Big Muff Help

[mmaatt25]

Hi,

I've just put together a Big Muff in as per the Torchy vero layout here http://www.aronnelson.com/gallery/main.php?g2_view=keyalbum.KeywordAlbum&g2_keyword=layout&g2_itemId=38880

It has some alternative Caps noted which I used.

I plugged it in and it's very quiet.

here are my voltages all 2n5088

Q1 E 0.04     Q2 E 0.04     Q3 E 0.05     Q4 E 1.12
      B 0.63          B 0.65          B 0.66          B 1.72
     C 3.93          C 2.81         C 1.28          C 4.07

I searched for some Big Muff voltages and comparing them it seems that Q1 is within range Q2 & Q3 emitter & base both within range, but collector voltages are both low and Q4 seems that both emitter & base voltages are high, but the collector is within range.

Is this just a biasing problem.

It seems from reading about his this layout it may need adjusting to get it to work. I just want to make sure I'm going in the right direction.

Thanks

Matt

[mmaatt25]

Just been comparing this BHPi schematic at Tonepad http://www.tonepad.com/getFile.asp?id=94

The only difference I can see is that there is an extra 100K resistor R13 on the tonepad schematic which is between Q3 base and ground?  I'll try adding this and see what happens!!

[Tonemonger]

Hi ! took me a while to find one of my old BMs and I noticed that no-one had helped you yet, so here goes.
It looks to be the same circuit as mine , yeah you're problem is at Q2- the collector should have about 4volts everything else is almost identical to mine . Q3 is fine . I'll find a clearer diagram and get back to you shortly .sorry.
Come on you other people help out here-It's a matter of life and Big Muff!!!

[mmaatt25]

After putting adding 100k resistor as above Q3 is now E 0.05 B 0.66 C 3.93 so all good with Q3

Q2 is still the problem, I've checked doubled check all resistor values around Q2 and all are correct E = 100r , B = 8K2, 100k & 470k and C = 470k to B

I've tried another couple of 2n5088's voltage is still 2.81 at Collector.

The only values that have been changed are some cap vales as per Torchy's not in his vero layout, I don't know if they would have an effect on Q2 and secondly for C1, C5 & C8 which are all 1uf I've used electrolytic caps as that's all I've got, again I'm not sure if getting the polarity of these would have an effect.

Just to clarify I'm getting an effect through it when hooked up, but when I turn up the volume and sustain it's not smooth Black Keys tone but gating/clipping!!

I tried changing R11 (emitter to Ground) from 100r to 180r, this did not bring up the voltage at the Collector?

Any help welcome.
Any help welcome

[Tonemonger]

O.K. an actual Electro-Harmonix circuit diagram dated Oct 15/1975 drawn by a guy named Howard Davis does'nt have a 100K resistor to ground at the base of Q3 (nor does my build-this has to be the diagram I worked off).
I'm assuming it's inclusion would see a higher voltage at Q3's collector , infact it would mirror Q2 seeing as both stages are otherwise identical.
I'll have to try it myself and see if there is any audible difference.
I have 4.06 volts at Q2's collector.(battery=9.22v)
The caps you listed are 'de-coupling' caps and would'nt affect your voltage readings ,their orientation should be guitar input -/+ to base of Q1 then collector +/- to following stage for all the others.
The stabilizing resistors off the emitters are 100r for Q1,2and 3- raising them would decrease  gain I think ?
Ah ! , I just found your problem - the resistor from B+ to Q2's collector should be 15K !
Take a deep breathe , solder carefully and get ready to rock !
That was fun !

[mmaatt25]

I've checked the tonepad schematic and torchy's vero layout Base to collector is 470K, Collector to V+ is 15K, Base to ground is 100K and Emitter to ground is 100r. This is the same in both!!

Edit. OK found the problem, checked all three electrolytic caps (as per tonemonger's explanation) I had two the reversed, one on the input and the other Q2. read the voltages again and Q2 Collector now 3.93v

A little bit of knowledge is truly a dangerous thing.

Many thanks for all your help.

[Tonemonger]

Right , O.K!  dc wasn't blocked so therefore followed the path of least resistance.
That would make sense ! Pretty hard doing this via text. cool though.

[mmaatt25]

OK it's up and running and this has generated some more questions:

It seems to be really compressing the highs, if i play A Major pent scale the four notes on E&B string you can hardly hear, then it kicks in for the G string all the way through DAE strings?

I had 2N5088 throughout but with sustain up just seemed to warble/wobble, it's smoother with 2N3904, but still have the compressed highs.

In the two diode sections I've used 1n914

All the cap values are the same as the tone pad schematic expect the two 500pf are 470pf (all I had) and also the 4nf cap (c10) closest I could find was 3.3nf.

So how do I get those highs to come through??

Many thanks

[Tonemonger]

Hmm, Not sure. Brain is mush ! ( I have an illness that requires venesections ( blood letting ) so short blood supply=lack of oxygen to brain !
But, you've answered your own questions so far !
Check diode orientation , off board wiring , does the tone control work smoothly through it's full rotation ?
When working properly those slightly smaller cap values would make it very bright .

[petemoore]

  Perhaps an audio probing from right to left [at points along the signal path] would reveal where the HF's are cut, if they're being shunted or otherwise lost at one point.

[mmaatt25]

I keep looking at this and now discovered that its passing a signal even when not connected to power!! Further more the volume pot effects the volume heard through the amp.

I've just made an audio probe, not probed the circuit yet, but if I'm getting a signal with no power  how will I know where the problem is!!

I appreciate these may seem like stupid questions, but I can't work it out.

Many thanks

Matt

[anchovie]

Is the no-power signal exactly like plugging straight into the amp, or different in any way?

Looking at the schematic, with no power supply there is an AC path from input to output via what would normally be the power rail but I would expect the output to be attenuated somewhat.

Audio-probe the circuit with it powered on to find the point where the highs disappear. With power to the transistors, they will become the preferred path for the audio signal.