Thermionic Deluxe gain issue
- Thread starter PKRPedals
- Start date
Chuck D. Bones
Circuit Wizard
Post a hi-res pic so we can verify all the parts. I built a Brown Betty (basically a single channel version of the TD) and it has plenty of gain. Some trimmers have their rotation reversed, did you try turning the trimmers the other direction? Unless you are playing a bass guitar thru this pedal, the Tight control is not going to behave like a volume control. Verify that C3A & C3B are 10nF. Both channels behave the same way?
chongmagic
Well-known member
My build had lots of gain on both channels, perhaps one of your TL072 ICs is wonky?
chongmagic
Well-known member
I had a bad ic on channel B and it would cause the volume to increase as I turned the volume knob counterclockwise.
Chuck D. Bones
Circuit Wizard
And you're sure all the resistor values are correct? You can verify most of them in-circuit with an ohmmeter, at least see if they are in the ballpark. R8, R10, R12 & the GAIN pot are the most critical for setting the gain & distortion. If TIGHT is not working right & C3 is correct, then I suspect R25 is way too low.
PKRPedals
Active member
I did check the orientation of the LED's and they are correct. I checked the voltages on my TL072's and they are all the same. I checked my pots and they are good. I checked a bunch of resistors and caps and they are all pretty close to what they should be. Channel B has more gain than channel A but it's not pleasant to the ear at all. Kind of sounds like a bad fuzz. Are the LED's on the left side supposed to light up when played? Mine don't, they only flicker when hooking up power.
Chuck D. Bones
Circuit Wizard
LED1 & LED2 do no light up in normal operation because there is so little current running in them. LED3 & LED4 will light up if the Gain, trimmer and Presence are set high enough. Max brightness will be governed by the Presence pot. They will not be as bright as the front panel LEDs.
It's possible that the colors did not all come across correct in the photo, but here's what I see that doesn't look right:
R3A is 1K, s/b 10K
R5A is 3.9K, s/b 39K
R8A is 2.2K, s/b 22K
R9A is 2.2K, s/b 22K
As best I can tell, everything else on the board looks OK.
I can't read R32-R37 because they are in the shadow. Verify that IC1-1 and IC1-7 are both close to +4.5V. If those are good, then check pins 1 & 7 on all of the other opamps. Should be between +4V and +5V.
Can't tell if the switch wiring is correct because the wires disappear out of view. I see an extra wire on the A/B switch, I assume you're doing something clever like a 2-color LED.
I see you are using a Mammoth C10K pot. I bought one from them that was very noisy, so there may be a lot quality problem. Try checking the resistance between pins 1 & 3 in-circuit, should go all the way down to zero when Presence is dimed.
If you get this far and haven't found the problem, then we have to suspect bad parts. The resistors I mentioned yesterday, measure them. It's rare, but resistors do get mismarked sometimes.
Finally, I never trust stamped-pin sockets. If you have an IC that's not working properly, it's more likely that the socket is bad than the IC is bad.
Good luck!
It's possible that the colors did not all come across correct in the photo, but here's what I see that doesn't look right:
R3A is 1K, s/b 10K
R5A is 3.9K, s/b 39K
R8A is 2.2K, s/b 22K
R9A is 2.2K, s/b 22K
As best I can tell, everything else on the board looks OK.
I can't read R32-R37 because they are in the shadow. Verify that IC1-1 and IC1-7 are both close to +4.5V. If those are good, then check pins 1 & 7 on all of the other opamps. Should be between +4V and +5V.
Can't tell if the switch wiring is correct because the wires disappear out of view. I see an extra wire on the A/B switch, I assume you're doing something clever like a 2-color LED.
I see you are using a Mammoth C10K pot. I bought one from them that was very noisy, so there may be a lot quality problem. Try checking the resistance between pins 1 & 3 in-circuit, should go all the way down to zero when Presence is dimed.
If you get this far and haven't found the problem, then we have to suspect bad parts. The resistors I mentioned yesterday, measure them. It's rare, but resistors do get mismarked sometimes.
Finally, I never trust stamped-pin sockets. If you have an IC that's not working properly, it's more likely that the socket is bad than the IC is bad.
Good luck!
PKRPedals
Active member
The pots on the bottom row all have dust covers. I only used the ones without covers because I knew they wouldn’t be touching anything. I’m thinking I may have put in a wrong cap. I went over all the resistors and they all measure as they are supposed to. I will go back over them again just to be sure though. I got to get it out of the box so I can access the back of the components and go from there. Thanks for all the replies and help. I hope this thing will be singing soon.
