Tube Scream(ish) circuit: Voltage questions

[patrick398]

Just threw together this layout for a Tube Screamer/Fulldrive based circuit i had on my breadboard; i like the clean boost side of Screamers, not so much the distortion so it's a reflection of that.
Anyway, got some issues around the output transistor which i'll figure out tomorrow but right now i'm wondering whether having 7ish volts on pin 7 is normal/expected. All other pins are sitting around half supply as expected and 8 and 4 are 8.6v and 0v respectively but 7 seems high?


No output because there's something fishy happening around the output transistor but if i probe at the base i get signal to that point...but it's way more distorted than it should be, wondering if that's related to pin 7 being high...shot in the dark really.






Note i'm not using the same output section of this schematic but it's based on everything before. Output section is from here:

[PRR]

> 7ish volts on pin 7

Pin 7 follows pins 5 and 6. Where are they sitting?

Some offset is expected because of R7 R8. R8 is not needed for "normal bias". It may be a leftover of a design revision. It may be a deliberate detail to introduce asymmetric clipping.

[patrick398]

Pin 5 and 6 seem normal to me but i have just noticed 3 seems a little low.

Pin 1: 4.12v
Pin 2: 4.82v
Pin 3 : 1.68v
Pin 4: 0.00v
Pin 5: 4.12v
Pin 6: 3.97v
Pin 7: 7.58v
Pin 8: 8.30v

Should alarm bells be ringing?
I'll check the board against the layout again as i can't see any issues in the vero layout myself

[patrick398]

Think i've found the bugger, the 1k between 6 and 7 is actually between 7 and 8. I'll report back when i've replaced

[patrick398]

All working now except for the tone pot. Doesn't seem to have any effect and i feel like i'm missing a lot of high end. It was pretty piercing when i had it on the breadboard, now it sounds dull and lifeless.
I've tried different values at C7 and C8 and tried a B10K as well as the B50K pot i'm using now (i know the tapers wrong)
Changing the values doesn't seem to have much effect either so i'm guessing it's a connection problem somewhere

[duck_arse]

what transistors are you using at the out buffer, and can we see photos of build, please?

[patrick398]

2N5088 on the output, i don't think it's critical, schem just calls for Si BJT and i think the 5088 is what i was using on the breadboard.






This is inside that old signal generator with a couple of other circuits so the wiring is a bit all over the place (this circuit is replacing the bassman pre that was in there previously but the off-board wiring is all dandy.

As i feared i've got continuity between pin 5 and 6...and between 5 and 7 weirdly but no sign of a short

EDIT: Continuity only between those pins when power is connected...say whaaa?

[duck_arse]

pins 5 6 and 7 are all connecting to the tone pot, no? might be a place to look. I see you've corrected the transistor orientation, the layout appears as for a BC54x.

[antonis]

<Continuity only between those pins when power is connected...say whaaa?>
say time for a new 4558..

[patrick398]

pins 5 6 and 7 are all connecting to the tone pot, no? might be a place to look. I see you've corrected the transistor orientation, the layout appears as for a BC54x.

Yeah that's where i've been looking, just disconnected the tone pot but still got continuity. It must be from somewhere else.

<Continuity only between those pins when power is connected...say whaaa?>
say time for a new 4558..

I was using TL022, swapped it for a TL072 just to check, same thing, tried another and there was continuity on basically all the pins so that went in the bin.

Am i destroying all my chips with my electric personality?

[patrick398]

No same thing with 4558

[antonis]

No same thing with 4558

meaning whaaa??

[patrick398]

With the chip in the socket there's continuity between 5 and 6 and 5 and 7 with power to the circuit. Without power there's no continuity.
With the chip out of the socket there's no continuity with or without power

[patrick398]

I should just clarify, the circuit is working fine...Bass, Drive, and Volume pots all working as expected and the sound is good it just needs some top end which i guess the tone control should be providing.

[thermionix]

I wouldn't check continuity with power applied.

[patrick398]

I did start to think that. I started getting continuity all over the place, between 9v and gnd. I guess in a way there is continuity...just a very round-a-bout one.

Still no closer on working out what's wrong with this tone section. Very frustrating 

[antonis]

Lift C8 (1μF) lower leg and R8 (1k) upper leg and short them to verify rest of circuit good operation..
(Distortion w output buffer but w/o tone control..)

[patrick398]

Good shout, it sounds good when doing that! Got more top end and mids through, still some missing but perhaps this is a good place to start.
In theory if i now unsoldered them and soldered the tone pot to each end of the components, bypassing the board, it should work. I might try that just to confirm there's a connection problem somewhere on the board

EDIT: no that won't work will it. Tone 3 goes to pin 6 not 7

[EBK]

Don't I wouldn't check continuity with power applied.

I had to fix that. It was bugging me. (Sorry!)
Carry on.

[patrick398]

Don't I wouldn't check continuity with power applied.

I had to fix that. It was bugging me. (Sorry!)
Carry on.

Why exactly is that? Don't get me wrong, i will blindly follow your advice, but interested to know. Is it that the continuity function sends a voltage between the probes and you might introduce that voltage somewhere it's not wanted...and deadly to some components?