Covington Fireplace Ultra Treble Booster Trace

[Gus]

So it is wired as a variable resistor?
Why would someone want to connect the C to 9VDC when the control is set to 0 ohms.
Did someone work on the effect before you worked on it?

Any other pictures of the PCB on the web of other builds?

[nickbungus]

I don't think this has been modded in any way or played with.

The guy who owns it says it's his favourite treble booster but he clueless to what is inside.

[idy]

Doesn't the schematic show the C attached by 100k to 9v?

[Gus]

Now the way this TB is wired is different it seems if the trace is correct the 10K is a variable resistor in the collector leg in parallel with a 22k?  They could have just used a 10k then the BIAS/GAIN CHANGE would be even greater. 0 to 6.8k vs 0 to 10k

Using a fixed resistor across the outer lugs make no sense to me, adjust the circuit to the potentiometer value you are using.  I will start a thread.

[nickbungus]

I suppose all I can do is make it and see what happens.

Anyway, I've knocked up a schematic and a copy of the layout to test.







Ref Name   Value
C1           22pf
C2           47uF
C3           10n
C4           47uF
C5           1n
C6           47n
C7           4n7
Q1           BC182
R1           4k7
R2           100ohm
R3           100K
R4           1M
R5           120K
R6           1K
R7           22K
R8           2k2
R9           1M
R10          150K
VR1          6k8

[duck_arse]

drop R4 and R9 (1M), they are both being swamped by 120k|150k. we don't really believe in mojo that much, do we? and make sure (make sure) of the bc184 BC182 pinout, it comes in the dreaded "L" variant.

http://pdf.datasheetcatalog.com/datasheet/MicroElectronics/mXuzsyu.pdf

[nickbungus]

I'm getting the feeling that opinion is, the person(s) who designed this didn't really know what they were doing?

I take it R4 is there to eliminate pop from switching?  Duck, what do you mean by 'being swamped'?

[duck_arse]

well, if you parallel 1M and 150k, which dominates the resulting resistance? it comes out a lot closer to 150k than to 1M, so the 150k has "swamped" the 1M.

you check that transistor pinout yet?

[nickbungus]

Thanks Ducksbum.

I think I've got some BC182s.  I was going to socket and see what sounds best/works anyway.

[anotherjim]

Are we absolutely sure the pot has 1&2 common? If true, and it certainly looks that way, how does the original behave? 'cause I'd expect min resistance to not only zero the output but greatly increase current consumption, although the increased voltage drop over the 100R supply filter will turn the Tr toward off - but maybe not enough.
Under DC conditions, there's the 2k2 emitter resistor to limit the current safely, but AC signal present will dump the supply via the collector-emitter to ground thru the 47u emitter bypass -  only the 100R in the supply is limiting that. Surely it's a battery killer?

Honestly, I'd put a minimum 470R (or even 1k) in series with the pot at the collector end for a little current limiting without changing the sound too much. Or the extra R at the supply end if you don't mind it not going to total silence (and when do we actually use a pedals volume knob as an off switch?)

Those extra 1M input and output resistors can be left out as Duck's says. A makers trick to catch out blatantly direct copies?
As if we would!

[nickbungus]

Hi Jim.  Thanks as always for your input.

Blatant copy!  That's exactly what I'm trying to achieve.  The lad I fixed it for really loves his so I said I'd clone it while I had it and make him (and myself) a (blatant) copy!

The pot definitely had 1 & 2 common.  The output is zero when set to min.  I never tested any current consumption.

[teemuk]

Under DC conditions, there's the 2k2 emitter resistor to limit the current safely, but AC signal present will dump the supply via the collector-emitter to ground thru the 47u emitter bypass -  only the 100R in the supply is limiting that. Surely it's a battery killer?

You ignore 47 uF of filtering capacitance post the "R". The circuit will suck up current from the path of least resistance and for a moment this current is limited only by capacitor's internal resistance. 100R only comes in play after some amount of energy is sucked dry from the capacitor and the parallel energy source is no longer swamped by a lower impedance source of energy. I would suspect it's not just a battery killer, it propably kills that transistor on a regular basis too.

