Help! LPB build (booster) is overdriving like crazy...

[ozzu2000]

Hi!
I built an LPB, but I can't get it to work as expected.
The lpb input is very sensible to the guitar output and will distort very easily.
If I set the LPB volume to maximum and the guitar to minimum, and then increase the guitar until I reach the desired volume, everything is ok and sounds fine.
But, if I have my guitars output set to maximum, it distorts like crasy, regardless of the LPB output volume. If I roll the guitars volume back to half, and increase the lpb output volume, it works fine.

-Tried with 12v and 9v (thanks PRR).
-I'm using the schematic below, with a BC549 instead of the 2N5088 (supposedly is a suitable replacement)
-Double checked values, connections, and BC549 pinout


THANKS!

[gcme93]

Just to clarify, your guitar has fairly low output pickups, like single coils rather than hot Alnicos or humbuckers?

[ozzu2000]

This happens with singles... and gets even worse with passive humbuckers...

[petey twofinger]

whats is the power supply . whats is it built on , vero , perf , breadboard , etc ?

[Mike Burgundy]

Voltages, debugging sticky!

[ozzu2000]

Supply is a 7812 regulator, putting out a constant 12.1v, also tryed with a fresh 9v bat.
Built on a perf board.

Voltages using the 12v supply:
12v = 12.10 v
gnd = 0v

BC549:
c: 3.90
b: 0.89v
e: 0.31v

Perfboard back


Parfboard top


Regulator (part of a valvecaster build)

[Mike Burgundy]

Are you sure you have E and C the right way round, both measuring AND the way you use them in the circuit?
Edit: I see a couple of loose connections and wires that may or may not have an influence on the LPB and/or the other circuit in that box. It may be a good idea to get it working *outside* the other circuit first. Make sure they both do what theyre supposed to do, on their own supplies (battery), then combine.

[ozzu2000]

The other circuit is just sitting there on the box, completely unconnected, the bypass switch is tested on the valvecaster and working. I unsoldered the input and output from the valvecaster and put the LPB in it's place so I could test it independently
I've checked the cbe connections on this build, the transistor is reversed on the BC549 vs the 2N5088.

PS: I checked on another post, and with 9.00V I should be getting about c: 7.12v ; b: 0.6v ; e: 0.07v... this is way different than what I have (c:0.31v b:0.89v e: 3.90v)

[ozzu2000]

Checked all the connections and remeasured the voltages (I had the values of the transistor wrong)

supply: 12.05

c: 4.32
b: .88
e: .29

Shouldn't the collector voltage be much higher?

[Mike Burgundy]

The pics were taken without the supply voltage hooked up to the board, I can see input (Yellow), output (white red cross) and ground (black).
That white wire with a red line, is that power? The tip on that looks like it was a very dry connection. Circuit looks ok, as far as I can decipher it (the board was rotated between pics which makes it a little harder to figure out ;P ). Except for the + connection, 'course.
Anyway, can't see the resistor values. Bias voltage looks ok, check the value of the collector resistor (ok, looks like a 10K, that one I can see) and are its solder connections ok? (reflow with flux to make sure). Could be the transistor overheated when soldering it in, on a circuit this simple I'd say build it again. Be very clean on your soldering, tack the transistor in last and don't overcook it. Test the transistor before putting it in to make absolutely sure the pinout is ok (although I'm pretty certain it'll be cbe) Leave it some more room on its legs, too. If you have some nice tiny needle nose pliers, you can use those as a heatsink to make absolutely certain the transistor lives.

[Mike Burgundy]

Bit of a weird one. I *think* an LPB1 should be able to handle "regular" pups just fine. Any idea what kind of voltage they put out? b and e look to be where I'd expect them, c seems a little bit off although that does differ between transistors. It should be at least working, you still have Volts of swing, so getting very LOUD, loud enough to overdrive anything after it while, when you roll back to a 1:1 volume compared to bypass being clean itself.

[ozzu2000]

Yes, the white and red was the power, unsoldered by the time I took the photo. I used a croc lead in parallel with it to make sure enough power was getting there.
I already did as you sugested, and put the transistor there only after all the other components were soldered so that I'd only had to heat it once.
I refluxed everything and triple checked all values and layout.

I'm starting to suspect this BC549 transistor is not such a good equivalent to the 2N5088...

The humbucker is the unit on a cheap squier strat hss, so nothing too hot. Same for the singles.
I tried to roll back the volume on the LPB (but keeping the guitar volume on max), so that I have the same volume than when bypassed. In this setting I get distortion (the bad kind one!).
If I put the LPB volume on maximum, and roll back the volume on the guitar, so that I have the same volume as above, it keeps clean (if not a bit brighter with the LPB).
(Btw- this amp is an ss with a huge headroom, I'm very sure this is not the amp's distortion)

I've build other much more complex pedals... I'm a bit frustrated that I can't debug this one.
My runoffgroove thor:

[Mike Burgundy]

It shouldn't distort AFAIK (measuring the pups output might yield a clue - the hottest pickup I ever encountered was on a cheap no-name brand ugly thing of a guitar...), the biasing isn't bad. You could decrease the 100k between base and ground to something like 82k to get the collector sitting more in the middle of your supply, but that does not explain the distortion the way you describe it. Reflowed every connection yet?

[ozzu2000]

I'll reflow again all connections incl connectors.
What about the transistor? Is the BC459 a suitable replacement for the 2N5088 without further changes to the circuit?

[Mike Burgundy]

Both general purpose NPN, no design limits are violated, it should work, even with the collector that low. You still have volts of clean swing either way, for a 100mV input. Any idea what your pups put out?
I'd say it's either solder related/mechanical or hot pickups, that's all I can come up with.
Current into base times Hfe equals current into collector. Ib*Hfe=Ic. Higher Hfe means more current into collector - so more voltage drop across the collector resistor. Higher Hfe will yield a lower bias point for the collector - I think you may just have a high Hfe transistor. No biggie.
Edit: if it turns out it is the combination between pickup output and LPB1, try upping the emitter resistor a bit - this will tame the gain a bit, so you'll still have oodles of output but it's a little more resistant to higher inputs. Say 560R.

[killerkev]

Why use a pump to power a LPB? Seems extreme to me. You voltages are good. Only requires a 9 volt power source not 12+. The guitar pick ups play a big part with this circuit. If they are hot pick ups then it will distort much more.

[ozzu2000]

I give up...
Reflowed every end, re-checked, soldered the circuid directly to the 12v supply, soldered the input directly to the input jack, tried values for the 100k R2 as low as 50k, and tried values for the 390ohm R4 as high as 1.5k (this last one mitigated the problem a bit). The pickups are low output, they have problems overdriving almost anything let alone a power booster, something is very wrong here to start with...

This was already a "tweaked to use more readily available values" circuit, this plus a not ideal transistor, plus a not very knowledgeable me with 2 coffees up his head... not a perfect combination.

Will try again some other time, with the exact components that are on the schematic

Thanks to all for the help!

[Mike Burgundy]

The LPB1 is reputed to be a bit dirty. Your description did suggest it is *very* dirty though, which led to the entire search ;P If you want a sparkly clean boost, maybe try something like the MOSfet booster: http://www.muzique.com/schem/mosfet.htm
Happy soldering!