Active tone control question?

Kennyg2209

Well-Known Member
20181225_172028.jpg 20190115_215339.jpg 20190115_215412.jpg I set up the 3 band tone control that you can love from Anglefire.com (Fun with tubes) on a breadboard. I'm having a couple issues . I'm feeding it with a 1khz sine wave . The signal coming out is not very clean. It almost appears like a few signals close together . The other thing is that I didn't think there would be a lot of gain in the circuit. It said on the website -1db. I'm still trying to understand the db's after 40 years. I'm putting in around 5vpp, and getting out 10 times that. I've gone over my circuit many times and pretty sure everything is correct. I shouldn't have any ground loops. I don't know if I would be getting interference from the filament wires or something. I'm using a 5814 (12au7) and 6eu7. My pinouts are all correct. Any thoughts on this?

Thanks

Kennyg
 
you are having it in open air. could be interference from basicaly anything in your room. try shielding it and see from there.
 
as for the over the top gain I don't know... Could be a bad cap or resistor somewhere. It's bein a few years since I've look at a gain circuit so yeah
 
I would build on a chassis not a breadboard, are the pots set to mid position?
does the mid pot adjust the output level ??
gain too high , remove the 50uf cathode bypass.
.....GC
 
1db loss is about a 20% loss, so if you fed in 1v you can expect about 0.8 volts out

http://www.fab-corp.com/pages.php?pageid=5

agreed about the shielding and such though. Tubes are a high impedance device and are very prone to picking up noise and interference. Lead dress issues also will play a lot into this. If the heater circuit isn't referenced to ground anywhere, that can cause troubles too. Easy way to do that is to use a pair of resistors somewhere in the 100-470 ohm range. Use one resistor from each heater leg to ground. I would also run a wire from the bracket where your pots are mounted to circuit ground.
 
Thanks for all the replies. My circuit on the breadboard is identical to the diagram. I'm going to check all my caps and resistances again. I really wasn't expecting any gain. Maybe some loss. I tried turning all my lighting and things off. Didn't make much difference. I'll ground that bracket and try a couple resistors on the heaters . I'll let you guys know how it goes.

Thanks again!

Kennyg
 
The 1 db loss specified for the circuit would be with the controls all centered for flat operation. Above that setting, there would be boost, which would show as gain. The circuit is basically a unity gain circuit when the controls are centered.

Dave
 
The 1 db loss specified for the circuit would be with the controls all centered for flat operation. Above that setting, there would be boost, which would show as gain. The circuit is basically a unity gain circuit when the controls are centered.

Dave
Hi Dave , I have that amount of gain with the controls centered. I'm going to pull the tubes and test all the resistances to ground. Just double check everything again. For some reason I didn't notice any change increasing or decreasing the mids pot. I should with a 1khz signal . I have to have something wrong. I always worry about getting a good connection on these breadboards.
Thanks


Kennyg
 
Kenny- You have way too much wiring to the pots. on this thing. I built his "Tone Control You Can Love" circuit and made the same mistake. The pot. wires were too long and can pick up whatever is floating by in the air. So I rebuilt the circuit on a Vectorboard with the tone control caps. on the edge of the board. This way, the leads to the pots. were kept at no more than 2". That made the difference and the circuit works great, good range and smooth control from mid. rotation to either side. Also, those plug in breadboards are too flaky, too easy for one connection to come unplugged, unnoticed. I use Vectorboard, 'flea clips' and bus wire.
You can test the mid. rotation of the controls by injecting a 400 Hz square wave and adjusting them until you get a good , square waveform. If they are not straight up, you have the wrong controls.
 
I am using linear pots. I actually found the problem. Makes me feel stupid. I'll post here shortly after I run some tests.

Kennyg
 
Ok, what I did was I ran the two 56k resistors and the 220pf cap to a single point on the breadboard and forgot to run the jumper from there to the output. I'm sure that will make it clear to you guys why I had so much gain. Now the signal is just attenuated a little when the pots are at midpoint. Increases and decreases from there. The signal is still a little dirty, but I'll be able to clean that up. Thanks for the help! I'm using this in a preamp I'm building. I'll post more on that. I'll probably need more help also.

