BeeBaa Mis-Bias

[rthryhorysak]

I've made a BeeBaa with a pcb I had fabricated. The first transistor stage sounds misbiased and kinda "farts" out, and in general doesn't have the heavy fuzz sound of a BeeBaa. I had previously built one on vero and I was comparing the voltages. I am getting
C - 8V
B - 1.02V
E - 0.664V

Which are not correct voltages. So I added a pot to bias the transistor to get the collector around 5.5V which is around what it should be and I still am getting a mis-biased transistor sound. I've double checked my pcb, and I cannot see what the problem is. I've jumpered the 1k across the 9V because I was getting some oscillation with it in. Basically, the first stage is the problem because when jumping the input to the next stage its sounds normal and I am getting the correct voltages on the other two transistors, which are 2n3904 and Q1 is a 2n5088.



I've highighted the signal path to make it easy to follow. There was a ground pour on both sides to connect the grounds which is why there are ratlines. and the purple path is going to a 10uf capacitor.

[PRR]

> not correct voltages. So I added a pot to bias the transistor to get the collector around 5.5V which is around what it should be and I still am getting

You put in a pot, don't say where, do not say *all* the voltages you got with the pot.

I have no clue what to say about where you are at now.

The before voltages: the stand-out is the 1.02V-0.664V= 0.35V base-emitter. This is "off" for practical purpose. However the 0.664V across the "1.8K" emitter resistor says the transistor is well on.

Check that "1.8K" again and again. You have 18K other places.... did one of those sneak into the wrong place?

> I've jumpered the 1k across the 9V

The 1K in the power line should *reduce* oscillation. Three gain stages in a row on the same supply point are prone to sneak-back at very low frequency and "motorboat". If you get the opposite effect, there may be more wrong.

[Gus]

Like PPR posted look for a 18K instead of a 1.8K at the emitter of Q1 will bias you at about 8VDC or even higher

[rthryhorysak]

> not correct voltages. So I added a pot to bias the transistor to get the collector around 5.5V which is around what it should be and I still am getting

You put in a pot, don't say where, do not say *all* the voltages you got with the pot.

I have no clue what to say about where you are at now.

The before voltages: the stand-out is the 1.02V-0.664V= 0.35V base-emitter. This is "off" for practical purpose. However the 0.664V across the "1.8K" emitter resistor says the transistor is well on.

Check that "1.8K" again and again. You have 18K other places.... did one of those sneak into the wrong place?

> I've jumpered the 1k across the 9V

The 1K in the power line should *reduce* oscillation. Three gain stages in a row on the same supply point are prone to sneak-back at very low frequency and "motorboat". If you get the opposite effect, there may be more wrong.

Sorry about that, I thought it was a more in depth post than in reality. So I checked and sure enough I had put an 18k in the 1.8K resistor. So, now that that has changed I an still getting the same sound. Here are my voltages

Q1
C - 0.056V
B - 0.652V
E - 0.02V

Q2
C - 5.32
B - 1.055
E - 0.442

Q3
C - 5.29
B - 1.063
E - 0.445

Currently I am in the process of checking all the component values. The other stuff going on is basically just splitting the clean signal and boosting it with an op amp based boost and summing it back. All that is working fine.

The oscillation was from the first transitor stage, but when I removed the 1k resistor it seemed to stop it.

[PRR]

> Q1 C - 0.056V

These are all standard common-emitter stages. You expect Collector to be designed to sit "halfway" between the zero and +9V power rails. "Halfway" may be 25% or 75%. But NOT 0.05V (0.6%!) of 9V.

Q1 is "slammed". Either it is flowing way too much current, or the collector is not actually connected to anything. (A BJT will show some teeny collector voltage as long as there is base bias.)

Looking at 100K and 1.8K, I note that full current in the 100K all going into the 1.8K would give 0.15V. The 15K diverts some of this. This is close enough to your measured 0.02V across 1.8K to make me suspect there is NO collector current. Likely causes are bad solder joint at collector or collector resistor, or grossly abused transistor.

However more wrong-value parts is a possibility. I did not ponder "every" possible wrong value.

Bright light is a good aid. (Sunlight or incandescent are better than fluorescent.) Ohm-meter is a good cross-check; in-circuit readings may be low, but way-wrong suggests further study.
____

> oscillation was from the first transitor stage

Single stages generally can't oscillate without more tricks than this one has. It probably involves more than one stage.

[rthryhorysak]

Ok, I just checked the 1.8k resistor again and my 1.8k seems to have been mixed in with 1.8R resistors. Everything is now good.