, to bright to look directly at.
My power supply for this amp is a DC power supply so it seemed a lot simpler to me. I just turn the pot to get my 6.3vdc filament voltage and turned the other pot and dialed in my 220vdc for my B+. Plugged in all my tubes and rechecked voltages and they were rocksteady 6.3 and 220.

[kagliostro]

Can you post the whole schem you are using ?

Franco

[EKDENTON]

It is this amp here. I could not find a suitable PT so I made my own dc power supply from ordered parts. The OT I used is the little 5watt fender OT. With the 6SL7 and the tone circuit lifted it may have a bit too much gain, but if you use a 6SN7, it should be pretty nice. They are cheaper anyway.

 I can say more once I get the tone circuit issue solved.

I thought about trying two 12v transformers and making a voltage doubler but that would have put the B+ over 300v unloaded so I wasn't sure if I would be able to get it down to 220v and a friend suggested trying the DC power supply.  I have links to the PS parts if anyone wants it.

[kagliostro]

A  choke input PS (no capacitor before the choke) is a way to drop B+

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With that configuration you can try a lot of different Tone Controls

Franco

[EKDENTON]

I finished modifying the tone circuit. Rather than lifting the tone completely I used a 500k to adjust how much of the tone circuit I want to lift. The tone pot is working now in the normal setting using the circuit from Steve's 18watt drawing. Now it is much nicer, Plenty of gain and boost when switched but easy to adjust ina little more or less. I still have the flashing led wired to come on when the tone is lifted and off when the tone is in the normal setting. I tried a 6SN7 also and it is pretty nice with that tube also, more clean volume before clipping. I re-drew the schematic as I have mine wired. If a correct voltage PT could be found I would not mind building one with a tube rectifier. It is a nice amp for the bedroom practice.

[sluckey]

Need to remove that coupling cap from the wiper of the vol pot.

[EKDENTON]

And put it directly after the plate resistor of the previous stage or from the junction of the 100k/.0047 cap over to pot left terminal (not ground terminal)

Okay I changed it to this:

Seems to be working great. I forgot what you said earlier in the midst of changing everything up.  Thanks for catching that.

[sluckey]

You need to completely remove that .01µF cap. I mean put it back in the parts bin. Look at the 18 watt schematic again.

[EKDENTON]

Do I not need the .01 cap because the two caps in the tone stack are blocking the dc voltage from the next grid?

 I went back and looked at the 18 watt and you have three caps including C1. Is C1 not needed in that 18watt circuit?

Guess I am confused. I can see why I think it doesn't need to be there but why is it there in the 18watt. What am I not seeing??

[kagliostro]

EKDENTON, I think all is around the presence of R9

a bit of the cathode bias voltage is taken and feed the grid via R9, so there is DC voltage there and the cap is to prevent it reaches the volume pot

Don't take this as an explanation, I'm only guessing and I'm curious to read the right answer

--

BTW, have you  a model for the boost circuit ?

Thanks

Franco

[EKDENTON]

it is ordered from ebay. The PS is $9 and the board is $6. All together with shipping I think it was still under $20 if you want the links PM me and I can get you the link for them. Takes about 7-10 days shipping.

[EKDENTON]

Sent you a PM.

Yes me too, I am curious what the difference is. Could be what you said. It sounds perfect as is but if it don't need it a guess a pc of wire is cheaper than a cap and I can use it somewhere else.

[EKDENTON]

Once again with the .01 cap removed. If you notice anything else let me know and I will edit and post it corrected, so someone wont build of the wrong drawing.

[PRR]

> Do I not need the .01 cap because the two caps in the tone stack are blocking the dc voltage from the next grid?

The point is you NEED a conductive DC path from grid to bias (usually ground).

If you don't have that, the grid "does not know where to be".

It could drift way negative and cut-off the tube. It could drift positive and slam plate to lowest possible voltage.

> Seems to be working great.

Yes, with many tubes (condition as well as type) the "unconnected grid" DC voltage *tends* to be a bit negative of cathode, which is a workable operating point.

That's at idle. A LARGE signal at the grid can't go much positive, so the grid will go very negative. Tube cuts-off, silent. It "will recover", at a time-rate of 0.01uFd times infinity Ohms; or maybe 200Megs due to tube and socket leakage, several seconds.

EVERY grid needs some DC path to bias it. Small tubes this may be a 1Meg resistor. Power tubes have lower limits.

This resistor CAN be a potentiometer. Your Volume pot. But your pot - 0.01u - grid system isolates the DC path through the pot!

[sluckey]

I went back and looked at the 18 watt and you have three caps including C1. Is C1 not needed in that 18watt circuit?

Guess I am confused. I can see why I think it doesn't need to be there but why is it there in the 18watt. What am I not seeing??

C1 is needed in the 18W amp because it couples into the grid of a LTP PI. This PI has a fairly large dc voltage on the grid. C1 blocks that dc voltage from appearing on the volume control, but more importantly, it prevents the volume control from affecting the bias of the LTP PI.

[EKDENTON]

Okay thanks to both of you. I understand now.