Lunar Module Oscillation.....was, BJT Emitter Pot and Bypass cap

[Mcentee2]

Been trying to find how this works with no success hence the question.

How does a common emitter amplifier work with regard to an emitter pot with a parallel bypass cap ?

I know how common emitter set ups work, how DC gain is controlled by the resistors etc and how the cap let's AC abive a frequency bypass that DC gain and basically have full gain.

What I don't get is how an emitter variable pot can affect any AC gain when there is already a parallel bypass cap ?

My reference example is the first stage of Skreddy's Lunar module - what exactly is that Q1 emitter pot doing?

I get for the DC part it is changing the gain factor of Q1 and affecting it's stability etc even at min there is 100r on the emitter..but...

...the bypass cap is always there for AC regardless of the pot setting...I don't get it....

Edit: actually I just noticed the cap isn't a bypass cap at all, it comes off the wiper of the pot.

So I can see that the wiper pot resistance and the fixed 100r before the cap set the gain that affects the AC, the fixed 100r and the whole pot added sets the DC gain and Q point.

[Kipper4]

This always helps

http://4.bp.blogspot.com/-jtoWoktqNs4/T0AQyiJsOUI/AAAAAAAAA8A/U5vbjO3-zig/s1600/lm_scheme.gif

[Mcentee2]

Many thanks, I was trying to work out how to include that as a link you beat me to it!

[PRR]

In that over-written diagram, the part of the pot *above* the wiper is not bypassed, and affects the AC gain.

[Mcentee2]

In that over-written diagram, the part of the pot *above* the wiper is not bypassed, and affects the AC gain.

Aha! Indeed, thankyou, that makes it clearer how it works:)

Reason for asking is to understand some oscillation I am getting, YouTube link here:

https://youtu.be/rUZQHHhJdUs

It seems to happen when the Q1 pre pot is turned for *less* gain ie there is more pot resistance before the cap.

I don't understand enough about these things but my simple brain tells me to expect things like that with more gain?

[antonis]

Perhaps you've a reverse wired pot..
(but Q1 shouldn't either oscillate due to 100pF feedback cap & rather "medium" gain..)

100μF bypass cap has negligible resistance compared to 180R + 55R (intrinsic emitter resistance at 450μA quiescent current) so Q1 maximun nominal gain should be about 42 (less than 33db) further reduced by R10 loading..

IMHO, something else oscillates but Kipper's over-written diagram doesn't help a lot...

[Mcentee2]

Perhaps you've a reverse wired pot..
(but Q1 shouldn't either oscillate due to 100pF feedback cap & rather "medium" gain..)

100μF bypass cap has negligible resistance compared to 180R + 55R (intrinsic emitter resistance at 450μA quiescent current) so Q1 maximun nominal gain should be about 42 (less than 33db) further reduced by R10 loading..

IMHO, something else oscillates but Kipper's over-written diagram doesn't help a lot...

I agree, I am working my way from left to right and thinking it through re each section.

Thick bassy settings and high fuzz settings are fine as long as pre is high has well.

As soon as pre gets lower, the oscillations creep in.

It is affected by the guitar volume knob as well.

Something that is related to the reduced input into the Q2/3 bit is the cause somewhere.

[Mcentee2]

Any tips to look what sorts of things can cause the oscillation?

I am planning on socketing and changing the transistors to see if it is one of them being "un-coperative".

My other thoughts are:

1) a bad solder, which I will check, but I would expect something more significant then oscillating.

2) a bad tolerance or incorrect resistor or cap or wired pot, not sure how to check these without removing, however I think these are unlikely?

3) wire lead dressing, I have wiggled these a lot with no difference.