The tapering... well, it obviously must work as some sort of "volume" control so overall gain should ideally change in logarithmic manner. Someone can probably calculate better than me how linear vs. "makeshift" logarithmic pot behave as variable collector resistances. Because it's not just a resistive attenuator for output signal, but also affects stage gain, the most ideal  taper function (for -consistent- gain/loudness control throughout the dial's range) is probably somewhat special.

[Cozybuilder]

Nick- While you have it on breadboard, why not simply connect the wiper to C6, and the pot with 22K across 1-3 as you described, with 1 to V+ and 3 to Q1C, using the VR as a 6K8 volume control?

[Gus]

Make VR1 10K forget about the 22K  gives a greater range of the strange gain/bias control.  Remember 6.8k is lower than 10k
Get rid of the 1megs and C1 they are a waste

You can make the collector a fixed 6.8k and change  R10 to a 100KA on top of a 47k, Or try a 100KA or 220KA volume

Or wire it like a RM with a 10KA in the collector leg, wiper as the output with a 3.3k emitter resistor

[anotherjim]

Nick, what was the repair you had to make to it? Was it the transistor?
I understand you want an exact clone, but expect some variation between builds. This is a highly interactive circuit. If the original transistor is good -  try to match it's Hfe for the copies.
Ok, that volume control. It would probably never be used at minimum resistance, so you avoid the potential for damage from that. Hmmm, if you use a chicken head knob, you could fit a pin in the case top to stop the control turning to minimum

[nickbungus]

The repair was very simple, a wire had come away from one of the pot lugs.  That was why I wanted to understand the role of the resistor across the pins.

The pedal would be useless with the pot at minimum as sound doesn't pass.  Sorry, I don't have the pedal anymore to check the Hfe.  I was just going to try a few transistors.  I've got a few BC182s as well as some potential substitutes.  The plan was just to see what sounds best.  Looking at the datasheet the BC182 can have an Hfe within the range of 125-500, so another lesson learned.  I should have checked that.

Question, do you think this pedal has been designed by an amateur? 

I was expecting it to be a 99.9% copy of a schematic easily available online but this seems to have some of the best on here scratching their heads.

[anotherjim]

Possibly amateur or else a pro who doesn't like anyone giving their design a once-over so it got missed. Peer review is a powerful check and a hallmark of a truly professional undertaking IMHO.

I try to resist "this all wrong" rants. If everything was done by the book, we wouldn't have much else of anything.

[Gus]

I would not worry much about the transistor hfe
If you look in another thread I posted a screenshot of a sim

When I tried different transistors in sims the voltages did not change much from 2N2222 to 2N5089 so most to92 etc package NPN Silicon transistors should bias up with not adjustment needed.  I added a bias note to the sim because I often build with an external bias control to change the sound

Note the use of a 2.2k emitter and a 22k base to ground and 101k base to +9 that makes a more stable bias because of the lower resistance values in the voltage divider bias.

[nickbungus]

So I've built my 100% replica and it seems pretty much exactly the same as 'original' I had.  It doesn't quite sound as dirty but I had my amp at a low volume because my boys are asleep.  I might try a few different trannies as an experiment if I get the same results with the amp driven harder (Vox AC4).

I subbed the 47nf for a 56nf (I've got some 47nfs on order so I may change this if anyone thinks it will make a difference).

Interestingly, the pot is crackly when adjusting.  This was also true of the original I had.  I just used a 10K pot and didn't bother with the resistor across the pins to give me 6K8.

I etched another board at the same time so I'll do another build, this time experiment with the suggestions from you guys. 

Any further suggestions are more than welcome too.

[nickbungus]

Just a quick update on this one.  I haven't got round to messing with this too much but my 100% clone (despite the unnecessary resistors and weird pot wiring), sounded amazing once I cranked my VOX AC4.  On my last post I said it didn't sound dirty enough at low volumes but the cranking of the amp made a great difference.

I'd go as far as saying its my favourite treble booster and I have built most of them that are available through the power of Google.