Sort of problem with sort of Stratoblaster

[thejoe]

Hello all,
I built this circuit by mashing a few things together. Basically a modified Stratoblaster. I just hooked it up and noticed that even with level turned all the way down it has a pretty heavy boost. Might someone be able to advise how to have more attenuation? I used the 1K pot because its original to the WWII comm box the pedal is built into.

Thanks in advance

[thermionix]

There is no volume control, you could add one at the end.  I don't see the point of the 12k on the emitter.

[thejoe]

The 12K resistor, or something similar, is on almost all of the schematics I found. According to circuits like the one I've linked below, that pot should control gain/boost which I assumed would also give me some level control.

http://www.diystompboxes.com/smfforum/index.php?topic=96291.0

[PRR]

> level turned all the way down it has a pretty heavy boost

At a glance, 4.7K/1K or almost 5X. The 12K adds some also.

Given the 12K, the pot should be 7.7K to get near unity gain.

Since there are no 7.7K pots, we resort to a 10K pot and make the 12K a 8.9K (8.2K, 9.1K, 10K) resistor.

Gains figured for "no" loading. With real guitar-cord loads, the emitter path should be about 10% lower. So 10K pot and 7.3K (6.8K, 7.5K) resistor.

All of this can be +/-50% (use what you got), with the understanding it won't hit exact unity gain.

[thermionix]

12k resistor in parallel with a 1k pot just makes a 923 ohm pot.  Within 10% tolerance.

Thread you linked shows this schematic:



Note the 10uF cap in series with the 50k pot.  It's important.

[PRR]

> Thread you linked shows this schematic:

Ah. I did not follow the reference. Kinda not the same, eh? Only the "12K" left.

I also missed that the plan in the first post has NO output blocking cap!! This is very rude to the next stage, throwing about 7V DC at it. Some inputs have blocking caps and won't mind (may POP if switched). Others will faint dead but survive. A very few will die.

[thejoe]

I'm not worried about nailing unity gain, just want to control the volume a bit. I can easily add in that cap after the pot. Should I still increase the value of my pot to 10k? I don't have a 50k pot lying around but I do have a 100k.

Also, in regards to the above reply, this pedal does not have a switch. It's always on and controlled by an external effects loop.

Thank you both for helping me out here.

[thermionix]

A 100k pot with a 100k resistor in parallel (across the outside lugs) becomes a 50k pot.  You do want that cap in series too.  It makes it so the gain pot is changing AC conditions and not DC.

You should probably add the DC blocking cap before the tone control.  I think your tone pot would be scratchy otherwise.

See the 100k resistor to ground at the output of the original schematic?  That can be a 100k pot instead, take the output from the wiper, now you have a master volume control.

[thejoe]

Noted. Thank you. I have added the cap and messed around with a few different pots. I settled on a 10k audio taper and left the resistor as a 12k. It definitely helps to lower the volume but seems like the first half of the pot rotation is prettt useless. I will have to continue to experiment. For now, at least it works as needed. You guys are the best!