Thanks again,

Kennyg 20190116_143158.jpg
 
Kenny- You have way too much wiring to the pots. on this thing. I built his "Tone Control You Can Love" circuit and made the same mistake. The pot. wires were too long and can pick up whatever is floating by in the air. So I rebuilt the circuit on a Vectorboard with the tone control caps. on the edge of the board. This way, the leads to the pots. were kept at no more than 2". That made the difference and the circuit works great, good range and smooth control from mid. rotation to either side. Also, those plug in breadboards are too flaky, too easy for one connection to come unplugged, unnoticed. I use Vectorboard, 'flea clips' and bus wire.
You can test the mid. rotation of the controls by injecting a 400 Hz square wave and adjusting them until you get a good , square waveform. If they are not straight up, you have the wrong controls.
I will keep that in mind. In the chassis I'm going to place all the caps right on the pots. Or as close as I can. Does that setup work good for you?

Kennyg
 
Keep us posted on your progress, You might consider building the power supply
on a separate chassis , this was common in 1950's designs keeps magnetic fields
& noise away from sensitive low level circuits.
 
Keep us posted on your progress, You might consider building the power supply
on a separate chassis , this was common in 1950's designs keeps magnetic fields
& noise away from sensitive low level circuits.
Keep us posted on your progress, You might consider building the power supply
on a separate chassis , this was common in 1950's designs keeps magnetic fields
& noise away from sensitive low level circuits.
Thanks, I'll keep you posted. I actually built a 2A3 SE amp with a separate power supply . I don't know if I'll do it with the preamp though. I should. I may just keep the chassis divided and shielded good. My amp sounds excellent. Can't use my turntable till I get the preamp done. I thought I'd post photos of everything when it's complete. I'm doing everything old-school, mainly point to point. I am using relays for the input selector though. Thanks again

Kennyg 20190109_094455.jpg
 
DIY is the best , you can select premium parts like ceramic tube sockets, teflon wire,
stepped attenuators , stainless steel hardware....
 
Kenny- You have way too much wiring to the pots. on this thing. I built his "Tone Control You Can Love" circuit and made the same mistake. The pot. wires were too long and can pick up whatever is floating by in the air. So I rebuilt the circuit on a Vectorboard with the tone control caps. on the edge of the board. This way, the leads to the pots. were kept at no more than 2". That made the difference and the circuit works great, good range and smooth control from mid. rotation to either side. Also, those plug in breadboards are too flaky, too easy for one connection to come unplugged, unnoticed. I use Vectorboard, 'flea clips' and bus wire.
You can test the mid. rotation of the controls by injecting a 400 Hz square wave and adjusting them until you get a good , square waveform. If they are not straight up, you have the wrong controls.
I checked out those vector boards. Pretty cool. Do you get them anywhere specific? I take it that they make different layouts.

Kennyg
 
20190117_174914.jpg
1db loss is about a 20% loss, so if you fed in 1v you can expect about 0.8 volts out

http://www.fab-corp.com/pages.php?pageid=5

agreed about the shielding and such though. Tubes are a high impedance device and are very prone to picking up noise and interference. Lead dress issues also will play a lot into this. If the heater circuit isn't referenced to ground anywhere, that can cause troubles too. Easy way to do that is to use a pair of resistors somewhere in the 100-470 ohm range. Use one resistor from each heater leg to ground. I would also run a wire from the bracket where your pots are mounted to circuit ground.
Very Cool! I used 390 ohms resistors on the heaters and it cleaned them right up. Top signal is going in. Bottom one is through one stage of the preamp and tone controls. Both on the same voltage scale. Just what I wanted. Thanks!

Kennyg
 
I will keep that in mind. In the chassis I'm going to place all the caps right on the pots. Or as close as I can. Does that setup work good for you?

Kennyg
Yes but you must keep the wiring to a minimum. Build out the connections near the edge of the board.
 
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