Thermionic Distortion issues. Squealing, Clipping LED's?

[TheRoo]

Hey everyone,

Just put together a Thermionic Distortion. First question is, when I play a note I only see 2 of the LED clipping diodes lighting up while the other 2 barely light or don't light at all, is this normal? FYI, it doesn't matter what I adjust the internal trimpot to. Also, I get A LOT of high pitch squealing unless I roll off the gain, volume, presence, or the controls on my Strat with noiseless single coils. However, I don't get any squealing at all when using active EMG's in my other guitar. Does anyone else have this issue? Ive checked all of the components and they are correct values.

Thanks!

 

[Chuck D. Bones]

Yes, the LEDs are acting normally. The LEDs in the 2nd stage don't get enough current to light up, but they are doing their job. The trimpot sets the gain for the 4th stage. The other 2 LEDs, the ones that light up, are at the end of the 4th stage, so they will respond to the GAIN, PRESENCE and trimpot settings. This pedal has a shitload of gain. If turning the guitar's volume down kills the squeal, then you're getting feedback from the speakers to the pickups. The feedback could be acoustic or magnetic. High-pitched squeals are usually magnetic feedback. Change the orientation between your guitar and speakers, see if that makes the squeal come and go. Hendrix experienced this problem too, so you're in good company.

 

[TheRoo]

Thanks a lot for clarifying on the LED's. The squealing however doesn't seem like normal feedback. It seems to be an issue with the pedal. I have a DS-1 and a hoof which don't do that at all with the same guitar. I either have to roll the tone control back, or the knobs on the pedal itself. I can't really push the pedal all.

 

[Chuck D. Bones]

Consider that the Thermionic has more gain, higher output and a different frequency response than a DS-1 or Hoof.

 

[mehfuzhoss]

Hello,

I have the same squeal issue but only with the tight pot set to 12 or higher. i can set it to 12 and be happy...
but the real issue is, when i switch on the pedal there is a huge volume drop and i set my volume on the pedal to max and gain to around 5/6 to get a decent amount of signal. i have used a lower value cap in c15 (2.2n) instead of 4.7n.
please advise.
Thanks

 

[Chuck D. Bones]

A lower value cap for C15 will make the TREBLE cut off at a higher freq. It won't cause low volume.
Questions for you:
1) Have you verified that all components (except C15) are correct?
2) Does changing the trimmer setting make a difference in volume?
3) Do the two LEDs to the left of the trimmer light up when you bang out a few chords?
4) When are you going to post a picture of your board?

 

[mehfuzhoss]

yes they are correct. (for c4 i used 200pf instead of 1000pf.)
trimmer turning clockwise makes very in audible difference to the sound.
yes. looking at the board from the back the two right leds are lit up when i bang a chord but the two left ones are seem to not liting up much
pic attached.

 

[TheRoo]

As for my issue, I have checked all of the values for each resistor, cap, etc. and all seem ok. The high pitch squeals happens even when the guitar is not plugged in. Even if I run a long 20ft cable and stand as far as I can from the amp it still squeals very loudly. The presence and treble knobs seems to adjust the overall volume of the squeal and the tight knob changes the pitch of the squeal. Pic attached show the board. FYI, the TL072 IC's are removed in the pic. Thanks again everyone.

 

[TheRoo]

Well, I figured out why I was getting so much squealing. The wires going to the input / output tips to switch were touching the pot legs. Even though they have a standard insulation jacket, it was causing interference. Once I moved the wires all the squealing stoped. Thanks for the input Chuck and good luck mehfuzhoss. Maybe check if anything (including wires) are not touching on your build.

 

[Chuck D. Bones]

"The high pitch squeals happens even when the guitar is not plugged in." A crucial piece of information.
Put two insulated wires up against each other and you get a capacitor. Maybe only a few pF, but that can be enough to turn a high-gain amplifier like this one into an oscillator. Some people shield the IN and OUT wires. Just run those two wires along the chassis wall, away from the board and everything else and you're good.
One more thing, I tell everyone else this so I'll tell you too: if you're going to use IC sockets, spend a few more pennies and get the machined pin sockets. The stamped pin sockets get loose after a few in/out cycles (ok, I can hear you snickering).

Mehfuzhoss, I spotted two resistors that are the wrong value, so you need to go back and check everything. One of them for sure is responsible for the huge loss in volume, but you need to correct both of them. Is there a reason you made C4 5x smaller? That 10-turn trimpot is overkill. Next build, you can save some $ and put in a 1 turn pot like TheRoo did.

 

[TheRoo]

Great info Chuck. Thanks again!

 

[Chuck D. Bones]

Nice work on the board assembly. When you get it all back together, put some pix in the Build Reports.

 

[mehfuzhoss]

Thank you chuck. Those two resistors fixed all the issues i had.